GB2094205A - Thread-cutting indexing unit - Google Patents

Abstract

The unit, for mounting on a lathe, comprises a first gear pinion 4 for continuous engagement with a leadscrew of a lathe. The first gear pinion 4 is mounted on a first shaft 2, and second and third gears 10, 11 are mounted on a second shaft 3. The second and third gears may be selectably engaged with a fourth gear 4 on the first shaft 2, and are arranged to rotate with an indexing disc 12, which carries two separate scales 18, 19. By moving the indexing disc 12 axially, either one of the second and third gears may be engaged with the fourth gear 14, to provide a selected gear ratio between the leadscrew of the respective lathe, and the indexing disc 12. The scales 18, 19 carry more information, particularly advantageous in the cutting of metric screw-threads. When a mark on one scale is lined up with a datum on the unit, a handle 6 is operated to engage a half nut 5 with the leadscrew for screw-cutting saddle traverse. The indexing unit is detachably mounted on a saddle of a lathe, which may be replaced by another indexing unit for cutting other screw-threads. <IMAGE>

Description

SPECIFICATION Thread-cutting unit This invention relates to thread-cutting indexing units, such as are used on lathers.

Conventionally, a lathe comprises a headstock, a tailstock, and a cross-slide mounted on a saddle. A workpiece is secured between the headstock and the tailstock. A tool on the cross-slide engages the workpiece, and carries out a desired work operation whilst the workpiece is rotated.

Often, it is necessary to synchronise movement of the saddle axially of the workpiece with rotational movement of the headstock about the axis of the workpiece. To this end, the saddle is provided with means whereby it may be engaged by a leadscrew extending longitudinally of the lathe, which leadscrew is driven in synchronism with the headstock.

Usually, gearing is provided between the headstock and the leadscrew.

Where it is desired to carry out a thread-cutting operation, it is important that the saddle is engaged with the leadscrew at a precise phase of rotation of the leadscrew. To this end, it is known to provide the saddle apron with an indexing disc which rotates synchronously with the leadscrew, and is provided with indexing marks thereon. Then, an operator simply has to monitor the index disc and when it is in a predetermined position, engage the saddle with the leadscrew, which is then in a desired phase relationship with the spindle.

Such indexing discs are conventionally built integrally with saddle aprons, and are also somewhat limited in the number of indexing marks that they can provide. This means that a given lathe usually has the facility for accurately screwcutting only a limited number of threads. If threads outside that limited number have to be cut, then the work process becomes unduly complicated.

The present invention aims to provide threadcutting units which can be improved in the foregoing respects.

According to one aspect of the present, there is provided a thread-cutting indexing unit for a lathe, the unit comprising gearing arranged to be driven by a leadscrew of a lathe, indexing means driven by said gearing and provided with two scales thereon, and means for selectively engaging either one of two gear ratios of said gearing, to provide a selected one of two gear ratios between the indexing means and a leadscrew, in use.

The indexing means is preferably in the form of a disc, and is provided with the scales around the periphery thereof. The thread-cutting unit may be formed integrally with an apron of a lathe.

Preferably, however, the thread-cutting unit is provided as a separate unit which can be attached and detached from an apron, as desired.

Preferably, the indexing means is axially slidable into and out of a housing on the unit, thereby to select either one of said gear ratios.

Said gearing may advantageously comprise a first gear pinion mounted on a first shaft, a half-nut member having an axis extending transversely of said pinion, and means for moving said half-nut means towards and away from said pinion, thereby to engage a leadscrew therebetween.

The gearing may further comprise second and third gears mounted on a second shaft, the indexing means being driven by said second and third gears.

Thus, by selectively engaging either the second or third gear with the fourth gear, a different gearing ratio may be provided between the leadscrew and the indexing means, in use, each of the different scales on the indexing means corresponding to a respective one of the different gear ratios. Thus, the indexing means can carry many more indexing marks than the simple indexing discs known from conventional thread-cutting apparatus provided integrally on a conventional apron. This can be especially important when cutting Metric threads, where typically a much larger range of indexing marks is required.

