EP0465357A1 - Device for machining contours in soft material and automatic machining process making use of such a device - Google Patents

Abstract

Device for machining contours (2) of soft material, such as plastic, comprising essentially a rotary tool (6) consisting of abrasive elastic lamellae, the flexibility of which makes it possible to damp the jolts of the member (33) carrying the tool (6). Preferably, there are used a pneumatic motor (9) and a tachometer (10) for controlling the tool (6) at a constant speed which guarantees a stable cutting or machining force. The invention is used particularly for the deburring and chamfering of parts. <IMAGE>

Description

The invention relates to an apparatus for machining contours of soft material such as polyurethane or other plastic materials, as well as to an automatic machining process suitable for such apparatus.

We seek above all to obtain a smooth and clean machined contour, which is difficult due to the tenderness of the material on which the jolts of the machining, due in particular to the irregularities of the cutting speed, the feed and of the depth of the pass and which are quite important in the case of automatic processes, have immediate repercussions and cause a poor surface condition or undulations. The primary object of the invention is therefore the supply of a tool, and more generally of an apparatus, which makes it possible to produce a machined contour of good quality on such materials, and this even if the machined contour is flexible, when for example we proceed to the removal of burrs in very fine ribs of a few tenths of a millimeter.

Another object of the invention is especially interesting for automatic processes and relates to the control of the position of the tool, that is to say of its depth of cut. On soft materials, the cutting force which can provide an estimate of the desired depth of cut is indeed very low and can hardly be captured correctly. We therefore planned another system to achieve a good result.

In its most general form, the device machining according to the invention comprises a tool consisting of elastic strips embedded at one end in a rotary block driven by a motor device. The slats are possibly made of an elastic canvas covered with abrasive.

If the burrs of the contour to be machined are flexible, it is advantageous to use a system of non-abrasive brushes pressing on two opposite faces of the contour in order to maintain the burrs around the tool in a state where they are suitable for deburring. . This avoids the folding of the burrs that the tool could cause and which would compromise its effectiveness by rejecting the burrs out of reach of the slats. Such a system can consist of two brushes carrying the radial elements constituting bending springs; the brushes then rotate around axes more or less parallel to the faces of the contour.

It is advantageous for the motor device to be pneumatic and for the device to include a sensor for the speed of rotation of the tool. It is then possible to adjust the cutting depth of the tool by slaving the rotation speed to a determined value, which can be periodically recalculated as a function of the idle rotation speed and its variations over time.

• les figures 1 et 2 sont deux vues de l'ensemble du système d'usinage représentant respectivement l'outil en vue de face et en vue de dessus ; et
• la figure 3 est un organigramme représentant le mode d'asservissement de l'appareil.

The invention will now be described in more detail with the aid of the following three figures, which are given by way of non-limiting illustration and which represent an embodiment of the invention:

• Figures 1 and 2 are two views of the entire machining system respectively representing the tool in front view and in top view; and
• Figure 3 is a flowchart showing the control mode of the device.

First of all, reference is made to FIGS. 1 and 2. The workpiece 1 comprises a flange 2, or a rib, the edge of which is machined 3 by a deburring operation, but the tool which will now be described is also usable for other types of machining, in particular chamfering.

It essentially comprises a rotary block 4 provided with lamellae 5 arranged in a radiating manner and which lie in planes through which passes the axis of rotation 7 of the tool 6. The rotary block 4 is subjected to a spindle 8 which drives under the action of a pneumatic motor 9. The slats 5 consist of an elastic fabric, one end of which is embedded in the rotary block 4 and the other end of which comes to rub against the edge 3, along which it is moved. This other end is covered with an abrasive powder which is responsible for the machining.

