CH714509B1 - Dimensional measurement method of a part and measuring tool to implement the method. - Google Patents

Abstract

The present invention relates to a method for measuring the dimensions and / or the position of a recess (3) or of a protruding part machined in a part (2), based on a change in the behavior of the machine. machining following contact between a probe (4) and the machined part. More precisely, the measurement method is based on the detection of a following error of a machine axis or of an increase in the motor current on a machine axis during contact. The present invention also relates to a method of dynamic correction of the wear of a machining tool involving the aforementioned measurement method and making it possible to compensate directly on the machining machine for machining inaccuracies. It also relates to the probe intended to implement the measurement method and the dynamic correction method.

Description

Object of the invention

The present invention relates to the field of machining. It relates more particularly to a method of measuring the position and / or dimensions of a recess or of a projecting part machined on a part. It also relates to a dynamic method making it possible to compensate during production for the machining inaccuracies induced by the wear of the machining tool, said dynamic method involving the aforementioned measurement method.

[0002] The present invention further relates to the measurement tool used to perform the measurement method and the dynamic compensation method according to the invention.

Technological background and state of the art

In machining, compliance with dimensions with a low level of tolerance is an essential parameter which will condition the quality of the workpiece. In particular, during an operation for driving a small component into a bore of the part, it is necessary to guarantee the interference between the component and the part. It is this interference which makes it possible to guarantee the resistance of the component in the part. If the interference is too low, the component will not be held, and if the interference is too large, the part and / or the component will be damaged during punching out.

The diameter of the bore intended to receive the component is directly impacted by the wear of the machining tool. As illustrated in FIG. 3, the wear of the tool will generate a reduction in the diameter of the bore over time. To ensure the accuracy of the bores, it is therefore necessary to continuously check the machined parts and monitor tool wear. Currently, production is either controlled by rudimentary, imprecise means such as a standard gauge where it is checked whether or not it passes through the bore or by metrological means external to production. In the event of non-compliance with the tolerances, the entire batch produced must be discarded, which generates substantial costs.

There are probes for measuring the bores on the machine tool during production. The probes are generally equipped with a spring trigger system which detects contact between the probe and the workpiece and prevents the probe from breaking. The operation of these probes is, for example, based on a change in the electrical resistance across contact elements of the probe or on the deformation of the probe induced by contact with the workpiece.

[0006] These conventional probes are not always suitable for micromachining applications, namely for the machining of bores having diameters of less than a few millimeters, whether for technological reasons or for reasons of space on the machine.

Summary of the invention

[0007] The object of the present invention is thus to propose a new method of dimensional measurement of a part suitable, without being limited thereto, for micromachining applications. This measurement method can be carried out directly on the machining machine. It uses as a measuring tool a simple rod which in contact with the workpiece generates a response from the machining machine which is used to determine the position and dimensions of a recess or a protruding part. More precisely, it is a following error on one or more of the axes of the machine and / or an increase in the motor current on one or more of these axes which is exploited to determine, on the basis of the position of the axes during contact. , the geometric characteristics of the recess or of the protruding part machined on the part. This measuring tool is thus, unlike the conventional probe, passive because it has no contact detection means. This measuring tool is more particularly suitable for measuring very small diameter bores having a diameter less than or equal to 6 mm, preferably 4 mm, more preferably 2 mm and even more preferably 1 mm, the minimum diameter possibly being as small as 0.3 mm.

[0008] The present invention also proposes a new method making it possible to guarantee the precision of the machining operations thanks to a dynamic correction of the wear of the machining tool. This new dynamic correction method is made possible by the aforementioned measurement method which can be implemented directly on the machining machine.

