GB2230363A - Tolerance analysis in cad system - Google Patents

Abstract

A system is provided which operates for performing tolerance analyses for mating parts, and generation of tolerance call-outs for fixed and floating fastener features on parts. Following choice of the fasteners, the user designates the positions on a part where the selected fasteners are to be used by moving a cursor over a displayed representation of the part. Hole sizes and positions are computed for the fasteners and mating parts. Upper and lower optimum hole sizes are computed to ensure that bearing surfaces of a bolt head contact the surfaces of a part through which it extends. The system investigates assembly of the parts if the worst case tolerances for part assembly exist at each part. Tool characteristics and material thickness are taken into account in the tolerances analyses. <IMAGE>

Description

COMPUTER INTEGRATED GAGING SYSTEM This invention relates to an inspec:ti##i Luul iui mechanical parts and more particularly, to such a part which utilizes part design data to construct an inspection gage and inspection data to construct a model of the inspected part for comparison with the gage.

According to one aspect of the present invention, a method of analysing data relating to a physical part having known design features and using a computer coupled to a multidimensionally movable position,measuring apparatus for determining the position of physical features of the part comprises the steps of: constructing data representative of an inspection gage using known design features; measuring the positions of corresponding physical features of the part; and, comparing the inspection gage data with the measured part data.

According to another feature of the present invention, a method of investigating tolerance call-outs on mating parts wherein design and tolerance data for the mating parts is available in memory, comprises the steps of: retrieving the design and tolerance data relating to the mating parts from the memory; comparing data representative of each part; and, indicating the results of the comparison.

According. to another feature of the present invention, apparatus for inspecting a physical part having known dimensional features and tolerance call-outs comprises: means for constructing a multidimensional model of an inspection gage using the part dimensional features and tolerance call-outs; means for measuring the part features to be inspected and for providing inspection datz representative thereof; means for constructing a multidimensional model of the inspected part using the inspection data; and, means for comparing the inspection gage model with the inspected part model, whereby compliance with design data tolerances is determined.

According to a yet further feature of the present invention, a computer controlled display for inspection and analysis of predetermined part features on a manufactured part coupled to CAD and tolerance data for the manufactured part comprises: a display; and means for simultaneously displaying a model of the manufactured part and inspection data relative to the part.

The above and other features of the present invention are illustrated, by way of example, in the drawings wherein: Page - 8 - follows Figure 1 is a block diagram showing the component parts of the system of the present invention.

Figure 2 is a flow diagram relating to the computer integrated gaging system of the present invention.

Figure 3 is a perspective view of a model of a manufactured part subject to inspection by the present invention.

Figure 4 is a perspective view of an inspection gage constructed through the use of the present invention.

Figure 5 is a diagram showing inspection path generation as used in the present invention.

Figure 6A is a plan view of the inspection gage of Figure 4.

Figure 6B is a plan view of the manufactured part of Figure 1.

Figure 7 is a flow diagram showing detail of initial portions of the flow diagram of Figure 2.

Figure 8 is a flow diagram showing detail of subsequent portions of the flow diagram of Figure 2.

Figure 9 is another flow diagram showing detail of the latter portions of the flow diagram of Figure 2.

Figure 10 is a data flow diagram of the system of the present invention.

Figure 11 is a chart of re resentative ANSI standard tolerance call-out symbols.

Figure 12 is a perspective view of a manufactured part depicting datums thereon.

Figures 13A - 13C are charts depicting inspection gages and datums for the manufactured part of Figure 12.

Figure 14 is a plan view of a part with a syntactically incorrect part feature call-out.

Figure 15 is a plan view of the part of Figure 14 with another syntactically inappropriate part feature call-out.

Figure 16 is a plan view of mating parts illustrating compatible part feature call-outs.

A short title for the function performed by the system disclosed herein is computer integrated gaging (CIG).

The system of the present invention may be seen with reference to Figure 1 wherein a computer 11, such as the VAXll/780*, is coupled to a display 12, such as the Textronix 4115*. A keyboard 13 is provided for entering information into the system for use by the computer in controlling system operation.

Visual reference for keyboard operation is provided at the display 12. A mechanism or robot 14 for providing three dimensional movement within a prescribed volume is exemplified by the Automatix robot designated AID 800*. A camera 16 is mounted in a known position overlying a working space and is utilized to determine the orientation of a part 17 resting on an underlying support surface 18. A sensor 19 is attached to the robot 14 and is exemplified by the non-contacting inspection (NCI) device shown in Figure 1 as a SELCOM* laser sensor. It should be noted that the position sensing device 19 could consist of a coordinate measuring machine (CMM) or a numerically controlled machine tool adapted with a touch probe. These devices would acquire inspection data by physically contacting various mechanical features on the part 17.

With reference to Figure 2 of the drawings * TRADEMARK the flow diagram depicted there indicates that the initial step in the process defining the invention involves the generation of a functional inspection gage. The manner in which this is accomplished includes the transmittal of computer aided design (CAD) data for a part 17 to the computer 11 as seen in Figure 1 and the subsequent display 20, in perspective as seen in Figure 3, of the designed part together with dimensional and tolerancing information in accordance with geometric dimensioning and tolerancing (GD & ) standards. The standard used for illustration here is U.S. Government designation ANSI Y14.5. There are shown three surface references, A, B and C.

Alternatively, the references may comprise the edge of a part, a point on a part, a hole, etc. As seen in Figure 3, the dimensional call-out is for four holes of one inch diameter plus 0.125 minus 0.0 inches on the display or model 20 of the part 17. This dimensional and tolerance call-out is considered to be a critical and major feature for the illustrated part. The holes are indicated to be located using the method of tolerancing termed wtrue positions (Figure 11) as indicated by the initial symbol in the tolerance block. Other drafting tolerancing methods may be selected such as reference to a surface profile or plus or minus tolerance dimensions. The holes in Figure 3 are required to be positioned so that their centers, as manufactured, will vary -nly within a 0.06 inch diameter circle at maximum material conditions (MMC, smallest holes). If the hole is larger than the MMC size, then the tolerance circle diameter grows in proportion. The true position of each hole is referenced to the three indicated surfaces A, B and C.

The operator of the system observes the ideal design or model 20 of the part 17 on the display 12 and is able to signify to the system through the keyboard 13 any one of several tolerancing conventions which appear on a menu on the screen. In the illustration of Figure 3, the true position tolerancing standard is indicated and a cursor which appears on the display is positioned to indicate the dimensional and tolerance call-out relating to the illustrated four holes in the part 17. The computer receives the dimensional and tolerancing indication.

The tolerance information is inspected through program instructions for syntactic correctness. Once the tolerance syntax is determined to be correct, the computer generates an inspection gage model 21 as seen in Figure 4. The consistency of the part design tolerance symbolism is therefore confirmed. With knowledge of the design description of the part and the tolerances applied to the described features, an inspection or functional gage using the same tolerance references as the part is constructed by the computer and shown on the display. Accomplishment of such a step is indicated at A in Figure 2. The inspection gage data is stored for future use.

