GB2136129A - Apparatus for Providing a Reference Position - Google Patents

Abstract

Apparatus for providing a reference indicating a position and orientation in space comprises a set of elements 10 mountable on a base plate 18. Each element 10 comprises an annular disc 11 having a plurality of apertures 12 which are spaced at unequal intervals around the axis 13 of the disc 11. The pin 14 extends from the underface of each disc and is dimensioned to be received in an aperture 12 of another element to the set. By stacking appropriately selected discs a particular height can be achieved. The orientation of the top disc can similarly be determined by selecting appropriate pin positions on the supporting disc 11 or base plate 18. The combination of the pin positions determines the particular position of the holes of top disc. Workpiece holding members 24 can then be mounted on the stack. <IMAGE>

Description

SPECIFICATION Apparatus for Providing a Reference This invention relates to apparatus for providing a reference and in particular, but not exclusively, to fixtures which incorporate such apparatus and may be used for locating workpieces.

In many fields it is desirable to be able to set an object, tool or the like at a particular position in space and at a particular orientation. Slip gauges are known for providing accurate measurement of length, but they give no information as to the orientation. The problem of accurate positioning and orientating is particularly acute in the setting up of machine tools and at present this is achieved by machining fixtures or by having standard components assembled by skilled operators. The fine adjustments required in this latter method make it extremely difficult to position a workpiece to the same degree of accuracy as can be achieved during machining by a numerically controlled machine. In addition the traditional method of holding workpieces cannot be operated by a robot.

It is an object of the invention to provide apparatus which gives an accurate determination of position and orientation and, in some embodiments of the invention, to provide a fixture for the workpiece having these features.

From one aspect the invention consists in an apparatus for providing a reference, comprising a set of elements having different heights or thicknesses, the elements being inter-engageable at different selectable predetermined orientations with respect to one another, so that by stacking appropriate elements in the correct respective orientations a reference is held in a desired location with a desired orientation.

In a preferred embodiment the elements are interengageable by means of a single projection on one element engaging in one of a set of unequally annularly spaced recesses, apertures, or the like, on another element. For example, the element may be an annular disc, or the like, having a series of recesses or apertures spaced unequally around the axis thereof and a pin projecting from the disc for engaging in a recess or aperture in an adjoining disc when the discs are stacked. Preferably the spacing between the recesses, or the like, increases in one direction around the disc.

The apparatus may further comprise locating means for holding a stack of elements at a predetermined location in a plane and additionally the locking means may further selectably locate the orientation of the first element in the stack.

For example, the locating means may include a post on which the discs can be threaded and the post may be surrounded by a series of equally spaced apertures or recesses for receiving the pin of an element.

The apparatus may further include means for locating a member on the top-most element in the stack such that its orientation is determined by that of the reference. In this instance the reference may be a particular recess on the disc and the member may have a pin which engages therein.

The invention also includes an element for use in the apparatus defined above.

From another aspect the invention consists in a fixture for holding a workpiece, comprising a base having an array of locations for locating posts to project from the base, a set of posts engageable in the locations, a set of elements stackable on the posts to provide a surface at a desired spacing from the base and to define a desired orientation about the axis of the post, a set of members for supporting, locking or clamping a workpiece, each being engageable on a post to sit on the surface at the height and orientation defined thereby, the arrangement being such that with a suitable choice of posts, locations, elements and members, the workpiece can be held at a desired position and orientation relative to the base.

The elements may be discs of the sort described above and each location may comprise a locating aperture for the post surrounded by an annular recess for receiving a disc and having a series of equally spaced recesses formed around its axis to locate the pin of that disc. Preferably a part of each location is frusto-conical to locate and support a corresponding portion of the post.

The set of members may be of any shape suitable for supporting, locating or clamping the workpiece, for example, they may be cuboid or spherical and preferably a range of shapes in included. In addition the set may include a member having a location for a post to support a further stack.

From a further aspect the invention consists in a measurement or reference device comprising a single disc or the like having a precise thickness, a .series of recesses or apertures spaced unequally around the axis thereof and a projection extending from one face to engage in a recess or aperature of a like disc when the two are stacked one on the other.

It will be appreciated that references to the elements being stacked do not indicate any particular orientation of that stack and thus the top-most element is that which is furthest away from the first element in the stack.

Although the apparatus of the invention has been defined above, the invention may also include any other inventive combination of the features set out in this specification.

