FIELD OF INVENTION

This invention relates to clamping of components on the workhoiding component of milling and milling/boring machine tools as well as on the workhoiding side of machining centres typically used for cutting metal, metal alloys, graphite, composite and other materials.

More in detail the present invention provides a method for limiting the deformations induced to a workpiece due to its clamping on said machine tools.

BACKGROUND OF THE INVENTION

Clamping is an operation required to ensure that the relative position between the workpiece and the workhoiding component of the machine tool is constant and sufficiently stiff to contrast static and dynamic forces induced by the machining operations.

The stiffness in said relative position is typically obtained by the friction generated by clamping forces applied to the workpiece towards the surface of a workhoiding component.

The workpiece may directly contact the said surface or the contact may be obtained through specific supporting elements.

In the state of the art are known tools and methods useful for positioning and clamping workpieces in a machine tool.

Some of these solutions are illustrated in the following patent documents:

• CN104252566 entitled "Case structure simplifying and clamping deformation simulation analysis method" filed on 28.09.2014, • CN104384984 entitled "A positioning and clamping tool and method" filed on 20.10.2014,

• CN 102765000 entitled "Self-adjusting clamping device" filed on 09.07.2012.

Patent application CN104252566 discloses a simplified method for clamping deformation analysis and simulation of a case structure, designed to impact analysis clamping force and the clamping force of the cylinder block machining precision layout, so as to control the engine block in the fixture layout and the clamping force is determined to provide a reference.

Patent application CN104384984 discloses a work table surface with four adjusting blocks uniformly placed, each adjusting block comprises a base and a supporting rod, it is equipped with a threaded hole in the middle of the base, a supporting rod is provided with an external thread, support rods through the external thread between the threaded hole of the base of the spiral movement of the lifting function. Through the adjustment of the adjusting blocks, working table and positioning plane are made parallel.

Patent CN102765000 discloses a self-adjusting clamping device, characterized in that comprises a clamping device, data receiving device, the data processing device and a driving device, the data receiving device receives the clamping device and the pressure value signal transmitted to the data processing device, the analysis of the data processing device through a feedback, the feedback signal is transmitted to the driving device. The clamping device comprises extrusion spring, pressure sensor, moving baffle, mechanical telescopic device.

In Figure 1 is depicted an example of a state of the art milling/boring machine tool. In Figure 2 an exemplary workpiece P to be clamped on the surface 3 of a workholding component 2 of a machine tool 1 through the state of the art supporting elements S1 , S2, S3 and S4 is depicted.

The clamping forces are provided by state of the art clamps C that are connected through rods R and it is applied in direct opposition of supporting elements S1 , S2, S3 and S4.

When supporting elements are being used, good practice suggests to apply the clamping forces in direct opposition to the supporting elements as depicted in Figure 3 where the clamping forces applied in opposition to support S1 are generated by the state of the art clamping system composed of clamp C connected through the rod R to the T-slot of the surface 3 of workholding component 2.

Please note that for simplicity, clamping systems for the generation of forces in opposition to supports S2, S3 and S4 are not depicted.

In many applications, a nominally flat bottom surface P1 on the workpiece P is clamped in contact with a nominally flat surface 3 on the workholding components 2, either directly or through the use of a number of supporting elements S1 , S2, S3 and S4, of nominally equal thickness.

Imperfections in the nominally flat bottom surface P1 of workpiece P will combine with imperfections of the nominally flat upper surface 3 of the workholding component 2 of machine tool 1 and/or with imperfections in the plane passing through the upper surfaces of supporting elements S1 , S2, S3 and S4.

If, applying state of the art good practice, the position of supporting elements S1 , S2, S3 and S4 was defined in order to provide favourable supporting points, the variation in the relative elevation between the upper surfaces of supporting elements S1 , S2, S3 and S4 and the corresponding points on the bottom surface P1 of workpiece P would typically result in a first condition A in which the workpiece P is prevalently supported on supporting elements S2 and S4 or result in a second condition B in which the workpiece P is prevalently supported on supporting elements S1 and S3. Due to the finite stiffness of workpiece P, conditions A and B will result in the deformations that are depicted applying augmented reality techniques in, respectively:

- Figure 4 (A), applied deformation (dashed) with workpiece P prevalently supported on supporting elements S2 and S4,

- Figure 5 (B), dashed, applied deformation (dashed) with workpiece P prevalently supported on supporting elements S1 and S3.

The subsequent state of the art application of clamping forces in direct opposition of supporting elements S1 , S2, S3 and S4 will induce additional deformations to workpiece P being that the magnitude of said deformations is proportional to the magnitude of the variations in the relative elevation between the points on the upper surfaces of supporting elements S1 , S2, S3 and S4 and the corresponding points on the bottom surface P1 of workpiece P.

Ideally, if the stiffness of the workpiece P would be very high, it would be advantageous to clamp the workpiece to the surface 3 of workholding component 2 of the machine tool 1 through the use of only three supporting elements, thus eliminating the effect of the combination of imperfections between the nominally flat bottom surface P1 of the workpiece P and of the nominally flat surface 3 of workholding component 2 of machine tool 1.