The part designer has made certain dimensional and tolerance call-outs for a part to be used in an assembly. Functional gauge data has just been created for the part as described hereinbefore.

The system then performs what is called design tolerance analysis as indicated in Figure 2. The purpose of the tolerance analysis is to determine if the designed part as toleranced will fit under all tolerance conditions with its mating part in the assembly. Details of this portion of the process are illustrated in Figure 7. A selection must be made by the operator to either analyze tolerances assigned to the part by the designer or define new optimal tolerances for the part. If existing tolerances are to be analyzed, then a worst case part is created by the computer wherein the part is in a "virtual" condition (reference ANSI Y 14.5), that is, the holes are all at the lower limit of the tolerances and any bosses, flanges, etc. are at the upper limit of the tolerances.Further, if holes are dimensioned with respect to true position, their size is further reduced by the amount of positional tolerance defined. This method simulates the condition of the holes as if placed, for purposes of the worst case part, at opposite limits of their allowable positioning tolerances.

Once the worst case part (virtual condition, having maximum material conditions and maximum positional deviation) is constructed by the computer, the tolerance call-out datums are aligned with those of the mating part. The mating part is also constructed by the computer in its virtual condition state. The computer checks for compatibility between the part undergoing tolerance analysis and its mating part. If the worst case parts fit, the design data together with the tolerance data is stored for future use. If the worst case parts do not fit, the process is returned to G as seen in Figure 7. The return of the process to this location occurs so that the design may be improved tolerancewise by creating the holes and/or bosses with different nomirzl sizes; i.e., a model geometry change.

As may be seen from Figure 7, if the analyzed existing tolerances do not fit, beginning at G , a new check or modification is made on the Part. In one instance new tolerances are obtained through the system input from the designer. A new tolerance syntax check is made relative to the new tolerances.

Alternatively, for model geometry change the tolerances are analyzed for the resulting model change. A new gage is then built by the computer, presuming tolerance syntax is correct. Analysis of any new set of existing tolerances involves repetition of the process described immediately hereinbefore.

CIGMA may analyze any set of GD and T specified tolerances. However, CIGMA is currently able to define new tolerances by itself for only two special design cases; fixed and floating fastener cases. In these specific instances where parts are fastened together (held in tension), the fixed and floating fastener analysis of Figure 7 is undertaken.

A bolt is an example of a fastener. It may go through a part or it may be threaded into a part. A fastener is generally selected from Federal standard H-28. The designer chooses the fastener based on stress requirements. The Federal standard provides the screw thread standards for the Federal services including body diameter, bearing area of the fastener and thread length. The target hole size is calculated together with the upper and lower tolerance on the hole size and a true position tolerance is then provided for the hole. The datums for the position of the hole are toleranced with flatness, straightness, roundness and cylindricity tolerance call-outs to guarantee that any possible positional error from the datums is below one-tenth of the true position error toleranced.For example, if the hole true position tolerance is .060, any error due to departure from flatness of the datums providing references for the hole position must contribute no more than .006 to the hole position error. This serves to guarantee part interchangeability.

The tolerances are thereafter analyzed by the program to ascertain that there is no diminution of fastener bearing surface by virtue of hole position error relative to the mating part for the part being analyzed. Diminution of fastener bearing surface refers to displacement of the fastener laterally in the hole in the part being analyzed to the extent that the bearing surface under the head of a bolt, for example, lies partially over the hole rather than on the material in the part surrounding the hole. In summary it is seen that optimal tolerance analysis (definition of new tolerances) is only performed by CIGMA for two special cases of GD and T tolerancing, fixed and floating fastener analysis, while worst cast analysis (analyze existing tolerances) is performed by CIGMA for all cases of GD and T tolerancing.

The ensuing step following the performance of design tolerance analysis may be seen from Figure 2 to involve the generation of an inspection path for the three dimensionally movable member which is a part of the robot 14 of Figure 1. The details of the step of generating an inspection path are set out in the flow diagram of Figure 7. The process cannot be entered until it is ascertained that an inspection gage for the part has been built and the analysis of the design tolerances show that the part fits properly with its mating part. After the gage has been built and the tolerance analysis successfully completed, the inspection path graphics are formed held displayed as shown in Figure 5. The x's indicate measurement points along surfaces A, B and C. Three inspection points on each surface A, B and C define the surface.

A probe 22 is shown on the display 12 having a number of tips 22a which are selected to contact one of the inspection points (shown at surface C) of the CAD part model 20. Item 22 of Figure 5 is termed a probe cluster. A probe vector extends from the probe cluster having a probe tip 22a on the free end thereof. A number of displays are available in the system. A probe vector may be caused to move on the cathode ray tube display to each of the inspection points indicated by the x's in Figure 5.

Alternatively, the tip 22a for the probe vector in use is caused to flash on the display. Also available is a display in which the probe cluster 22 moves around the part model 20, placing a probe tip in sequence at each inspection point. The path of the probe tip in whatever display is in use in a particular CIGMA system is a logical progression from one point x to another, considering the shortest distance between points and avoidance of obstacles. The progression is meant to inform the user of the direction from which the probe will physically approach the surface to be inspected and to provide information which may be used to avoid collisions between the probe cluster and the part. Measurements are generated for each specific part feature at each probe contact point.It may also be seen that there are three measurement points associated with each of the four holes in the part 17, whereby each of the holes is fully defined. Having monitored the graphic display of the path, the computer, upon command, forms a path program in accordance with the depicted path on the display. The path program is converted to a program intelligible to the robot 14, and the inspection path data is then stored for future use. The combination of this portion of the process is indicated at B in both Figures 2 and 7.

If a modification of the inspection path is desired, a function is entered through the keyboard 13 and the cursor or vector on the display 12 is controlled by the user. A menu of desired changes in the inspection path is presented to the user who may wish to add an inspection point to a surface, or to reroute the movement of the movable member to avoid an obstacle. In the event an additional inspection point on a surface is to be designated for inspection, such a function is selected, the cursor is moved to the additional inspection point and the program is informed through the keyboard of the addition.In the instance where the path of the movable member is to be altered for purposes of avoiding an obstacle, the indicated function is selected and the cursor is moved to a point or points in succession on the display through which it is now desired that the movable member shall pass to avoid the obstacle. The new points are entered into the path program through the keyboard and the program descriptive of the path of the movable member is thus altered. Following creation and/or modification of the inspection path, the path program may be called up and displayed as the cursor undertakes motion throughout the entire inspection path which is indicative of the motion sequence followed by the movable member on the robot 14.

As seen in Figure 2, following generation of the inspection path and any desired modifications thereto, the next portion of the process relates to job execution. Job execution refers ~ any job which may be performed by CIGMA including cutting parts, performing statistical process control, etc. Jobs may be executed manually by inputs through the keyboard or automatically under the control of the computer.

