GB2156077A - Apparatus for providing a reference - Google Patents

Abstract

This invention relates to apparatus for providing a height and orientation reference and in particular, but not exceptionally, to fixtures which incorporate such apparatus and may be used for locating workpieces. The apparatus 30 comprises a base element 33, an intermediate element 34 and a top element 35 which are essentially concerned with providing an orientation although they contribute to defining the height. The apparatus also includes a second group of elements 32 comprising a number of shims or washers 41 of different thickness which allow 'fine tuning' of the height. The particular orientation is selected by appropriate interengagement of the pins 33a in recesses 36 and the teeth 37, 38 of elements 33 to 35. <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. Our co-pending British Patent Application No.8305392 described one particular construction for overcoming the problem of accurately positioning and orientating a workpiece for machining by a numerically controlled machine.

Whilst the apparatus described in that application is very satisfactory for many uses, it does not readily provide the degree of angular resolution which is required in certain uses.

It is an object of the invention to provide apparatus which gives good angular resolution in the defined orientation of the reference.

From one aspect the invention consists in apparatus for providing a reference, comprising a set of elements, 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, at least a pair of the elements being interengageable by means of respective sets of regularly angularly spaced formations.

In a preferred embodiment at least some of the elements have different heights or thicknesses and the formations are constituted by teeth or co-operating projections and holes or recesses. When there is plurality of co-operating pairs of sets of formations, each pair preferably has a unique number of formations in its set and conveniently those numbers have few or no common factors.

The apparatus further comprises locating means for holding a stack of elements at a predetermined location in a plane and the locating means may selectably locate the orientation of the first element in the stack. For example the locating means may include a set of regularly angularly spaced formations for engaging the co-operating set on the first element. Conveniently the locating means formations are projections, holes or recesses.

It is preferred that the number of formation means set is unique, vis-a-vis the other sets of formations and that the number of formations in the locating means set has few or no common factors with the number in the other sets.

The elements may have apertures, in which case the locating means may include a post on which the elements can be threaded.

In any of the above cases means may be provided for locking the member on the top of the stack such that its orientation is determined by that of the reference.

The invention also 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 constituted by the apparatus defined above 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 fixture may have a locating aperture for the post having a series of equally spaced formations formed around it to locate the first element in the stack.

From another aspect the invention consists in a measurement or reference device comprising a single disc or the like having a precise thickness and a respective set of regularly angularly. spaced formations on each face for engaging co-operating formations on other discs or surfaces, the number of formations in each set being different and having no or few common factors.

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.

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

The invention may be performed in various ways and a specific embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an exploded view of the set of elements constituting reference providing apparatus and a locating base plate; and Figure 2 is an isometric view showing an assembled stack having an adaptor mounted thereon with a further stack and locating member supported by the adaptor.

As mentioned above the set of elements described in our co-pending application No.8305392 are simple to make and ideal for many purposes, however for certain uses they do not provide a sufficient degree of angular accuracy for the reference. This problem may be overcome by using an alternative set of elements generally indicated at 30 in Figure 1. The elements 30 are divided into two groups 31 and 32: the first, 31, is essentially concerned with providing orientation although it contributes to defining the height, whilst the second, 32, is purely concerned with height.

The first group 32 comprises a base element 33, one or more intermediate elements 34 and a top element 35. The base element is provided, at its lower surface, with five equally angularly spaced pins 33a, which locate in recesses 38 in the base plate 1 8 as will be described below. Its upper surface has a number of upwardly projecting teeth 37 which engage a corresponding number of teeth 38 on a lower face of the intermediate element 34. The upper face of the intermediate element 34 in turn has a different number of teeth which engage with the same number of teeth on the lower face of the top element 35. Six equally angularly spaced pins 40 project upwardly from the upper face of the top element 35.

The second group of elements 32 comprises a number of shims or washers 41 of different thickness which are apertured so that they can be slid on to the pins 40 to sit on the top element 35. The thicknesses of these shims are determined in accordance with the "postage stamp" problem described in copending application No.8305392 If there are 29 teeth 37, 38 then there are 29 different rotational positions in which the intermediate element 34 can be placed on the base element 33. Similarly, if there are 31 teeth on the top face of the intermediate element 34 and the bottom face of the top element 35 there are 31 rotational positions in which the top element can be located on the intermediate element 34. Thus it can readily be seen that the top element 35 can sit in 29 x 31 rotational positions relative to the base element 33.The set of rotational positions is further increased because the base element 33 can be located in six different rotational positions in the recesses 36. Further, the locating member 24 is provided with recesses (not shown) for receiving the pins 40 and there are thus five orientations in which it can sit with respect to the top element 35.