Nevertheless, in real applications, it is common practice to use more supporting elements (S1 , S2, S3, S4, ... Sn), selected in order to minimize local deflections of the workpiece P generated by the application of forces deriving from its own mass. In these real applications with more than three supporting points, variations in the relative elevation between supporting elements S1 to Sn and points on the bottom surface P1 of workpiece P could have a magnitude of some 0,060 mm.

The subsequent application of clamping forces in direct opposition to the supporting elements S1 to Sn will make the points on the bottom surface of workpiece P to contact said supporting elements thus inducing significant deformations to the workpiece in its clamped conditions as depicted in Figure 4 and Figure 5.

These deformations will adversely affect the accuracy of the finished machined workpiece.

SUMMARY OF THE INVENTION

The present invention addresses the problems above and, in particular, provides a method to minimize workpiece deformations due to its clamping by:

i. defining the number and location of supporting points to be applied in order to minimize the deformation of a workpiece submitted to forces deriving from the application of its own mass,

ii. calculating the forces that would insist on each individual supporting point if the relative elevation between supporting points and points on the workpiece surface would be exempt from imperfections,

iii. interposing adequate force gauges between the adjustable supporting points and points on the workpiece surface and

iv. adjusting the elevation of supporting points in order to equalize the reading of individual force gauges to the calculated forces deriving from the application of the workpiece own mass.

When the condition in (iv) is attained, the workpiece can be clamped to the workholding side of the machine tool by applying a measurable clamping force in direct opposition to the individual supporting points, thus minimizing workpiece deformations. BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the method according to the present invention is described hereinafter with reference to the Figures and enclosed drawings that illustrate respectively:

Figure 1 - Example of a state of the art milling/boring machine tool;

Figure 2 - State of the art exemplary workpiece to be clamped to the surface of a workholding component of the machine tool through four supporting elements of equal thickness;

Figure 3 - Example of a state of the art system to clamp an exemplary workpiece to said surface of said workholding component.

Figure 4 - Augmented representation of deformations on the exemplary workpiece when it is prevalently supported on two diagonally opposite elements illustrated in Figure 3;

Figure 5 - Augmented representation of deformations on the exemplary workpiece when it is prevalently supported on the other two diagonally opposite elements illustrated in Figure 3;

Figure 6 - Flow diagram of the method that is the object of the present invention; Figure 7 - Representation of the exemplary workpiece to be clamped to the workholding surface of the machine tool according to the present invention through four known adjustable supporting elements and four known interposed force gauges. DETAILED DESCRIPTION OF THE INVENTION

Hereafter the logical steps depicted in Figure 6, for the application of the method that is the object of the present invention and its application, depicted in Figure 7, for clamping an exemplary workpiece P on the workholding surface 3 of a machine tool 1 modified with the addition of the devices according to the present invention, are explained.

Step 1 - The number, shape, size and position of the supporting elements that are adequate for the appropriate supporting and subsequent clamping of workpiece P to the surface 3 of workholding component 2 are determined starting from the Computer Aided Design (CAD) data of workpiece P.

Step 2 - The design of the supporting elements foresees the use of known adjustable supporting elements as those identified with 4, 5, 6 and 7 in Figure 7. Known force gauges are interposed between said adjustable supporting elements and the bottom surface P1 of workpiece P. In this way, the force gauges identified as 8, 9, 10 and 1 1 in Figure 7 can be considered as integrating the adjustable supporting elements.

Step 3 - The forces that would be applied to individual supporting elements defined in Step 1 and depicted in Figure 7, deriving from the known mass of workpiece P, if there were no variations in the relative elevations between contacting points on upper supporting elements surfaces and the corresponding points on the bottom surface P1 of workpiece P are computed.

Step 4 - The effective forces that are applied to individual supporting elements are measured.

Step 5 - Individual supporting points are adjusted so that the forces measured as in Step 4 are proportional to the nominal forces computed in Step 3. When this condition is attained there is no variation in the relative elevation between contacting points on upper supporting elements surfaces and the corresponding points on the bottom surface P1 of workpiece P.

Step 6 - When the adjustments described in Step 5 are completed, workpiece P can be clamped to the surface 3 of workholding component 2 by applying measured additional forces generated by (non depicted) clamping systems located in opposition to the adjustable supporting elements 4, 5 and 6 similar to the depicted system composed of clamp C and rod R located in opposition to adjustable supporting element S1 .

The present invention has been described for illustrative but not limitative purpose. For example:

- the depicted adjustable supporting elements 4, 5, 6 and 7 can be of many different types, including any mechanical, hydraulic or pneumatic adjustable supporting elements that might include strain gauges to integrate force measurement;

- the depicted exemplary force gauges 8, 9, 10 and 1 1 are cylindrical force gauges with external power supply and wire signal transmission but they can be replaced by wireless and/or battery-less force gauges.

It has to be understood that further variations or modifications may be made by those men skilled in the art without departing from the relative scope of protection, as defined by the attached claims.

The work leading to this invention has received funding from the European Community's Seventh Framework Programme FP7/2007-2013 under grant agreement n° 285489