When automatic control is desired, first the job control language is defined as hereinafter described.

Thereafter, job execution is simulated by a display on a screen. All steps to be run are simulated on the display. Auto job control is then called by the operator subsequent to a determination that the job simulation is acceptable. The criteria for acceptance is that all analysis runs are correct, with zero deviation from perfection. The operator makes the determination to call auto job control by referring to a menu on the system display called 'run jobw from which he chooses either manual or automatic.

With reference once again to Figure 2 the next undertaking in the process of the present invention is to measure data from the manufactured part 17. Measurement of the physical features of the manufactured part 17 is only undertaken after the inspection gage has been built and the inspection path has been generated as described hereinbefore and as seen in Figure 8. Further, a determination is made as to whether the job is to be executed manually by the operator or automatically by the system, as also hereinbefore described. In the event automatic job control is implemented, the stored job control program is called up as indicated at E (Figure 8) and the process continues under control of the computer.

Otherwise, the subsequent functions are sequenced manually by keyboard selection of various menu items on the part of the operator.

The orientation of the part 17 on the support surface 18 is sensed by the camera 16 attached in a known location over the working volume. The part orientation is used to orient the inspection path as it is to be carried out by the movable member. The movable member is moved along the oriented inspection path by operation of the robot 14. Position data for the physical features of interest on the manufactured part 17 are obtained by the sensor 19 (NCI or CMM) attached to the robot and the measured data is converted to a form which may be brought up visually as the model 17a of the manufactured part on the display 12. The measured model 17a of the manufactured part 17 is then placed in storage for future use.This is indicated in Figures 2 and 8 at C As indicated in Figure 2 following C , the measured data for the manufactured part 17 (used to construct the measured model 17a) is analyzed statistically, as will hereinafter be described in detail in conjunction with Figure 9 and a determination is made either by the operator or by the job control program (whichever is in control) as to whether the measurement data will be analyzed relative to the inspection gage constructed at A or from the standpoint of the measurement history of the population of those parts, or both. In the case of reference to the measurement history of the population of those parts, a statistical analysis of the measurement is performed and a determination is made from the measurement as to whether the process is in control, as hereinafter described. An out of control process is stopped and the reason for the statistical aberration is identified. In the case of analysis relative to the constructed inspection gage, the measurement data is compared to the inspection gage 21 of Figure 4. Both analysis relative to the functional or inspection gage and statistical analysis of the measurements proceed simultaneously. As seen in Figure 9, data representing the inspection gage 21, the inspection path between inspection points of Figure 5, and the measurements from the parts 17 must be complete before the comparison step or the statistical survey may be undertaken. Statistical data from the process is updated considering the measured part data. This updates the parts fabrication history. The type of analysis to be undertaken, gage or statistical, is decided by the operator or otherwise in the job control language.

When gage analysis is selected, the inspection gage 21 and the measured manufactured part 17a data are called up and compared graphically on display 12 as well as mathematically in the computer 11. The gage 21 of Figure 6A is generally shown in the color green on the display and the model 17a of the manufactured part 17 as measured may be brought up on the display in the color cyan (light blue). The gage and manufactured part models are then caused to overlie so that a visual depiction of the manufactured part in comparison with the gage is shown. Mathematical analysis also takes place. The colored visual picture comparison is only for the visual comfort of the operator and for verification. It may readily be determined visually if the gage and the part have any intersecting surfaces, because of the different colors assigned to each.However, it is the mathematical comparison results generated by the computer which are subsequently used and are at this time stored as indicated at D . The comparison results are held for availability to other systems which may function in conjunction with the disclosed integrated gaging system.

The comparison results are then formulated in the form of an error report as seen in Figure 9. The error report is then called up on the graphic display 12. If there are no errors, a green light is illuminated to indicate that the manufactured part is within tolerance. If there are out of tolerance measurements, they are investigated to see if they can be reworked, so that the manufactured part may be saved. This is done in the illustrated instant by graphically enlarging the holes to their largest allowable size (least material condition) and comparing once again the reworked holes on the manufactured part model to the gage 21. If the gage fits the part, a yellow light is illuminated which indicates that the part is reworkable.If the gage does not fit the reworked model of the manufactured part, a red light is illuminated indicating that the manufactured part should be scrapped as not reworkable.

When statistical analysis is selected, the statistical history of a measured dimension of a specified part is reviewed. A constant monitor is provided for measured dimensional quantities for statistical purposes. The last entered part measurement is reviewed to determined if the process is in control. That means a determination is made as to whether the measurement is included within the area defined under a normal distribution bell curve and within plus or minus three standard deviations (plus or minus three sigma) from the mean of the normal distribution. If the last measured dimensional quantity is within the plus or minus three sigma limits of the normal distribution, the program returns to measure further data from the part.If a maverick point occurs falling outside the plus or minus three sigma limits defined under the bell cure, the process is stopped and the statistical ranges for that measurement quantity are displayed. The cause of the error is thus determined by analyzing trends in the statistical process history. The process is then repaired so that maverick points are less likely to occur.

The following is an abbreviated program listing depicting one manner in which a program may be formulated for operating the disclosed system in performing the disclosed gage and inspection module processes.

With reference now to the data flow diagram of Figure 10 of the drawings, structure is shown in which the CIGMA modules execute. The user or operator interacts with the CIGMA system through one or more input/output devices as represented by the user I/O device 30 in Figure 10. This device can be any interactive graphics terminal which can display, manipulate and identify three dimensional wire frame images as well as alpha numeric text. The drivers for the I/O devices and associated software routines are provided by a CAD data base generator shown at 31. A CIGMA I/O processor, seen at 32 in Figure 10, is a further link in the interaction between a user and CIGMA. Interaction between a user and CIGMA is provided by the CAD data base generator 31 of Figure 10, as well.Such interaction is exemplified by I/O processor performance of the functions of selecting individual CIGMA modules, entering numbers through a keyboard or by picking or selecting geometry, needed as input for the creation of a gage. The CAD data base generator capabilities are actually used in creating the gage graphics, but the CIGMA I/O processor creates the commands to activate appropriate CIGMA routines and to retrieve data from the data base. An interface specification is provided to the CAD vendor who uses it to write subroutines which allow the CAD data base generator to plug directly into CIGMA. The routines resulting from the interface specification provide the means by which CIGMA gets data from the user and by which it presents information back to the user.

Information (data) is exchanged throughout the CIGMA system in one of the following ways as seen in Figure 10.

1. Intermodule communication is achieved through the CIGMA main data base 33.

2. Inspection, analysis and statistical results are written to and read from the input/output files 34.

3. Positional data is sent to and read from various electro-mechanical inspection devices, such as coordinate measuring machines, vision systems, numerical control machine tools and laser range finding devices represented at 35 in Figure 10.

