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Pinching collet calculation |
The pinching collet for general use is the simplest since it doesn’t require special design configurations (like axial stop). The pinching collet is composed basically by the collet body a cylindrical part with an inner conical surface and a hardened steel sleeve with an outer conical surface. The collet sleeve is moving relative to collet body and its movement is called drawback. The collet sleeve has slits cut partway along its length and forming flexible jaws whose ends are brought close to each other. The form and size of the hole in a collet sleeve must correspond to the workpiece profile. The pinching collet is actuated by various ways (cam-type, air-mechanic actuator, hydraulic, manual). When the workpiece is loaded on the collet, the actuating mechanism pulls back (drawback) the sleeve and the workpiece is pushed through the jaws of the collet sleeve. The jaws are pressed by elastic force against the workpiece surface. As the collet sleeve is pull back inside collet, the jaws close under the action of wedge forces produced by the conical surface, and clamp the workpiece.
The required actuating force N of the collet sleeve is given by formula (1) where Q is the clamping force of the workpiece and Q’ (2) is the force to compress the collet sleeve tabs in order to take up the clearance between the collet jaws and the work. The angle a is collet angle cone; b is the angle of friction between the collet and the clamping sleeve; E the modulus of elasticity of the collet sleeve material; L is the length of tab from the point of restraint to mid-cone; n the number of tabs; f is the tab sag whish is half the diametric clearance between the work and collet.
J (3) is the moment of inertia of the thin circle sector (see section A) at the point of tab restraint where R is the outside radius of tab surface(D/2); s is the tab wall thickness; a1 is half the tab sector angle in rad.