The dynamic correction method according to the invention consists: first of all to precisely measure the dimensions of the new machining tool, then machining one or more recesses / protruding parts with said tool in a first part, then measuring on the machining machine the dimensions of the last recess or the last protruding part machined, to calculate a corrective factor making it possible to compensate for the differences between the nominal dimensions of the required recess or protruding part and those actually measured after machining, to machine again one or more recesses / protruding parts on this same first part or on subsequent parts, measuring again on the machining machine the dimensions of the last recess or of the last protruding part machined in order to adjust the corrective factor and, so on, until the end of the tool's life.

[0010] This dynamic compensation method thus makes it possible to guarantee precise machining over the entire life of the tool with a low tolerance, without having to measure all the parts. Indeed, an adjustment of the corrective factor is not required after each machining but can be carried out, according to the desired tolerance level, at a greater or lesser frequency.

Brief description of the figures

The present invention will be better understood in the light of the following description, referring, by way of example, to Figures 1 to 14.

[0012] FIG. 1 schematically illustrates a method of machining by interpolation allowing from a single tool to machine bores of different diameters.

FIG. 2 illustrates, as a function of the X, Y coordinates, the radius of the tool (Routil) and the interpolation radius (Rip) required for the machining of a bore diameter (ΦBore) given in the case of interpolation machining as shown in figure 1.

[0014] FIG. 3 graphically illustrates the impact of the wear of the machining tool on the diameter of the bore over time. The Y axis represents the diameter of the bore as a function, on the X axis, of the number of bores for a tool with a nominal diameter of 0.5 mm.

FIG. 4 represents the modeling curve for the points of FIG. 3.

FIG. 5A represents the curve of the corrective factor to be applied as a function of the number of bores to compensate for the wear of the tool. FIG. 5B represents as a corollary the interpolation radius to be programmed in order to maintain a constant bore diameter.

[0017] FIG. 6 represents the bore diameter obtained as a function of the number of bores after correction with the corrective factor of FIG. 5A.

[0018] FIG. 7A shows a three-dimensional view of the probe according to the invention mounted on a tool holder facing the part with the previously machined bores.

[0019] FIG. 7B represents a front view of the probe according to the invention. FIG. 7C is a partial view of the probe of FIG. 7B at its end.

Figures 8A to 9B are different representations of the results obtained during a measurement with the probe according to the invention mounted on a machine with parallel kinematics. During measurement, the probe moves within the bore along the X axis. In FIG. 8A, the position of the three axes during measurement is shown. In Figure 8B, the corresponding tracking error for all three axes is shown. In FIG. 8C, the increase in motor current on these same axes during the measurement is shown. In addition, the trigger which triggers the stopping of the axes during contact between the probe and the part is shown. Figures 9A and 9B are enlargements of Figures 8B and 8C respectively. The horizontal line in dotted lines in FIG. 9A represents the detection threshold of 0.3 μm for this example with the concomitant triggering when this threshold is reached (vertical line in dotted lines).

Figures 10 and 11 respectively show the maximum force and bending to which the probe can be subjected as a function of its diameter.

FIG. 12 is a three-dimensional view of the probe in contact with a point on the circumference of the bore machined in the part and a plan view of this same part during contact with the probe.

FIG. 13 illustrates, as a function of the X, Y coordinates, the various geometric characteristics of the bore for 6 measuring points.

FIG. 14 represents comparative measurements of the diameter and of the circularity of the bore made respectively with the probe according to the invention on the machining machine and on Zeiss equipment external to the machining machine.

Legend

[0025] (1) Machining tool (2) Part (3) Recess and, in particular, bore (4) Measuring tool also called probe (5) Spindle of the machining machine

Detailed description of the invention

The present invention relates to a method for measuring the dimensions and the position of a projecting part or of a machined recess in a part. It also relates to a method of dynamic compensation of the wear of the machining tool involving the aforementioned measurement method. The two methods are more specifically described for the machining of bores but they can be extended to the machining of recesses of any shape as well as to the machining of protruding parts. The methods are applicable for any machining process and, in particular, for an interpolation machining process where different bore diameters are produced with the same standard tool (referenced 1), as shown in FIG. 1.