In another aspect, the present invention provides a lathe provided with a headstock, a tailstock, a saddle, a leadscrew driving the saddle, and two threadcutting indexing units each comprising indexing means arranged to be driven by the leadscrew, each of the thread-cutting indexing units being provided with means whereby it may be detachably secured to the saddle, and the two thread-cutting indexing units having different gear ratios between the leadscrew and the respective indexing means in use.

Thus, the thread-cutting indexing units may be interchangeably mounted on the saddle, for differing requirements.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, Figure 1 of which is perspective exploded view of a preferred thread-cutting indexing unit embodying the invention.

The illustrated thread-cutting indexing unit comprises a main mounting 1 in which first and second shafts 2 and 3 are mounted, in respective apertures.

At one end of the first shaft 2, there is provided a first pinion gear wheel 4, which is located above a "half-nut" member 5 having an axis extending perpendicularly to that of the first shaft 2. A control lever 6 is mounted on a further shaft 7, which drives an eccentric 8 connected to a mounting plate 9 upon which the half-nut 5 is secured. Upon movement of the lever 6, the half-nut 5 on the mounting plate 9 can be moved upwardly and downwardly towards and away from the pinion 4, thereby selectively to engage or disengage a leadscrew positioned therebetween.

Second and third gear wheels 10 and 11 are mounted for common rotation upon the second shaft 3, and an indexing disc 12 is connected to the gear wheels 10 and 11, for rotation therewith. The indexing disc protudesthrough an aperture 13 in a faceplate ofthe unit. Afourth gearwheel 14 is mounted on the first shaft 2, and is arranged to rotate with the pinion 4. The fourth gearwheel 14 can mesh selectively either with the second gear wheel 10 or the third gear wheel 11. The second and third gear wheels 10 and 11 have different numbers of teeth.

Bolts 15 pass through respective apertures 16 in the main mounting 1, to secure the indexing unit to the saddle apron of a lathe, which apron is adapted to receive the illustrated unit. Thus, the indexing unit can be detachably secured to such an apron, and interchanged with an alternative indexing unit, it desired.

Operation of the illustrated indexing unit is as follows.

The indexing unit is so arranged that the pinion 4 is constantly in mesh with the leadscrew of the lathe.

Thus, as the leadscrew rotates, the pinion 4 and the fourth gear wheel 14 rotate in synchronism therewith. Drive from the fourth gear wheel is transmitted to the indexing disc 12 by either the second gear wheel 10 or the third gearwheel 11. The indexing disc 12, together with the gear wheels 10 and 11, can be pushed axially along the shaft 3 between two positions, in which a respective one of the gear wheels 10 and 11 meshes with the gear wheel 14. A spring loaded ball 17, mounted in the shaft 3, is urged radially outwards towards the inner surface of the indexing disc 12, to engage either one of two indentations therein, in the manner of a spring detent, to locate the indexing disc 12 positively in each of its two optional positions.

The indexing disc 12 has two separate scales, each imprinted on a respective one of two peripheral surfaces 18 and 19. Each scale corresponds to a respective one of the gear wheels 10 and 11. Thus, when the gear wheel 10 is engaged with the gear wheel 14, the scale on the surface 19 of the indexing disc 12 is read, in order to determine predetermined phase angles of the leadscrew of the lathe, in rotation. When the gear wheel 11 engages the gear wheel 14, the scale on the surface 18 of the disc 12 is read, instead. Because of the different number of teeth on the gear wheels 10 and 11, a different gear ratio is provided respectively between the leadscrew of the lathe and the respective scale on the indexing disc 12. Thus, the indexing disc 12 can carry considerably more information than a conventional indexing disc.This can be of particular importance when cutting Metric thread screws, where a relatively large number of scales needs to be considered.

In use of the indexing unit, when a scale on the indexing disc 12 is at an appropriate point (usually determined against a fiducial mark on the faceplate 13), the lever 6 is operated to bring the half-nut5 into engagement with the leadscrew, which is then in the desired phase relationship. Drive from the leadscrew is then transmitted via the indexing unit to the saddle of the lathe, which then moves axially of the workpiece on the lathe.

