BACKGROUND ART Conventional cam-operated lathes for threading operations rely on a purpose made cam for advancing and retracting the threading burr. Each different component requires such a cam which must be carefully designed by an expert, accurately made by a cam manufacturer and then fitted and properly timed by a competent toolsetter.

In addition the indexing to and from the threading station involves the setting of trip dogs and in many cases the design, manufacture and setting of a further purpose-made cam.

Any alteration in the spindle speed, or cycle time of the lathe will affect the proper functioning of this threading process and may well require the redesign and remanufacture of the cams.

DISCLOSURE OF THE INVENTION According to the invention a lathe arrangement includes a pair of quills, one adapted to hold a drill bit and the other a threading bit, with means to interchange the quills precisely with respect to a work piece.

The quills may be carried on a rocking tail stock and in a preferred form of the invention the relative rate of rotation of the quills and the spindle for the work piece are adjustable-for example by means of suitable clutches.

WO 2004/012890 PCT/ZA2003/000104 The interchange of the two quills may be achieved through one or more suitable cams and these may be actuated by a single driving shaft through pulleys, gears or the like. to the cam shaft/s. Thus, a shaft on the rocking tail stock may be provided with a cam or the like which controls the angular movement of the rocking tail stock so that the non-rotating quill and the rotating quill are alternatively aligned with the lathe spindle holding the work piece.

The latter camshaft may also carry a cam to control the changeover from one clutch to another. This camshaft may also include means to limit the rotation to only one revolution of the camshaft for each threading sequence.

The axial movement of the non-rotating quill is controlled by a cam or the like on the lathe while the axial movement of the rotating quill is controlled by a cam on a camshaft in the rocking tail stock.. This camshaft may rotate at a rate proportional to that of the lathe spindle during the threading operation and is stationary during the drilling operation.

The cam controlling the axial movement of either quill may be adjustable. Thus, the axial movement rate of the rotating quill for the threading operation may be varied to allow the cutting of threads of different pitches.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention is described below with reference to the accompanying drawing which is an isometric diagrammatic representation of the essential integers of the lathe according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION In the drawing a lathe 10 may be cam-operated and has a spindle 12 for holding a work piece 14 and a means, preferably a cam, for moving a shaft 18.

WO 2004/012890 PCT/ZA2003/000104 In addition the lathe drives a rotating shaft 20 at a rate proportional to that of the spindle 12.

All of the components shown in the drawing, with the exception of the bushes 22, 22a, the actuating shaft 18, an actuating arm 24, and the lathe 10 with its spindle 12, are carried by a rocking tail stock (not shown for reasons of clarity of the essential integers).

This tail stock is caused to rock to and fro and is pivoted about the shaft 20 so that quill 30 (for the drill bit 32) and 34 (for the threader bit 36) may be alternately aligned with and presented to the work piece.

The operation of the lathe comprises the following sequence: a. Quill 30 carrying a drill bit 32 or other cutting tool returns to its rearmost position having completed its work, and in so doing activates switch 40. b. Switch 40 activates clutch 42 such that camshaft 44 begins to rotate being driven via belt drive 46, clutch 42, and reduction gearbox 48. c. Indexing bolt 50 is withdrawn from locating bush 22a by means of cam 52. d. Rocking tailstock is caused to rock by means of cam 90 acting against roller 54 fixed to frame of lathe, such that rotating quill 34 is moved into alignment with spindle 12. e. Indexing bolt is allowed by cam 52 to drop into locating bush 22. f. Rotating quill 34 is moved axially toward workpiece at a rate appropriate to the thread being cut, this rate being set by means of the pitch adjuster element 66 on ratio arm 68.

At this stage, the tap 36 or other threading tool is being rotated at a speed somewhat slower than that of the spindle 12 by means of belt drive 70 driven through clutch 72.

This differential rotation will tend to screw a right-handed threading tool into the workpiece thus causing a thread to be cut. g. When the threading tool has reached the required depth, switch 76 is activated by cam 78 causing clutch 72 to be disengaged and clutch 82 to be engaged. This causes the speed of the rotating quill 34 to increase to a speed higher than that of spindle 12 such that the differential rotation will now tend to screw the threading tool out of the workpiece.

WO 2004/012890 PCT/ZA2003/000104 h. When the threading tool has been extracted from the workpiece, and the rotating quill has returned to its rearmost position, switch 76 is de-activated thus de-activating clutch 82 and activating clutch 72 such that the rotating quill returns to its slower speed. i. The indexing bolt 50 is withdrawn from locating bush 22 and the rocking tailstock is caused to rock back by means of cam 90 acting on roller 54 until quill 30 is once again in line with spindle 12. j. Indexing bolt 50 is allowed by cam 52 to drop into locating bush 22a. k. Switch 100 drops into notch on cam 102 which causes clutch 42 to be de-activated thus stopping the rotation of camshaft 44 and completing the threading sequence.

It will be appreciated that the present invention a. Obviates the need for cams to be designed, made or fitted. The device incorporates universal cams suitable for a wide range of components and threads both internal and external. b. No timing is involved as all events are pre-timed as factory settings and the threading sequence is automatically triggered on completion of the previous drilling or turning operation. c. The device is unaffected by changes in spindle speed and cycle time as its operation is synchronised with the rotation of the main lathe spindle.