CH714509A1 - 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 projecting part machined in a workpiece (2), based on a change in the behavior of the machine. machining following the contact between a probe (4) and the machined part. More precisely, the measurement method is based on the detection of a tracking error of a machine axis or an increase of the motor current on a machine axis during the contact. The present invention also relates to a method of dynamically correcting the wear of a machining tool involving the above-mentioned measuring 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

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 the dimensions of a recess or of a protruding 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.

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 a driving operation of a small component in a bore of the part, it is necessary to guarantee the interference between the component and the part. It is this interference that guarantees the behavior of the component in the room. If the interference is too weak, the component will not be maintained, and if the interference is too large, the part and / or the component will be damaged during hunting.

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 cause a reduction in the diameter of the bore over time. To guarantee the accuracy of the bores, it is therefore necessary to continuously monitor the machined parts and monitor the wear of the tools. Currently, production is either controlled by imprecise rudimentary 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 case of non-compliance with tolerances, the entire batch produced must be discarded, which generates significant costs.

There are probes for measuring the bores on the machine tool during production. 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 electrical resistance through contact elements of the probe or on the deformation of the probe induced by contact with the workpiece.

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 bulk. on the machine.

SUMMARY OF THE INVENTION The object of the present invention is therefore to propose a new method of dimensional measurement of a part suitable, without being limited thereto, for micromachining applications. This measurement method can be performed directly on the machining machine. As a measuring tool, it uses 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 specifically, it is a tracking 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 used 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 measurement tool is thus, unlike the conventional probe, passive because it does not have contact detection means. This measurement tool is more particularly suitable for measuring bores of very small diameters 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 being as small as 0.3 mm.

The present invention further provides a new method for ensuring the accuracy of machining through 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 of:

- first of all to precisely measure the dimensions of the new machining tool,

- then to machine one or more recesses / protruding parts with said tool in a first part,

- then to measure on the machining machine the dimensions of the last recess or of the last machined projecting part,

- to calculate a corrective factor making it possible to compensate for the differences between the nominal dimensions of the recess or of the protruding part required and those actually measured after machining,

- to re-machine one or more recesses / projecting parts on this same first part or on subsequent parts,

- to measure again on the machining machine the dimensions of the last recess or the last machined protruding part in order to adjust the corrective factor and, thus, until the end of the tool's life.

CH 714 509 A1 [0010] This dynamic compensation method thus guarantees precise machining with a low tolerance over the entire life of the tool, 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 [0011] The present invention will be better understood in the light of the following description, referring, by way of example, to FIGS. 1 to 14.

Fig. 1 schematically illustrates a process of interpolation machining allowing from a single tool to machine bores of different diameters. Fig. 2 illustrates as a function of the coordinates X, Y the radius of the tool (R _or tii) and the interpolation radius (R _ip ) required for the machining of a bore diameter (üAiesage) given in the case of interpolation machining as shown in fig. 1. Fig. 3 illustrates graphically 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 in fig. 3. Fig. 5A represents the curve of the corrective factor to be applied as a function of the number of holes to compensate for the wear of the tool. Fig. 5B is a corollary to the interpolation radius to be programmed to maintain a constant bore diameter. Fig. 6 represents the bore diameter obtained as a function of the number of bores after correction with the correction factor in fig. 5A. Fig. 7A shows with a three-dimensional view the probe according to the invention mounted on a tool holder facing the workpiece with the previously machined bores. 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. Figs. 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 the measurement, the probe moves within the bore along the X axis. In fig. 8A, the position of the three axes during the measurement is shown. In fig. 8B, the corresponding tracking error for the three axes is shown. In fig. 8C, the increase of the 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. Figs. 9A and 9B are enlargements of Fig. 8B and 8C respectively. The horizontal line in dotted lines in fig. 9A represents the detection threshold of 0.3 pm for this example with the concomitant triggering when this threshold is reached (vertical line in dotted lines). Figs. 10 and 11 represent respectively the maximum force and bending to which the probe can be subjected according to 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 when in contact with the probe. Fig. 13 illustrates according to the X, Y coordinates the different geometrical characteristics of the bore for 6 measurement points. Fig. 14 represents comparative measurements of the diameter and 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 [1 012] (1) Machining tool

CH 714 509 A1 (2) Part (3) Recess and, in particular, bore (4) Measuring tool also called probe (5) Spindle of the machine tool

Detailed description of the invention The present invention relates to a method of measuring the dimensions and the position of a projecting part or a recess machined in a part. It also relates to a method of dynamic compensation for the wear of the machining tool using 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 projecting 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.

Furthermore, the measurement method is more specifically adapted to measure bores having a diameter which can drop below a millimeter. The measurement method combined with the dynamic compensation method according to the invention makes it possible to measure the diameter of the bores during production on the machining machine and to compensate in real time for any drift due to 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 system when the bores are first sketched on a machine and then finished on another machine.

