Abstract:
A machine collet is provided for mounting and centering tools with minimum circular runout. The collet has a single pieces body formed with integral resilient flexures having lands for gripping the tool shank. The flexures are machined to the same dimensions and yield equally, thus maintaining centrality regardless of tool shank diameter.

Description:
This invention relates generally to collets, and refers more particularly to collets for mounting and centering tools such as mills, drills, reamers, grinding arbors and the like with minimum circular runout. 
     BACKGROUND OF THE INVENTION 
     The mounting of tools concentric with the axis of rotation is necessary for good machining practice. If the tool is mounted in a machine collet eccentric to its center, several undesirable effects result. Cutting flutes at the high point of eccentricity will cut oversize. These same flutes will take a heavier cut than flutes at the low point of eccentricity and fail earlier. These same flutes will cut deeper than the other flutes, resulting in a rough micro-finish. 
     The mounting shanks of tools vary in diameter, but the tools must be centered in the collet regardless of tool shank diameter. However, it has been found that collets which clamp these tool shanks against sliding surfaces lack repeatability. This is because of variations in friction and also because of a slip-stick motion, both of which are characteristic of sliding surfaces. 
     Collets which are clamped by manual tightening or by the application of hydraulic or pneumatic pressure vary the circular runout since these methods are not exactly repeatable. For example, an operator may not tighten with the same pressure each time. As a result, the compliant members of the machine collet do not react the same and centering does not repeat. 
     SUMMARY OF THE INVENTION 
     This invention provides a machine collet for mounting and centering tools with minimum circular runout by means of a very repeatable clamping action of relatively stiff flexible, resilient flexural elements or flexures which do not involve frictional or sliding clamping forces. The clamping action is self-energizing. The operator is involved only with the tool release. 
     Preferably, the clamping of the tool shanks is provided by land-forming V blocks at the front and at the rear of the collet. These land-forming V blocks are machined integrally with the supporting flexures. The flexures are preferably machined to the same dimensions and, therefore, yield equally, thus maintaining centrality regardless of tool shank diameter. 
     The flexures provide sufficient stiffness to resist displacement of the tool by cutting forces. 
     Objects, features and advantages of this invention are to provide a collet having the above features, and one which is easy to operate, of a rugged and simple design and economical to manufacture. 
     These and other objects of the invention will become more apparent as the following description proceeds, especially when considered with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of a collet constructed in accordance with the invention. 
     FIG. 2 is a sectional view taken on the line 2--2 in FIG. 1. 
     FIG. 3 is a sectional view taken on the line 3--3 in FIG. 1. 
     FIG. 4 is a fragmentary sectional view taken on the line 4--4 in FIG. 3. 
     FIG. 5 is a fragmentary sectional view taken on the line 5--5 in FIG. 3. 
     FIG. 6 is a fragmentary sectional view taken on the 
     line 6--6 in FIG. 2. 
    
    
     DETAILED DESCRIPTION 
     Referring now more particularly to the drawings, there is shown a machine collet 10 in the form of a single piece cylindrical body 12 of a suitable spring steel or other strong, relatively stiff or rigid yet resilient and somewhat flexible material. A central blind bore 16 extends into the body 12 through the rear face 17. The body 12 is secured to a spindle 18 by bolts 20, with its rear face 17 rigidly clamped against the spindle face and with its central axis coinciding with the axis of rotation 21 of the spindle. 
     The body 12 is machined to provide three spaced apart, generally parallel slots 22, 24 and 26. The slots are relatively long in comparison to their width as seen in FIG. 1, with mutually generally parallel sides 28, 30, 32, 34, 36 and 38. Sides 32 and 34 of slot 24 are necked inwardly at the mid-portion where indicated at 40. The slots are parallel to the axis of rotation 21 and extend axially from the front face 42 of the body to the bottom of the blind bore 16. The center of the slot 24 coincides with the axis of rotation 21 of the spindle and of the body 12. The slots 22 and 26 are spaced equal distances from slot 24 on opposite sides thereof and form two integral flexural elements or flexures 44 and 46. The flexures 4 and 46 are substantially mirror images in size and shape. The flexures are capable of resilient flexing when placed under load. 