A tachometer 10 fixed to the pneumatic motor 9 measures the speed of rotation of the spindle 8 and of the tool 6. Furthermore, the casing of the pneumatic motor 9 carries a console 15 extended by a beam 16 parallel to the spindle 8 and which passes near the tool 6. The beam 16 carries the axes of two parallel brushes 17 and 18 provided with groups of nylon bristles 19 that the rotation of the brushes 17 and 18 rubs on the two flat and opposite faces 20 and 21 of the collar 2 and the edge 3; the axes of rotation of the brushes 17 and 18 are parallel to the faces 20 and 21. The beam 16 is arranged so as to extend a little next to the flange 2. The bristles 19 of the two brushes 17 and 18 have bearing surfaces which partially overlap or merge, but the brushes 17 and 18 are slightly axially offset and thus remain separate.

A motor 22 fixed to the console 15 drives, by a first belt 23, one of the brushes 17, and a second belt 24 interconnects the axes of the brushes 17 and 18 so that the second brush is driven.

The bristles 19 are bent by rubbing on the faces 20 and 21 and then straighten out thanks to their elasticity. The opposing forces that the brushes 17 and 18 thus exert on the flange 2 and the edge 3 have the effect of bending the flexible burrs which would otherwise bend under the force of the tool 6 and would be difficult to eliminate. Such a burr bears the reference 29. It is folded around its root (where it is connected to the collar 2) and glued against the upper edge of the collar 2 by the upper brush 17. The tool 6 then attacks it by the root and thus detaches it from the collar 2 rather than eliminating it. It will be noted that the tool 6 is placed obliquely with respect to the surfaces of the flange 2 and only removes material from the edge which carries the burr 29, that is to say it requires chamfering. The tool 6 with blades 5 is much preferable in such a situation to a rotary brush furnished with abrasive bristles because the elastic abrasive structures must be close enough to the region to be deburred to exert a sufficient abrasion force. Hair would therefore be brought to portions of the surface of the flange 2 adjacent to the region to be deburred, which would therefore be scratched, which can be accepted for reasons of aesthetics in many cases, while the strips 5 are curved. over their entire height.

The lower brush 18 is intended to raise the burr 29 if necessary, in order to put it within the reach of the upper brush 17.

The pneumatic motor 9 is disposed at one end of a slide 30, the other end of which is pushed back by an eccentric 31 actuated by a motor 32. A return system such as a spring 35 holds the end of the slide 30 against eccentric 31; moreover, all of these parts are housed in a terminal member 33 of a robot arm which is not shown in detail and which is programmed to follow approximately the edge 3 to be machined. The slide 30 is held substantially in a direction where its sliding allows it to approach or move the tool 6 away from the edge 3, and therefore to modify the cutting force; the machining passes consist in moving the terminal member 33 along the contour 2 while adjusting the depth of the pass by constantly sliding the slide 30.

A potentiometer 34 locates the displacements of the motor 32 and therefore of the eccentric 31.

Reference is now made to FIG. 3 to examine how the servo-control of the tool 6 takes place, but it is previously necessary to devote some considerations to the machining conditions in the technical context of the invention.

To guarantee a uniform depth of pass, we want to work at constant effort, but if the material being machined is soft, the effort is very low, of the order of a few grams, and therefore could not be measured without excessive noise by a force sensor linked to spindle 8.

It is actually possible to estimate with acceptable precision the cutting force using the speed of rotation of spindle 8, especially when this spindle 8 is driven by a pneumatic motor 9.

où C est le couple moteur, J est l'inertie des parties entraînées autour de l'axe de rotation 7, B est un coefficient représentatif des pertes par frottement visqueux, C_ext est le couple exercé par la matière usinée, qui dépend directement de l'effort de coupe, et ω′ et ω désignent la vitesse de rotation de l'outil 6 et son accélération. Comme le couple moteur C est proprotionnel à la pression d'alimentation du moteur pneumatique 9, il peut être connu avec une grande précision, et on peut de même estimer J et B avec précision grâce à un calibrage préliminaire.We can indeed write the equation $${\text{C = J}}_{\text{ω}}{\text{′ + B}}_{\text{ω}}{\text{+ C}}_{\text{ext}}\text{,}$$

where C is the driving torque, J is the inertia of the parts driven around the axis of rotation 7, B is a coefficient representative of the viscous friction losses, C _ext is the torque exerted by the machined material, which directly depends on the cutting force, and ω ′ and ω denote the speed of rotation of the tool 6 and its acceleration. As the engine torque C is proportional to the supply pressure of the pneumatic engine 9, it can be known with great precision, and it is likewise possible to estimate J and B with precision thanks to a preliminary calibration.