The CAD data base generator 31 of Figure 10, is essential to many of the CIGMA operations providing 3D CAD geometry as input. In addition, many of the CIGMA operations create and display 3D CAD geometry as output. CIGMA was designed so that a CAD data base generator (i.e., *Anvil-4000, *Unigraphics, *CADAM) could be 'plugged' into the system. The CAD data base generator allows the user to generate basic 3D geometry and allows CIGMA to use the intrinsic CAD functions to create and display CAD geometry as needed. Through the CAD system, CIGMA performs the basic I/O functions of terminal display driving, menu display and data entry. Since CIGMA operates using many of the capabilities of the CAD system, users interacting with CIGMA may not realize at any point in time whether they are operating the CAD vendor's software or they are executing the CIGMA software.

A more detailed description of each of the five CIGMA modules hereinbefore described will now be undertaken. The data flow diagram of Figure 10 shows the five modules as follows: the gage module 36, the inspection module 37, the analysis module 38, the job control module 39, and the tolerance module 40.

Description will hereinafter proceed for each module including: 1. The module inputs.

2. How the module works, what it does, and what algorithms are used.

3. The module outputs.

A description of the gage module 36 of Figure * Trademarks 10 begins with reference to the inputs for the module. Inputs include drafting notes and three dimensional geometry. The drafting notes are those depicted in Figure 11 which is a chart taken from the American National Standard for Dimensioning and Tolerancing, ANSI Y14.5M, together with plus and minus dimensioning. The plus and minus dimensioning is considered to be all dimensioning outside of the geometric tolerancing of ANSI Y14.5M. The three dimensional geometry is obtained from the CAD data base generator 31 plugged into the CIGMA system.

The gage module 36, besides asking for three dimensional geometric information from the CAD data base 31, also asks for drafting note information from the data base for the purpose of establishing tolerances. ~ The CIGMA software asks for the information in a specific sequence as represented by a menu displayed to the user. The menu prompts the user to initially define the datums on the three dimensional geometric display of the part to be dimensioned. Datum definition involves assigning a symbol to the datum (plane, hole, etc.) and then identifying the datum feature by designating the feature for the program; i.e., identifying the edges and the location of a datum plane. CIGMA understands the ANSI Y14.5M drafting text.Therefore, the datums are further defined by the four form characteristics (straightness, flatness, circularity or roundness, and cylindricity) seen in Figure 11. The tolerances on the form characteristics assigned to the datums, as hereinbefore described, must never be allowed to be greater than about ten percent of the tolerance allowed on the other part features which are referred to the datums. For example, if a location tolerance of another part feature is 0.006, the flatness tolerance of a plane used as a datum must be no more than 0.0006.

CIGMA understands all of the other drafting text of Figure 11 which may be assigned to the various part features. Simultaneously with the input of drafting text to the CIGMA system, syntax checks are taking place, definitive examples of which will be presented hereinafter.

Profile tolerances, orientation tolerances, location tolerances and runout tolerances (Fig. 11) are all determined with respect to one or more datums. When specifying these tolerances one or more datums need to be referenced in the feature control symbol. An example of a feature control symbol as it appears on a drawing depicting a part is as follows: 0 0 .060 M A B C The foregoing is expressed to the CIGMA system by the user as: TP, CZ .060 M, A, B, C. In this example, the positional tolerance of .060 must be considered with respect to three datums, A, B and C. The datums referenced by these feature control symbols, serve to define the functional requirement of the features being controlled. This means that the degrees of freedom of the controlled feature are defined.

Examples of the application of datums to a part having certain controlled features may be seen. with reference to the controlled part 41 of Figure 12. A number of datums are depicted in Figure 12 as shown on the part 41 designated A through E, and a number of part features are also shown. Part 41 has a rectangular solid base with similar length and width dimensions and a smaller height dimension. The upper surface 42 of the base is designated datum A. Part 41 also has four similar bosses 43 extending upwardly from datum A and a fifth boss 44 designated datum E. One vertical side of the base is designated datum C as shown.

Another vertical side is designated datum B as shown.

A hole 46 centrally located in the base 42 is designated datum D.

Figures 13A, B and C are chart diagrams of gages which the CIGMA system can construct for checking various features of the part 41 of Figure 12. Each of the Figures 13A - C has four columns a, b, c and d and four horizontal rows e, f, g and h. It may be seen that if a gage seen at Figure 13A, a, e for the four bosses 43 of the part 41 is constructed using only datum A in the geometric tolerances, then the gage will have four holes 47 and the dimension will have three remaining degrees of freedom in the X translational direction (XTT), the Y translational direction (YTT), and the Z rotational direction (ZTR). Reference only to the A datum does not tie the pattern bosses 43 down in the X or Y translational directions, nor in the Z rotational direction.The remaining gages of Figures 13A, B and C have indications of the datums applied to the geometric dimensioning of the part 41, and indicate the remaining degrees of freedom as a result of that geometric dimensioning. Sometimes the datum indications also contain modifying symbols such as M (maximum material condition) and S (regardless of feature size) which have an effect on the remaining degrees of freedom as will hereinafter be described.

The CIGMA system automatically determines what the functional requirement of a given set of features are. The system then displays this functionality by generating a three dimensional model of the worst case mating part, sometimes called a functional gage, a number of which are seen in Figures 13A, B and C. The CIGMA system determines the underlying functionality for any set of datums and modifying symbols in a particular order or precedence by applying the following rules.

With reference to Figure 13A, gages are shown for inspection of various features of the part 41 of Figure 12 where the datum referenced for geometric tolerancing is a plane. If the datum referenced is the primary datum, the CIGMA system forces three points of contact between this datum and the mating part. If the datum referenced is the secondary datum, the CIGMA system forces two points of contact between this datum and the mating part. If the datum referenced is the tertiary datum, the CIGMA system forces one point of contact between this datum and the mating part. The primary, secondary and tertiary datums are the first, second and third datum symbols respectively to appear in the feature control block.

They appear in the right-hand end of the feature control block as seen in the gage charts of Figures 13A through 13C.

If the datum referenced in the feature control block is a datum feature of size, such as a hole or a boss, then the gages of Figures 13B and 13C apply. If the material condition referenced is at maximum material condition (MMC or M ) as seen in Figure 13B, then if the datum is the primary datum, the CIGMA system forces the axis of the mating part to be parallel to the axis of this datum in three dimensions. If the datum is the secondary datum, then the CIGMA system forces the mating part feature to fall within this datum if the datum is a hole or to totally surround the datum if the datum is a boss.

Similarly, if the datum is a tertiary datum then the CIGMA system forces the mating part feature to fall within this datum if the datum is a hole, or to totally surround the datum if the datum is the boss.

Alternatively if the material condition referenced is at 1regardless of feature size (RFS or S as seen in Figure 13C) then if the reference datum is the primary datum, the CIGMA system forces the axis of the mating part to be parallel to the axis of this datum in three dimensions, and prevents the mating feature from translating within this datum. In other words, the mating feature is simulated by a tapered pin on an axial compression spring which forces the mating feature to take up space between itself and the datum. This may be seen in Figure 13C where a tapered pin 48 is shown constructed on the gage depictions when the datum D (centrally located hole 46) of the part 41 of Figure 12 is used in the feature control block. This may be contrasted with the boss 49 shown on the gages of Figure 13B where the MMC symbol M is used.