     The flexures 44 and 46 are formed to receive and grip the shank of a tool T on the axis of rotation 21. To grip the shank of the tool, the flexures have integral lands which will now be described. 
     Flexure 44 has circumferentially spaced arcuate lands 48 and 50 near the front face 42 of the body which are separated by a relief 52. Flexure 46 has circumferentially spaced arcuate lands 54 and 56 near the front face of the body which are separated by a relief 58. The lands 48, 50, 54 and 56 are equally spaced from the axis of rotation 21 and are ground to a diameter somewhat smaller than the smallest tool shank they are required to receive. 
     The flexure 44 has circumferentially spaced arcuate lands 60 and 62 near to the bottom face of bore 16 in the body which are separated by a relief 64. Flexure 46 has circumferentially spaced arcuate lands 66 and 68 near the bottom face of bore 16 in the body which are separated by a relief 70. The lands 60, 62, 66 and 68 are equally spaced from the axis of rotation 21 and are ground to the same diameter as the lands 48, 50, 54 and 56. 
     Land 48 is separated from land 60 by a relief 72. Land 50 is separated from land 62 by a relief 74. Land 54 is separated from land 66 by a relief 76. Land 56 is separated from land 68 by a relief 78. 
     Together, lands 48 and 50 in effect form a V block. Similarly, lands 54 and 56, lands 60 and 62, and lands 66 and 68 form V blocks. The V block formed by lards 48 and 50 is opposed to and cooperates with the V block formed by lands 54 and 56 at the front of the body to grip the shank of a tool. The V block formed by lands 60 and 62 is opposed to and cooperates with the V block formed by lands 66 and 68 at the rear of the body also to grip the shank of a tool. 
     The body is formed with aligned radially extending bolt holes 79 drilled on a line which extends through the axis of rotation 21 at right angles thereto. These holes extend through the slots 22 and 26 and into the flexures 44 and 46. Bolts 80 and 82 extend into these holes with bolt 80 threaded into the flexure 44 and bolt 82 threaded into flexure 46. The holes are counterbored to receive the heads of the bolts. When the bolts are tightened, the flexures 44 and 46 are flexed away from one another. The bolts may be provided with self-locking nuts 84 which limit the distance that the bolts may be backed out. 
     An adaptor 86 bolted to the spindle extends into the blind bore 16 in the body and has an elongated slot 88 to receive flats on the end of the tool shank to act as a driver for the tool. Four radial, equally-spaced set screws 90 are threaded in body 12 and bear on the outside diameter of adaptor 86 to provide means to centralize the body 12 on spindle 18. 
     In use, the bolts 80 and 82 are tightened to retract the flexures 44 and 46 and the shank of the tool is then loosely inserted into the mid-portion of the slot 24 with the flats on the ends of its shank extending into the slot 88 in the adaptor 86. The bolts 80 and 82 are then loosened to allow the flexures to return toward their natural free state condition enabling the V blocks provided by lands 48, 50, and lands 54, 56 at the front of the body 12 to securely grip the shank of the tool under a resilient clamping pressure. At the rear of the body 12, the V blocks provided by lands 60, 62 and lands 66, 68 also similarly grip the shank of the tool. The flexures grip the tool shank with a self-energizing action since they merely need to be released to resiliently clamp the tool shank. Since the flexures are machined to the same physical dimensions, any variation in the diameter of the tool shank will cause equal displacement of the V blocks and maintain centrality of the tool. The tool shank is firmly gripped at two axially spaced points by the two sets of V blocks. The tool may be easily released by tightening the bolts to retract the flexures and unclamp the tool shank.