EP0315060A1 - Self-ganging equipment for a plane grinding machine - Google Patents

Abstract

The invention relates to self-gauging equipment for plane grinding machines, which comprises a measuring head (12) fixed to the table (4) of the grinding machine with which the equipment is associated in order to measure the distance between this table and the grinding wheel (6), and an electronic measuring and control apparatus (14) which receives the output signal of the measuring head and which produces signals in order to control the advancing and withdrawing movement of this grinding wheel. <??>In the equipment according to the invention, the measuring head comprises a pneumatic circuit with a first branch of very small internal volume, equipped with an inlet nozzle and with a measuring outlet nozzle (48) directed so as to blow towards the edge of the grinding wheel, a second branch equipped with an inlet nozzle and with a reference nozzle and with means for connecting the inlet nozzles of these branches to an installation (18) supplying compressed air at a regulated pressure, and a differential pressure transducer, with a semiconductor element, for detecting the difference between the pressures prevailing in the two branches. <??>As compared with a mechanical measuring head, this head has the advantage of not becoming worn and of not causing wear on the grinding wheel. Furthermore, it makes it possible to prevent the measurements being falsified by spraying chips and grains from the grinding wheel. <IMAGE>

Description

The present invention relates to self-calibrating equipment for flat grinders which conventionally comprise a horizontal table for carrying workpieces and a grinding wheel disposed above this table, which has a slice by which it machines these parts and which, by a movement of advance and retreat, can respectively come into contact with and deviate from them.

More precisely, the subject of the invention is self-calibrating equipment which comprises a measuring head fixed to the table of the grinding machine to produce, during the machining of the parts, an electrical output signal which passes through an extreme value each time the grinding wheel passes over it, this extreme value being a function of the distance which then separates the grinding wheel from the table, means for memorizing values which correspond to at least some of the values taken by the output signal from the head measurement and means for producing signals for controlling the movement of the grinding wheel forward and backward when the stored values correspond to set values.

In known equipment of this kind the measuring heads are mechanical heads which include a measuring key which palpates the edge of the grinding wheel by exerting on it a certain measuring force created by a spring and an inductive or capacitive transducer which produces the electrical output signal just discussed, in response to movement of this key.

As a result, these known devices have several drawbacks:

First, the edge of the grinding wheel is worn irregularly by the measuring key and conversely the key is worn by the grinding wheel, which inevitably leads to measurement errors which increase over time.

Second, as the grinding wheel spins very quickly, grains which detach from it and shavings which come from the machined parts are projected and often strike the key, which makes it oscillate, sometimes until resonance, and like the memorized values, which in this case are minimum values, are then, therefore, lower than they should be we can pass prematurely from a rectification phase to the other and the back of the wheel can be controlled before the parts have reached their exact final dimension. This is all the more true since one generally arranges so that the measurement key exerts on the grinding wheel only a weak measuring force so that the reciprocal wear of this key and this grinding wheel is limited on no more possible.

Third, when the table of the grinding machine is of the "sweeping" type, that is to say when it performs a linear movement back and forth under the wheel, the moving parts of the measuring head such as the key, the connection piece between it and the transducer, the movable part of it and the spring which creates the measuring force, are subjected to positive and negative accelerations and vibrations of the table and this is another source of errors for measures.

The object of the invention is to provide self-calibrating equipment which does not have these drawbacks and it is achieved thanks to the fact that in the equipment according to the invention the measuring head comprises a pneumatic circuit with a first branch of very small internal volume, provided with an inlet nozzle and a measurement outlet nozzle oriented so as to blow towards the edge of the grinding wheel, a second branch provided with an inlet nozzle and an outlet nozzle reference, means for connecting the inlet nozzles of these branches to an installation supplying compressed air at a regulated pressure, and a differential pressure transducer with semiconductor element for detecting the difference between the pressures prevailing in said branches and produce the aforementioned electrical output signal.

Thus, the air stream which leaves the measuring nozzle does not wear out the grinding wheel and since there is no longer any contact between this grinding wheel and the measuring head the latter is also not subject to wear.

On the other hand, in the case of a sweeping table, the accelerations and vibrations thereof have no measurable effect on the air stream in question.

Finally, as the measuring head no longer has moving parts capable of oscillating and entering into resonance, the shavings and seeds of the grinding wheel can no longer have a harmful influence on the measurements.

That said, in self-calibrating equipment according to the invention the measuring head preferably comprises, in addition, at least one cleaning nozzle also oriented so as to blow towards the edge of the grinding wheel in order to separate the coolant used for the machining of the parts from the place to which the measurement nozzle blows and from means for supplying compressed air to this cleaning nozzle.

