GB2110127A - Lathe chuck with centering means - Google Patents

Abstract

A lathe chuck has an adjustable mounting which makes it easy to centre any workpiece on the axis of the lathe with high precision. Two or more eccentric adjusters (4) act between the chuck and the mounting (1) and thus, by precise, controlled movement of the chuck in at least two directions normal to the chuck axis the chuck can be accurately positioned in any position within the range of adjustment. Tapered screws engaging offset bores can be used instead of adjusters (4). <IMAGE>

Description

SPECIFICATION Lathe chuck mounting This invention relates to the correction of errors in the concentricity of workpieces held in self-centring chucks as fitted to machine tools used in engineering.

The self-centing chuck is a convenient and inexpensive work-holding appliance in wide use on centre lathes and other machine tools.

An important feature is that it can be used to hold workpieces of a wide range of sizes. For many operations the concentric accuracy provided by its self-centering characteristic is sufficient but, due to manufacturing tolerances and wear in service some discrepancy, referred to as the chucking error, will exist between the axis of work held in the chuck and the true centreline of the spindle on which the chuck is mounted. For many precision operations the chucking error cannot be tolerated and this can generate a requirement for larger machine tools and specialized, expensive work-holding appliances. Features of the chucking error in self-centering chucks are that, although the error varies randomly with different diameters of workpiece the error remains substantially constant at any one workpiece diameter.Thus, for precision work in a wide range of sizes, it is of particular value to be able to compensate for the chucking error at any particular working diameter and the ability to re-chuck work of the same diameter with consistently high concentric precision can save on capital costs.

In one type of self-centring chuck there is provision for correcting the chucking error.

This is achieved by providing for a small radial movement between the front part of the chuck, which contains the work-holding mechanism and the rear part which is mounted rigidly on the machine spindle. In this arrangement the radial movement between the two parts of the chuck is controlled by three chordally mounted adjusting screws, each threaded into the front part of the chuck and each having a shallow taper formed in its thicker, unthreaded part; the taper engages between correspondingly formed parts in the front part of the chuck and an internal extension of the rear, fixed part of the chuck. Thus, by screwing in an adjusting screw a taperwedging action is induced between the two parts of the chuck which causes the required radial movement of the front part of the chuck.This type of adjustment mechanism is prone to certain disadvantages: due to the high mechanical advantage stresses can accidentally be generated in the chuck and, under these conditions, with the actions of the three adjusting screws opposing each other, final adjustment can be imprecise and lacking in feel. Furthermore, the front and rear parts of the chuck are secured together by screws which, in order to make any adjustment, have to be sufficiently loose to permit radial movement between the two parts: in many installations these securing screws are inaccessible when the chuck is fitted to a machine and thus cannot be tightened after an adjustment has been made: in these circumstances working loads are inevitably transmitted through the adjustment mechanism which may already be highly stressed.

This invention provides a chuck which is mounted on a rigidly fixed mounting so as to be slideable in a plane normal to the axis of the chuck and having at least two cam or eccentric means acting between the chuck and the mounting for adjusting the chuck in at least two directions normal to the chuck axis.

Three embodiments of the invention will now be described: In a first embodiment, three cylindrical adjusters are fitted in axial bores in the chuck mounting and are spaced equally about the chuck axis. Each adjuster has a forward extension which, ideally, is cam shaped: alternatively, as a satisfactory compromise, the forward extension is formed as an eccentric. The adjusters are exposed at the rear of the mounting where they are suitably formed for manipulation with a small tool.The eccentric or cam shaped forward extension of each adjuster is arranged to bear in a register formed inside the chuck body and the relative dimensions are arranged so that, with the chuck body centred, each eccentric or cam is in contact with the chuck register when the eccentric or cam is in the mid-position so that one quarter turn of an adjuster would displace the chuck from the truly centred position by one half throw of the eccentric or cam. In a variant of this embodiment, arranged so that a range of different models of chuck can be fitted to a single design of mounting, the adjusters are arranged to operate on an intermediate register ring which is individually machined to suit the locating register of any chuck of the same nominal size as the mounting.

In a second embodiment of the invention, three threaded adjusters are screwed axially into equally spaced threaded holes in the rear of the chuck. A taper is formed in the unthreaded body of each adjuster. Correspondingly tapered holes are formed in the chuck mounting and said tapered holes are radially offset from the threaded holes in the chuck.

