Machining fixture for centering and holding workpiece

A centering and holding fixture includes a base, a piston disposed within a piston chamber at least partially located within the base, and a radially flexible ring connected to the piston by force transmission apparatus operable to contract and expand the radially flexible ring by upward and downward motions respectively of the piston. The flexible ring may be slotted and may include alternating inwardly and outwardly extending radial slots extending axially through the flexible ring and extending radially inwardly and radially outwardly from radially outer and inner surfaces of the flexible ring respectively to axial stress relief holes extending axially through the flexible ring. The apparatus may further be operable to contract the radially flexible ring when the piston chamber is pressurized and to expand it when the piston chamber is depressurized. Spring loaded clamps mounted to the base may be used to secure a workpiece to the holding fixture.

FIELD OF THE INVENTION

This invention relates generally to multi axis machine tools such as vertical turret lathes (VTL's) with rotary tables and, more particularly, to fixtures and chucks used to position and hold workpieces on the rotary table.

BACKGROUND OF THE INVENTION

In most machining operations involving turning a workpiece, it is necessary to securely fix the workpiece in place upon a rotary table or other rotary work surface. Examples of such surfaces to which workpieces are secured include rotary tables, mill tables, fixtures or face plates and lathes, etc. Usually, the workpiece must be secured upon the surface to expose a selected portion of the workpiece upon which work such as cutting or machining is to be performed. In addition to securely fixing the workpiece in place, it is often necessary or desirable to provide fine adjustment in the position and alignment of the workpiece. For example, when a workpiece is secured to a rotary table, accurate alignment of a selected portion of the workpiece with the axial center of the rotary table is desired to facilitate and accurately conduct machine operations upon the workpiece.

Prior to any machining operations, it is necessary to align the part to be machined with the center of rotation of the machine tool. This is particularly difficult for machining large diameter workpieces, such as turbine rotors, on vertical turret lathes. Typically, such workpieces are held in 3-jaw or 4-jaw chucks which are mounted to rotary tables and aligned to the axis of the rotary table. Because of the critical nature of such workpieces, it may be necessary to align them to small tolerances such as within 0.002″ or less. This procedure is very time consuming and, therefore, very costly.

SUMMARY OF THE INVENTION

A centering and holding fixture includes a base, a piston disposed within a piston chamber that is at least partially located within the base. A radially flexible ring is connected to the piston by a force transmission apparatus operable to contract and expand the radially flexible ring by upward and downward motions respectively of the piston. The flexible ring may be slotted and may include alternating inwardly and outwardly extending radial slots extending axially through the flexible ring and extending radially inwardly and radially outwardly from radially outer and inner surfaces of the flexible ring respectively to axial stress relief holes extending axially through the flexible ring. The apparatus may further be operable to contract the radially flexible ring when the piston chamber is pressurized and to expand it when the piston chamber is depressurized. Spring loaded clamps mounted to the base may be used to secure an annular workpiece to the holding fixture.

The flexible ring may be slotted and an exemplary embodiment of the flexible ring has alternating inwardly and outwardly extending radial slots extending axially through the flexible ring and extending radially inwardly and radially outwardly from radially outer and inner surfaces of the flexible ring, respectively, to axial stress relief holes extending axially through the flexible ring.

An exemplary embodiment of the force transmission apparatus includes an annular sleeve having a central bore coaxial with an upper counterbore in the annular sleeve. The central bore is wider than the upper counterbore and the central bore is separated from the upper counterbore by an upper annular ledge. A plurality of piston springs are circularly disposed within the upper counterbore in compression between the upper annular ledge and the piston. A piston extension rests on the piston and has a cylindrical extension shaft at least partially disposed within the bore and a downwardly tapering radially outer conical edge on the extension shaft. The radially flexible ring rests on a top of the annular sleeve and is disposed around the piston extension and at least a portion of the flexible ring has an upwardly tapering shape substantially conforming to the radially outer conical edge of the piston extension.

One more particular exemplary embodiment of the centering and holding fixture further includes an upper annular sleeve wall extending upwardly from the top of the annular sleeve. An extension head attached to the extension shaft radially overlaps the upper annular sleeve wall and has a downwardly tapering radially outer conical edge. The radially flexible ring resting on the top of the annular sleeve is disposed around the extension head. An annular lower portion of the flexible ring abuts and conforms to the upper annular sleeve wall and an upwardly tapering conical annular upper portion of the flexible ring substantially conforms to the radially outer conical edge of the extension head.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated inFIG. 1is a fixtured machining assembly8including an annular workpiece14mounted in a centering and holding fixture10fixedly attached to a horizontal rotary table12designed for rotation about a vertical axis C2of a machine tool such as a vertical lathe (not illustrated). The fixture10is designed to releasably center and carry an annular workpiece14having a center axis C. The fixture10is designed for aligning the center axis C of the workpiece14with the vertical axis C2of the rotary table12. The fixture10includes a cylindrical base16relatively fixedly mounted to the rotary table12.

