To drill these boards, as is known, they have to be clamped all the time they are being drilled, to prevent inaccuracies and ensure that the minute chips which are formed cannot get in between two superposed layers. For this reason, each spindle is provided with a workpiece clamp which ends towards the board, that is towards the work to be drilled, with a compression ring, hereinafter referred to as the"foot", containing a hole through which the tool bit advances.

This foot performs the function described above of maintaining constant pressure on the workpiece, and, provided its hole is the correct size, it facilitates and optimizes the extraction of the chips by an extraction system, the extraction pipe of which terminates near this hole.

In the current state of the art, during a drilling operation, the above-described workpiece clamp is essentially integral with the spindle and moves translationally with the latter towards the workpiece until it makes contact with it, thereafter pressing it with a force which increases as the tool advances into the workpiece. This force is controlled by a system of springs which, by their elastic yield deflection, allow the spindle to advance further in the axial direction than the foot which is already pressing on the workpiece.

Clearly, when working as described, two kinds of problem arise: in the first place, the spindle bearings and devices that take the axial loads must withstand not only the axial reaction of the drilling but also that generated by the workpiece clamp transmitted by the abovementioned springs, resulting in an increase in stresses and a reduction in the service life of precise operation; and secondly, the fact that a force of variable intensity is applied to the workpiece, that is, as we have seen, the force increases as the tool advances, compromises the accuracy of the work, owing to minute slipping and settling movements between the superposed sheets, which, because of the special type of machining and the very high speeds of rotation of the tool (even exceeding 100,000 revolutions per minute), can give rise to sometimes unacceptable defects of quality.

To avoid these problems, the inventor of the present invention decided that it would be necessary to completely separate the movement of the workpiece clamp towards the workpiece from the advancing movement of the actual tool, that is from the axial advance of the spindle carrying the tool.

For this purpose the inventor devised a workpiece clamp that moves relative to the said reference support to which the spindle is moveably connected, in such a way as to be totally independent of the latter and moved by a pneumatic system which acts independently, with timing that is co-ordinated with the machining program of the machine tool and is made possible by a high-frequency solenoid valve, with the air, which pneumatically moves the workpiece clamp, passing through it, and by a system of return springs that act on the workpiece clamp.

The subject of the present invention is thus a workpiece clamp as described in the appended Claim 1.

A more detailed description will now follow of a preferred illustrative embodiment of the workpiece clamp according to the invention, which embodiment is chosen from the many that can be realized by those skilled in the art in keeping with the teachings of the said Claim 1.

In the course of this description reference will also be made to the appended figure, which shows a longitudinal section through the said illustrative embodiment, with the workpiece clamp connected to a reference support, to which a spindle (not shown) is also moveably connected.

In the figure mentioned above it can be seen that the workpiece clamp 1 according to the invention, in the embodiment examined here, comprises firstly a pneumatic piston 4 of annular shape which is arranged outside a cylindrical part 2s, coaxial with the spindle, which projects from a reference support 2 of known type (this last being depicted generically and not in section) towards the workpiece 5 that is to be drilled.

The connection between this cylindrical part 2s and the said pneumatic piston 4 of the workpiece clamp 1 is such as to allow relative coaxial movement between the two along an axis that coincides with the drilling axis x-x of the spindle.

Attached to the bottom of the workpiece clamp 1 is a foot 7 containing a hole 7f for the drilling tool to pass through as already explained, its axis therefore coinciding with the axis x-x of the spindle.

The axial movement of the workpiece clamp 1 by which it performs its function is caused, by known methods, by injecting air into the annular space of the said

cylindrical part 2s in which the said pneumatic piston 4 slides.

Before the air, which is generated by a compressor or similar appliance (not shown in the drawing) reaches the annular pneumatic piston 4, it passes (see arrows C) through a high-frequency solenoid valve 8, which can carry out up to 15-20 complete cycles of opening and closing per second, causing the said annular piston 4 to execute, with the aid of a series of return springs 6i interposed between itself and a contact surface 4r on the said cylindrical part 2s, the same number of strokes in both directions, thus fulfilling the production rate requirements of the latest machine tools, and making it possible to achieve a level of accuracy and reliability never hitherto achieved.

As in workpiece clamps of conventional type, the chips are extracted by suction from the central hole 7f of the foot 7 and expelled (arrow U) by an extraction system (not shown) which has one or more extraction pipes 9 with their open ends close to the said hole 7f.

With a workpiece clamp according to the invention, the object addressed by the inventor is therefore achieved, with lower stresses on the spindle and constant pressure on the workpiece that is to be drilled.