• - la figure 1 montre schématiquement une rectifieuse plane, représentée partiellement, et l'équipement d'autocalibrage qui lui est associé;
• - la figure 2 est une vue schématique en coupe longitudinale de la tête de mesure qui fait partie de cet équipement; et
• - la figure 3 est le schéma d'une mémoire qui est utilisée dans l'appareil électronique de mesure et de commande qui forme une autre partie de cet équipement.

The invention will be better understood on reading the following description of an embodiment which has been chosen as an example to illustrate it. This description refers to the attached drawing in which:

• - Figure 1 schematically shows a plane grinder, partially shown, and the self-calibrating equipment associated therewith;
• - Figure 2 is a schematic view in longitudinal section of the measuring head which is part of this equipment; and
• - Figure 3 is a diagram of a memory which is used in the electronic measuring and control device which forms another part of this equipment.

The plane grinding machine which has been shown both partially and schematically in FIG. 1 comprises a frame 2 which carries, for example by means of an oil film, a horizontal sweeping table 4 which can perform on this frame 2 a linear back-and-forth movement between two extreme positions, in the direction of the double arrow F.

Above the table 4 is obviously a circular grinding wheel 6 which can rotate around a horizontal axis 8, orthogonal to the direction of movement F of this table 4, and which is mounted on a support not shown so as to be able be moved vertically and possibly parallel to the axis 8 in order to be able to machine parts whose width is greater than the thickness of the grinding wheel and / or several rows of parts arranged side by side.

Figure 1 shows only two of these parts which are designated by the reference 10 and which are located at the end of a single row or not.

The self-calibrating equipment according to the invention which is also shown in this figure comprises three parts: a measuring head 12 which is fixed to the table 4 of the grinding machine permanently or temporarily, for example by screwing, gluing or magnetization, in the extension of the row of parts 10 and in the field swept by the grinding wheel 6, an electronic measuring and control device 14 which will be discussed later, which is connected to the measuring head 12 by an electric cable 16, and an installation 18 for supplying this head 12 with compressed air.

As can be seen, the installation 18 is mainly constituted by a source of compressed air 20, capable of supplying air under a pressure ranging for example from 4 to 10 bars, a filter 22 placed at the outlet of this source. 20, a pipe 24 which directly brings part of the air transmitted by this filter 22 to the measuring head 12, a pressure regulator 26 which receives the other part of this air, another pipe 28 which connects this regulator 26 at the head 12 and a pressure gauge 30 to indicate the pressure in this line 28.

FIG. 2 schematically shows how the measuring head 12 is made.

This head 12 comprises a body 32, for example parallelepipedic or cylindrical, which has at one of its ends two end pieces 34 and 36 for the connection of the pipes, respectively, 28 and 24 which have just been mentioned and a terminal 38 for the connection of the electric cable 16 which, unlike the latter, is not visible in the figure.

There is an internal pipe 40 from the end piece 34 which divides at the other end into two branches 42 and 44.

The first, 42, of these branches is delimited by an inlet nozzle 46 and a measurement outlet nozzle 48 which is on the side or around the body 32, so as to be able to blow towards the edge of the grinding wheel 6 when the head 12 is fixed on the table 4 of the grinding machine.

The second branch 44 is delimited by another inlet nozzle 50 and an adjustable reference outlet nozzle 52.

As shown in Figure 2, this second branch opens on the side of the end pieces 34 and 36 but this is obviously not an obligation. The main thing is that the compressed air which circulates there can always escape freely.

On the other hand, it can be seen by looking at FIG. 2 that the two branches 42 and 44 each have a branch 54, respectively 56. These branches 54 and 56 lead to a differential pressure transducer with semiconductor element 58 which is used to measure the pressure difference between the two branches 42, 44 and which is electrically connected to the aforementioned connection terminal 38 in order to be able to be supplied by the cable 16 and transmit to it the electric signal which it is responsible for producing.

This transducer 58 is essentially constituted by a semiconductor wafer in which a membrane has been produced, by chemical machining methods, and a bridge of piezoresistors formed on this membrane. Transducers of this kind are now well known. However, if one wishes to have more information on their structure, their operation and their manufacture one can refer to the French patent application No. 2 266 314.

Finally, it can also be seen from FIG. 2 that the head 12 has another internal pipe 60 which starts from the end piece 36 and which terminates in another nozzle 62 placed near the measurement nozzle 48, so as to be able to separate the coolant, for example the oil, which is on the edge of the grinding wheel 6 from the place towards which this measuring nozzle 48 blows when the grinding wheel 6 passes over it.