[0027] Furthermore, the measurement method is more specifically suitable for measuring bores having a diameter which may fall below one millimeter. The measurement method combined with the dynamic compensation method according to the invention makes it possible to measure, during production on the machining machine, the diameter of the bores and to compensate in real time any drift following the wear of the tool. It also makes it possible to measure the position of the bores and to use this information to recalculate the reference frame when the bores are first roughed on one machine and then finished on another machine.

The dynamic compensation method, also called the correction method according to the invention, is based on the tool wear law. As shown in Figure 3, the decrease in the diameter of the bore as a result of tool wear is fairly regular. This wear is of the type (Eq.1): with U which is the wear for bore n U0 which is the nominal wear of the tool n which is the number of the bore τ which is a variable. machining by interpolation, the diameter of the bore (fig. 2) is given by (Eq. 2): ØBore = 2 (Rip + Routil) with ØBore which is the diameter of the bore Rip which is the radius of the interpolation Tool which is the radius of the tool. The Rout is reduced due to its wear during machining operations (Eq. 3): Tool = Tool_0 + U with Routil_0 which is the radius of the new tool, with Ø Bore which is a fixed value that is desired constant Rip which is a quantity that can be varied by programming. Consequently, to maintain a constant bore diameter, it is the interpolation radius must be varied according to the following law (Eq. 5):

For the example of Figure 4 with a nominal tool diameter of 0.5 mm, the corrective factor U is shown in Figure 5A as a function of the number of bores. By virtue of this correction and by correspondingly varying the interpolation radius as shown in Fig. 5B, it is possible to guarantee stable machining throughout the life of the tool as shown in Fig. 6.

Unfortunately, this law of attrition is not universal. It varies from tool to tool. It is therefore necessary to adapt the corrective factor during the life of the tool. According to the invention, measurements of the diameter of the bore are carried out at certain intervals in order to adjust the correction to be applied, which makes it possible to maintain a substantially constant bore diameter over the entire machining period. For this purpose, a specific measuring tool, also called a probe, and referenced 4 in FIGS. 7A-7C, has been developed for measurements of small diameter bores. This probe is intended to be mounted on the spindle 5 of the machining machine. It is about a probe having at its end a shape which can be any (square, rectangular, ...). More specifically, it is a spherical probe or preferably a cylindrical probe having, as shown in FIGS. 7A-7C, one end with a circular section of diameter D. The cylindrical or spherical probe has a circular section with a smaller diameter or equal to 6 mm, preferably to 4 mm, more preferably to 2 mm, and even more preferably to 1, or even 0.5 mm, with a minimum diameter which can go down to 0.2 mm. Preferably, the ratio between the contact length L and the diameter D is between 1 and 4. The probe is made of a material capable of abutting the part during contact without being deformed. Indeed, a deformation of the probe during contact could lead to an erroneous measurement. For example, the probe can be made from a ceramic material such as tungsten carbide, from a metallic material such as steel or else from composites with a metal matrix reinforced with ceramic particles such as tungsten carbide.

Unlike conventional probes, the probe is triggered on the basis of a response from the machine following contact of the latter with the probe and not a modification of the behavior of the probe following contact. Thus, the feeler according to the invention is a simple rod without internal contact detection means. Contact between the probe and the part is detected on the basis of changes in the behavior of the machine and more precisely on the basis of a following error of one or more of the axes and / or on the basis of an increase in the motor current d 'one or more of the axes as shown in Figures 8 and 9. These are different representations for a measurement performed on a 701S machining machine from Willemin Macodel. This is a measurement of the bore diameter along the X axis. This example is based on a measurement made with a Delta type parallel kinematics machine, the three axes (q1, q2, q3) move during probing along the X axis. In FIG. 8A, the position of the three axes is displayed and in FIG. 8B, the following error on these three axes, ie. the difference between the commanded position and the resulting position. In the example, the detection value Δq was set at 0.3 µm. Preferably, this detection value is less than or equal to 2 μm, more preferably to 1 μm and, even more preferably to 0.5 μm with a minimum value which is of the order of 0.1 μm.