The dynamic compensation method, also called correction method according to the invention, is based on the wear law of the tool. As shown in fig. 3, the reduction in the diameter of the bore following wear of the tool is fairly regular. This wear is of the type (Eq.1):

u = U _o (l - e-?) with

U which is wear for bore n

Uo which is the nominal wear of the tool n which is the number of the bore τ which is a variable.

During interpolation machining, the diameter of the bore (fig. 2) is given by (Eq.2):

Φ Bore 2 '(^ Rip + Tool) with θ Bore which is the diameter of the bore

R _ip which is the radius of the interpolation

Routii which is the radius of the tool.

The Routii decreases due to its wear during the machining operations (Eq.3):

Tool ~ Tool_0 U with

Routii which is the radius of the new tool, which gives (Eq.4):

$ Bore ~ 2 ‘^ Rip + Routil_0 + ^ 0 (l ® with e Bore which is a fixed value that we want constant

Ri _p which is a quantity which can be varied by programming.

Consequently, to maintain a constant bore diameter, the interpolation radius must be varied according to the following law (Eq.5):

£ _ip = - (r _ouM0 + i / o (l - e “?)) For the example of fig. 4 with a nominal tool diameter of 0.5 mm, the corrective factor U is shown in fig. 5A depending on the number of bores. Thanks to this correction and by varying the radius accordingly

CH 714 509 A1 interpolation 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 wear law 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 made at certain intervals to adjust the correction to be applied, which makes it possible to maintain a bore diameter that is substantially constant throughout the machining period. To this end, a specific measurement tool, also called a probe, and referenced 4 in FIGS. 7A-7C, Has been developed for small bore measurements. This probe is intended to be mounted on pin 5 of the machining machine. It is a probe having at its end a shape which can be arbitrary (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 diameter less than 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 of up 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 against the part during contact without deforming. Indeed, a deformation of the probe during contact could lead to an incorrect measurement. For example, the probe can be made from a ceramic material such as tungsten carbide, from a metallic material such as steel or from composites with a metal matrix reinforced with ceramic particles such as tungsten carbide.

Unlike conventional probes, the triggering of the probe is performed on the basis of a response from the machine following the contact of the latter with the probe and not of a modification of the behavior of the probe following the contact. Thus, the probe according to the invention is a simple rod without internal contact detection means. The contact between the probe and the workpiece is detected on the basis of changes in the behavior of the machine and more precisely on the basis of a tracking 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 figs. 8 and 9. The latter are different representations for a measurement carried out on a 701S machining machine by Willemin Macodel. This is a measurement of the diameter of the bore along the X axis. This example being based on a measurement carried out with a parallel kinematic machine of the Delta type, 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 tracking error on these three axes, i.e. the difference between the commanded position and the resulting position. In the example, the detection value Aq was set to 0.3 pm. 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 tracking error is observed a little before 0.1 s, this first error is linked to the starting of the axes. After starting, the axes move before reaching the stop against the part resulting in the abovementioned tracking error. More specifically, in fig. 9A, we see that as soon as the detection value is reached (cross in dotted lines), the trigger is triggered and all the axes stop (plate on fig. 8A) in the shortest possible time or, in others words, over the shortest possible distance, to avoid damage to 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 fig. 8C and 9B, we observe concomitantly with the tracking 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 workpiece. 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 fixed as a function of the characteristics of the machine, and more particularly of its reactivity, and of the mechanical properties of the probe. Indeed, to avoid breaking the probe which, due to its small dimensions and the materials chosen, is fragile, the machining machine must have sufficient reactivity only to trigger the stop of the probe as soon as there is contact between the part and the probe. In this regard, performance is all the better if the following conditions are met:

1. 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 rapid 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 responsiveness 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.

Figs. 10 and 11 represent, by way of illustration only, the maximum force and the maximum bending to which the probe can be subjected before rupture for a circular probe in tungsten carbide and for a useful length L

CH 714 509 A1 probing, i.e. the length of the probe which can 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 being too slow which would compromise commercial profitability and, on the other hand, not being too fast in which case the machine does not would not have time to detect and stop before the probe ruptures 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 workpiece 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 to 0.002 mm and, even more preferably to 0.001 mm. For example, for a distance of 0.001 mm and a detection threshold s set at 0.0003 mm, this 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:

s 0.0003 t = - = ———— = 0.0015 seconds v 0.2 [0027] And the machine is stopped with a uniformly decelerated movement with time t:

2-s 2-0.0007 t = - = -—- = 0.007 seconds v 0.2 [0028] This demonstrates that for such distances and detection thresholds, the speed of processing of the machine must be high.