Remaining with Figure 13C wherein the RFS or S call-out is used, if the datum to which the material condition applies is the secondary datum, then the CIGMA system forces the mating part feature to fall within the datum if the datum is a hole or to totally surround the datum is a boss. As explained for the primary datum in this case hereinbefore, this prevents the mating feature on the gage (Fig. 13C) from translating within the datum, in this case datum D on part 41 of Figure 12. In like fashion, if the datum to which regardless of feature size condition applies is the tertiary datum, then the CIGMA system forces the mating part feature to fall within the datum if the datum is a hole or to totally surround the datum if the datum is a boss.As with the primary and secondary datums, such a material condition assigned to a tertiary datum prevents the mating gage feature, tapered pin 48 on the gages of Figure 13C, from translating within the datum, D on the part 41 of Figure 12 in this example.

When datums are referenced in a feature control block, the foregoing rules can be applied to determine the precise functionality of the mating part with respect to the features being controlled. If the primary datum is a plane, then if the feature control block appears as true position, diameter, .060 M A, where datum A is a plane, then physically this plane A controls the orientation of the mating part. What that means is, the mating part must make contact on the three high points of the plane A referenced as the primary datum, and the mating surface will be allowed to translate and rotate, but will be constrained to remain coplanar to the datum surface A.An example of such a dimensioning result may be seen in Figure 13A,g,a which depicts a mating part (gage in this instance) for the part 41 and which may translate in the XT and YT directions and may rotate about the ZTX. It may also be seen that for a feature control block call-out of true position, diameter .060 datum A, where A is a plane, the mating part or gage of Figure 13A,e,a applies which allows translation of the mating part relative to the part 41 of Figure 12 in the XT and YT directions and rotation about the ZT access. Mathematically the datum A reduces the amount of allowable motion from a totally uncontrolled motion (three directions of plus and minus translation and three directions of plus and minus rotation) to three degrees of freedom, translation along XT and YT and rotation about ZT.

If the primary datum is a hole or a boss, the symbols M or S are used as hereinbefore described. If the material condition specified on the primary datum is at MMC ( M ) the feature control block might look as follows: 0 0 .060 M D In the foregoing the hole 46, shown as datum D in Figure 12, physically controls the orientation of the mating part for the part 41. The axis of the mating part is forced to be parallel to the axis of the datum hole D. Similarly, the pin 49 on the gage of Figure 13B,e,a is forced to be parallel to the axis of the datum hole D. Once the datum and the mating part (or the gage) are oriented correctly, the mating features are allowed to translate and rotate within the datums where they are holes such as D, and to surround the datums and translate and rotate around the datums where they are bosses.Geometrically the mating part is allowed to translate along the XT and YT axes, and to rotate about the ZT axis as seen in Figure 13B,f,a, for example. However, the mating part is always held within the datum for datum holes (D) or is always held surrounding the datum for datum bosses (E).

Mathematically datum D reduces the amount of allowable motion from six degrees of freedom, affording no control at all, to three degrees of freedom.

Additionally, a datum hole or boss lunits the amount of XT and YT translation by the amount of the deviation between the datum feature and the mating part feature.

If the material condition on the primary datum is at RFS, seen as S , then the feature control block would appear as 0 0 .060 S D where D is a hole or boss. In the examples set forth herein, datum D is a hole as seen in Figure 12.

Physically the hole 46 controls the orientation of the mating part by forcing the axis of the mating part to be parallel to the axis of the datum hole (or boss).

Once the datum and the mating part are oriented correctly, the mating feature is only allowed to rotate about the axis established by the datum. No translation is allowed as with the MMC modifier M hereinbefore described. This is illustrated in Figure 13C,e,a wherein rotation about the ZT axis only is allowed. Thus, the RFS feature specification, in this instance, mathematically reduces the amount of allowable motion between the part 41 and its mating part from an uncontrolled six degrees of freedom condition to a single degree of freedom condition ZTR.

The CIGMA system checks the syntax of the ANSI standard dimension call-outs as mentioned hereinbefore. Referring to Figure 14, a machined part 54 is depicted having an array of 7 holes indicated at 56. As seen in Figure 14 the datum B is a lip or boss on the part 54. Referring to the box call-out, it may be seen that datum B is not modified by either a maximum material condition M or a regardless of feature size S symbol. This is error since the condition of the boss is not defined completely without one such call-out and it cannot therefore be a useful datum. The same would hold if B was a datum hole. The box must therefore appear as 0 00 M D B S F . The system recognizes the error, indicates it on the system display and prompts the user to correct the tolerance call-out to appear in the aforementioned proper form.

With reference to Figure 15, an ANSI standard call-out is shown for two threaded holes 57 in the part 54 wherein the hole diameters are toleranced at maximum material conditions. While this is not outright error from the standpoint of the tolerance standards, the holes are threaded and the M call-out would require measurement of the thread peaks and the thread roots for conformance. This is clearly impractical from both a measurement and a use standpoint. The generated system inspection gage will not recognize the maximum material call-out. All that is needed is proper positioning of the fixed fastener which will engage the threads. The system therefore displays a warning that the system inspection gage will be generated at 'regardless of feature sized and prompts the user to substitute S for M at the hole diameter true position tolerance.

The gages of Figures 13A - C are shown on the system display with an XYZ coordinate system and only depict the controlled features on the part they are constructed to mathematically inspect. That is why the relatively simple call-out which references only datum A for the part 41 of Figure 12 causes the CIGMA system to construct a relatively simple gage as seen in Figure 13A,e,a. The gage just mentioned consists only of four holes 47 in the datum plane A . It may be seen that the more restrictive call-out of Figure 13A,h,d uses as datums the plane A, the hole D and the boss E of the part 41 in Figure 12. Therefore, the holes 47 appear together with the tapered pin 48 (because the datum D is modified by the RFS symbol S ), and a tapered hole 51 (because the datum boss E is also modified by the RFS symbol).The gage for feature control call-outs for primary, secondary and tertiary planes A, B and C is shown in Figure 13A,f,b, wherein flanges 52 and 53 are provided on the gage for forced contact with datums C and B respectively. A coordinate system is also displayed with each of the gages of Figures 13A - C displaying the three axes along which translation and about which rotation may be made in accordance with remaining degrees of freedom "r) after tolcranc#ng.