The fixture10includes a radially outer support ring30which vertically supports the annular workpiece14and rests within a radially outer groove31illustrated herein as an outer annular rabbet32along a radially outer edge33in the base16. An exemplary cylindrical piston34is disposed within an exemplary cylindrical piston chamber36. A pressurized fluid supply passage42extends from an outer periphery40of the base16to the piston chamber36. A fitting may be used for connecting a pressurized fluid hose to the supply passage42. The fixture may use pressurized hydraulic fluid or pressurized air. An annular sleeve50fixedly attached to the base16rests in a radially inner annular rabbet52formed in a central cylindrical recess54in the base16, the inner annular rabbet52being located radially inwardly of the radially outer annular rabbet32.

The annular sleeve50has a central bore56and upper and lower counterbores58and60, respectively, separated by a lower annular ledge61. The lower counterbore60is upwardly bounded by a downwardly facing annular lower surface62of the lower annular ledge61in a lower portion67of the annular sleeve50. The upper counterbore58is wider than the lower counterbore60. The piston chamber36is bounded upwardly by a downwardly facing piston surface63of the piston34and downwardly by an upwardly facing inner circular surface66of the base16located within the recess54, respectively.

The piston chamber36is bounded radially by a lower counterbore wall70that circumscribes the lower counterbore60in the annular sleeve50. An annular gap102lies between the piston34and the lower counterbore wall70. A piston ring37is disposed in a piston ring groove39in the piston34to seal against the lower counterbore wall70. The upper counterbore58is upwardly bounded by an upper annular surface74on an upper annular ledge75separating the upper counterbore58and bore56. A plurality of piston springs76are circularly disposed within the upper counterbore58in compression. The piston springs76are loosely mounted on piston spring pins78which are relatively fixed with respect to the piston springs76and illustrated as attached to the piston34.

An upper annular sleeve wall80extends axially upwardly from a top82of the annular sleeve50and is radially inwardly bounded by the bore56. A piston extension86includes a cylindrical extension shaft88disposed within the bore56and resting on the piston34. An extension head90attached to the extension shaft88radially overlaps the upper annular sleeve wall80. A radially outer conical edge92of the extension head90tapers downwardly. A radially flexible ring94rests on the top82of the annular sleeve50and is disposed around the extension head90attached to the extension shaft88and the upper annular sleeve wall80of the annular sleeve50.

The flexible ring94includes an annular lower portion96that abuts and conforms to the shape of the upper annular sleeve wall80. The flexible ring94includes a conical annular upper portion98that tapers upwardly and abuts and substantially conforms to the tapered shape of the radially outer conical edge92of the extension head90.FIG. 2illustrates an exemplary embodiment of the flexible ring94that is slotted so that it is flexible in the radial direction. The flexible ring94includes alternating inwardly and outwardly extending radial slots20and22extending radially inwardly and radially outwardly from a radially outer and inner surfaces24and26, respectively, to axial stress relief holes28. The inwardly and outwardly extending radial slots20and22and the stress relief holes28extend axially through the flexible ring94. Thus, fixture10provides a force transmission means for causing the radially flexible ring94to contract and expand by upward and downward motions respectively of the piston34illustrated inFIG. 1.

Referring toFIG. 1, the flexible ring94is pre-loaded along a conical surface100between the conical annular upper portion98of the flexible ring94and the radially outer conical edge92of the extension head90. The tapered conical annular upper portion98of the flexible ring94is pre-loaded against the radially outer conical edge92of the extension head90such that when there is no pressure supplied to the piston chamber36or the piston chamber36is depressurized, the flexible ring94is radially expanded and centers and holds annular workpiece14in place for machining. When there is no pressure supplied to the piston chamber36, the piston34is urged downwardly by the piston springs76which are compressively loaded between the annular sleeve50and the piston34. The sleeve is fixedly attached to the base16of the fixture10, thus, the piston springs76urge the tapered conical annular upper portion98of the flexible ring94against the radially outer conical edge92of the extension head90which radially expands the flexible ring94.

The flexible ring94contracts when pressure is supplied to the piston chamber36. The piston34moves upwardly working against a compressive force of the piston springs76when pressure is supplied to the piston chamber36causing the piston extension86to move upwardly. This causes the radially outer conical edge92of the extension head90to move away from the conical annular upper portion98of the flexible ring94allowing the flexible ring94to radially contract such that the annular workpiece14may be mounted in the fixture10attached to the rotary table12. Thus, fixture10provides the force transmission means with the ability for causing the radially flexible ring94to contract when the piston chamber36is pressurized and to expand when the piston chamber36is depressurized.

Spring loaded clamps110have lugs108pivotably mounted on threaded pins112extending upwardly from the base16. Clamp springs114are disposed in compression between the lugs108and clamp nuts120that are screwed on to the threaded pins112. Once the workpiece14is positioned in place, the lugs108are pivoted to engage the workpiece and hold the workpiece in place. When the pressure to the piston chamber36is removed, the spring loaded lugs108hold the workpiece14in place.

While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims.