GB2568906A

GB2568906A - Thread checking

Info

Publication number: GB2568906A
Application number: GB1719897.9A
Authority: GB
Inventors: Silva Pedreira Jonathan; Yeoman Stephen; Müller Heinrich; Grimm Thomas; Staudenmaier Timm
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2017-11-30
Filing date: 2017-11-30
Publication date: 2019-06-05
Also published as: GB201719897D0

Abstract

A method of checking whether a thread produced on a workpiece conforms to a target thread. The method comprises determining a three dimensional position of a predefined feature of the thread on the workpiece; comparing the determined 3D position of the predefined feature with reference data in order to determine a deviation, if any, of the thread on the workpiece from the target thread; and providing an output indicating the determined deviation. The reference data may define a 3D position of an equivalent feature on the target thread. The deviation may be determined from the difference between the measured 3D position of the feature and 3D position of the equivalent feature in the reference data. The predefined feature may comprise a stationary point of a thread or a point equidistant from two opposing thread flanks. The output may be used to set a start position and orientation of a thread producing tool relative to a further workpiece. The workpiece may be a engine bore for a spark plug. The target thread may be designed to retain the spark plug with a predefined orientation when screwed to a predefined tightness.

Description

The present disclosure relates to thread orientation checking. In particular, but not exclusively it relates to thread orientation checking in a bore of an engine cylinder head for receiving a spark plug.

Aspects of the invention relate to a method of and a computer program, apparatus, and system for checking whether a thread produced on a workpiece conforms to a target thread.

BACKGROUND

For some applications, threads, either internal or external, need to be produced with reliably oriented terminating positions so that when two components are screwed together to a desired torque, one has a predetermined orientation with respect to the other.

In internal combustion engines for motor vehicles, for example, a spark plug is screwed into a bore associated with a cylinder head of the engine so that the spark plug may ignite fuel injected into a combustion chamber associated with a cylinder of the engine. Modem engines have fuel injectors that inject fuel at the last possible moment before ignition. The injected fuel may be arranged in directed sprays or jets that pass close to the spark plug gap (i.e., the gap between the central live electrode and the arm of the ground electrode which projects from the main body of the spark plug and overhangs the central live electrode).

It is desirable to avoid such sprays being directed between the gap or else they can disrupt the development of the spark. If this were to happen, the spark would not be as hot as intended and this would lead to inefficient and or incomplete burning of the fuel. This could result in the engine failing to meet emissions targets.

If the spray should hit the arm, the spray may be deflected between the electrodes and/or displace gas between the electrodes which is ionised as a precursor for the electric arc that follows shortly and provides the requisite heat to ignite the fuel. Therefore it is desirable for 1 the thread produced in the bore in the cylinder head for receiving the spark plug to be designed such that the orientation of the spark plug within the combustion chamber of the cylinder does not place the arm in the path of the fuel sprays.

It is therefore desirable to implement a check of the threads produced in bores for receiving spark plugs to ensure that they conform to a target thread orientation which is designed such that, when the spark plug is screwed into the bore with the desired torque, the orientation of the spark plug within the combustion chamber does not place the arm in the path of the fuel sprays.

The example of the spark plugs is merely an example. It is in general desirable to check that threads produced on any workpiece conform to a target thread for that workpiece.

It is known to perform such checks using a manual gauge that accurately replicates the design and structure of a component which is intended to be screwed together the workpiece using the produced thread. Thus when the manual gauge is manually screwed together with the workpiece, the orientation of the manual gauge relative to the workpiece corresponds to the predicted final relative orientation of the workpiece and the component when assembled in final products.

In the example of checking a thread produced in a bore for receiving spark plugs, the suitable manual gauge is designed to imitate the thread of a spark plug and includes, or acts in conjunction with, an inclinometer which is zeroed for an orientation of the gauge which matches the orientation of the spark plug within the cylinder which does not place the arm of the spark plug in the path of the fuel sprays. A deviation of the produced thread from the target thread is reflected by a deviation of the value given by the inclinometer from 0 degrees.