The invention may be performed in various ways, specific embodiments of which will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view from above of an element for use in the apparatus of the invention; Figure 2 is a cross-sectional view along the line Il-Il; Figure 3 is a plan view from above of a mixture for a workpiece, incorporating the element of Figure 1; Figure 4 is a side view of the fixture of Figure 3; Figure 5 is a plan view of a detail of a locating aperture of the fixture of Figure 4 on a different scale; and Figure 6 is a perspective view of an assembly of a post, elements and a member of the fixture of Figure 5.

Figures 1 and 2 show one of a set of elements 10 for providing a reference indicating a position and orientation in space. Each element 10 comprises an annular disc 11 having a plurality of apertures 12 (numbered 1 to 8) which are spaced at unequal intervals around the axis 13 of the disc 11. Each disc has a pin 14 which extends from the underface 1 5 thereof and which is dimensioned to be received in aperture 12 of another element of the set. The disc 11 has a cut 1 6 to avoid binding in certain uses.

The spacing of the holes is selected to enable a continuous series of angular values to be achieved when a minimum number of elements are astacked together. The actual angles of the centre lines passing through the apertures depends on the number of apertures used and the resolution required. The coordinates given in the table below gave a resolution of T of a degree:: Pin and Hole Coordinates (Pitch Circle Radius=14.5 mm) Angle X Y 16 (Pin) (0) 14.5 1 (36) 11.731 8.523 2 (72.25) 4.420 13.810 3 (109.25) -4.780 13.689 4 (146) -12.021 8.108 5 (186.75) -14.399 -1.704 6 (2223.25) -10.561 -9.935 7 (262.5) -1.893 -14.375 8 (298.75) 6.974 -12.712 MM MM Similar considerations apply to the location of the thicknesses of the discs in a set and here the selection depends on the number of discs required, the minimum thickness of the disc, the desired resolution and the minimum length to be determined by the discs.The table below gives one possible set of disc thicknesses, which give a resolution of 0.02 mm in stacks of four or more.

Disc Thicknesses 'W' A 3.24 MM B 3.26 MM C 3.32 MM D 3.42 MM E 3.72 MM F 3.94 MM G 4.22 MM H 4.26 MM 4 off of each In principal the selection of the thickness and angles are determined by using an ampirical solution of what is known as the "postage stamp" problem.

It will be seen that if the discs are stacked with their respective pins in a selected sequence of holes then the upper surface of the stack is a known height from the underface of the bottom element in the stack and each of the apertures 12 of the top-most element is at a known orientation.

Normally, it would be desired to locate the stack at a particular position in space and this can be achieved by threading the stack on a column or post, which should be encircled, at its base, with equally spaced recesses for receiving the pin of the bottom-most element in the stack. The apertures in the element can provide an angular variation for a given aperture for about 300 and thus by correctly orientating the bottom element with respect to the post it is possible to provide a reference aperture through the whole 3600.

This arrangement is extremely useful for the setting up of any machine or element, but the apparatus described above can be particularly usefully incorporated into a mixture for a workpiece.

Such a fixture is generally indicted at 1 7 in Figures 3 and 4 and comprises a base plate 18, which has an array of locating holes 1 9, a set of posts 20, a set of elements 10 and a set of locating, supporting or clamping members 21.

When it is desired to support the workpiece 30, for example for machining, above the base plate 18, posts 20 are inserted in the appropriate locating holes 1 9 and suitably selected sets of discs 11 are threaded on to the posts with their pins inserted in appropriate apertures 12. A member 21 is then itself threaded on to each post and locked thereto by a nut 22, see Figure 6. Each member 21 has a pin (not shown) which is identical to pin 14 and which can be located in an appropriate aperture 1 2 in the top-most disc 11 in the stack so that the stack determines both the position and orientation of the member.

Different shaped members are provided for different purpose. For instance, supporting members 23 have flat upper surfaces with a recess to receive nuts 22, whereas locating members 24 may have flat or curved surfaces, or projecting pins for engaging the sides of the workpiece. A further set of clamping members 25 may be provided to secure the workpiece.

Thus for any particular workpiece and any machining operation there is a particular combination of post positions, elements and members which will hold a workpiece rigidly in the correct position for the operation. All of these factors can be readily calculated and can be fed as an instruction to an industrial robot of other machine which can assemble the necessary fixture from a store of posts, elements, members and nuts.

The details of the locating holes 1 9 are shown in Figure 5 and it will be seen that the holes 19 comprise an annular recess 26 which surrounds a frusto-conical portion 27, which terminates in a threaded aperture 28. Each post can be threaded into the aperture 28 and has a frusto-conical portion which engages that of the hole 1 9 so that the location of the posts is precisely defined and is not determined by the engagement of the threads. This arrangement could be further improved by locking the bottom end of the post 20 with a further nut (not snown) and inserting a resilient washer between the nut and the base plate 1 8. The recess 26 is dimensioned to receive the bottom-most element 10 in the stack and has equally spaced holes 26a for receiving the pin 14.

