WO1982000513A1

WO1982000513A1 - Measurement of radial dimensions of cylindrical form threaded workpieces

Info

Publication number: WO1982000513A1
Application number: PCT/AU1981/000106
Authority: WO
Inventors: Scient Ind Res Org Commw
Original assignee: Beardsley R
Priority date: 1980-08-07
Filing date: 1981-08-07
Publication date: 1982-02-18

Abstract

An instrument and method for measuring a peripheral contour of a cylindrical-form, helically threaded workpiece (14) wherein the workpiece is mounted for rotation about its longitudinal axis and a measuring stylus (34) contacts the periphery of the workpiece so as to be able to move both parallel to, and perpendicularly of, the longitudinal axis of the workpiece, and as the workpiece is rotated about its longitudinal axis rotary movement of the workpiece and radial movement of the stylus as it follows a peripheral contour of the workpiece are converted to respective electrical signals representative of the angular rotational position of the workpiece and the radial position of the stylus.

Description

ME_AS_UREMENT_OFRADIALDIMENSIONSOFCYLINDRICALFORMTHREADED ORKPIECES - -

This invention concerns the measurement, recording and/or display of the radial dimensions of cylindrical-form threaded wor pieces, such as rotary cutting tools, for example threading taps, including taper taps and fluteless taps, milling cutters, drills, reamers, D-bits and the like.

Because of the importance and difficulty of the problem, the present invention is particularly concerned with the measuremen and recording of the radial relief of taps, but the instruments and techniques suitable for this purpose are readily adaptable to the measurement and recording of ovality (or lack of radial symmetry) in precision bolts and the like. Thus, while the following explanation and description of the invention will be largely directed toward the measurement of the radial relief of thread cutting taps, it will be appreciated that the techniques, instruments and devices of this invention are not confined to that application or purpose.

While the correct radial relief of the clearance surfa of a rotary tool such as a tap or milling cutter is critical for the performance of the tool, radial relief is difficult to measure, display or record accurately in the vicinity of the cutting edge, making quality control in production correspondingly difficult. Similarly, while the dimensional accuracy of the cylindrical portion of a precision bolt can be easily checked, it is more difficult to measure thread ovality or other radial non-uniformity. Tap radial relief is normally measured on a floating carriage thread measuring machine equipped with a protractor and datum pointer.

Taps with an even number of lands are measured across the diamet using two thread measuring wires or cylinders which contact the thread flanks close to the pitch diameter. The relief is assessed by measuring the diameter near to the cutting- edge, rotating the tap in increments of a few degrees and re-measuring

In the case of odd numbers of lands, one measuring cylinder is used together with a fixed stop and the change in radius of each land is measured.

Another method is to use an instrument having thread form rolls to contact the tap. One roll is movable laterally an its linear displacement is detected by an indicator or transduce

⁽e.g. Johansson Threadometer) . With an odd number of lands three rolls are used giving a satisfactory location for the tap helix angle, but a correction has to be made to the indicator reading as it does not show the true changes in diameter. Even numbered lands are measured with two thread form rolls and a plain roll to locate the tap axis centrally. During measure¬ ment the tap helix angle is displaced in relation to the thread form rolls and the tap axis wanders during rotation due to the radial relief on the land contacting the plain roll. Thus measurement of radial relief with this type of instrument is restricted to taps having an odd number of lands.

The abovementioned methods cannot measure the tap geometry in its critical region, i.e. close to the cutting edges. When the measurements are plotted against the incre¬ ments of rotation the laboriously obtained result does not display the form of relief satisfactorily. Other, less laborious techniques employ complex and expensive instruments which involve the use of a highly-accurate master tool against which production tools can be gauged by comparative measure¬ ment. Such instruments, though accurate and capable of checking other parameters of the tool, are expensive and dependent upon the availability of a precision master for every product to be checked.

By contrast, the present invention seeks to provide a simple instrument and technique which does not require a master for comparison and yet is capable of measuring the radial relief of a variety of rotary-form tools both rapidly and accurately.