A description of the functions of the inspection module 37 of Figure 10 will now be undertaken. The inspection gages of Figures 13A - 13 C are stored in the computer as hereinbefore described in conjunction with the description of the gage module 36. A three dimensional CAD presentation of the part to be inspected is also resident in the computer. The computer is aware of the part shape so that it may generate a convenient inspection path. The sensor configuration (probe array) will depend on the shape of the part. The cluster 22 of Figure 5 uses standard hardware obtained from Renishaw Corporation. One type of probe 22a is a shank with a ruby tip. The sensor is pressure sensitive and is moved from point to point about the part being inspected on the robot ram.

The CIGMA software now goes into an inspection path definition. There exists two options for the definition of the inspection path. In the first option, the previously defined critical and major features on the part to be inspected as represented by the stored inspection gage are used.

The inspection gage, as hereinbefore described, uses the GD and T call-outs from the part drawing as they exist in the CAD presentation of the part in the computer. The software picks an appropriate tip in the cluster 22 (Figure 5) and creates a logical path in three dimensions for inspection of the features required. The required features are those which are envisioned as critical and major in the inspection gage. Thus, in this option the inspection gage models are used to determine the inspection path.

In an alternate option for defining the inspection path, the user or operator picks the part feature to be inspected. The software, having knowledge of the cluster configuration, then designates the appropriate probe tip 22a in the cluster 22 to be used for inspection of that part feature and creates the inspection path with reference to the CAD model contained in the computer. At this point five physical part features may be selected by the user in the user definable mode of inspection path definition; i.e., threaded features, bores, bosses, planar surfaces and edges.

The inspection path may be modified in a number of ways. The user may indicate the portion of the path to be modified on the CRT screen for the system and enter new coordinates for any such path point through the system keyboard. Alternatively, a new point or coordinate may be added in the inspection path by positioning the cursor on the CRT face at the new point and entering it through a keyboard election. Additionally, inspection path points may be deleted by designating the point to be deleted by the cursor on the CRT face and electing deletion at the keyboard. Modification may also be made to the inspection path with regard to approach distance'.

Every contact between a probe 22a and a part involves appropriate positioning of the probe at a nominal distance from the inspection point known as the 'approach distance. After inspection the probe 22a is withdrawn through what is called a retract distance. Both of these distances may be altered by selection at the keyboard to thereby modify the inspection path manually.

Now that the inspection path is defined, the CIGMA software enters the inspection path orientation process. The location of the part is within certain bounds called the machine envelope. Some approximate predetermined orientation of the part is required within the machine envelope as depicted on the CRT screen so that the part is in an orientation approximately known. The probe cluster is moved to touch the part on certain easily reached known features of the part while the part is in such an orientation. Examples of such feature combinations which will provide orientation identification are any three planes, a plane and two holes, a plane and a cylinder with a known axis orientation, etc.

Following the orientation process for the inspection path, a calibration process for the probe cluster is entered. It may be imagined that the probe cluster itself is constructed with certain tolerances on the actual location of the probe tips 22a relative to the cluster body 22. A calibration artifact is located on the inspection machine bed. The dimensions of the calibration artifact are known precisely. The probe cluster is brought over to the artifact by the machine and each probe tip is brought into contact with the artifact. With knowledge of the dimensions of the calibration artifact and the measurements as sensed by the cluster, errors are identified and compensation values are stored for subsequent application to actual inspection results.

Description of the job control module 39 seen in Figure 10 will now be undertaken. The job control portion of CIGMA defines sequentially the steps which are desired for a specific job prior to any job execution. First the CIGMA system is informed of the identity of a certain kind of a machine which will be attached to the system. For example, a Cincinnati numerically controlled milling machine may be attached.

ATTACH command is illustrative of job control language utilized in the system. The ATTACH command is used to connect the CIGMA system to the specified CMM or DNC machine. When the ATTACH command is encountered in the JOB file, the specified machine is first 'connected' to the CIGMA system. The device name used in the computer allocation procedure must be defined by the logical name wCIG MACHINE . This is done externally to the job. For example, the LOGIN.COM procedure file might contain the following command: ASSIGN TXC3: CIG MACHINE. Some operator instructions are given at the time the 'ATTACH' is performed. These instructions are machine type dependent. When the requested actions are completed, then the job execution continues.If the machine cannot be successfully attached, then the job execution terminates. The following illustrates job control language used in conjunction with the ATTACH command.

FORMAT: ATTACH [machine typei PARAMETERS: [machine typel The machine type specified in the ATTACH command may be one of the following: o CINCINNATI for cincinnati milicron 5VC machines o DEA for the DEA CMM machine o AUTOMATIX for the AUTOMATIX laser robot CMM o SIMULATE for testing and debugging JOBS.

The simulate machine prompts for data to simulate measured data collected from a machine. This is useful for quality testing the software.

o ECHO for testing JOBS. The ECHO machine echos back a perfect measurement. Useful for verifying that a job will run correctly when a part is made correctly.

n WALDRICH for the WALDRICH CORrJP5 machine.

QUALIFIERS: /TOOL NUMBER=nnnn /TOOL ~ NUMBER specifies the tool number to be selected during the ATTACH. If supplied, the requested tool is loaded into the SPINDLE when the machine is first attached. This may be useful if the NC data file does not contain a tool change or if the CAD model NC tool path does not specify a TOOL to the post processor. This option is for use on DNC/CMM or DNC machines only. It is ignored on all other machines.

RELATED OPERATIONS: An ATTACH must be used before any DNC or CMM type command can be used. If it is not used, then a system related error will be reported. The DISCONNECT command may be used to free the device for use by another process.

EXAMPLE: ATTACH/TOOL=99 99 SIMULATE This information with regard to the machine type attached to the system serves as a ~wake upw for the system. The system then executes the calibration process described in conjunction with the inspection module 37.

CALIBRATE is also illustrative of the system job control language. CALIBRATE is used to measure the actual geometry of a probe cluster 22 prior to use. The system requires that all probe tips 22a be calibrated before they are used to measure a part. If the exact probe geometry is known, and the design of the probe cluster in the system is exact, or if testing is desired, then a probe may be calibrated to the design values contained in the CAD system. If a previous calibration is to be used, the calibration results may be read in from a data file. The calibration table is defined as the vector from the cluster reference point to the center of the ball tip contained on each probe. The following illustrates job control language used in conjunction with the CALIBRATE command.

FORMAT: CALIBRATE processnumber CALIBRATE/DESIGN CALIBRATE/FILE= [ filename ] CALIBRATE process number PARAMETERS In this form of the CALIBRATE command, the process number to use is given as a parameter. This form is used when an actual CLUSTER calibration is to be performed. Note that the use of FILE, DESIGN and any other qualifier is not allowed (i.e., there are three different forms of the calibrate command.) QUALIFIERS: /OUTPUT ~ FILE=tfile~ name] The calibration results are stored in the file specified.

This file may be read in later by the CIGMA system to calibrate a probe rather than using machine time to calibrate the probe.

/MAXTIPERR= [ real value ] . The MAXTIPERR value is used to control how far from design the top of each probe calibrated may be. Each tip location relative to the design location is checked to see if it is within this MAXTIP ERR of the design location. If the error exceeds this value, the CIGMA system terminates with an error. If the MAXTIP ERR qualifier is not specified, or if the value specified is 0.0, then no check is made.

