Technical Field

The present invention relates generally to collets and other clamping tools, such as but not limited to, expanding arbors, tool holders of various types and similar tooling, and, more particularly, to an adjustable clamping device for minimizing angular clamping error of a collet retained in the adjustable clamping device.

Background

A collet is a device having two or more "jaws" between which an object such as a tool or a workpiece can be positioned, the jaws forming a collar around the object. The jaws of the collet are configured to constrict around the object and exert a clamping force. In one type of collet, constriction of the jaws may be effected by pushing a suitable mechanism around the circumference of the collet. In another type of collet called a draw-in collet, the collet is tapered and drawn into a sleeve to cause the jaws to constrict around the tool or workpiece. Collets are typically fabricated from steel, which allows the jaws to be sufficiently flexible relative to each other to facilitate the constriction of the tool or workpiece.

With regard to either type, the collet is held in a clamping device, which is connected to a shaft that can be rotated, which in turn causes the tool or the workpiece to rotate. In instances in which a tool is clamped in the collet and rotated, the tool is made to engage or be engaged by a workpiece. In instances in which the workpiece is clamped in the collet and rotated, the workpiece engages or is engaged by a tool.

The rotational accuracy of the tool or the workpiece affects the process of removing swarf from the workpiece. In particular, the rotational accuracy of the tool or the workpiece is typically a function of concentricity along the axis of rotation. Once clamped into a machine tool such as a lathe or a grinding machine, the tool or workpiece is adjusted to attain concentricity upon rotation. The concentricity achieved with mechanical clamping is often inadequate or compromised during rotation, thereby resulting in runout, which will detrimentally affect the finished workpiece. Concentricity is often adjusted numerous times during an operation of a machine tool.

Summary In one aspect, the present invention resides in a device for clamping a collet in a machine tool having a rotatable spindle and a spindle nose thereon. The device comprises a mounting portion fastenable to the spindle and a clamping portion extending from the mounting portion. A bore is defined through the clamping portion and the mounting portion. At least one movable ring is located within a portion of the bore defined by the clamping portion, the movable ring being configured to receive the collet. At least one differential screw is mounted in the clamping portion and can be tightened against the movable ring. The collet is attachable to the spindle nose of the machine tool and can be drawn into the bore of the clamping portion and clamped therein.

In another aspect, the present invention resides in an adjustable clamping system for a machine tool. This clamping system comprises collet for holding either a tool or a workpiece and a collet adapter that is locatable on the machine tool. The collet adapter comprises a mounting portion fastenable to a spindle of the machine tool, a clamping portion extending from the mounting portion, and a at least one movable ring located within a bore extending through the mounting portion and the clamping portion. The movable ring is configured to receive the collet therein. At least one differential screw is in communication with the movable ring, the differential screw being adjustable to allow for an adjustment of the position of the collet.

In another aspect, the present invention resides in a method of clamping a collet in a machine tool. This method comprises providing a clamping device defining a bore extending therethrough, the bore including at least one movable ring that is engageable with one or more differential screws. The clamping device is attached to a spindle o the machine tool, and the collet is inserted into the clamping device and held in place at a nose portion defined by the spindle. The collet is then drawn or pushed into the clampmg device. The position of the collet can be adjusted using the one or more differential screws.

Brief Description of the Drawings

FIG. 1 is a perspective view of a clamping device, of the present invention, suitable for having a collet located therein.

FIG. 2 is side sectional view of the clamping device of FIG. 1.

FIG. 3 is a sectional view of the clamping device of FIG. 1. FIG. 4 is a side sectional view of a differential screw of the clamping device of FIG. 1 showing thread pitches.

FIG. 5 is one exemplary embodiment of an automated grinding machine incorporating the clamping device of FIG. 1.