In this example, the collision between the probe and the surface of the bore is detected by the axis q3 at a time of +/- 0.6 s on the abscissa axis. A first following error is observed a little before 0.1 s, this first error is linked to the starting of the axes. After start-up, the axes move before they come to a stop against the workpiece resulting in the aforementioned following error. More precisely, in FIG. 9A, it can be seen that as soon as the detection value is reached (cross in dotted lines), the trigger is triggered and all the axes stop (plateau in FIG. 8A) in the shortest possible time or in other words, over the shortest distance possible, to avoid damaging the probe. They then return to their position when triggered to record the position of the axes and, on this basis, calculate the position and dimensions of the bore as explained below. In FIGS. 8C and 9B, one observes concomitantly with the following error an increase in the motor current of the order of 0.1 A on the axis q3. This data can also be used to detect contact between the probe and the part. The detection threshold value can preferably be set between 0.02 A and 0.5 A and, more preferably, between 0.05 and 0.2 A.

This detection threshold value is set as a function of the characteristics of the machine, and more particularly of its reactivity, and of the mechanical properties of the probe. In fact, to avoid breaking the probe which, due to its small dimensions and the materials chosen, is fragile, the machining machine must have sufficient reactivity to trigger the stop of the probe as soon as there is contact between the part and the probe. In this respect, the performances are all the better if the following conditions are met: 1. A great sensitivity of the advance controls in the event of an external disturbance with, for example, a direct drive by linear motor and very low friction. 2. Very low inertias to guarantee high dynamics with, for example, a machine with parallel kinematics. 3. A very fast reaction of the advance commands with a regulation loop having, for example, a frequency between 10 and 50 kHz.

Preferably, in order to optimize the reactivity of the machine during contact, the detection software is done directly in the advance commands and the communication between the advance commands is done by hardware.

Figures 10 and 11 represent, purely by way of illustration, the maximum force and the maximum bending to which the probe can be subjected before rupture for a circular probe made of tungsten carbide and for a useful probing length L, ie .d. the length of the probe which may be in contact with the material, fixed at twice the diameter D of the probe at its end.

According to the invention, the probing speed is suitable for, on the one hand, not to be too slow which would compromise commercial profitability and, on the other hand, not to be too fast in which case the machine does not would not have time to detect and stop before the probe breaks or the part is damaged. A probing speed of between 0.02 and 1 mm / s, and preferably between 0.1 and 0.5 mm / s is a good compromise.

To avoid damaging the part and the probe, it is preferable to detect the collision and stop the machine over a distance less than or equal to 0.005 mm, more preferably 0.002 mm and, even more preferably 0.001 mm. For example, for a distance of 0.001 mm and a detection threshold s set at 0.0003 mm, that leaves a distance of 0.0007 mm to stop the machine. Thus, for a probing speed v of 0.2 mm / s, the detection is carried out over a time t:

And stopping the machine with a uniformly decelerated movement is achieved with a time t:

This shows that for such distances and detection thresholds, the processing speed of the machine must be high.

The measurement of the diameter of the bore is carried out from 6 measurement points on the circumference of the bore as illustrated in Figures 12 and 13. In theory, 3 measurement points are sufficient, but the calculation of the Circularity "O" with 6 measuring points allows to detect measuring errors due to dirt and to make the measuring method more robust by eliminating the contentious points. The diameter D and the circularity O of the bore are calculated on the basis of the following equations (see also fig. 13): D = 2 · R + Dp 0 = max (Ri) - min (Ri) With: Xp; Yp = nominal position of the center of the bore in X-Y, Xc; Yc = recalculated position of the center of the bore in X-Y, Xmi; Ymi = position of the center of the probe during contact between the probe and the part for measurement i with i between 1 and 6 in this example, R = radius of the bore, Dp, Rp = probe diameter and probe radius.