/MAXRADERR= [ real ~value]. The MAXRAD ERR value is used to control how far off of design the radius of the ball tip may be from the design value. If this value is not specified, then no checks are made.

/MAX VARIATION. This is used to control what the maximum deviation of computed probe tips can be. The calibration process generates five points around a sphere 10 (Figure 1) to calibrate each tip. This results in five computed diameters for each sphere tip.

These values are averaged. If the deviation from the average for any probe tip exceeds MAX~VARIATION, then CIGMA terminates with an error message. /TOOL= [ tool number] If /TOOL is given, then the specified tool is loaded into the spindle before the calibration process is executed.

CALIBRATE/DESIGN. There are no other parameters or qualifiers used with this form of the CALIBRATE command. This command specifies that the design of the PROBE CLUSTER is to be used to calibrate the cluster.

CALIBRATE/FILE= [ file ~name ] . There are no other parameters or qualifiers used with this form of the CALIBRATE command. This command specifies that the calibration is to be read in from a calibration file. NOTE: the probe cluster name is contained in the calibration file, and must match the probe cluster that is to be used in the operation of the machine during the inspection operations that follow.

RELATED OPERATIONS: The ORIENT and INSPECT commands rely on the cluster calibration. If an INSPECT or ORIENT is attempted with an uncalibrated probe, an error message is given and the CIGMA system terminates. If an INSPECT or ORIENT uses a different CLUSTER than the CLUSTER that was calibrated previously, then an error message is generated, and the CIGMA system terminates.

EXAMPLE: CALIBRATE/DESIGN CALIBRATE/FILE=STAR CLUSTER.CAS CALIBRATE/OUTPUT=START CLUSTER. CAL/MAXTIP ERR=.0001 901 Following execution of the calibrate command in job control, a point is found on the CAD model stored in the computer by aligning the cursor crosshairs manually on the desired point on the CAD model. A corner is a useful point for manual designation because it is easier to align the cursor accurately thereon. The CAD depiction of the srittio of the part for which the job control sequence is being generated is shown on the CRT.

Thereafter, the orientation process described in conjunction with the inspection module is run. The orientation process may be for alternate uses. The job may relate to machining new features on a part or to inspecting machined features. It is possible to perform either of these functions from the initially defined datums. Moreover, in some instances it may be desirable to machine new features followed immediately by inspection of the newly machined features from the aforementioned datums. in this fashion a part may be literally built step-by-step and inspected step-by-step with reference to the datums contained in the CAD model and the inspection gages hereinbefore described.

Having run the inspection process in a step wise make - inspect manner or for the entire manufactured part all at once, or any combination thereof, job control now turns to analysis of the inspection results. Analysis proceeds for simulation in the fashion to be described hereinafter for the analysis module 38. subsequent to the analysis step in the job control definition a command is given to detach the machine and the system is turned off.

Other functions are sprinkled throughout the generation of the job control sequence which may be required during any specific job. Certain displays may be provided for specific purposes during the running of a job. Operator messages may be provided which are specific to that job. When all of the foregoing is accomplished including the other or special functions for a particular job, that job control is simulated by executing the job sequence in a fashion so that it may be observed by the operator who has just generated the job control sequence. When the operator is satisfied through observation of the sequence, job control may thereafter be called up by the operator at will.

In the actual performance of job control in the shop the identification of the attached machine provides information to the CIGMA software with regard to the tools available and/or the inspection devices available. The operator then selects ~Run the jobw and the calibration process is entered for the cluster probes as hereinbefore described. The designated point for orientation on the part after it has been approximately oriented in accordance with the CRT depiction of the part flashes on the CRT screen and the operator goes to that corresponding point on the part manually with the probe. Run orientation is entered and the CIGMA software takes control back from the operator.The predetermined part features as designated by the job control are thereafter manufactured on the part if that is included in this job control and/or the inspection of those manufactured features ensues. The results of the inspection are taken into the computer data, and analysis, to be hereinafter described, is run by the analysis module 38 of Figure 10. At the end of the job control sequence the command to detach the machine is entered and the process is turned off.

The analysis module 38 of Figure 10 to which reference was made hereinbefore will now be described. Two functions are performed by the analysis module, gage analysis and -statistical process control (SPC) analysis. These analyses may be provided simultaneously or separately by the system.

Gage analysis will be described wherein the query is 'Is this part alright?". The gages that apply are designated by the job control routine. The gages are placed on the part as constructed by the inspection results and the system attempts to fit the gages through the allowable degrees of gage freedom to the inspected part. If the gage fits, inspection is complete. If the gage does not fit, analysis is undertaken for rework capability. If it is determined, as hereinbefore described that rework is possible, the manner in which such rework may be undertaken is communicated to the operator. If the gage does not fit, no rework is possible and the machine is detached and that job is shut down.

With regard to statistical process control analysis, the query is "Is the machine tool making parts the way they were made in the past when they were acceptable?". A record of inspection quantities for each inspected feature on each part is kept in the system file. This record provides a distribution which is contained within the defined part tolerances for the population which has been inspected. This population is used as a reference for the same features inspected on parts thereafter. A normal distribution, within which plus or minus three sigma is acceptable (99.7% of the population), is thereby defined within the defined part tolerances. When one inspected feature goes outside the plus or minus three sigma range (3 out of 1,000), an out of control flag comes up for that process.This occurs even though the part may still be within the part feature tolerances. An investigation is immediately entered.

Possible causes of the maverick point outside the plus or minus three sigma range may be due to a number of causes. These causes include a new operator, a loose fixture, bad/wrong materials, a worn out tool, etc.

Something is changed to correct the out of control condition. About five parts are made by the process thereafter and if all are good, the process is considered to be back in control and is continued. If one or more of the five parts are bad, the investigation is continued.

When an out of control process indication is made, the operator can recall some depiction of the historical data. He may call up a run chart which shows how that specific manufactured feature is appearing as a result of the inspection process or he may call up what is called a X-Bar Chart which is a depiction of the mean of the inspection samples.

Alternatively, an R-Chart may be called up which depicts the range of inspection points for that feature in that run. With this information the operator is better equipped to designate one of the potential sources hereinbefore mentioned for the out of control condition. Thus, an intelligent means is provided for making the aforementioned change to the process prior to running the five part sample to determine if the process is back in control.