This method is labour-intensive and time-consuming. It is therefore disruptive to the efficiency of the engine manufacturing production line.

It is an aim of the present invention to address disadvantages of the prior art.

SUMMARY OF THE INVENTION

According to an aspect of the invention there is provided a method of checking whether a thread produced on a workpiece conforms to a target thread, the method comprising: determining a three-dimensional position of a predefined feature of the thread produced on the workpiece; comparing the determined three-dimensional position of the predefined feature with reference data in order to determine a deviation, if any, of the thread produced on the workpiece from the target thread; and providing an output indicating the determined deviation.

In some examples the reference data defines a three-dimensional position for an equivalent feature of the target thread.

Determining the deviation, if any, may comprise determining a difference between the determined three-dimensional position of the predefined feature and the three-dimensional position for the equivalent feature defined by the reference data.

The predefined feature may comprise a stationary point of a thread form or a point equidistant from two opposing thread flanks of the thread form.

In some such examples, determining the three-dimensional position of the stationary point of the thread form may comprise determining the three-dimensional positions of two opposing thread flanks of the thread form.

In some examples, the provided output indicating the determined deviation comprises an alert if the determined deviation is outside a predetermined range of values.

The provided output may be used to set a start position and orientation, relative to a further workpiece, of a thread-producing peak of a thread-producing tool which is moved from the start position in a predefined manner to produce a further thread on the further workpiece.

In some examples, the workpiece on which the thread is produced comprises a bore in an engine cylinder head for receiving a spark plug having a complementary thread.

In some such examples, the target thread is designed to retain the spark plug within the bore in the engine cylinder head with a predefined orientation when the spark plug is screwed into the bore to a predefined tightness.

According to another aspect of the invention there is provided a computer program, which, when run on a computer, controls performance of the method of checking whether a thread produced on a workpiece conforms to a target thread, comprising controlling performance of: determining a three-dimensional position of a predefined feature of the thread produced on the workpiece; comparing the determined three-dimensional position of the predefined feature with reference data in order to determine a deviation, if any, of the thread produced on the workpiece from the target thread; and providing an output indicating the determined deviation.

According to another aspect of the invention there is provided a non-transitory tangible physical entity embodying a computer program comprising computer program instructions that, when executed by at least one electronic processor, controls performance of the method of checking whether a thread produced on a workpiece conforms to a target thread, comprising controlling performance of: determining a three-dimensional position of a predefined feature of the thread produced on the workpiece; comparing the determined three-dimensional position of the predefined feature with reference data in order to determine a deviation, if any, of the thread produced on the workpiece from the target thread; and providing an output indicating the determined deviation.

According to another aspect of the invention there is provided an apparatus configured to check whether a thread produced on a workpiece conforms to a target thread. The apparatus comprises at least one processor; and at least one memory including computer program code. The at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to control performance of: determining a three-dimensional position of a predefined feature of the thread produced on the workpiece; comparing the determined three-dimensional position of the predefined feature with reference data in order to determine a deviation, if any, of the thread produced on the workpiece from the target thread; and providing an output indicating the determined deviation.

According to another aspect of the invention there is provided an apparatus configured to check whether a thread produced on a workpiece conforms to a target thread, the apparatus comprising: means to determine a three-dimensional position of a predefined feature of the thread produced on the workpiece; means to compare the determined three-dimensional position of the predefined feature with reference data in order to determine a deviation, if any, of the thread produced on the workpiece from the target thread; and means to provide an output indicating the determined deviation.

In some examples, the apparatus comprises an electronic processor having one or more electrical inputs and an electronic memory device electrically coupled to the electronic processor and having computer program instructions stored therein.

In such examples, said means to determine a three-dimensional position of a predefined feature of the thread produced on the workpiece comprises the processor being configured to access the memory device and execute the instructions stored therein such that it is operable to determine a three-dimensional position of a predefined feature of the thread produced on the workpiece.

