GB2190497A - Measuring pitch circle diameters - Google Patents

Abstract

A method of measuring a pitch circle diameter comprises positioning a pair of locating devices 12 on the pitch circle, the locating devices of the pair being spaced apart by a predetermined distance, moving a third locating device 15 along a predetermined path 24 with respect to the said pair until the third locating device is also on the pitch circle, and determining the pitch circle diameter utilising the distance between the said pair and the third locating device.The third locating device 15 can be moved along the path 24 by means of an elongate slide to which is attached a cursor 20. Fine adjustment, eg by Vernier scale, is controlled by adjusting screws, eg 14. <IMAGE>

Description

SPECIFICATION Measurement of pitch circle diameters It is frequently desirable to be able to measurethe diameter of a pitch circle, for example when machining an annular groove in aflangewhich isto receive a sealing member having particular dimensions. However such measurement is not easy and generally involves accurately positioning two locating devices on the pitch circle, spaced apart by exactly 1800, locating a central point, and then measuring the distance between the centres ofthe locating devices to give the pitch circle diameter.

We have now realised that it is possible to measure accurately pitch circle diameters by a much simpler method.

Accordingly, the invention provides a method of measuring a pitch circle diameter comprising positioning a pair of locating devices on the pitch circle, the locating devices of the pair being spaced apart buy a predetermined distance, moving a third locating device along a predetermined path with respect to the said pair until the third locating device is also on the pitch circle, and determining the pitch circle diameter utilising the distance between the said pair and the third locating device.

The invention also provides apparatus for measuring a pitch circle diameter, the apparatus comprising a pair of locating devices positionable on a pitch circle and spaced apart by a predetermined distance, a third locating device guided for movement along a predetermined path with respect tothesaid pair, and meansto measurethe movementofthethird locating device along the said path.

Preferably the apparatus is generally T-shaped, the said pair being positioned respectively on the arms of the T, and the leg of the T comprising the said path.

Preferably the leg of the T is provided with at least onescaleto assist in the said measurement.

Preferably the leg of the T is provided with at least one scale arranged and calibrated such that the pitch circle diameter can be read-offonthe leg oftheT without the need to make subsequent calculations.

When the a pparatus is for use in measuring the pitch circle diameter of a groove, the said locating devices may comprise balls arranged to seat in the groove.

The leg ofthe T may have a presetting device thereon for presetting the device to check the diameter of a desired size of pitch circle.

The presetting device may comprise a slide having a first scale comprising a plurality of spaced apart markings each corresponding to a desired size of pitch circle, the slide being movable adjacent to a second scale on the leg of the T, the second scale having a plurality of similar spaced apart markings, the presetting being brought about by moving the slide until the marking corresponding to the desired size of pitch circle on the slide is in register with the same marking on the leg oftheT.

Preferablytheslidehasathirdscalewhich co-operates with a fourth scale on the leg of the Tfor use in reading offthe pitch circle diameter.

The said fourth scale may comprise a Vernier scale.

Byway of example, a specific embodiment of the invention will now be described, with reference to the accompanying drawings, in which Figure lisa side view of an embodiment of measuring apparatus according to the invention; and Figure2 is a plan view of the apparatus shown in Figure 1.

The apparatus is generally T-shaped, the T having arms 10 and 11 and a leg 24. Projecting downwardly from the free end of each ofthe arms 10 and 11 is a hardened steel ball 12.

Housed in one side face of the leg 24 there is an elongate slide member 13 which can be moved longitudinallywith respecttotheleg24byturninga knurled roller 14. The roller 14 is mounted in the end ofthe leg 24 and is prevented from moving longitudinallywith respecttothe leg. The roller 14 has a threaded hole (not shown) therethrough which engages with a threaded projection (not shown) on the end ofthe slide 13 such that when the roller 14is rotated the slide 13 moves longitudinally.

Mounted to slide on the outer periphery of the leg 24 is a cu rsor 20 which has mounted on its underside a third hardened steel ball 15. The cursor can be locked in position using a locking screw 16. It has a conventional Vernier scale 17 thereon and a conventional fine adjustment device comprising a smaller cursor 18with its own locking screw 19, the smallercursor 18 being connected to the larger cursor 20 by a threaded rod 21 which engages with a rotatable roller 22 captive in the smaller cursor 18.

The apparatus is for checking the pitch circle diameter of annular grooves which areto be machined in the flanges of lengths of oil pipeline, in orderto receive a sealing ring of standard dimensions. These grooves are desired to have standard dimensions. The grooves have a flat base and outwardly inclined walls. The standard groove having the code number 47 for example, should have a pitch circle diameter of 9.000, a depth of .500 and a mouth width of .781. These dimensions are in inches.