The present invention provides an instrument for measuring a peripheral contour of a cylindrical-form helically threaded workpiece; said instrument having means for mounting the workpiece for rotation about its axis; means for bringing and maintaining a measuring stylus in contact with the periphery of the workpiece as the latter is rotated; a first mechano-eleetrie transducer operatively connectable to the workpiece and adapted to generate an electrical signal related to the angular position of the work- piece; and a second mechano-eleetrie transducer operatively associated with the stylus and adapted to generate an electrical signal related to the radial position of the stylus; the means

- UmlEX OMPI for bringing and maintaining the stylus being adapted to enable the stylus to move both parallel to, and perpendicu¬ larly of, the axis of the workpiece so as to follow a peripheral contour of the workpiece. The invention also provides a method for measuring a peripheral contour of a cylindrical-form, helically threaded workpiece wherein the workpiece is mounted for rotation about its axis and a measuring stylus is brought into and maintained in contact with the periphery of the workpiece so as to be able to move both parallel to, and perpendicularly of, the axis of the workpiece, and wherein the workpiece is rotated about its axis and rotary movement of the workpiece and radial movement of the stylus as it follows a peripheral contour of the workpiece are converted to respective electrical signals related to the angular position of the workpiece and the radial position of the stylus.

Basically, the present invention may involve turning the tool or workpiece about its axis, using a first mechano- electric transducer to derive an electrical signal related to the angular position. of the tool, using a second mechano- electric transducer to derive an electrical signal related to the radial position of a stylus capable of following radial movements of the tool surface, and employing an X-Y recorder to produce an accurate plot of relief with rotation of the tool in the region of stylus contact when the tool is rotated. The invention thus comprises an instrument which may incorporate the X-Y recorder or, alternatively, a method, device or jig for mounting the tool or workpiece which is capable of rotation about and for deriving the signals for use with such a recorder.

In an instrument according to the invention, the stylus may be mounted on a carriage assembly movable parallel to the workpiece axis, with the carriage assembly being on a support means resiliently mounted for movement perpendicularly of that axis. In one form, the support means is an arm extending substantially parallel to the workpiece axis, the carriage assembly being mounted adjacent one end of the arm with the arm being pivotally mounted at its other end so as to

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OMFI be resiliently pivotable in a plane in which the workpiece axis is located. In that form, the arm preferably is pivotally mounted by means of a leaf spring.

In an alternative form, the support means is resiliently mounted for movement perpendicularly of the workpiece axis by means of leaf springs spaced in the direction of that movement and extending perpendicularly with respect to both that direction and the workpiece axis. In each of those forms, the carriage assembly may include a carriage on which the stylus is mounted and guide means for guiding the carriage in movement parallel to the workpiece axis between opposed stop means. In one arrange¬ ment the guide means includes a lapped pin extending through the carriage and between the stop members, and a ball bearing mounted on the carriage to one side of the pin and riding along a guide surface parallel to the pin. In an alternative arrangement, the guide means includes a pair of guide members between which the carriage is guided, each guide member defining a vee-way in which is located ball bearings on which a respective side of the carriage is slidable.

The first transducer preferably is a rotary transducer connectable to the workpiece such as via the means for rotating the latter. One suitable such first transducer is a multi-turn potentiometer having a shaft connected to and rotatable by the means for rotating the workpiece.

The second transducer preferably is a linear transducer positioned in contact with the stylus so as to be responsive to movement of the stylus perpendicularly of the axis of the workpiece. In such case, the second transducer may be in contact with the stylus via a member movable with the stylus in the direction of. perpendicular movement of the latter but fixed against movement parallel to the axis of the workpiece.

Where the instrument includes an "X-Y" recorder, its "X" input preferably is connected to the first transducer and its "Y" input connected to the second transducer for receiving the respective signals generated by the transducers. The record may be one adapted to provide a visual representation of the measured profile, such as to provide the representation as a plot on X-Y co-ordinate chart.

It will be found simplest to employ rotary and linear transducers (respectively) as the said first and second trans^¬ ducers, though it will be appreciated by those skilled in the art that mechanical linkages can be designed to generate an angular position signal from a linear transducer. It will also be appreciated that the technique of the present invention allows the measurement of the radial dimension of any given point on the surface of the tool or other workpiece accessible by the stylus without the necessity of an X-Y recorder (or othe display device) to graphically portray the profile or contour of the workpiece surface. Where the tool has grooves, cutting faces or lands which are not orthogonal to the axis (such as the helical grooves of a threading tap or precision bolt) , the instrument in accordance with the present invention allows the stylus free axial movement under pressure from the tool or workpiece itself (as it is rotated) . As indicated, the stylus may be mounted on a carriage which, in one arrangement, is in turn mounted upon a arm arranged adjacent to the tool or workpiece so that the styl may be brought into contact therewith, the carriage being free to move axially on the arm under pressure from the stylus but being constrained against other movements with respect to the ar The arm may be pivoted close to a support column to enable the carriage and stylus to move cojointly in the radial direction. The stylus should be confined to move in the plane of the tool axis at all times but free to move radially and axially within that plane so as to follow the surface of the tool, the radial movement being employed to operate the first transducer which remains essentially unaffected by the axial movement of the stylus .

It is necessary to arrange the pivoted arm (including the pivot) for adjustment to and from the workpiece axis so that the arm itself can be set parallel with the tool axis when the stylus is in contact with the workpiece surface. The pivot may conveniently comprise a leaf spring.

_Having portrayed the general nature of the present invention, two particular examples will now be described by way of illustration only. The chosen examples relate to the 5 measurement of radial relief in taps and are described in reference to the accompanying drawings, in hich:

Figures 1A and IB show an ordinary tap in side elevation and transverse section, respectively, for the purpose -of identifying the conventional parameters (including ^■ _j_0 radial relief) ,

Figure 2 is a perspective view of the first example of a measurement jig formed in accordance with the present invention,

Figures 3 and 4 are a front elevation and plan view ^■ _]_5 of the jig of Figure 2,

Figure 5 is a part sectional view on line V-V of Figure 3,

Figure 6 is an underside plan view of the stylus and carriage assembly of. the jig of Figure 2, 2o Figure 7 is a sectional view on line VII-VII of

Figure 6,

Figures 8 to 10 are views corresponding to Figures 2 to 4, respectively, showing details of a second^'embodiment of the invention, 5 Figure 11 is a part sectional view taken on line

XI-XI of Figure 9,

Figure 12 is an underside plan view of the stylus and carriage assembly of the jig of Figure 8,

Figure 13 is a sectional view on line XIII-XIII of 0 Figure 12, and

Figures 14 to 18 show typical traces obtained with either of the embodiments of Figures 2 to 7 or Figures 8 to 13.

Precision measurement of tap geometry is vital to tap manufacturers and research^"staff engaged in performance 5 testing of new tap designs. One element of tap geometry requiring extreme care in measurement is the radial relief of the thread form illustrated in Figures 1A and IB in relation to the thread form 2 of tap 4. This relief (or clearance) consist of a gradual reduction in radius from the cutting edge 5 of eac

OM land to the heel 6, denoted by the. clearance angle θ. The purpose of this relief is to reduce the contact area between the tap and workpiece, resulting in less friction being generated during tapping. Except for small diameter taps and certain types having spiral flutes, nearly all precision ground taps are produced with some form of thread relief. The radial relief is not necessarily linear with rotation and is dependant on the cam design of the thread grinding machine. From an examination of Figure 1, it will be appreciated that practically all thread cutting is performed by the chamfer 7 at the front of the tap and, therefore chamfer 7 will also need to be relieved. behind the cutting face in the radial direction. Desirably, therefore, an instrument designed for the measurement of radial relief in the thread form of taps should be capable of measuring the relief in the chamfered portion of the tap. Both the chosen examples described below will permit -the measurement of radial relief at any position along the "thread length" 8 of the tap, including the chamfered portion at the front of the tap. Threads can be produced in ductile materials by a forming process using fluteless taps which are not circular in cross section but have four lobes to produce the thread form. These taps can also be readily measured with the present invention. Taper pipe taps also must have radial relief of the thread form to enable them to cut, as -without relief the increasing diameter would cause severe rubbing at the back of each land. Radial relief of these taps -is extremely difficult to measure by traditional methods but can readily be measured with the present invention. The chosen jigs are also capable of measuring eccentricity of the threaded surface of a precision bolt or the shank or the unthreaded portion of either workpiece.

Turning now to the first embodiment of the present invention as illustrated in Figures 2 to 7 , the instrument comprises a ground base-plate 10 having a pair of precision centre sub-assemblies 11 and 12 fixed to and accurately aligned thereon by means of dovetail connection 13. The tap 14 to be measured is located between the male or female centre points 15 of centre assemblies 11 and 12. A pulley 18 is mounted via a centre split-collet 20 on the shank of tap 14. The pulley 18 is turned by hand and carries a belt 22 which drives a pulley 24 supported on a ball bearing 25 coupled to the shaft of a multiturn potentiometer 26 fixed to the base-, plate 10. Potentiometer 26 serves as the first or angular- position transducer.

The second - or linear - transducer 28 is fixed to column 30 via a bracket 32 so as to be vertically adjustable. The sensing tip of transducer 28 is located above a stylus 34 mounted on a carriage assembly 36 for axial movement along the undersurface of the distal end of arm 38. The promixal end of arm 38 is mounted for vertical adjustment on column 40, via bracket 41 to which the distal end of arm 38 is connected by leafspring 42. Stylus 34 is able to move vertically by pivoting of arm 38 in a vertical plane at hinge 42; the arrangement being such that due to the length of arm 38 relative to the vertical movement of stylus 34, the vertical movement is for all practical purposes linear. Also, the rigidity of hinge 42 laterally of arm 38 prevents lateral movement of stylus 34.

Referring more particularly to Figures 5 to 7, it will be seen that the sensing tipor plunger 43 of transducer 28 presses against a ground button or pad 44 mounted on the upper surface of arm 38, above stylus 34. The stylus is mounted for movement longitudinally of arm 38 on a carriage block 45 slidable between end walls 46 of a steel block 47 screwed to the lower face of arm 38. Carriage block 45 is slidable on a ball bearing 48 at the rear and, at the front, on a lapped pin 49 extending longitudinally of arm 38 between walls 46 of block 47. Walls 46 limit the extent of movement of carriage 45 and, hence, stylus 34; while keeper plate 50 extending between walls 46 support carriage 45 when the stylus is not in contact with a tool such as tap 14. During a measurement, described in more detail below, a very small vertical movement of stylus 34 results from the detected radial relief, or variations in relief, which may be as small as 0.001 mm or less, and display at high magnification therefore is necessary, for example 1000X, by means of a recorde Thus, it is highly important to have precise control of arm 38 and the stylus, and this is provided by means of a sensitive lifting device 52 which contacts arm 38 when the stylus is not supported in a thread form of a tool to be measured. Device 52, which also limits movement of stylus 34 to prevent damage to the recorder, is cam operated; rotation of knob 53 and, with it, a cam (not shown) in support 54 raising or lowering rod 55 and its arm 56 to move support block 57 into or out of contact with depending stud 58 on the under-side of arm 38. In operation, stylus 34 is lowered by device 52 into the thread form of tap 14, after which arm 38 is set parallel to the tap by adjusting bracket 41 and using a dial gauge (not shown) on the two ground buttons 60. Transducer 28 then is lowered to bring its sensing tip or plunger 43 to contact stylus 34 through pad 44 and arm 38. Transducer 28 has its leads 62 connected to the amplifier unit 64 and, via unit 64, to the "Y" input of the "X-Y" recorder 66; while leads 68,69 of potentiometer 26 are connected respectively to the "X" input of recorder unit 66 and a low voltage power supply unit 70. Thus, as pulley 18 is turned by hand, a signal proportional to the rotation of pulley 18 and, hence, tap 14 is generated at^' the "X" input of unit 66. Simultaneously, the tip of stylus 34 follows the thread form and, in so doing, stylus 34 and carriage 45 are drawn axially of tap 14 by virtue of the helical form of the thread; while stylus 34 and arm 38 move vertically due to the radial relief of the thread form. This vertical movement is transmitted to and activates linear transducer 28, with the latter consequently generating a signal proportional to the radial relief which is applied to the "Y" input of recorder 66.