Thus, with the chuck in the truly centralized mid-position the centrelines of the tapered holes in the mounting are radially eccentric to the adjusting screws so that the action of screwing an adjuster into the chuck will cause a corresponding radial movement of the chuck against the mounting as the taper bore in the mounting forces the taper body of the adjuster to line up coaxially.

In a third embodiment of the invention, two cylindrical adjusters are fitted in axial bores in the chuck mounting with their centrelines located in planes normal to each other. Each adjuster has a forward extension which, as aforesaid comprises either a cam or an eccentric and is provided for manipulation from the rear as aforesaid: each adjuster is provided with a detent to limit rotation of the adjuster in its bore to one half turn; the angular location of the detent is arranged so that one half turn of the adjuster rotates the point of maximum lift on the eccentric or cam from a minimum to a maximum position on the radius of the chuck which passes through the axis of the adjuster and vice versa, and so that the sense of operation of both adjusters is the same.Each adjuster is made double acting by means of its eccentric or cam bearing in a close fitting circular grouve or tangential slot formed in the rear of the chuck, the centreline of said grouve or slot passing through the axis of each adjuster when the chuck is truly centralized, ie in the mid point of the available range of adjustment. In a variant of this embodiment, arranged so that a range of models of chuck can be fitted to a single design of mounting, the eccentrics or cams of the adjusters are arranged to operate in a grouve or slot formed as aforesaid in an intermediate register ring which is individually machined to fit the locating register of any chuck of the same nominal size as the mounting.

The invention will now be further described by way of example with reference to the accompanying drawings in which:~ Figure 1 is a cross-sectional view of a chuck mounting having three cylindrical bodied adjusters and an intermediate register ring as described aforesaid in the first embodiment of the invention.

Figure 2 is a plan view of the forward face of the chuck mounting shown in Fig. 1.

Figure 3 is a cross-sectional view of a cutaway chuck and mounting with three taper bodied adjusting screws as described aforesaid in the second embodiment of the invention.

Figure 4 is a cross-sectional view of a chuck mounting with two double-acting cylindrical bodied adjusters and intermediate register ring as described aforesaid in the third embodiment of the invention.

Figure 5 is a plan view of the forward face of the chuck mounting shown in Fig. 4 with the intermediate register ring removed.

It will be appreciated that the chuck itself, comprising body, work-holding jaws, operating scroll and gears is conventional in the art and it is not therefore shown in detail.

Referring firstly to Fig. 1, there is shown a chuck mounting (1) bored and threaded (2) for fitting to a lathe spindle (not shown) with a boss (3) formed at its rear face. Three cylindrical bodied adjusters (4) bear in reamed, partially blind bores (5) in the mounting (1) and said bores (5) partially break through at the junction (6) of the mounting flange and its boss (3) so that the rear end of each adjuster (4) is exposed. Said exposed parts of the adjusters (4) are radially drilled with a number of holes (7) for manipulation with a tommy bar (not shown). A cam or eccentric (8) of lesser diameter than the body of the adjuster (4) is formed at the forward end of each adjuster (4) and said cam or eccentric (8) protrudes from the bore (5) where it bears on the inner circumference of a register ring (9) which, during operation, is loosely attached to the mounting (1).Each adjuster (4) is held captive in its bore (5) between the register ring (9) and the partially blind end of the bore (5). A chuck (not shown) is fitted to the mounting (1) by machining the outer circumference (11) of the register ring (9) to a close fit to the locating register in the chuck (not shown) and drilling clearance holes (12) for the chuck fixing screws (not shown) which pass through the mounting (1) and screw axially into the chuck (not shown).To ensure that the inner and outer circumferences of the register ring (9) are concentric with the pitched circle diameter of the three adjusters (4), and thus that the chuck when fitted is nominally centred in the middle of the available range of adjustment, the mounting (1) is provided with a concentric spiggot (1 3) which is at first made a close fit to the inner circumference of the register ring (9); After fitting the chuck to the mounting assembly the assembly is dismantled and the spiggot (13) is further machined so that there is sufficient radial clearance for the register ring (9) to move over the full available range of adjustment.The dimensions of the internal diameter of the register ring (9) and the eccentrics or cams (8) are arranged so that when the points of maximum lift on the peripheries of all three eccentrics or cams (8) are circumferentially in line with the axes of the adjusters (4) and when the register ring (9) is centralised in the mid-position of its available range of movement the inner circumference of the register ring (9) is in firm contact with all three eccentrics or cams (8) and is thus fixed radially in relation to the mounting (1); thus by rotating any one cam or eccentric (8) away from the register ring (9) by one quarter turn the register ring (9) is freed to move radially by one half throw of the eccentric or cam (8); conversely; once freed by rotating any two of the eccentrics or cams (8) as aforesaid movement of the third cam or eccentric (8) by up to one quarter turn in the appropriate direction will move the register ring outwardly along an approximate radius of the mounting through the axis of the said third cam or eccentric (8) by up to one half throw of the said cam or eccentric (8).

