Method and apparatus for adjusting a collet in a machine tool

A collet adaptor for a split, draw-back collet having a cylindrical part for mounting in the collet adaptor and a tapered split part as its front end for gripping a workpiece, the collet adaptor comprising:

• a body with a cylindrical bore for receiving the cylindrical part of the collet and tapered surface at the front end of the adaptor for engagement with the tapered, split part of the collet;
• and
• anti-rotation means for preventing rotation of the collet relative to the collet adaptor;
• wherein the collet adaptor includes a plurality of adjustment pins engageable with the cylindrical part of the collet at circumferentially spaced locations around the collet to provide a fine adjustment of the angle of the central longitudinal axis of the collet relative to the central longitudinal axis of the collet adaptor;
• and
• wherein the body of the collet adaptor has an accurately ground planar rear face, and a rear flange with a plurality of apertures extending therethrough for receiving mounting bolts for securing the body of the collet adaptor to the planar front face of a spindle headstock.

A collet adaptor for a split, draw-back collet having a cylindrical part for mounting in the collet adaptor and a tapered split part as its front end for gripping a workpiece, the collet adaptor comprising:

• a body with a cylindrical bore for receiving the cylindrical part of the collet and tapered surface at the front end of the adaptor for engagement with the tapered, split part of the collet;
• and
• anti-rotation means for preventing rotation of the collet relative to the collet adaptor;
• said collet adaptor including a plurality of adjustment pins engageable with the cylindrical part of the collet at circumferentially spaced locations around the collet to provide a fine adjustment of the angle of the central longitudinal axis of the collet relative to the central longitudinal axis of the collet adaptor, said adjustment pins being movable in a substantially radial direction relative to the central longitudinal axis of the collet adaptor, each adjustment pin having a head with a curved surface engaged by a cam surface of a respective elongate adjustment member which is movable relative to the collet adaptor in a longitudinal direction substantially parallel to the central longitudinal axis of the collet adaptor.

A method of compensating for runout errors in a CNC machine having a spindle rotatable about its central longitudinal axis, a collet adaptor, and a split draw-back collet having a cylindrical part mounted in the collet adaptor and a tapered split part at its front end for gripping a workpiece, said method including:

• providing the collet adaptor with an accurately ground planar rear surface, and internal tapered surface at its front end, a cylindrical bore to receive the cylindrical part of a collet, anti-rotation means to prevent rotation of the collet relative to the adaptor, and a plurality of adjustment pins engageable with the cylindrical part of the collet;
• mounting the collet adaptor and collet to the headstock of a rotatable spindle with the planar rear surface of the collet adaptor adjacent an accurately ground front surface of the spindle headstock;
• clamping a workpiece in the collet;
• adjusting the radial position of the collet adaptor and collet relative to the central longitudinal axis of the spindle to compensate for errors in concentricity between the central longitudinal axis of the spindle and the central longitudinal axis of the internal tapered surface of the collet adaptor;
• and
• adjusting one or more of said adjustment pins to compensate for errors in angularity between the central longitudinal axis of the workpiece clamped by the collet and the central longitudinal axis of the spindle,
• wherein the collet adaptor mounted to the spindle headstock by mounting bolts which extend through apertures in a rear flange of the collet adaptor, the apertures being of slightly larger diameter than the mounting bolts to permit the adjustment of the radial position of the collet adaptor relative to the central longitudinal axis of the spindle.