The second embodiment of the invention is illustrated in Figures 8 to 13. In this, features corresponding to those of Figures 2 to 7 are identified by the same reference numeral plus 100. The second embodiment differs principally in use of an alternative method of mounting the stylus and the second, linear transducer. The centre assemblies 111 and 112, the pulleys 118 and 124 and the angular transducer 126 are largely as described for the first embodiment.

OMPI As shown in Figures 11, 12 and 13, carriage assembly 136 for stylus 134 consists of a base plate -76 to which are secure front and rear vee-ways 77. A carriage 78, from which stylus 134 depends, is slidable sideways, parallel to the axis of tap 114, between the vee-ways. This sliding movement is facilitated by ball bearings 79 between each vee-way 77 and the adjacent side of carriage 78; with each bearing being located in a respective aperture in a plate 80 received in its vee-way and the adjacent side of carriage 78. As shown most clearly in Figure 12, travel of carriage 78 and, hence, stylus 34 is limited by stop plates 81 at the ends of vee-ways 77, such as to a distance of 5 mm.

Carriage assembly 136 is screwed to the bottom of angle plate 82; with the latter being adjustably mounted via bracket 83 on rectangular column 130 which also carries transducer 128. Plate 82 and bracket 83 are resilient inter¬ connected by upper and lower sets of twin parallel leaf springs 84. This form of resilient mounting is found to provid a stable height setting for stylus 134 to which it is returned, after vertical displacement, with a high degree of precision. However, a calibration check or re-adjustment for use with a tool 114 of different size, is provided by means of jacking screw 85 fitted at the top of column 130 and operable directly on bracket 83; while locking of bracket 83 and, hence, setting of plate 82 and stylus 134 is provided by screw 86.

An arm of bracket 83 also supports linear transducer 128 with its tip or plunger 143 contacting steel pad 144. The latter is carried by angle plate 82; while transducer 128 is vertically adjustable relative to, and lockable on, bracket 83 by screw 87.

In operation, stylus 134 is lowered with jacking screw 85 into the thread form and the tap 114 is rotated to bring the high point of the land under the stylus. The transducer 128 is lowered to contact the steel pad 144 and locked in position with screw 87, and jacking screw 85 finely adjusted to give sufficient loading of the stylus 134 in the thread form. The movable bracket 83 on column 130 is locked in position with screw 86.

As in the first embodiment, the tap is rotated by mean of pulley 118 in the opposite direction to normal cutting until the stylus starts to drop over the cutting edge. Tap 114 then is rotated towards the heel of the land and the stylus follows the groove in the thread form in the axial direction of tap 114 by virtue of sliding carriage assembly 136, and any vertical displacement of stylus 134 will generate the desired "Y" signal by transducer 128.

In order to measure the radial relief of taper taps with either of the two embodiments of the present invention, th centres 11,12 or 111,112 are tilted in the vertical plane throu half the taper angle. As shown in Figure 9, the centres are tilted by mounting them on a tapered wedge 88 fitted on top of the base 10. This makes the thread form of the taper tap 114 (shown in an exaggerated form in Figure 9) horizontal and the radial relief of the thread can then be measured as described above for non-tapered taps. Alternatively, the front portion o the base of the instrument can be made in two horizontal pieces with the top piece pivoting at the right-hand end of the bottom piece (nearest the pulley 18) and a suitable block fitted betwee the two base pieces to give the required angle of tilt.

