1. Field of the Invention
The present invention relates to a polyvalent clamp chucks for machine-tools, i.e. a rotating part having retractable jaws able to rigidly hold a workpiece in a very precise position. Numerous types of chuck are already known, each being designed for a specific use.
2. Discussion of Background Information
A first type of chuck has a cylindrical body with two, three, four or six concentric gripping jaws guided radially in its front face. By their design, arrangement and large degree of movement, the jaws of these chucks can grip large workpieces. These jaw-type chucks are differentiated according to the type of drive means provided for radially actuating the jaws in response to axial displacement of a concentric internal control cylinder in the chuck body.
A first kind of jaw-type chuck, the so-called wedge-type, as, for example, described in French patent publication FR-2 447 768, comprises a control cylinder terminating with a sleeve having, facing each jaw, an oblique outwardly-directed wedge in the form of a groove of inverted T cross-section, in which wedge is engaged the rear heel of a main jaw, also of T shape. As the main jaw is mobile only radially in the front face of the chuck body, it is easily understood that when the control cylinder is withdrawn, the lower part of the sleeve's wedge causes lowering of the jaw's correction heel. FR-2 431 338 describes a device for compensating for centrifugal force for this wedge-type of chuck, comprising a lever situated to the rear of and perpendicular to the jaw, with a short arm bearing in a recess in the rear face of the jaw and a longer arm that is pulled outwards by an inertia block under the action of centrifugal force.
In a second kind of jaw-type chuck, the so-called sliding block chuck, such as that described in European patent publication EP-0 215 350, the main jaw is engaged on one side (or on both sides) by an oblique tenon and mortice coupling with a part sliding parallel to the chuck axis, this block being actuated by a rear control cylinder also via a tenon and mortice coupling. To avoid jamming effects, the tenon and mortice coupling between the sliding block and the main jaw may have a trapezoidal cross-section. Alternatively, the oblique coupling of jaws/sliding block may also be provided by a series of parallel, oblique and interpenetrating toothings.
Another jaw-type chuck of the so-called lever type, such as that sold by Messrs GAMET under reference "MX", comprises a lever for each jaw, situated in the radial plane passing through the jaw, with a short arm engaged in a recess provided in the jaw's rear face, and a longer arm at right angles to the other, that engages in a recess provided in the external face of the control cylinder. Thus, by withdrawing the control cylinder, the short arms are caused to move down, thus bringing the jaws together. Advantageously, one or all of the levers further comprises, in extension of their short arm and beyond the pivot, an inertia block that enhances gripping of the jaws under the action of centrifugal force.
A second general type of chuck of the so-called pincer-clamp type, such as that described in European patent publication EP-0 258 771, comprises several pincers in the form of segments of a hollow truncated cone, whose external surfaces come to bear in an also-conical fixed part of the chuck body. The ends of these pincers are provided with hooks by means of which they can be pulled by the control cylinder towards the inside of the fixed conical part of the chuck. This type of chuck is more particularly designed for machining workpieces from bars or parts of medium dimensions requiring uniform gripping over a large surface.
Another general type of chuck, the so-called "front-driving" type, comprises a coaxial body whose end is fitted centrally with a centering point and, adjacent to this point, a series of radial driving knives. The centering point may be fixed with the mobile driving knives driven by a rearwardly-located jack, or vice versa. Cooperating with a similar facing body, these parts are designed to grip by its lateral faces a workpiece to be machined over its entire external surface. Frequently, these chucks are mounted on a jaw-type chuck, either by being directly fixed against the flat front face by bolts, or by being gripped in the jaws themselves. However, the centering of this fitted jaw may create some difficulties, which may slow down the installation operation.
Another general type of chuck, the so called expansible type, comprises a body fitted against the chuck's front face. An external part of this body can expand, or an internal part can contract, under the action of a displaceable control body or under the action of hydraulic pressure, so as to seize a workpiece to be machined on its external surface.
A first type of expansible mechanical chuck comprises a simple external cone forming an integral part of the basic body and over which a slit part is pushed or pulled, according to the inclination of the cone. The converse arrangement can also be envisaged, namely a fixed pincer with an inner cone that is pushed or pulled.
A second type of expansible mechanical chuck of the so-called "double cone" type comprises a pincer gripped between a conical part of a main body and a second mobile conical part situated at the end of a pull-piece. By pulling the pull-piece, thus bringing the two conical parts towards one another, it is possible to radially displace the pincers so they can grip from the inside a hollow part to be machined on its outer surface.
A third type of expansible mechanical chuck comprises, also in a fitted body, an elastic membrane deformed by hydraulic pressure produced by displacement of a piston in a cylinder integrated in the fitted body, which piston is manually or automatically actuated by a jack.
From the above, it can be understood that a mechanic's workshop having to machine numerous small series of different parts, as, for instance, is usual for subcontractor's workshops, must imperatively be equipped with several types of chuck, which requires a large investment.
Moreover, changing from one type of chuck to another is often long and intricate, and increases the dead time between production runs. Furthermore, it would be possible to allow entirely automatic machines to run continuously, even when the operator is absent during breaks, the workpieces being loaded and unloaded by the machine as long as no manual adjustment is required when passing from machining with orthogonal jaws to machining with oblique jaws, or vice versa.
Lastly, upon each change of the clamping chuck, the position of the gripping jaws on the main jaws must accordingly be set to a procedure that may vary from one chuck manufacturer to another.