Detailed Description

As is shown in FIG. 1, an adjustable device for clamping a collet into a machine tool is designated generally by the reference number 10 and is hereinafter referred to as "collet adapter 10." The collet adapter 10 comprises a mounting portion 11 and a clamping portion 14 extending forward from the mounting portion. The clamping portion 14 is substantially cylindrical and tapers to a truncated conical configuration at a forward end thereof. The mountifigportion 11 attaches to a spindle of the machine tool to effect the rotation of the collet adapter 10.

As is shown in FIG. 2, a bore is defined through the collet adapter 10 such that a portion of the bore proximate to the end of the col let adapter 10 having the closing portion 12 is of a first diameter and a portion of the bore proximate the opposing end having the truncated conical configuration is of a second diameter. A shoulder 18 defines a transition between the first diameter and the second diameter. The shoulder 18 includes a groove in which a first o-ring 32 is located

The collet adapter 10 is locatable on a spindle of the machine tool such that the bore of the first diameter is located on a drawbar 16 of the machine tool which is movable relative to the collet adaptor 10 in opposing directions indicated by an arrow 22. Attachment screws 26 (e.g., four attachment screws shown in FIGS. 1 and 3) are used to attach the mounting portion 11 to the spindle. A pin 28 extends from a rearward-facing surface of the mounting portion 11 to facilitate the alignment of the collet adapter 10 for attachment to the spindle via the attachment screws 26.

A guide bushing 30 is located in the bore of the first diameter and is positioned to abut the shoulder 18. An outer surface of the guide bushing 30 is positioned against the surface of the bore of the first diameter. A sleeve 33 is located at least partially in the guide bushing 30 and in the bore of the second diameter.

The sleeve 33 is received in the guide bushing 30 from the front of the collet adapter 10. The sleeve 33 is sized to accommodate the insertion of the collet (show at 40). The collet 40 can be mounted in the collet adapter 10 by being inserted through the forward end of the clamping portion 14 and by turning the drawbar 16, the collet is threaded into the drawbar 16. Upon attaching the collet 40 in such a manner, the piston of the machine tool is operated such that the drawbar 16 retracts in the rearward direction, thus causing the collet to be clamped in the truncated conical configuration of the clamping portion 14.

Differential screws 44 are located a in threaded inserts 45 in the clamping portion 14 and in communication with an adjusting sledge 38 to facilitate the adjustment of the collet 40 clamped in the collet adapter 10. The differential screws 44 are generally located equidistantly around the circumference of the clamping portion 14. The adjusting sledge 38 can be adjusted to center and stabilize the collet 40 in the clamping portion 14 by tightening or loosening the differential screws 44 using a tool such as a screwdriver, a Hex-key or the like. The application of force by the differential screws 44 via the adjusting sledge 38 resiliently deforms the guide bushing 30 acting on sleeve 33 (e.g., in the micrometer range), thus reducing clearance in relation to the bore of the collet adapter 10 and thereby improving repeat accuracy. In certain cases or variants of this invention, the adjusting sledge may be acting directly on sleeve 33.

The differential screws 44 can be made to engage the adjusting sledge 38 via the guide bushing 30 acting on sleeve 33 to compensate for centering and angle errors. By engaging the differential screws 44 with the adjusting sledge 38 (as opposed to allowing the differential screws to directly engage surfaces of the collet 40 when the collet is inserted into the clamping portion), direct contact between the differential screws 44 and the collet is prevented, thus minimizing wear on the collet adapter 10 and the collet. The differential screws 44 are replaceable within the collet adapter 10.

As is shown in FIG. 3, six differential screws 44 in the threaded insert 45 are located equidistantly around the circumference of the clamping portion 14. However, it should be understood that the present invention is not so limited. For example, additional (or fewer) differential screws may be located around the circumference of the clamping portion. Furthermore, the differential screws 44 may be aligned along the length of the clamping portion in an axial direction to allow for the adjustment of the angle of a tool or workpiece in the collet 40 or for compensating for error due to excessive (or shorter) length of the tool, the workpiece, or the collet. It should also be noted that although four attachment screws 26 are equidistantly located around the mounting portion or plate 11 of the collet adapter 10, the present invention is not so limited as additional (or fewer) attachment screws may be used.