If it is desired to check the pitch circle diameter of such a groove,for example during machining, the roller 14 is rotated to bring a scale markdesignated 47 on the slide 13 into registerwith the scale mark designated 47 on the leg 24 ofthe T. Although only the two scale marks designated 47 are shown in Figure 1, it should be understoodthatthe slide 13 and leg 24will in practice carry a plurality of spaced apart scale marks, each marked with one of the code numbers ofthe standard grooves.

Once the slide 13 has been preset in this way to suit the particular standard groove, it is only necessary to position the two balls 12 in the groove, release the locking screws 16 and 19, and slide the cursors 18 and 20 until the third ball 15 also seats neatly in the groove. The atual pitch circle diameter can then be read-offbycomparingtheVernierscale 17 on the cursorwith a Linear scale 23 on the slide 13. If fine adjustment of the position of the ball 15 is required, the locking screw 19 can be turned to lock the small cursor 18 on the leg oftheT, and the roller22 can then be rotated to move the cursor 20 and hence ball l5witha high degree ofcontrol.

It will be appreciated that the apparatus can be used rapidly and very simply. lttakesjust a second or two to preset the slide 13 using the roller 14, seatthe three balls in the groove, and make the necessary reading on the Vernier scale.

The invention has been made possible by the realisation that it is not necessary to actually measure the diameter of the pitch circle by accurately positioning two locating devices at 1 80" interval and measuring the distance between the centres ofthe locating devices. It has been realised that even though the two balls 12 are maintained a predetermined distance apart,the distance between those balls 12 and the ball 15will vary in a predetermined manner according to the diameter of the pitch circle. Thus the pitch circle diameter can be read-off by applying appropriate calibrated scales to the leg 24 and the slide 13.

Although the mathematics required to calculate the scales and their positions is lengthy, it is a routine matterwhich can be carried out by any person skilled in the artandthe mathematics will therefore not be stout in detail here.

The invention is not restricted to the details ofthe foregoing example. Forinstancethe balls 12 and 15 may be interchangeabie with balls of different diameters to suit different widths ofgroove.

Claims (12)

1. A method of measuring a pitch circle diameter comprising positioning a pair of locating devices on the pitch circle, the locating devices ofthe pair being spaced apart by a predetermined distance, moving a third locating device along a predetermined path with respect to the said pair until the third locating device is also on the pitch circle, and determining the pitch circle diameter utilising the distance between the said pair and the third locating device.

2. Apparatus for measuring a pitch circle diameter the apparatus comprising a pair of locating devices positionable on a pitch circle and spaced apart by a predetermined distance, a third locating device guided for movement along a predetermined path with respectto the said pair, and means to measure the movement ofthe third locating device along the said path.

3. Apparatus as claimed in Claim 2, being generallyT-shaped, the said pair being positioned respectively on the arms of the T, and the leg oftheT comprising the said path.

4. Apparatus as claimed in Claim 3, in which the leg of the T is provided with at least one scale to assist in the said measurement.

5. Apparatus as claimed in Claim 4, in which the leg of the T is provided with at least one scale arranged and calibrated such that the pitch circle diameter can be read-off on the leg of the Twithout the need to make subsequent calculations.

6. Apparatus as claimed in any one of the Claims 3to 5, in which the leg ofthe T has a presetting device thereon for presetting the deviceto checkthe diameter of a desired size of pitch circle diameter.

7. Apparatus as claimed in Claim 6, in which the presetting device comprises a slide having afirst scale comprising a plurality of spaced apart markings each corresponding to a desired size of pitch circle, the slide being movable adjacent to a second scale on the leg ofthe T, the second scale having a plurality of similar spaced apart markings, the presetting being brought about by moving the slide until the marking corresponding to a desired size of pitch circle on the slide is in register with the same marking on the leg of the T.

8. Apparatus as claimed in Claim 7, in which the slide has a third scale which co-operates with a fourth scale on the leg of the Tfor use in reading off the pitch circle diameter.

9. Apparatus as claimed in Claim 8, in which the fourth scale comprises a Vernier scale.

10. Apparatus as claimed in any one ofthe preceding claims, for use in measuring the pitch circle diameter of a groove, in which the said locating devices comprise balls arranged to seat in the groove.

11. A method of measuring a pitch circle diameter, substantially as described herein with reference to the accompanying drawings.

12. Apparatus for measuring a pitch circle diameter, substantially as herein described with reference to the accompanying drawings.