GB2078563A

GB2078563A - Work Holding Fixture

Info

Publication number: GB2078563A
Application number: GB8020193A
Authority: GB
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1980-06-20
Filing date: 1980-06-20
Publication date: 1982-01-13
Also published as: GB2078563B

Abstract

A machine tool fixture comprises a pair of spaced facing turntables 14, 15 each having a vice between which a workpiece blank 18 is held. The fixture can be indexed to any angular position about an indexing axis by a proprietary electronic indexing head 27 for machinery of the workpiece by a tool 11. A platen arrangement has fixing means for attaching a platen to the turntables so that it abuts the already held workpiece and supports it during subsequent machining. The platen may also have an array of tapped holes to which clamps can be secured to hold the workpiece to the platen before the tool 11 parts the workpiece from its held ends, after which it is removed with the platen. <IMAGE>

Description

SPECIFICATION Machine Tool Fixture ; This invention relates to machine tool fixtures 4and in particular to a fixture providing an additional degree of movement rotatably about an indexing axis extending at right angles to the rotational axis of the tool.

It is common practice to produce components for apparatus, such as avionic apparatus, by machining solid slabs or plates of aluminium alloy on numerically controlled milling machines, control of the machine as to cutting position being determined as a function of the position relative to the machine bed of a platen to which the slab or workpiece is secured. For each face of the slab to be worked, it has to be secured to same or a different platen with a new face directed towards the tool. Repositioning the workpiece on the tool is a skilful and time consuming operation as great care is needed to ensure that positional tolerances are not exceeded after several loading operations.

Furthermore the time spent on handling the work between operations frequently exceeds the time of active machining.

It is an object of the present invention to provide a machine tool fixture which enables the machining of several faces of a workpiece to be performed with a single loading operation.

According to one aspect of the present invention a machine tool fixture providing an additional degree of movement rotationally about an indexing axis extending at right angles to the rotational axis of the tool comprises a pair of turntables facing each other and rotatable about said indexing axis, each turntable being provided with holding means to hold one of two opposing ends of a workpiece such that an axis through the workpiece extends along said indexing axis and forms a datum axis in the workpiece relative to which the workpiece is machined indexing means operable to index at least one of the turntables about said axis and to lock it at any one of a specified number of angular positions for a machining operation to be performed on the workpiece, a platen arrangement adapted to extend between the turntables and be secured to each after a workpiece is held by the turntables such that the platen abuts one face of the workpiece during a machining operation in which said face is not worked, said platen being operable to support the workpiece against deformation during working of a substantially opposing face of the workpiece and adapted for clamping the workpiece thereto in order to separate the machined workpiece from its end portions held in the holding means.

The platen arrangement may comprise a plate machined to the contour of the face of the workpiece which it will abut and at each side thereof mounting means by which it is secured to the turntables.

Each mounting means may comprise a mounting block adapted to be carried by the turntable in radially slidable relationship with the indexing axis, clamping means operable to secure the block with respect to the turntable at a desired location, a platen block secured to an edge of the platen and articulation means comprising a pair of spaced bars secured at one end to one of the mounting or platen blocks and each extending into an individual recess in the other block each bar having a portion thereof, contained within the recess, enlarged to form a spherical surface about which the recessed block can undertake limited rotational movements and having a portion of reduced cross sectional area into which a retaining pin can extend through the wall of the recessed block to permit limited translational movements of the rod relative to the block.

The workpiece holding means may comprise a vice having a pair of self-centring jaws movable radially of said axis and at least one adjustable positioning stop movable radially of the indexing axis and perpendicular to the vice jaws. The vice may have grooves extending down each side thereof parallel to the direction of movement of the jaws and the mounting block of the platen arrangement may then be formed as a channel section with lugs extending into the channel and adapted to locate in said grooves. The indexing means may be a proprietary electronically controlled and hydraulically lockable indexing head. One turntable alone may be indexed, the other being free running; it may, however, have hydraulic locking means.