Additionally said means to compare the determined three-dimensional position of the predefined feature with reference data in order to determine a deviation, if any, of the thread produced on the workpiece from the target thread comprises the processor being configured to access the memory device and execute the instructions stored therein such that it is operable to compare the determined three-dimensional position of the predefined feature with reference data in order to determine a deviation, if any, of the thread produced on the workpiece from the target thread.

Additionally said means to provide an output indicating the determined deviation comprises the processor being configured to access the memory device and execute the instructions stored therein such that it is operable to provide an output indicating the determined deviation.

According to another aspect of the invention there is provided a coordinate measuring machine (CMM) system comprising the apparatus configured to check whether a thread produced on a workpiece conforms to a target thread, and further comprises a measuring 5 device configured to determine a three-dimensional position of a predefined feature of the thread produced on the workpiece.

According to another aspect of the invention there is provided a coordinate measuring machine (CMM) system comprising the computer program which, when run on a computer, controls performance of the method of checking whether a thread produced on a workpiece conforms to a target thread, and further comprises a measuring device configured to determine a three-dimensional position of a predefined feature of the thread produced on the workpiece.

According to another aspect of the invention there is provided a coordinate measuring machine (CMM) system comprising the non-transitory tangible physical entity embodying a computer program comprising computer program instructions that, when executed by at least one electronic processor, controls performance of the method of checking whether a thread produced on a workpiece conforms to a target thread, and further comprises a measuring device configured to determine a three-dimensional position of a predefined feature of the thread produced on the workpiece.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates an example of a method;

Figure 2 schematically illustrates examples of a thread produce on a workpiece and a target thread;

Figure 3 schematically illustrates an example of a cross-section of a portion of a thread;

Figure 4 schematically illustrates an example of a threaded bore in a cylinder head of an internal combustion engine, in which a spark plug is received;

Figure 5 schematically illustrates an example of a system; and

Figures 6A and 6B schematically illustrate examples of a measurement device of the system.

DETAILED DESCRIPTION

Figure 1 schematically illustrates a method 100 for checking whether a thread 12 produced on a workpiece 10 conforms to a target thread 22. The target thread 22 is a nominal thread which, if produced on the workpiece 10, would enable the workpiece 10 and a part which receives or is received by the workpiece 10 to be assembled with a predetermined or required orientation when they are screwed together, using the thread, and tightened to an appropriate torque value.

For the purpose of describing figure 1 reference will be made to figure 2 which, on its left hand side illustrates a workpiece 10 having a thread 12 and identifies a predefined feature 16 in a particular three-dimensional (3D) position 14. The thread 12 has been produced with a pitch 15 equal to that of the target thread 22. The right hand side of figure 2 schematically illustrates the target thread 22 which has an equivalent feature 26 at a particular 3D position 24. The 3D position 24 of the equivalent feature 26 is defined by reference data 40 (shown in figure 5).

It is to be understood that while figure 2 illustrates a continuous target thread 22, in some examples only a limited number of points on the target thread 22 illustrated in figure 2 may be defined by the reference data 40. In some examples the reference data 40 may define only a single point, that being the 3D position 24 of the equivalent feature 26.

The thread 12 conforms to the target thread 22 and there is no deviation 30 if the 3D position 14 of the predefined feature 16 of the thread 12 matches the 3D position 24 of the equivalent feature 26 of the target thread 22. The thread 12 also conforms to the target thread 22 and there is no deviation 30 if the ratio of the distance between the 3D position 14 of the predefined feature 16 of the thread 12 and the 3D position 24 of the equivalent feature 26 of the target thread 22 in a direction parallel to the axes on which the thread 12 and the target thread 22 are centred to the angular offset (in radians) between the 3D position 14 of the predefined feature 16 of the thread 12 and the 3D position 24 of the equivalent feature 26 of the target thread 22 about the respective axes on which the thread 12 and the target thread 22 are centred is equal to the pitch 15 of the thread 12 and the target thread 22 divided by 2π radians. If there is some other misalignment between the 3D position 14 of the predefined feature 16 of the thread 12 and the 3D position 24 of the equivalent feature 26 of the target thread 22, then there is a deviation 30.