Referring to Fig. 2, this shows the relative locations of the three adjusters (4) whose bores (5) are equidistantly spaced on a concentric pitched circle diameter of the mounting (1). Clearance holes (12) for the chuck fixing screws are spaced approximately equidistantly between the adjusters (4) and three small screws (14) are provided to retain the register ring (9) to the mounting (1) said small screws (14) pass through clearance holes and counterbores (not shown) in the mounting (1) and are threaded into the register ring (9).

The small screws (14) are tightened to secure the register ring (9) rigidly to the mounting (1) while the register ring (9) is being machined to fit the locating register on the chuck (not shown) but when finally assembled the small screws (14) are left loose to permit the required radial movement of the register ring (9). This view shows the mounting after a chuck (not shown) has been fitted and therefore a radial clearance (15) is shown between the internal circumference of the register ring (9) and the spiggot (13). This clearance (15) and clearances in the screw holes (12) and clearance holes and counterbores (not shown) for the small screws (14) are all arranged to be sufficient to permit radial movement of the chuck (not shown) and its locating register ring (9) over the full designed range of adjustment.

Referring now to Fig. 3, there is shown a cut-away part of a chuck (16) fitted to a mounting (1) and one of three adjusting screws (17), said adjusting screws (17) are fitted into tapped and partially reamed holes in the chuck (16) and said holes are equidistantly spaced on a pitched circle diameter of the chuck (16). Said holes are interspaced between tapped holes provided in the chuck for the usual chuck fixing screws, one of which is shown at (24). Each of the adjusting screws (17) has a cylindrical, unthreaded portion (19)which is made to be a close fit in the reamed part of the corresponding threaded hole in the chuck. Each of the adjusting screws (17)also has a tapered portion (20) and has a socket (23) formd in its outer end for manipulation with a socket wrench (not shown).The chuck (16) is not registered to the mounting (1) but is radially located by the adjusting screws (17) whose tapered portions (20) partially bear in correspondingly tapered bores in the mounting (1) of which one is shown at (21) said tapered bores (21) are radially in line with the location of the adjusting screws (17) but the centres of the tapered bores (21) are on a greater pitched circle diameter than and hence offset from the centres of the adjusting screws (17). The relative locations of the centres of the tapered bores (21) and adjusting screws (17) are indicated respectively at (22) and (18). Clearance holes (12) in the mounting (1) are provided for the chuck fixing screws (24) as aforesaid.Thus, with any two adjusting screws (17) slackened off the action of tightening the third adjusting screw (17)will move the chuck outwardly along a radius passing through the centre of the third adjusting screw (17) by an amount up to the offset between the centres of the adjusting screw (17) and its corresponding tapered hole (21).

Referring now to Fig. 4, there is shown a chuck mounting (1) as described aforesaid with one of two cylindrical body adjusters (4).

Each said adjuster (4) is essentially as described above as regards its eccentric or cam shaped part (8), its rearwardly exposed part with drilled holes (7) for manipulation with a tommy bar (not shown) and its retention within the partially blind bore (5). In this embodiment, however, the two adjusters are fitted in axial bores (5) in the mounting (1) with centres on the same pitched circle diameter and spaced apart by 90 . The protruding cam or eccentric (8) is arranged to be a close fit in a circumferential groove (24) in an intermediate register ring (9) and the diameter of the centre line of said groove (24) is equal to the pitched circle diameter on which the centres of the adjusters (4) are located.In this example of the embodiment rotation of each adjuster (4) is restricted to approximately 1 80' by a detent pin (25) formed on the rear face of the adjuster (24) by said detent pin (25) engaging on the periphery of the boss (3). Said detent pin (25) is located, in relation to the cam or eccentric (8) so that one half turn of the adjuster (4) moves the point of maximum lift on the cam or eccentric (8) from a minimum to a maximum position on the radius of the mounting (1) which passes through the adjuster (4) and vice versa. The detent pin (25) is also arranged so that the sense of operation of both adjusters (4) is the same. Each adjuster (4) is thus made doubleacting and therefore only two adjusters (4), 90 apart are required to locate the register ring (9) in any position within its designed range of adjustment.In this example the register ring (9) extends to the full diameter of the chuck (not shown) and mounting (1). The chuck (not shown) is fitted to the mounting assembly by machining a register (26) in the register ring (9) and locating and drilling clearance holes (12)for the chuck fixing screws through the register ring (9) and mounting (1) while the register ring (9) is temporarily centred on the spiggot (13) as aforesaid.