According to another aspect of the present invention, a method of machining a workpiece on a machine tool comprises holding the workpiece at opposite ends thereof between turntables of an indexing arrangement of the machine arranged to rotatably index the workpiece about an indexing axis extending perpendicularly to the axis of rotation of a tool of the machine and securing a platen to the turntables after the workpiece and abutting the workpiece face at appropriate times when that face is not being worked to support the workpiece against deformation perpendicularly to the indexing axis by the tool and/or clamping to the workpiece prior said workpiece being parted by the tool from the ends held by the turntables.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a machine tool bed showing a part of a fixture according to the present invention and a workpiece held thereby in relation to a rotary milling tool, Figure 2 is a perspective view of a platen arrangement according to the present invention, and Figure 3 is an exploded perspective view of the mounting arrangements for the platen of Figure 2.

Referring to Figure 1, a machine tool 10, which is conveniently a numerically controlled (NC) milling machine capable of moving the milling tool 1 1 and machining bed 12 relatively to each other along three co-ordinate axes, designated x, y and z. The fixture according to the present invention is indicated generally at 13 and carried by the machine bed 12. The fixture comprises a pair of identical turntables 1 4, 1 5 (only 14 being shown in detail) facing each other and both rotatable about a horizontal indexing axis 16 extending between them and perpendicular to the axis of the milling tool 11.

The turntable 14 (and 15) is provided with holding means 17 for supporting a workpiece 18, in this example a blank slab. The holding means comprises a vice 19 having a pair of self-centring jaws 20 which move radially of the indexing axis and two adjustable positioning stops 21 and 22 which are movable radially of the indexing axis in grooves 23 and 24 respectively which extend through the turntable at right angles to the line of movements of the vice jaws. The grooves are undercut so that the stops may be secured by screwing them to a plate carried in the groove.

The vice body 1 9 also has a pair of grooves 25, 26 extending along opposite sides and parallel to the line of the jaw movement. The purpose of the grooves will become apparent from later description.

The turntable 14 is covered by a shaft (not shown) of an electronic indexing device 27. This is a proprietary indexing arrangement such as manufactured by Messrs. Jones and Shipman, and in connection with the numerically controlled machine is able to index the turntable 14 to any desired angle and there to lock the turntable hydraulically. The turntable is freely rotatable, that is undriven, in the manner of a conventional tailstock, but it may be provided with means to lock it hydraulically with the turntable 14 for machining.

Referring also to Figure 2 the fixture includes a platen arrangement 30 which comprises a platen 31 of steel or aluminium machined flat and drilled with an array of tapped holes 32 over its surface.

The platen 31 is one of a range of sizes which correspond to a range of standard blocks used for the workpiece 18. Each platen has at opposite ends thereof amounting means which is adapted to be attached to the turntables. Referring also to Figure 3 the mounting means comprises a mounting block 33 of channel section having lugs adapted to engage the grooves 25, 26 in the vice 19. The block also has a pair of clamping screws 35 by which the block can be clamped in position on the vice. The mounting means also comprises a platen block 36 secured to the plate 31 and articulation means 37. The articulation means comprises a pair of rods 38 and 39 (shown incomplete) the ends of which are embedded in recesses in the platen block 36 and secured thereto by screws 40 in flanges 41.The rod 38 has a portion 42 enlarged to form a spherical surface and a portion 43 of reduced cross sectional area. The mounting block 33 has a recess 44 into which the rod 38 slides with the enlarged spherical portion making contact with the recess walls. A retaining screw 45 extends through the wall of the block and locates in the portion 43 to prevent the rod from leaving the recess while permitting small translational and rotational movements by virtue of the spherical portion. The rod 39 is similarly formed and contained in a corresponding recess (also not shown).Mounting means is provided at each end of the platen and enables the platen arrangement to be slid easily onto the vice bodies 19 of both turntables while accommodating any small differences in position, and for the platen 31 to be brought into contact with a face of the clamped workpiece before the platen arrangement is secured by clamping. Similarly the platen may be removed by releasing the clamps and sliding along the vice bodies. The clamping actions takes up any translational movement and effectively locks the platen block with respect to the mounting block.