In some instances it may be useful to have an assembly of posts, by providing further members 21, which have locating holes 19 formed in them.

Such a chain of stacks could be used to provide a spatial location not limited by the plane of the base plate or the frequency of the locating features incorporated into it.

It will be appreciated that the construction of the elements 10 may be varied, for example, it is not essential that they form a closed loop nor does the pin have to extend from the under-face 15 of the disc 11. It could for example be cranked from the edge of the disc and have an inwardly projecting tooth, which could engage in angularly spaced detents on the edge of the disc. This could have the advantage of allowing the disc to lie flat on a surface, without the need for any further recesses. Further, particularly where the elements 10 are being used merely to provide a reference and are not supporting a member, it would be possible to dispense with the posts by having cup-shaped centre portions to the elements so that the centre portion sat one within the other as the elements were built up. The bottom-most portion could then sit in a simple recess in the base plate.

Similarly the fixture 1 7 need not be restricted to use with machine tools, but can be used to clamp any object, which requires accurate location, for example a prism in a physics experiment.

Claims (20)

1. Apparatus for providing a reference, comprising a set of elements having different heights or thicknesses, the elements being interengageable at different selectable predetermined orientations with respect to one another, so that by stacking appropriate elements in the correct respective orientations a reference is held in a desired location with a desired orientation.

2. Apparatus as claimed in claim 1, wherein the elements are inter-engageable by means of a single projection on one element engaging in one of a set of unequally angulariy spaced recesses, apertures or the like on another element.

3. Apparatus as claimed in claim 1 wherein the elements are annular discs, or the like, each having a series of recesses or apertures spaced unequally around the axis thereof and a pin projecting from the disc for engaging in a recess or aperture in an adjacent disc when the discs are stacked.

4. Apparatus as claimed in claim 3, wherein the spacing between the recesses increases in one direction around the disc.

5. Apparatus as claimed in any one of the preceding claims, further comprising locating means for holding a stack of elements at a predetermined location in a plane.

6. Apparatus as claimed in claim 5 wherein the locating means further selectably locates the orientation of the first element in the stack.

7. Apparatus as claimed in claim 5 or claim 6, when dependant on claim 3, wherein the locating means include a post on which the discs can be threaded.

8. Apparatus as claimed in claim 7, when dependant on claim 5, wherein the post is surrounded by a series of equally spaced apertures or recesses for receiving a pin.

9. Apparatus as claimed in any one of the preceding claims further including means for locking a member to the top-most element in a stack such that its orientation is determined by that of the reference.

10. An element for use in the apparatus in any one of claims 1 to 9.

11. A fixture for holding a workpiece comprising a base having an array of locations for locating posts to project from the base, a set of posts engageable in the locations, a set of elements stackable on the posts to provide a surface at a desired spacing from the base and to define a desired orientation about the axis of the post, a set of members for supporting, locking or clamping a workpiece, each being engageable on a post to sit on the surface at the height and orientation defined thereby, the arrangement being such that with a suitable choice of posts, locations, elements and members, the workpiece can be held at a desired position and orientation relative to the base.

12. A fixture as claimed in claim 11 wherein the elements are annular discs, each having a series of recesses or apertures spaced unequally around the axis thereof and a pin projecting from the disc for engaging in a recess or aperture in an adjacent disc when the discs are stacked.

13. A fixture as claimed in claim 12 wherein each location comprises a locating aperture for the post surrounded by an annular recess for receiving a disc and having a series of equally spaced recesses formed around its axis to locate the pin of a disc.

14. A fixture as claimed in claim 13 wherein a part of each location is frusto-conical to locate and support a corresponding portion of a post.

1 5. A fixture as claimed in any one of claims 12 to 14 wherein the set of members also includes a member having a location for a post.

16. A fixture as claimed in any one of the preceding claims wherein a member has a generally spherical surface for engaging the workpiece.

1 7. A measurement or reference device comprising a single disc or the like having a precise thickness and a series of recesses or apertures spaced unequally around the axis thereof and a projection extending from one face to engage in a recess or aperture of a like disc when the two are stacked one on the other.

1 8. Appartus for providing a reference substantially as hereinbefore described with reference to the accompanying drawings.

1 9. A fixture for a workpiece as hereinbefore described with reference to the accompanying drawings.

20. A measurement or reference device substantially as hereinbefore described with reference to the accompanying drawings.