Referring back to figure 1, at block 102 of the method 100, a 3D position 14 of a predefined feature 16 of a thread 12 produced on the workpiece 10 is determined. In some examples, determining the 3D position 14 of the predefined feature 16 comprises searching for the predefined feature 16 in the locale of the 3D position 24 defined by the reference data 40.

In some examples, determining the 3D position 14 of the predefined feature 16 comprises measuring the longitudinal distance (defined parallel to the axis on which thread 12 is centred) from a datum face 301 (as illustrated in figure 4) to the predefined feature 16. In some examples the azimuthal position (defined about the axis on which thread 12 is centred) at which the predefined feature 16 is found is also measured. Alternatively the azimuthal position may be predefined and the 3D position 14 of the predefined feature 16 is sought along a line of 3D positions having this predefined azimuthal position in common.

At block 104 of the method 100, the determined 3D position 14 of the predefined feature 16 is compared with reference data 40 in order to determine a deviation 30, if any, of the thread 12 produced on the workpiece 10 from the target thread 22. Determining the deviation 30, if any, comprises determining a difference between the determined 3D position 14 of the predefined feature 16 and the 3D position 24 for the equivalent feature 26 defined by the reference data 40. In some examples, the difference may be calculated as an angular offset about the axis of the thread 12, as a distance in a direction parallel to the axis on which the thread 12 is centred, or as a combination of the two.

In some examples, the reference data 40 comprises the longitudinal distance between the datum face 301 and the equivalent feature 26. The reference data 40 may also comprise the azimuthal position at which the equivalent feature 26 is found.

In some examples, the longitudinal distance between the datum face 301 and the equivalent feature 26 is given along the line of 3D positions having the predefined azimuthal position (at which the predefined feature 16 is to be measured in some examples of block 102) in common. Determining the deviation 30, if any, in such examples, comprises firstly determining whether or not the difference between the measured longitudinal distance between the datum face 301 and the predefined feature 16 and between the longitudinal distance comprised in the reference data 40 is a multiple of the pitch 15. If this difference is a multiple of the pitch 15, no deviation 30 or, in other words, a value of the deviation 30 equal to zero is determined. If this this difference is not a multiple of the pitch 15, the deviation 30 is determined to be equivalent to this difference. In some examples the deviation may be expressed as an angular offset (in radians) by multiplying this difference by a factor of 2π divided by the pitch 15.

Though described above in relation to a datum face 301, in some examples, the method 100 uses a point or line as a datum.

At block 106 of the method 100, an output 2011, 201₂ (shown in figure 5) indicating the determined deviation 30 is provided.

Figure 3 schematically illustrates a cross-section of a portion of the thread 12 on the workpiece 10 along a direction parallel to the axis about which the thread 12 is centred. The cross-section comprises a periodically repeated shape. This shape is the thread form 13 and the period of repetition is the pitch 15. The thread form 13 comprises two stationary points 17, 19, one a local minimum and one a local maximum, separated by flanks 18. The local minimum is a root 17 (i.e. a point of the thread 12 with the minimum protrusion from the workpiece 10, hence a maximum diameter when speaking of an internal thread and a minimum diameter when speaking of an external thread). The local maximum is a crest 19 (i.e. a point of the thread 12 with the maximum protrusion from the workpiece 10, hence a minimum diameter when speaking of an internal thread and a maximum diameter when speaking of an external thread).