In the two embodiments illustrated, it is to be under¬ stood that the carriage assemblies are inter-changeable. Thus, an assembly 36 as described in Figures 2 to 7 could be used in place of assembly 136 in the embodiment of Figures 8 to 12, and vice-versa.

In the foregoing, specific description is not directed to stylus 34 or 134, although suitable configurations are illus¬ trated and variations will be readily apparent. The principal functions of the stylus are an ability to follow the contour to e measured and to transmit radial variation in the contour to be measured to the linear transducer. For the first function, the stylus is to have a tip of suitable dimensions, and it is found that an arrangement in which the stylus tapers down to a small ball tip, as illustrated, is most satisfactory. For the second function, it is necessary that the stylus have sufficient rigidity to avoid flexing under the loads applied.

The tip of the stylus can however have a variety of forms. The ball tip arrangement is well suited to measuring radial relief in a range of thread sizes. It is preferred that the ball tip bear on the opposed flanks of successive thread portions rather than at the thread root since the root is more likely to be somewhat irregular. A ball diameter such that the ball tip rides between central zones of the flanks of successive thread portions is preferred, although the ball diameter may be greater so that the ball rides between the crests of successive thread portions.

The nature and operation of transducers 26,126 will be readily understood from the foregoing description, as will the nature and form of alternatives for these transducers. Similarl the nature and operation of transducers 28,128 and of alternativ will be readily understood. However, a preferred form for trans ducers 28,128 is that known as linear variable differential transformers, such as available from Tesa S.A. of Switzerland. Such transducer is typified by a ferrite core, movable in respon to the movement of the stylus, and circuitry providing a modifie output derived from a signal generated in a sensing coil around the core.

It additionally is to be understood that while pulleys 18,118 of the illustrated arrangements are manually rotatable, an indexing motorized drive could be used for rotating taps 14,1 However, given the small rotation frequently required, manual rotation is highly desirable, particularly if manual adjustment of the stylus and recorder is necessary prior to each measuremen being taken. Also, the manual pulleys 18,118 can be indexed, if required, for successive measurements by use of suitable means engageable in a selected one of the circumferentially spaced apertures shown in pulleys 18,118.

Tapered wedge 88 and tap 114 are shown in Figure 9 with an exaggerated taper angle. Typically, the tap will have a taper of 1 in 16 such that half ^"the included angle will correspond to a taper of 1 in 32. Wedge 88 has its upper surface inclined to its horizontal lower surface at half the included angle of tap 114, such that for a half angle taper of 1 in 32 wedge 88 will taper at 1°48'. The lower surface of wedge 88 can be in a dovetail fit with base 10, while the centres 111,112 are similarly mounted on the upper surface of the wedge.

Figures 14 to 17 show typical traces obtained with either of the chosen embodiments. The traces of Figure 14 are from successive lands of a taper pipe tap, taken using a taper wedge as illustrated in Figure 9. The traces show substantial uniformity in radial relief in each land, although the third trace from the top showing slight deformities, possibly due to burrs, at the leading and trailing ends.

Figure 15 shows similar traces taken on the chamfer or lead portion of a tap, which is unthreaded, and were obtained after first locking the carriage to prevent movement of the ισ stylus along the tap axis. The radial relief is of a good quality and uniform and it will be appreciated from the scale that the substantial relief shown is typical of that for the chamfer portion of such tap.

Where, as in obtaining traces as in Figure 15, it is 5 necessary to lock the carriage, a variety of means can be employed. Thus, in the case of Figures 2 to 7, a screw can be inserted through a threaded bore in one wall 46 of block 47 so as to hold carriage block 45 firmly against the other wall 46. Similarly, in the arrangement of Figures 8 to 13, carriage 78 0 and the horizontal portion of angle plate 82 can be provided with apertures which are aligned with carriage 78 against one pair of stop plates 81, and a headed pin then inserted through the aligned apertures to prevent carriage 78 from moving away from those plates 82. 5 Figure 16 shows traces showing relief in a con-eccentric tap. This is of good quality, the tap showing virtually no relief over the initial interval of about 5°, although a slight negative relief of little consequence is shown over that interval in the second 0 trace. Where positive relief is provided after the initial interval, this is not uniform but would not detract from the performance of the tool.