The diameter of the bore is measured from 6 measurement points on the circumference of the bore as illustrated in figs. 12 and 13. In theory, 3 measurement points are sufficient, but the calculation of the circularity "O" with 6 measurement points makes it possible to detect measurement errors due to dirt and to make the measurement method more robust by eliminating the disputed points.

following equations (see also fig. 13):

Xc =

The diameter D and the circularity O of the bore are calculated on the basis of

Yc = _n Σ Ymi-è, _z 2 --- Y p

J (Xm; - Xc) ² + (Ym; - Yc) ²

Ri = _ Σ # 1-6 ^R ~~ 6 ~

D - 2 R + Dp 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,

Xmij; Ymij = 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 = diameter of the probe and radius of the probe.

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

Comparative measurements were carried out with the probe 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 entirely comparable to those obtained with the Zeiss equipment.

Based on the measurement of the bore with the probe according to the invention, the corrective factor U of the equation 1 mentioned above is calculated. Beforehand, the nominal radius R _or tii_o, i.e. the radius before the first use of the machining tool must be known. For example, this measurement of the radius can be carried out 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

CH 714 509 A1

R _ip of the machining tool is then calculated on the basis of the corrective factor U and the desired bore diameter (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 machine tool with the probe according to the invention of the geometric characteristics of at least the last machined bore,

4. Determination of the corrective factor of equation 1,

5. Machining of one or more bore (s) on the same first part or on subsequent parts,

6. Measurement on the machine tool with the probe according to the invention of the geometric characteristics of at least the last machined bore,

7. Adjustment of the correction factor of equation 1 to guarantee that the following machining is correct,

8. Repeat steps 5 to 7 over the life of the tool.

It will be noted that during the machining of the first bore, test bores may possibly be made on parts of the part intended for scrap. 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.

When machining 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 projecting part. The wear of the machining tool will cause 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

1. Measuring method for determining the position and dimensions of a projecting part or a recess (3), in particular of a bore, machined in a part (2), said method being implemented on a machine machining and suitable 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,

- Displacement 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 the contact between the measuring tool (4) and the part (2) on the basis of a change in behavior of the machining machine,

- Determination of the dimensions and / or the position of the recess (3) or of the projecting part on the basis of the position of the axis or axes of the machining machine during contact between the measuring tool (4) and the part (2).

2. Method according to claim 1, characterized in that the contact between the measuring tool (4) and the part (2) is

Claims (15)

detected on the basis of a tracking error in one or more of said axes and / or on the 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 pm, preferably between 0.05 and 1 pm, and more preferably between 0.1 and 0.5 pm. 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 any one of the preceding claims, characterized in that the speed of displacement of the axis or axes 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 any 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 distance less than or equal to 0.005 mm, more preferably to 0.002 mm and, even more preferably to 0.001 mm, the or CH 714 509 A1 the axes then returning to their position during contact between the measuring tool (4) and the part (2) to determine the dimensions and / or the position of the recess (3) or of the part in projection. 7. Method according to any one of the preceding claims, characterized in that at least three measurements, and preferably six measurements, at separate contact points on the part (2) are carried out to calculate the dimensions and / or the position of the recess (3) or of the projecting 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 relationships for n measurements: ΣΧ ™ · ι-η Xc = 2 --— ~ Xp Σ Ym ^ -n Yc = 2 --— n - Yp Ri = J (Xmi - Xc) ² + (Yrrii - Yc) ² R = ^^ n D - 2 R + Dp O = 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, Xm ,; Ym ,, = position of the center of the measuring tool (4) during contact between the measuring tool (4) and the part (2) for the measurement / with / 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. Measuring the dimensions of the machining tool (1), B. Machining 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. Based on 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 lifetime of the machining tool (1). 10. Method according to claim 9, characterized in that, when the recess (3) is a bore having the dimensions of a diameter, the corrective factor has the formula: U = u _o (l - _e -?) Where U _o and τ are the two parameters to be adjusted to compensate for the reduction in diameter caused by wear of the machining tool (1). 11. Measuring tool (4) suitable for implementing the measuring method according to any one of claims 1 to 8, characterized in that it is devoid of means for detecting the contact between the measuring tool (4 ) and the 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 of any shape having each of its dimensions less than or equal to 6 mm , preferably 4 mm, more preferably 2 mm, and even more preferably 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 CH 714 509 A1 spherical having a diameter D less than or equal to 6 mm, preferably to 4 mm, more preferably to 2 mm, and even more preferably to 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 any one of claims 11 to 14, characterized in that it is made of a material capable of not being deformed during contact. 16. Measuring tool (4) according to any one of claims 11 to 15, characterized in that the material is chosen from the list comprising ceramics, alloyed metals and composites with a metal matrix reinforced with ceramic particles. CH 714 509 A1 CH 714 509 A1 0 senior 0 * 00 I ·············· ü.W · [·· --- 0 * 00 Ovr 0.7î «ï ·: ·· o,? Oo -h; 0.784: 0.