The predefined feature 16 of the thread 12 for which the 3D position 14 is determined is, in some examples, selected to be one of the stationary points 17, 19 or a point equidistant from two opposing flanks 18. In the example of the thread form 13 illustrated in figure 3 and for other symmetrical thread forms, the point equidistant from two opposing flanks 18 has a common longitudinal (defined parallel to the axis on which thread 12 is centred) and angular (defined about the axis on which thread 12 is centred) coordinate with one of the stationary points 17, 19. Therefore, in some examples, the 3D position 14 of the stationary points 17, 19 of the thread form 13 may be determined from the 3D position of two opposing points on the opposing flanks 18. With knowledge of the thread form 13 the 3D position of the intervening stationary points 17, 19 can be determined from the 3D position of the two opposing points on the opposing flanks 18.

Although illustrated in figure 3 with a symmetrical trapezoidal cross-sectional shape, it will be understood that the thread form 13 may be produced with other cross-sectional shapes. For example, the thread form 13 may belong to a square thread or a round thread or an asymmetrical thread such as a buttress thread.

Figure 4 illustrates an example in which the workpiece 10 on which the thread 12 is produced comprises a bore 302 in an engine cylinder head 300 for receiving a spark plug 304. In this instance the target thread 22 is designed to complement the thread of the spark plug 304. The target thread 22 is designed to retain a spark plug 304 within the bore 302 in the engine cylinder head 300 with a predefined orientation when the spark plug is screwed into the bore to a predefined tightness (i.e., tightened to a desired torque). In this way, the orientation of the spark plug extending into a combustion chamber of the engine can be controlled to a tight tolerance.

In the example of a thread 12 produced in a bore 302 in an engine cylinder head 300 for receiving a spark plug 304 (as per figure 4), the datum face 301 may be a seating face around the threaded bore 302.

Figure 5 schematically illustrates an example of a coordinate measuring machine (CMM) system 200 comprising a controller 202, a measuring device 206 and a database 204.

The controller 202 provides means for implementing the method 100 of checking whether the thread 12 produced on the workpiece 10 conforms the target thread 22. The controller 202 comprises processing circuitry 210 and memory circuitry 212.

The processing circuitry 210 may comprise one or more processors. The processing circuitry 210 may also comprise an output interface (not shown) via which data and/or commands are output via the processing circuitry 210 and an input interface (not shown) via which data and/or commands are input to the processing circuitry 210.

The memory circuitry 212 may be configured to store a computer program 214 comprising computer program instructions (computer program code) that controls the operation of the controller 202 when loaded into the processing circuitry 210. The computer program instructions of the computer program 214 provide the logic and routines that enable the controller 202 to control performance of the method 100. The processing circuitry 210 by reading the memory circuitry 212 is able to load and execute the computer program 214.

In some examples the controller 202 is comprised in an apparatus 201 which may be connected and disconnected, in some cases physically, from the other components of the CMM system 200 such as the measuring device 206 and, in some cases, the database 204. In some examples, though not shown, the database 204 may also be comprised in the apparatus 201. The apparatus 201 may be, for example, a personal computer such as a desktop or laptop computer and may be connected into the CMM system 200 for controlling a CMM which comprises the measuring device 206.

The database 204 stores the reference data 40 used in at least block 104 of the method 100 for the purpose of determining the deviation 30, if any, of the thread 12 produced on the workpiece 10 from the target thread 22. The reference data 40 may be read from a lookup table stored in the database 204 or defined by a three-dimensional model of the target thread 22 stored in the database 204. The database 204 may comprise reference data 40 corresponding to a plurality of target threads having different physical properties.

The measurement device 206 comprises a sensor (an example of which is shown in figures 6A and 6B) configured to detect the 3D position 14 of the predefined feature 16 of the thread 12. The sensor may in some examples be an electronic touch trigger probe 207 (see figure 11

6A) configured to detect the position of a feature (e.g., the predefined feature 16 of the thread 12) by being brought into contact with said feature. On contact with said feature, the probe sends coordinate information to the controller 202. In some examples the probe comprises a stylus 209 (see figure 6B), for example a ruby ball stylus, sized to fit between opposing flanks 18 of the thread 12. The stylus 209 may be sized to contact both opposing flanks 18 simultaneously or alternatively to fit between the opposing flanks 18 so as separately contact both. The sensor may alternatively be any sensor suitable for detecting a position of a feature or surface including for example optical proximity sensors, capacitive displacement sensors, ultrasonic sensors etc.