Traces (a) of Figure 17 are from a fluted tap, with the final three traces being taken from the one land and showing the high level of reproducibility obtainable with the

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OMPI invention. The relief is non-uniform but, more importantly, show the tap to be of poor quality to initial negative relief producing high points of lands behind the cutting edges.

Traces (b) of Figure 17 are taken from a helical fluted tap without radial relief. These show the lands to be slightly variable, with the land from which the second trace was taken in fact showing a degree of negative relief.

Figure 18 -shows respective, traces over a full revolution, taken from two different fluteless taps. These show the taps generally to be of acceptable quality, although the second tap has slight under-cutting after each pea while the third peak., shows the absence of a desirable uniform interval.

It will be appreciated from the foregoing description of chosen embodiments, that a simple and accurate instrument and technique has been devised for the direct recording of the radial relief of cylindrical-form tools such as taps. It will be appreciated by those skilled in the art, however, that many modifications and additions can be made to the technique, ^' the instrument or the jig described without departing from the principles or scope of the present invention.

It is not essential, for example, to mount the workpi between centres as some workpieces may have no centres provided (either female or male) and some other known method of mounting the workpiece will need to be employed - for example the use of diaphram or collet-chuck mounted on a rotatable spindle. It i vital, however, that the method of holding should be extremely precise as any run out of either the chuck or workpiece will be reproduced in the trace at high magnification. Where a chuck and rotatable spindle are employed it would be convenient to mount the rotary transducer on the spindle itself, thereby avoiding the need for the pulleys and belts as illustrated.

Though not shown inthe particular embodiments illustrated, a simple locking device, such as a set screw, may be used to lock the stylus carriage in a fixed position for certain types of measurement. This would be appropriate where ovality or surface finish of a plain cylindrical workpiec was to be measured or, more particularly, where the radial relief of a tap chamfer or of an unthreaded cutting tool was to be measured .

These and many other variations and modifications can be made without departing from the scope of this invention.

Claims

1. An instrument for measuring a peripheral contour of a cylindrical-form helically threaded workpiece; said instrument having means for mounting the workpiece for rotation about its axis; means for bringing and maintaining a measuring stylus in contact with the periphery of the workpiece as the latter is rotated; a first mechano-electric transducer operatively connectable to the workpiece and adapted to generate an electrical signal related to the angular position of the workpiece; and a second mechano- electric transducer operatively associated with the stylus and adapted to generate an electrical signal related to the radial position of the stylus; the means for bringing and maintaining the stylus being adapted to enable the stylus to move both parallel to, and perpendicularly of, the axis of the workpiece so as to follow a peripheral contour of the workpiece.

2. An instrument according to claim 1, wherein said stylus is mounted on a carriage assembly movable parallel to the workpiece axis, the carriage assembly being on a support means resiliently mounted for movement perpendic¬ ularly of that axis.

3. An instrument according to claim 2, wherein the support means is an arm extending substantially parallel to the workpiece axis, the carriage assembly being mounted adjacent one end of the arm with the arm being pivotally mounted at its other end so as to be resiliently pivotable in a plane in which the workpiece axis is located.

4. An instrument according to claim 3, wherein the arm is pivotally mounted by means of a leaf spring. 5. An instrument according to claim 2, wherein said support means is resiliently mounted for movement perpendicu¬ larly of the workpiece axis by means of leaf springs spaced in the direction of that movement and extending perpendicularly with respect to both that direction and the workpiece axis. 6- An instrument according to any one of claims 2 to

5. wherein the carriage assembly includes a carriage on which

OM the stylus is mounted and guide means for guiding the carriage in movement parallel to the workpiece axis between opposed stop members.