As is shown in FIG. 4, each of the differential screws 44 incorporates two different thread pitches, namely, a first thread pitch 50 and a second thread pitch 52. The difference between the first thread pitch 50 and the second thread pitch 52 gives the radial correction per rotation. The closer the first thread pitch 50 is to the second thread pitch 52, the finer the possible adjustment of the differential screw 44 is.

Because of the two different thread pitches, the friction produced in the threads allows for the differential screws 44 to be self locking.

Referring now to FIGS. 1-4, rotational adjustment of a tool or workpiece clamped in the collet 40 is performed by adjusting the differential screws 44, thereby moving the differential screws in or out of the clamping portion 14. In an undamped position, i.e. where the differential screws 44 are not engaging guide bushing 30 via the adjusting sledge 38, the differential screws can be turned in a narrow range with slight resistance, and the guide bushing 30 can be freely adjusted within the available tolerance. By adjusting the differential screws 44 inward, the differential screws engage the adjusting sledge 38, friction and resistance increase, the guide bushing 30 and the sleeve 33 and the collet 40 inserted therethrough are held in place, and the adjustment is thereby stabilized. To ensure that the adjustment is stabilized, the differential screws 44 engage the guide bushing 30 under either tension or

compression stress.

In addition to the fine possible rotational adjustment, it is also possible to compensate for larger rotational concentricity irrespective of the original accuracy of the clamping device (e.g., trued attachment of the mounting portion 12 to the spindle of the machine tool). Likewise the collet adapter 10 permits wear to be compensated and can thus considerably increase the service life of clamping devices. Adjustment of the collet 40 via the differential screws 44 can be performed with a tool or workpiece clamped in the collet prior to processing of the workpiece or between two processing phases. Readjustment is possible at any time without the removal of individual components such as the collet adapter 10, the collet 40, or the tool or workpiece.

In the collet adapter 10, consideration may be given during the design thereof to permitting or preventing the passage of coolants, lubricants, flushing agents, or the like. Depending on the application in which the collet adapter 10 is used, the collet adapter 10 may be configured to provide free access for cleaning purposes or sealed variants that prevent dirt from entering.

Also, the components of the collet adapter 10 can be fabricated using different materials depending on the application in which the collet adapter is to be used. For example, various types of metals and alloys may be used. Metals that may be used to fabricate the collet adapter 10 include, but are not limited to, titanium and nickel. Alloys that may be used include steels such as hardenable steels, non-hardenable steels, stainless steels, and low-corrosion steels. Other alloys are also within the scope of the present invention. Various materials and coatings may be applied and used to improve surface properties. Likewise materials and coatings used in the chemical and foodstuffs industries may be used.

The collet adapter 10 of the present invention can be employed in any suitable machine tool application. Suitable machine tool applications include, but are not limited to, tool grinding machines for volume production and regrinding operations, grinding machines, lathes and automatic lathes for high precision manufacturing using the main spindle and sub-spindles of such lathes, transfer machines, and dividing attachments, tail stocks, spindle noses, and other clamping devices.

As is shown in FIG. 5, one exemplary type of machine tool application is an automated grinding machine 60 in which the collet adapter 10 is used to hold a workpiece 62. The workpiece 62 is movable via movement of a table 63, carriage, carousel, or arm to which the tool is attached. The movement of the table 63 is controllable using a programmable logic circuit 64 (PLC 64) such as a computer. The PLC 64 incorporates a keypad 66 through which an operator can interface with the PLC 64 and a display screen 68 on which data can be viewed. By interacting with the PLC 64 using the keypad 66, the operator can program movements of the workpiece 62 and control the action thereof

Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the appended claims.