The controller 202 is configured to receive data from the measurement device 206 indicative of a 3D position 14 of the predefined feature 16 of the thread 12 produced on the workpiece 10 or of the 3D position of additional thread features (such as, for example, the flanks 18) indicative of the presence of the predefined feature 16. The controller 202 is configured to determine the 3D position 14 of the predefined feature 16 of the thread 12 produced on the workpiece 10, as per block 102 of the method 100, from the data received from the measurement device 206.

In some examples the movement of the probe 207 into contact with features of the thread 12 is controlled by the controller 202. The controller may process the reference data 40 received from the database 204 to determine a 3D position at which the predefined feature 16 is likely to be located and control the probe 207 to move towards that 3D position. In some examples the controller controls the probe 207 to move towards the 3D position 24 defined by the reference data 40. Subsequently, if a predefined feature 16 is not found at the 3D position 24 defined by the reference data 40, the controller 202 controls the probe 207 to search for the predefined feature 16 by rotating the probe about the axis of the thread 12 or by moving the probe parallel to the axis of the thread 12. Under usual circumstances the predefined feature 16 will be found within a rotation of 2π radians or a distance equal to the value of the pitch 15.

In other examples the measurement device 206 is manually operated by a user in order to move the sensor (e.g., the probe 207) into a position at which the predefined feature 16 of the thread 12 can be detected and its 3D position 14 determined.

In some examples movement of the sensor (e.g., the probe 207) may be restricted to moving parallel to the axis on which thread 12 is centred and, where contact is required for position detection, towards and away from the axis in order to facilitate contact with features of the thread 12. As such the 3D position 14 of the predefined feature 16 may be defined solely by a longitudinal coordinate (defined parallel to the axis on which thread 12 is centred). In some such examples, the 3D position 24 defined by the reference data 40 consists only of a longitudinal coordinate.

The controller 202 is further configured to compare the determined 3D position 14 of the predefined feature 16 with the reference data 40, which defines the 3D position 24 of an equivalent feature 26, received from the database 204 in order to determine a deviation 30, if any, of the thread 12 produced on the workpiece 10 from the target thread 22, as per block 104 of the method 100.

In some examples the controller 202 is configured to determine whether the deviation 30 is outside of a predetermined range of values. This predetermined range of values may be reflective of the tolerance of the predetermined orientation. If the determined deviation 30 is outside of this predetermined range of values, the controller 202 provides an output 2011 which comprises an alert, for example a visual or audio alert.

In some examples the controller 202 is configured to provide an output 201₂ to a machine in which a thread-producing tool 208 is comprised. The machine may be a computerised numerical control (CNC) machining centre or CNC lathe. The output 201₂ is used in setting a start position and orientation, relative to a further workpiece, of a thread-producing peak of the thread-producing tool 208. The thread-producing tool 208 is moved from the start position, by the machine in which it is comprised, in a predefined manner to produce a further thread on the further workpiece.

In some examples, the machine comprising a thread-producing tool 208 also comprises or is operably connected to the CMM system 200. In some examples the CMM system 200 is embedded into a CNC machining centre. The controller 202 may be common to the CMM system 200 and the machine comprising the thread-producing tool 208. The controller 202 may determine the start position and orientation and provide an output 201₂ to control actuators which adjust the start position and orientation of the thread-producing tool 208.

In other examples, not illustrated in figure 5, the controller 202 is configured to provide other outputs which indicate the determined deviation 30, as per block 106 of the method 100. In some examples the output may be a visual or audio output of the deviation 30 regardless of whether or not it is outside of a predetermined range of values. The outputs may involve presenting the deviation 30 as an angular offset of the thread 12 from the target thread 22.