The tolerance module 40 of Figure 10 will now be described. The tolerance module is written in the CIGMA system for use by design engineers as opposed to quality control or process engineers. Two separate functions are performed by the tolerance module, the first of which is the less complex function. It has long been recognized that it is difficult for the design engineer to design two mating parts with tolerancing on the part features which will guarantee assembly without interference for any condition of the two parts within the recited part tolerances. Often one engineer designs and tolerances one part while another engineer designs and tolerances the mating part.The CIGMA system takes in data descriptive of each of two mating parts together with the tolerancing according to the ANSI standard and investigates assembly of the parts if the worst case tolerances for part assembly exist at each part. The CIGMA system also checks whether one of the mating parts is described with the correct GD and T dimension and tolerance description relative to the GD and T part description of the mating part. In this fashion the mating parts may be identified with regard to (1) potential material interference, and (2) datum definition inconsistencies between the parts.In summary, the first function of the tolerance module checks tolerance values which have already been called out by the design engineer or engineers and indicates to the system user if there is potential material interference of if there is inconsistency in the datum call-outs which would allow an otherwise correctly toleranced mating part to achieve a 'within tolerance' but wno fit' condition.

The second function of the tolerance module 40 in Figure 10 is performance of fixed and floating fastener analysis. A high percentage of tolerances on mechanical drawings are there to show the location of features which function to hold parts together with fasteners. It should be noted that a fixed fastener is represented by a threaded bolt which passes through a clearance hole in one part and engages a threaded hole in a mating part. A floating fastener is represented by a bolt which passes through a clearance hole in one part and a corresponding clearance hole in the mating part, and serves to fasten the two parts together by means of a nut, for example, applied to the threads of the fastener on the opposite side of the mating part.This second tolerance module function serves to create the tolerance values to be called out by the design engineer on the drawings for the part and the mating part.

The procedure undertaken by the user in performing floating fastener analysis in the second function of the tolerance module involves initially choosing a fastener to be- used. Fasteners are described having standard body diameters and head sizes (on bolts, for example) which describe defined bearing areas on the underside of the bolt head. Such fastener descriptions may be obtained from mechanical engineering tables. The user then designates the positions on a part where the selected fasteners are to be used. This is done by placing a cursor at a fastening point on a displayed depiction of the part and entering the information through the keyboard, as hereinbefore described for other functions of the CIGMA system.The user now designates the datums on the displayed part which are to be utilized in locating the features on the part, such as holes, where the fasteners will be placed and enters the datums into the system. The CIGMA system now computes the optimum size of the holes for the fastener and the true position of the holes on the mating part while the system simultaneously investigates the CAD models of the part and the mating part stored therein. Upper and lower optimum hole sizes for the holes in both parts are computed such that all the bearing surface of a fastener bolt head is in contact with the surface of the part through which it extends. It may be recognized that it is detrimental to the design of the assembly if holes in a part receiving a fastener are so large as to extend outside the dimensions of the holding portion of the fastener (the bolt head).

The CIGMA system also takes into consideration the characteristics of the tool to be used to create the part feature. For example, e drill as it wears out will make a larger hole and mechanical engineering tables provide an indication of the magnitude of such enlargement. A 0.593 diameter drill bit, for example, will never create a hole over 0.625 diameter even when the drill bit reaches a dull condition. The CIGMA system, knowing these facts, uses them to tolerance the part and the mating part.

By way of example of the hole tolerance generation by the CIGMA system for floating fasteners, reference is made to Figure 16 wherein a part 57 is shown having four clearance holes 58 therethrough. In this example a bolt having a 0.500 body diameter and a 0.750 head size is chosen by the design engineer to fasten part 57 to a mating part 59 also having four clearance holes 61 therethrough. If the holes 58 never exceed 0.625, the bearing surface of the bolt head will cover the holes 58. A 0.593 drill, incapable of drilling a hole larger than 0.625 as mentioned hereinbefore, is selected and the four holes are called out at 0.593 diameter plus 0.032, which allows a maximum hole size of 0.625. The minimum hole size is the difference between 0.593 and the bolt body diameter, whereby the minus tolerance on the hole becomes 0.093 so that the hole may never be less than 0.500.The ANSI standard call-out therefore appears as true position, diameter, zero tolerance at maximum material conditions relative to datum A (the top face of part 57) as seen in Figure 16.

When CIGMA is advised that a fixed fastener is being toleranced with regard to the mating parts, the user inputs are as designated hereinbefore when tolerancing for a floating fastener. Additionally, CIGMA asks for the thickness of the part containing the clearance holes and the mating part containing a corresponding pattern of threaded holes as hereinbefore described. In this instance the member containing the clearance holes will have a clearance hole tolerance on the plus side which is the same as for the floating fastener analysis , but the negative tolerance on the clearance holes will be diminished, because the fastener when fixed in the threaded portion of the part containing the threaded holes clearly cannot move. The clearance holes in the floating part must therefore be more tightly controlled. The CIGMA system recognizes this necessity during fixed fastener analysis and, for purposes of comparison, the tolerance on the holes 61 in part 59 of Figure 16, presuming they are for this example threaded holes for receiving the fastener, would be 0.062 at maximum material conditions where the thickness of the part 57 is taken into consideration. The ANSI call-out for the four threaded holes 61 of Figure 16 would therefore appear as follows: 1/2 13 UNC-2B 0 0 0.062 M A 0.510 P The following is an abbreviated program listing depicting one manner in which a program may be formulated for operating the disclosed system in performing the disclosed analyses, job control and tolerance module processes. C FMC Corporation 1987.

Although the best mode contemplated for carrying out the present invention has been herein shown and described, it will be apparent that modification and variation may be made without departing from what is regarded to be the subject matter of the invention.

HMS:smb

Claims (7)

1. CLAIMS 1. A method of investigating tolerance call-outs on mating parts wherein design and tolerance data for the mating parts is available in memory, comprising the steps of retrieving the design and tolerance data relating to the mating parts from the memory; comparing data representative of each part; and, indicating the results of the comparison.
2. 2. The method of claim 1 wherein the tolerance data include datums on each of the mating parts, comprising the steps of determining if there is inconsistency in the datum call outs in the tolerance data for the mating parts; and, indicating alternatively no inconsistency if there is none and a location of such inconsistency if some exists.
3. 3. The method of claim 1 or 2 comprising the step of investigating the worst case tolerance conditions for material interference between mating parts; and indicating alternatively no interference where none exists and a location of interference where some exists.
4. 4. The method of claim 1 and for determining tolerance call-outs for fixed and floating fastener features on mating parts comprising the steps of selecting a fastener; designating the position on a part where the fastener is to be used; designating the datums on the part from which the fastener locations are to be referenced; selecting a tool for forming the part features to receive the fasteners; determining the part feature maximum and minimum sizes considering the tool and the selected fastener; and, displaying the true position tolerance for the fastener part features.
5. 5. The method of claim 4 wherein the fastener is a floating fastener and the step of displaying comprises the step of showing a true position tolerance zone of zero at maximum material conditions.
6. 6. The method of claim 4 wherein the fastener is a fixed fastener and wherein the part feature in a floating part is a clearance hole comprising the steps of determining the thickness of the floating part; and, reducing the size of the clearance hole tolerance in accordance with such thickness.
7. 7. A method of investigating tolerance call-outs on mating parts as claimed in claim 1 and substantially as described.