The advantages to be gained from a removable platen arrangement will be seen from a simple operating sequence.

For the purpose of this example it is assumed that it is desired to mill recesses and through pockets in both major faces of the blank 1 8, to profile the four edges and to drill into the edges parallel to the indexing axis.

Initially the blank, which may already be flycut is loaded into the fixture in the manner described above and shown in Figure 1. The machine (which is numerically controlled) indexes the fixture until the blank is horizontal. The milling tool descends and performs the relevant operations on the upturned face, including milling through pockets.

The indexing means then rotates the blank through 900 and one longitudinal edge is milled and/or drilled. The workpiece is then rotated a further 180Q and the process repeated for the other edge. The workpiece is indexed so that the worked face is approximately 300 to the horizontal (as shown in Figure 1) at which time the platen arrangement (Figure 2) is placed on the fixture with the platen resting on the worked face and the mounting means clamped relative to each turntable.

The workpiece is then indexed until the uncorks 4 face is upturned and machining of it completed. The workpiece is then clamped to the platen by way of conventional clamps (not shown) which extend over the edge of the edge of the workpiece and are screwed into convenient tapped holes 32 of the array. The workpiece is then parted from the end-pieces contained in the vice jaws by milling through all but a skin of metal (so as not to damage the platen) and performing any milling of these two edges which extend perpendicularly to the indexing axis.

The platen arrangement is then unclamped and slid from the fixture (only slight pressure being required to rupture the web between the workpiece and the end pieces). The vice jaws are then parted to release the end pieces and the end stop 22 of each turntable removed to permit the next blank to be inserted.

The removed platen arrangement is parted from the machined workpiece by releasing the clamps, and this may be done away from the machine.

At this stage any drilling required into the two Sedges which extended perpendicular to the indexing axis and which were inaccessible to the tool may be carried out.

It will be appreciated that the ability to perform operations on a workpiece held directly in an indexing fixture enables many operations to be carried out without having to load and unload the fixture, saving time and improving accuracy as the workpiece never moves from the datum line.

Furthermore the ability to attach the workpiece to a platen at a late stage in the operation enables the final machining of the ends and separation from the end pieces to be carried out.

Also the addition of a platen supports the workpiece agains distortion or diaphragming of the workpiece under pressure of the machining tool.

Clearly, where the material demands, it all machining may take place with a platen supporting the side not being worked. The ease of attachment and removal of the platen means that when another side is to be worked, one platen can be added to the other side before the first one is removed. All these platen changes may be performed quickly and without the requirement for setting up the machine as the datum axis of the workpiece is not moved relative to the machine.

A number of variations are clearly possible. For instance the arrangement shown with a horizontal indexing axis may be made with a vertical indexing axis for a horizontal axis machine tool.

Also while the fixture is suited for use with a numerically or other controlled machine it may be used also with a manually controlled machine with similar benefits in respect of loading and setting times. Furthermore the fixture is not limited to use with a 3-axis machine. It may be used with existing 1- or 2-axis machines.

The arrangement is not limited to use with flat plates stock as shown and any shape of material blank may be used, be it slab, bar or polygonal (even circular) in section, the jaws of the vice being arranged appropriately to grip it.

The platen will always have a face to abut even if it is not a major face of a plate and if the face is not flat the platen may be given a suitable profile to support the face for as much of its length as necessary.

The turntables 14 and 15, although shown circular may be of any shape which provides the necessary function.