For other non-circular geometries of the machined recess, the equations are adapted accordingly.

Comparative measurements were carried out with the feeler according to the invention and Zeiss equipment which is a three-dimensional metrology measuring machine.

FIG. 14 shows that the results of the measurements carried out with the probe according to the invention are quite comparable to those obtained with the Zeiss equipment.

On the basis of the measurement of the bore with the probe according to the invention, the corrective factor U of equation 1 mentioned above is calculated. Beforehand, the nominal radius Routil_0, i.e. the radius of the machining tool must be known before first use. For example, this radius measurement can be performed with an optical sensor such as a Conoptica camera. This technology has the advantage of measuring tools without contact and thus avoiding damage to small tools (diameter <1 mm). In the particular case of an interpolation machining process, the interpolation radius Rip of the machining tool is then calculated on the basis of the corrective factor U and the desired bore diameter Φ (Eq. 5). It should be noted that depending on the wear of the tool and the desired tolerance level, the measurement of the bore is carried out with a greater or lesser frequency. Thus, purely by way of example, a measurement can be carried out after having machined several tens of bores.

In summary, the compensation method according to the invention comprises the following steps: 1. Provision of a machining tool and measurement of its geometry, 2. Machining of bores on the first part, 3. Measurement on the machining machine with the probe according to the invention of the geometric characteristics of at least the last bore machined, 4. Determination of the corrective factor of equation 1, 5. Machining of one or more bore (s) on this same first part or on subsequent parts, 6. Measurement on the machining machine with the probe according to the invention of the geometric characteristics of at least the last machined bore, 7. Adjustment of the corrective factor of equation 1 to ensure that the following machining is correct, 8. Repeat steps 5 to 7 throughout the life of the tool.

It should be noted that during the machining of the first bore, test bores can optionally be made on parts of the part intended for disposal. The test bores are measured with the probe according to the invention and, if necessary, a correction is applied. In this way, the diameter of the bore is correct from the first machining. The method according to the invention thus makes it possible to guarantee, from the first to the last bore, an almost constant bore diameter.

During the machining of protruding parts, the measurement method as well as the compensation method take place in a similar manner with a measurement of the position and / or dimensions of the protruding part taking place on the basis of the contact between the measuring tool and the periphery of the protruding part. The wear of the machining tool will generate an increase in the dimensions of the protruding part which, according to the compensation method according to the invention, will be compensated during machining.

Claims (16)