7. An instrument according to claim 6 , wherein the 5 guide means includes a lapped pin extending through the carriage and between the stop members, and a ball bearing mounted on the carriage to one side of the pin and riding along a guide surface parallel to the pin.

8. An instrument according to claim 6, wherein the 10 guide means includes a pair of guide members between which the carriage is guided, each guide member defining a vee-way in which is located ball bearings on which a respective side of the carriage is slidable.

9. An instrument according to any one of claims

15 1 to 8, wherein the first transducer is a rotary transducer connectable to the workpiece via the means for rotating the latter.

10. An instrument according to claim 9 , wherein the first transducer is a multi-turn potentiometer having a

20 shaft connected to and rotatable by the means for rotating the workpiece.

11. An instrument according to any one of claims 1 to 10, wherein the second transducer is a linear transducer positioned in contact with the stylus so as to be responsiv

2 to movement of the stylus perpendicularly of the axis ofthe workpiece.

12. An instrument according to claim 11, wherein the second transducer is in contact with the stylus via a membe movable with the stylus in the direction perpendicular

³⁰ movement of the latter but fixed against movement parallel to the axis of the workpiece.

13. An instrument according to any one of claims 1 to 12, further including an "X-Y" recorder having a "X" input connected to the first transducer and a "Y" input connected

^■a tr to the second transducer for receiving the respective signals generated by the transducers, the 'recorder being adapted to provide a representation of the measured profile.

14. An instrument according to claim 14, wherein the recorder is adapted to provide the representation as a plot ⁰ on X-Y co-ordinate chart. _^-c-τ _$p^*- f ____. OMPI

15. An instrument according to any one of claims 1 to 14, wherein the mounting means comprises .a spaced pair of precision centres each having a centre point locating the workpiece therebetween, the means for rotating the workpiece including a pulley, on the axes of the centre points and providing a driving connection between the workpiece and the first transducer.

16. A method for measuring a peripheral contour of a cylindrical-form, helically threaded workpiece wherein the workpiece is mounted for rotation about its axis and a measuring stylus is brought into and maintained in contact with the periphery of the workpiece so as to be able to move both parallel to, and perpendicularly of, the axis of the workpiece, and wherein the workpiece is rotated about its axis and rotary movement of the workpiece and radial movement of the stylus as it follows a peripheral contour of the workpiece are converted to respective electrical signals related to the angular position of the workpiece and the radial position of the stylus.

17. A method according to claim 16, wherein the stylus is resiliently maintained in contact with the workpiece so as to be movable radially of the workpiece, the stylus being mounted on a carriage assembly movable parallel to the axis of the workpiece and moved with the assembly, in following the profile of the workpiece, during rotation of the latter.

18. A method according to claim 17, wherein the carriage assembly is movable at one end of a support arm pivotally mounted at its other end to enable resilient movement of the assembly and stylus, the stylus being brought into contact with the workpiece by adjusting the support arm to a required parallel relationship with the workpiece axis. 19- method according to claim 17, wherein the carriage assembly is movable on a support block mounted for pivotable movement perpendicularly of the workpiece axis to enable resilient movement of the assembly and stylus, the stylus being brought into contact with the workpiece by adjusting the support block to a required spacing from the workpiece. 20. A method according to any one of claims 16 to

19. wherein rotary movement of the workpiece is converted to the one of the respective signals related to angular position by a first, rotary transducer and radial movement of the stylus is converted to the other of the. signals by a second, linear transducer, said signals being used to obtain a representation of the measured profile of the workpiece.

20. A method according to claim 20, wherein the signal related to angular position is passed to the "X" input of an "X-Y" recorder and the other signal passed to the "Y" input of the recorder to obtain a representation of the measured profile.

21. A method according to claim 19 or claim 20, wherein the workpiece is rotated by a pulley co-axial therewith, the first transducer being a multi-turn potentiometer having a shaft rotatable with and driven by the pulley.

OMPI