In the arrangement shown two edge stops 21 and 22 are shown. In practice, where a workpiece 5 to be machined and parted from the ends as described the vice jaws may be serrated to grip the workpiece from lateral movement. Only one stop, 21, is then used for the purposes of loading a blank in the fixture at a position in which the indexing axis passes approximately through the centre of the blank. Where it is desired not to grip the workpiece so securely, say where the end pieces are subsequently used, or where the centre of mass of the workpiece is at some time removed from the indexing axis large centrifugal forces may be set up during indexing and the use of a second stop 22 provides extra security for the workpiece position.

Claims

1. A machine tool fixture providing an additional degree of movement rotationally about an indexing angle extending at right angles to the rotational axis of the tool comprising a pair of turntables facing each other and rotatable about said indexing axis, each turntable being provided with holding means to hold one of two opposing ends of a workpiece such that an axis through the workpiece extends along said indexing axis and forms a datum axis in the workpiece relative to which the workpiece is machined indexing means operable to index at least one of the turntables about said axis and to lock it at any one of a specified number of angular positions for a machining operation to be performed on the workpiece, a platen arrangement adapted to extend between the turntables and be secured to each after a workpiece is held by the turntables such that the platen abuts one face of the workpiece during a machining operation in which said face is not worked, said platen being operable to support the workpiece against deformation during working of a substantlally opposing face of the workpiece and adapted for clamping the workpiece thereto in order to separate the machined workpiece from its end portions held in the holding means.

2. A fixture as claimed in claim 1 in which the workpiece holding means comprises a vice having a pair of self-centring jaws movable radially of said axis and at least one adjustable positioning stop movable radially of the indexing axis and perpendicularly to the vice jaws.

3. A fixture as claimed in claim 1 or claim 2 in which the platen arrangement comprises a plate machined to the contour of the face of the workpiece which it will abut and at each side thereof mounting means by which it is secured to the turntables.

4. A fixture as claimed in claim 3 in which the mounting means comprises a mounting block adapted to be carried by the turntable in radially slidable relationship with the indexing axis, clamping means operable to secure the block with respect to the turntable at a desired location, a platen block secured to an edge of the platen and articulation means comprising a pair of spaced bars secured at one end to one of the mounting or platen blocks and each extending into an individual recess in the other block each bar having a portion thereof, contained within the recess, enlarged to form a spherical surface about which the recessed block can undertake limited rotational movements and having a portion of reduced cross sectional area into which a retaining pin can extend through the wall of the recessed block to permit limited translational movements of the rod relative to the block.

5. A fixture as claimed in claim 3 or claim 4 when dependent from claim 2 in which the vice has grooves extending down each side thereof parallel to the direction of movement of the jaws and the mounting block of the platen arrangement may then be formed as a channel section with lugs extending into the channel and adapted to locate in said grooves.

6. A fixture as claimed in any one of claims 3 to 5 in which platen includes an array of tapped holes to which clamps may be secured.

7. A fixture as claimed in any one of the preceding claims in which one of the turntables comprises a free running tailstock caused to rotate by rotation of the workpiece.

8. A fixture as claimed in claim 7 in which said one turntable is provided with locking means by which its indexed position can be maintained.

9. A fixture as claimed in claim 8 in which said locking means is hydraulic and coupled for operation with that of the other turntable under the control of the indexing means.

10. A machine tool fixture substantially as herein described with reference to, and as shown by, the accompanying drawings.

11. A method of machining a workpiece on9 machine tool comprising holding the workpiece at opposite ends thereof between turntables of an indexing arrangement of the machine arranged to rotatably index the workpiece about an indexing axis extending perpendicularly to the axis of rotation of a tool of the machine and securing a platen to the turntables after the workpiece and abutting the workpiece face at appropriate times when that face is not being worked to support the workpiece against deformation perpendicularly to the indexing axis by the tool and/or clamping to the workpiece prior said workpiece being parted by the tool from the ends held by the turntables.

12. A method of machining a workpiece substantially as herein described with reference to the accompanying drawings.