For purposes of this disclosure, it is to be understood that the controller(s) described herein can each comprise a control unit or computational device having one or more electronic processors. An apparatus 201 or CMM system 200 may comprise a single control unit or electronic controller or alternatively different functions of the controller(s) may be embodied in, or hosted in, different control units or controllers. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) to implement the control techniques described herein (including the described method(s)). The set of instructions may be embedded in one or more electronic processors, or alternatively, the set of instructions could be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present disclosure is not intended to be limited to any particular arrangement. In any event, the set of instructions described above may be embedded in a computerreadable storage medium (e.g., a non-transitory computer-readable storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions.

The blocks illustrated in figure 1 may represent steps in a method and/or sections of code in the computer program 214. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

For example, the method 100, and particularly block 102 thereof, need not be carried out by the CMM system 200. In some examples the 3D position 14 of the predefined feature 16 is determined using an optical pre-setter (not shown) or using other measurement methods in conjunction with processing circuitry such as the processing circuitry 210.

Although described in the foregoing predominately in terms of a method of checking internal threads such as a thread 12 produced in a bore 302 in an engine cylinder head 300 for receiving a spark plug 304 (as per figure 4), it is to be understood that the method is equally applicable to external threads such as those produced on a spark plug 304.

Features described in the preceding description may be used in combinations other than the combinations explicitly described.

Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. In particular, whist the examples above have been to measuring and controlling the orientation of a thread for a spark plug, it will be appreciated that this approach may be equally useful in measuring and controlling the orientation of a thread arranged to receive a fuel injector, especially for direct fuel injected engines such as direct injected gasoline engines. The precise measurement and control of the orientation of a thread arranged to receive a diesel fuel injector may equally benefit from the aforementioned method and apparatus.

Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant 15 claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. A method of checking whether a thread produced on a workpiece conforms to a target thread, the method comprising:

determining a three-dimensional position of a predefined feature of the thread produced on the workpiece;

comparing the determined three-dimensional position of the predefined feature with reference data in order to determine a deviation, if any, of the thread produced on the workpiece from the target thread; and providing an output indicating the determined deviation.

2. The method according to claim 1, wherein the reference data defines a threedimensional position for an equivalent feature of the target thread.

3. The method according to claim 2, wherein determining the deviation, if any, comprises determining a difference between the determined three-dimensional position of the predefined feature and the three-dimensional position for the equivalent feature defined by the reference data.

4. The method according to any preceding claim, wherein the predefined feature comprises a stationary point of a thread form or a point equidistant from two opposing thread flanks of the thread form.

5. The method according to claim 4, wherein determining the three-dimensional position of the stationary point on the thread form comprises determining the three-dimensional positions of two opposing thread flanks of the thread form.

6. The method according to any preceding claim, wherein the provided output indicating the determined deviation comprises an alert if the determined deviation is outside a predetermined range of values.

7. The method according to any preceding claim, wherein the provided output is used to set a start position and orientation, relative to a further workpiece, of a thread-producing peak of a thread-producing tool which is moved from the start position and orientation in a predefined manner to produce a further thread on the further workpiece.

8. The method according to any preceding claim, wherein the workpiece on which the thread is produced comprises a bore in an engine cylinder head for receiving a spark plug having a complementary thread.

9. The method according to claim 8, wherein the target thread is designed to retain the spark plug within the bore in the engine cylinder head with a predefined orientation when the spark plug is screwed into the bore to a predefined tightness.

10. A computer program, which, when run on a computer, controls performance of the method of one or more of the preceding claims.

11. An apparatus configured to check whether a thread produced on a workpiece conforms to a target thread, the apparatus comprising:

at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to control performance of the method of one or more of claims 1 to 9.

12. A coordinate measuring machine (CMM) system comprising the apparatus according to claim 11 or the computer program according to claim 10, and a measuring device configured to determine the three-dimensional position of the predefined feature.