As there are no common number or factors in the sets of formations defined by the interengaging teeth or pins and recesses there should be no redundancy in the orientations defined, so that the total number of orientations available should be approximately equal to 5 X 6 > c 29 x 31,oralmost27,000 positions. A limited number of common factors would reduce the number of available positions but would be acceptable in many instances. Thus, a very high degree of angular resolution is available. Obviously, the elements in group 31 contribute to the height and it may be necessary to provide a range of such elements, but it is desirable to keep these to a minimum because of their difficult construction. Accordingly, the "fine tuning" of the height is carried out with the shims of set 41.

It is desirable to lock the elements against axial movement and accordingly, as in our previous application No.8305392, posts are provided which could be threadingly engaged at their lower end in holes 1 9 in the base plate 1 8. The upper end of the post (not shown) has a head 43 or is threaded to receive a nut. The holes 1 9 are surrounded by the recesses 36.

It is the nature of interengaging teeth of this sort described that they are self-centring and there is therefore no need for the post (not shown) or the holes 1 9 to be extremely accurately located; the post 20 merely serves to lock the elements in the stack either by its head 43, or a nut, bearing down on an internal shoulder in an adaptor 44 or a workpiece locating member 47 placed on the top of the stack (see below). The recesses 36 in the base plate 1 8 of course need to be very accurately located. The holes 19, and the sets of recesses 36 which surround them can be arranged in any grid, but an hexagonal grid of the type shown in Figures 1 and 2 is preferred. In this arrangement recesses 36 lying between holes 1 9 are shared between the holes.

In order to provide good coverage of the whole area of the base plate 18, an adaptor 44, as shown in Figure 2 may be provided.

This sits on top of a first stack of elements 45 and carries at the free end of its arms a further stack of elements 46. A workpiece locating member 47 sits on the top of the stack 46.

It will be appreciated that holes and recesses can be substituted for the teeth if desired, but the construction shown in the drawings is believed to be particularly advantageous. Similarly the pins 33a may be reduced to two; a first which locates the base element 33 in a horizontal plane and a second which restrains the element 33 from rotation about the first pin. The second pin may be diamond shaped in cross section or otherwise suitably cut away.

Claims (25)

1. Apparatus for providing a reference comprising a set of elements, 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, at least a pair of elements being interengageable by means of respective sets of regularly angularly spaced formations.

2. Apparatus as claimed in claim l,wherein at least some of the elements have different heights or thicknesses.

3. Apparatus as claimed in claim 1, wherein the formations are teeth.

4. Apparatus as claimed in claim 1, wherein one set of formations is constituted by projections and the other by holes or recesses.

5. Apparatus as claimed in any one of claims 1 to 4, wherein there is a plurality of co-operating pairs of sets of formations and wherein each pair has a unique number of formations in its set.

6. Apparatus as claimed in claim 5 wherein the numbers have few or no common factors.

7. 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.

8. Apparatus as claimed in claim 1 wherein the locating means further selectably locates the orientation of the first element in the stack.

9. Apparatus as claimed in claim 7 or claim 8, wherein the locating means includes a set of regularly angularly spaced formations for engaging a co-operating set on an element.

10. Apparatus as claimed in claim 9, wherein the set of formations in the locating means are projections, holes or recesses.

11. Apparatus as claimed in claim 7 or claim 8, wherein the location means includes a round pin or the like locating the first element and a second pin or the like for preventing rotation of the element about the first pin.

12. Apparatus as claimed in any one of claims 7 to 11, wherein the number of formations in the locating means set of formations is unique vis-a-vis the other sets of formations.

1 3. Apparatus as claimed in claim 12, wherein the number of formations in the locating set means has no or few common factors with the number in the other sets of formations.

1 4. Apparatus as claimed in any one of claims 7 to 13, wherein the locating means include a post on which the elements can be threaded.

1 5. Apparatus as claimed in claim 14, wherein the post is surrounded by a series of equally spaced apertures or recesses for receiving a pin.

1 6. Apparatus as claimed in any one of the preceding claims further including means for locking a member on the top of the stack such that its orientation is determined by that of the reference.

1 7. An element for use in the apparatus in any one of claims 1 to 16.

18. 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 constituted by the apparatus of any one of claims 1 to 1 5 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.

1 9. A fixture as claimed in claim 18, wherein each location comprises a locating aperture for the post having a series of equally spaced formations formed around it to locate the first element in the stack.

20. A fixture as claimed in claim 1 8 or 1 9 wherein the set of members also includes a member having a location for a post and/or for a stack of elements.

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

22. A measurement or reference device comprising a single disc or the like having a precise thickness and a respective set of regularly angularly spaced formations on each face for engaging co-operating formations on other discs or surfaces, the number of formations in each set being different and having no or few common factors.

23. Apparatus for providing a reference substantially as hereinbefore described with reference to the accompanying drawings.

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

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