1. Measurement method for determining the position and dimensions of a protruding part or of a recess (3), in particular of a bore, machined in a part (2), said method being implemented on a machine machining and adapted for micromachining applications, said method comprising the following steps: - provision of a measuring tool (4) and mounting of said measuring tool (4) on a spindle (5) of the machining machine, - movement of the measuring tool (4) mounted on the spindle (5) within the recess (3) or on a periphery of the projecting part along one or more axes of the machining machine, - detection of contact between the measuring tool (4) and the part (2) based on a change in the behavior of the machining machine, - determination of the dimensions and / or the position of the recess (3) or of the protruding part on the basis of the position of the axis (s) of the machining machine during contact between the measuring tool (4) and part (2). 2. Method according to claim 1, characterized in that the contact between the measuring tool (4) and the part (2) is detected on the basis of a following error of one or more of said axes and / or on basis of an increase in the motor current of one or more of said axes. 3. Method according to claim 2, characterized in that the detection threshold for a tracking error of one or more of said axes is between 0.01 and 2 µm, preferably between 0.05 and 1 µm, and more preferably between 0.1 and 0.5 µm. 4. Method according to claim 2 or 3, characterized in that the detection threshold for an increase in the motor current of one or more of said axes is between 0.02 and 0.5 A and, preferably, between 0.05 and 0.2 A. 5. Method according to one of the preceding claims, characterized in that the speed of movement of the axis (s) of the machining machine is between 0.02 and 1 mm / s and, preferably, between 0.1 and 0.5 mm / s . 6. Method according to one of the preceding claims, characterized in that as soon as the contact between the measuring tool (4) and the part (2) is detected, the axis or axes involved in the movement stop over a smaller distance. or equal to 0.005 mm, more preferably 0.002 mm and, even more preferably 0.001 mm, the axis or axes then returning to their position upon contact between the measuring tool (4) and the part (2) to determine the dimensions and / or position of the recess (3) or the protruding part. 7. Method according to one of the preceding claims, characterized in that at least three measurements, and preferably six measurements, at distinct contact points on the part (2) are carried out to calculate the dimensions and / or the position. of the recess (3) or of the protruding part. 8. Method according to claim 7, characterized in that, for a recess (3) which is a bore, the diameter D and the circularity O of the bore are calculated on the basis of the following relations for n measurements: D = 2 · R + Dp 0 = max (Ri) - min (Ri) with: Xp; Yp = nominal position of the center of the bore in X-Y, Xc; Yc = recalculated position of the center of the bore in X-Y, Xmi; Ymi = position of the center of the measuring tool (4) during contact between the measuring tool (4) and the part (2) for measurement i with i between 1 and n, R = radius of the bore, Dp = diameter of the measuring tool (4). 9. Method for compensating during a machining process of recesses (3) and / or protruding parts in one or more parts (2) a modification of the dimensions of the recesses (3) or of the protruding parts generated. by wear of a machining tool (1), the method comprising the following steps: A. measurement of the dimensions of the machining tool (1), B. machining of one or more recesses (3) and / or one or more protruding parts in the part (s) (2), C. measuring the dimensions of at least the last recess (3) and / or at least the last protruding part machined using the method according to any one of claims 1 to 8, D. on the basis of the measurement made in step C, calculation of a corrective factor taking into account the wear of the machining tool (1), E. machining of one or more recesses (3) and / or one or more protruding parts in the part (s) (2), F. measuring the dimensions of at least the last recess (3) and / or at least the last protruding part machined using the method according to any one of claims 1 to 8, G. adjustment of the corrective factor based on the measurement in step F, H. repetition of steps E, F and G over the life of the machining tool (1). 10. Method according to claim 9, characterized in that, when the recess (3) is a bore having a diameter as dimensions, the corrective factor has the formula: where U0 and τ are the two parameters to be adjusted to compensate for the reduction in the diameter. diameter caused by the wear of the machining tool (1). 11. Measuring tool (4) suitable for implementing the measuring method according to one of claims 1 to 8, characterized in that it is passive, since it does not have any means for detecting contact between the measuring tool. (4) and part (2). 12. Measuring tool (4) according to claim 11, characterized in that it has at its end intended to be in contact with the part (2) a surface each having its dimensions less than or equal to 6 mm, preferably at 4 mm, more preferably at 2 mm, and even more preferably at 1 mm, with a minimum dimension greater than or equal to 0.2 mm. 13. Measuring tool (4) according to claim 11 or 12, characterized in that it has at its end intended to be in contact with the part (2) a cylindrical surface or a spherical surface, the cylindrical surface or the surface. spherical having a diameter D less than or equal to 6 mm, preferably 4 mm, more preferably 2 mm, and even more preferably 1 mm, with a minimum diameter greater than or equal to 0.2 mm. 14. Measuring tool (4) according to claim 13, characterized in that the ratio between the length L of the measuring tool (4) intended to be in contact with the part (2) and the diameter D of the measuring tool (4) is between 1 and 4. 15. Measuring tool (4) according to one of claims 11 to 14, characterized in that it is made of a material capable of not deforming during contact. 16. Measuring tool (4) according to one of claims 11 to 15, characterized in that the material is chosen from the list comprising ceramics, alloy metals and composites with a metal matrix reinforced with ceramic particles.