The illustrated indexing unit can readily be modified for use with a conventional indexing disc, and this option is also illustrated in the drawing. In this case, an alternative indexing disc 20 is secured directly to the first shaft 2, such that it is driven directly by the pinion 4. The indexing disc 20 protrudes through an alternative aperture 21 in the faceplate 13. Such an arrangement can be suitable where it is desired only to cut Imperial threads. The indexing unit when constructed in such a way still has the extremely useful advantage that it can be detached from the suitably adapted apron of the lathe, and readily replaced with an alternative indexing unit, as desired. Thus, a Metric indexing unit can be used with a Metric leadscrew, and an Imperial indexing unit with an Imperial leadscrew, either unit being detachably secured to the one apron.

The accompanying Figure 2 illustrates how scales can be applied to the indexing disc 12. Figure 2 shows a development of scales on the peripheral surfaces 18 and 19 of the disc 12.

On the surface 18 of the indexing disc 12, there are in fact two scales a and b superposed on top of one another. Scale a represents the case wherein there are four revolutions of the leadscrew between each successive two marks on the scale. At the very bottom of Figure 2, the development of such a full scale is shown. Immediately above this development, scale b illustrates the case wherein there are five revolutions of the leadscrew between each successive two marks on the scale. (Gear 11 has 20 teeth).

At the very top of Figure 2, there are shown the developments of scales c and d. These represent respectively the cases wherein there are seven and three revolutions of the leadscrew between each successive two marks on the scale (Gear 10 has 21 teeth). The scales c and dare superposed one on top of the other on the surface 19 of the indexing disc 12, as shown in Figure 2. To avoid overcrowding of the superposed scales, only four indexing marks are actually made on the surface 19 for the scaled.

However, this is quite sufficient to enable operation.

In a modification of the indexing unit of Figure 1, the two gears 10 and 11 can be mounted on the first shaft 2, and the other gear 14 on the second shaft 3, with the indexing disc 12.

Claims (11)

1. Athread-cutting indexing unitfora lathe, the unit comprising gearing which comprises two gear ratios and is arranged to be driven by a leadscrew of a lathe, indexing means driven by said gearing and provided with two scales thereon, and means for selectively engaging either one of said two gear ratios, to provide a selected one of two gear ratios between the indexing means and a leadscrew, in use.

2. A unit according to Claim 1, wherein the indexing means is in the form of a disc, and is provided with said scales around the periphery thereof.

3. A unit according to Claim 1 or 2, formed integrally with an apron of a lathe.

4. A unit according to Claim 1 or 2, formed as a separate unit which can be attached and detached from an apron of a lathe, as desired.

5. A unit according to any preceding claim, wherein the indexing means is axially slidable into and out of a housing on the unit, thereby to select either one of said gear ratios.

6. A unit according to any preceding claim, wherein said gearing comprises a first gear pinion mounted on a first shaft, a half-nut member having an axis extending transversely of said pinion, and means for moving said half-nut means towards and away from said pinion, thereby to engage a leadscrew therebetween.

7. A unit according to Claim 6, wherein said gearing futher comprises second and third gears mounted on a second shaft, and a fourth gear driven by said first shaft, the indexing means being driven by said fourth gear and by either said second or third gear.

8. A unit according to any preceding claim, adapted for use in cutting metric threads.

9. Athread-cutting indexing unit for a lathe, the unit being substantially as hereinbefore described with reference to the accompanying drawings.

10. A lathe provided with a headstock, a tailstock, a saddle, a leadscrew driving the saddle, and two thread-cutting indexing units each comprising indexing means arranged to be driven by the leadscrew, each of the thread-cutting indexing units being provided with means whereby it may be detachably secured to the saddle, and the two thread-cutting indexing units having different gear ratios between the leadscrew and the respective indexing means in use.

11. A lathe according to Claim 10, wherein at least one of the thread-cutting indexing units is in accordance with any one of Claims 1 to 9.