Referring to Fig. 5, the relative locations of the two adjusters (4) in their bores (5) and of the clearance holes (12) for the chuck fixing screws are shown. Three small screws (14) are used to secure the register ring (9) to the mounting (1) as aforesaid.

Each of the three embodiments described is relevant in different circumstances. For instance, a mounting with the three cylindrical adjusters (first embodiment) can readily be adapted to fit any existing chuck whereas the mounting with three taper-bodied screws (second embodiment) is more appropriate for fitting as orginal equipment with a new chuck.

The double acting, two adjuster mounting (third embodiment) is, for relatively unskilled operators, somewhat easier to understand and use but, because the adjusters are asymmetrically disposed in the mounting, there can be problems of balance if the assembly is required to run at high speed and this embodiment is therefore more appropriate to certain specialised, low speed applications. The operation of each of the three embodiments of the invention is essentially similar. When high concentic accuracy is required a Dial Test Indicator (DTI) is used to determine the degree of eccentricity of a work piece gripped in the jaws of the chuck. To make a correction, the chuck fixing screws are eased slack to permit the chuck and register ring if fitted to move against the mounting and the adjusters are then manipulated to cancel out the error with reference to the DTI. Finally, the chuck fixing screws are firmly tightened to secure the chuck to the mounting in the adjusted position. With each embodiment of the invention the adjustment is rapid, positive, and precise to an extent that it is likely to prove more acceptable to operators than the existing taper-wedging system described above the rejection of which often creates a requirement for over size machine tools with relatively sophisticated and expensive work-holding appliances. The invention therefore offers substantial savings in capital equipment in situations requiring repeatable high concentric accuracy of work pieces over a wide range of diameters.

Claims (9)

1. A chuck which is mounted on a rigidly fixed mounting so as to be slideable in a plane normal to the axis of the chuck and having at least two cam or eccentric means acting between the chuck and the mounting for adjusting the chuck in at least two directions normal to the chuck axis.

2. A chuck mounted as in Claim 1 in which three adjusters fitted in axial bearings spaced equidistantly in the mounting have eccentric or cam-shaped portions which bear internally on the chuck to effect the adjust ment.

3. A chuck mounted as in Claim 1 in which three adjusters fitted in axial bearings spaced equidistantly in the mounting have eccentric or cam-shaped portions which bear in an intermediate register ring to which the chuck is registered.

4. A chuck mounted substantially as here in before described with reference to Fig. 1 and 2 of the accompanying drawings.

5. A chuck mounted as in Claim 1 with three threaded adjusters which screw axially into the chuck body and which have tapered bodies which act in similarly tapered bores in the mounting with the centre lines of said tapered bores parallel but radially offset and thus eccentric to the centre lines of the threaded adjusters.

6. A chuck substantially as hereinbefore described with reference to Fig. 3 the accompanying drawings.

7. A chuck mounted as in Claim 1 in which two adjusters fitted in axial bearings in the mounting with centre lines in planes at right angles to each other and to the axis of the chuck each have an eccentric or camshaped portion which acts in two directions by bearing in a close fitting groove or slot in the chuck.

8. A chuck mounted as in Claim 1 in which two adjusters fitted in axial bearings in the mounting with centre lines in planes at right angles to each other and to the axis of the chuck each have an eccentric or camshaped portion which acts in two directions by bearing in a close-fitting groove or slot in an intermedite register ring to which the chuck is registered.

9. A chuck mounted substantially as hereinbefore described with reference to Fig. 4 and 5 the accompanying drawings.