où t₁ désigne l'instant considéré, t₀ l'instant du début des mesures et A est un correcteur d'avance de phase. Le résultat de ce calcul analogique est fourni à un système d'actionnement 43 qui actionne le moteur 32 et modifie ainsi la position réelle P_r de l'excentrique 31 qu'il connaît grâce au potentiomètre 34 jusqu'à la faire coïncider avec la position angulaire P_c prédédemment calculée. La modification de la position de l'outil 6 qui en résulte se traduit par une convergence de la vitesse réelle ω₁ vers la vitesse de consigne ω_c.The tachometer 10 is connected by its output to a filtering system 40 which calculates the real speed ω₁ at each instant. A setpoint speed supply system 41 simultaneously supplies the setpoint speed ω _c to be observed. A calculation system 42 collects the two speeds ω₁ and ω _c and calculates the angular position P _c of the eccentric 31 for which the real speed ω₁ would become equal to the set speed and which is equal to a coefficient near to

where t₁ designates the instant considered, t₀ the instant of the start of the measurements and A is a phase advance corrector. The result of this analog calculation is supplied to an actuation system 43 which actuates the motor 32 and thus modifies the real position P _r of the eccentric 31 which it knows thanks to the potentiometer 34 until it coincides with the position angular P _c previously calculated. The resulting modification of the position of the tool 6 results in a convergence of the real speed ω₁ towards the set speed ω _c .

This system has a very short reaction time.

The set speed ω _c can be chosen proportional to the no-load speed of the tool 6. As this no-load speed varies over time, in particular following variations in supply pressure, temperature or lubrication conditions, the set speed ω _c must be recalculated from time to time by the set speed supply system 41. In practice, a measurement can be made each time the tool 6 is released to carry out a new machining pass.

At these release moments, the servo described above is interrupted and the eccentric 31 is brought back to a fixed position where the slide 30 is halfway. When a new machining pass is made, the tool 6 is brought to a few millimeters from the edge 3, then the end member 33 is brought closer to the edge 3 at slow speed until it touches it (in the direction of the arrow shown). in FIG. 3), after which a decrease in the speed of rotation is detected and the trajectory which has been taught to the robot is traversed by resuming the servo-control.

Claims (10)

Contour machining apparatus made of soft material, characterized in that it comprises a tool (6) consisting of elastic strips (5) embedded at one end in a rotary block (4) driven by a motor device (9). Machining apparatus according to claim 1, characterized in that the lamellae consist of an elastic fabric covered with abrasive. Machining apparatus according to any one of claims 1 or 2, characterized in that it comprises, for machining flexible contours (2), an elastic system pressing on two opposite faces (20, 21) of the contour. Machining apparatus according to claim 3, characterized in that the elastic system consists of two brushes (17, 18) carrying elastic radial elements (19), the brushes rotating around axes substantially parallel to the faces of the contour. Machining apparatus according to any one of claims 1 to 4, characterized in that it comprises a device for adjusting (30, 31) the position of the tool (6) in the direction of the contour (2) according to the indications of a sensor (10) of a magnitude representative of the machining effort. Machining apparatus according to claim 5, characterized in that the sensor (10) is a sensor measuring the speed of rotation of the tool (6). Machining device according to any one of Claims 1 to 6, characterized in that the motor device is pneumatic. Method for automatically machining contours of soft material using a machining device according to claims 6 and 7, characterized in that the machining device is moved along the contour, the tool (6) being approached and distant from the contour with determined movements by controlling the speed of rotation of the tool to a determined value. Automatic machining method according to claim 8, characterized in that the determined value is periodically recalculated as a function of the idle speed of the tool. Automatic machining method according to claim 9, characterized in that it breaks down into machining passes between which the determined value is recalculated, each pass being then preceded by a slow approach of the tool (6) from the contour to 'a decrease in the speed of rotation of the tool (6) is detected.

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