In general, the position of a cleaning nozzle like the nozzle 62 relative to the measuring nozzle 48 obviously depends on the way in which the head 12 is placed on the table 4, on the position it occupies and on the direction in which the grinding wheel 6 rotates.

In the present case, the two nozzles 62 and 48 are substantially aligned in the direction of the length of the head 12 and the nozzle 62 is closer to the end where the end pieces 34 and 36 are located, but it could be that the measuring nozzle that is closer to this end. Furthermore, if the measuring head was arranged transversely to the direction of movement of the table and not longitudinally as in FIG. 1, the cleaning nozzle 62 should then be offset laterally with respect to the measuring nozzle 48.

Note that one could also provide not a single cleaning nozzle but two or more distributed around the measuring nozzle.

That said, as we are currently talking about how the head 12 is placed on the table 4, we can take this opportunity to indicate that it does not need to be at the end of the or a row of pieces 10. It could also be next to or among pieces 10 and if these pieces were too thin for it to be placed on the table it could be hung on the side wall thereof. The main thing is that the grinding wheel 6 passes above it or at least above the measuring nozzle and the cleaning nozzle (s) and that the pipes and the cable which connect it respectively to the installation 18 compressed air supply and the electronic measuring and control device are not subjected to excessive deformation and are not likely to cause or be damaged during a machining operation.

The measuring head that has just been described operates on exactly the same principle as the pins or rings that are currently used to pneumatically measure interior or exterior diameters.

When there is nothing in front of the measurement nozzle 48 the compressed air escapes freely from the latter at a constant flow and if it is supplied the transducer 58 then produces an equally constant electrical output signal representative of the algebraic value of the pressure difference between branches 44 and 42 or 42 and 44, depending on how it is connected.

On the other hand, when the grinding wheel 6 passes over the nozzle 48, from a certain moment the compressed air can no longer exit from it except by radiating around its outlet orifice, as shown by the arrows represented on figure 2, and if the condition which one will indicate thereafter is respected the flow of this air becomes a function of the area of the annular surface which is between the edge of this orifice and the edge of the grinding wheel and consequently of the average distance between this edge and this edge.

In other words, the flow of air that leaves through the nozzle 48 decreases, passes through a minimum value when the axis of the grinding wheel is located at the right of the center of the opening of the latter and then increases to resume the value that he had before the wheel went through.

Conversely, the air pressure in branch 42 and its branch 54 increases to a maximum value which corresponds to the minimum value of the flow rate and then decreases.

On the other hand, it is clear that the more the wheel advances the lower this minimum value of the flow and the higher the maximum value of the pressure.

Suppose first of all that the transducer 58 is connected so as to provide an output signal which represents the difference between the pressure in the branch 44 and the pressure in the branch 42. Under these conditions, each time that the grinding wheel 6 passes to the above the measuring nozzle 48, this output signal passes through a minimum value which is lower the lower the grinding wheel.

The difference with the flow rate of the air which leaves through the nozzle 48 is that this minimum value of the signal can in principle be negative. For this reason it cannot be said that it is always truly representative of the distance between the grinding wheel and the table. By cons it is always a function of it.

In practice, we will generally manage in this case to achieve the same result as with the mechanical measuring heads that we currently use, that is to say that we will adjust the reference nozzle so that the pressures in the branches become substantially equal when the parts have reached their final dimension and we will finish adjusting the zero of the transducer output signal by means of a potentiometer provided in the measuring and control device to obtain a signal whose minimum values represent exactly the excess thickness of the parts compared to this final dimension.

Theoretically, we could also adjust the reference nozzle and the potentiometer so that the signal zero corresponds to a zero distance between the grinding wheel and the table so that the minimum values obtained represent the nominal dimension of the parts. But what we prefer to know in general when we rectify parts is their extra thickness and not their rating.

Suppose now that the transducer 58 is connected so as to provide an output signal which represents the difference between the pressure in branch 42 and the pressure in branch 44.

At this time, this signal no longer passes through a minimum value when the grinding wheel 6 is above the measuring nozzle 48 but, on the contrary, through a maximum value and this value becomes increasingly larger as and when that the grinding wheel is lowered.

As will be seen later, it is easy then to obtain the same information as in the previous case by slightly modifying the electronic circuit of the measurement and control device.

In fact, for the measuring head 12 to function correctly, as has just been indicated, two conditions must be met.

The first, which has already been alluded to, is that from the moment we want to make measurements the area of the outlet of the measuring nozzle 48 is larger than the area of the smallest surface which can be between the edge of this orifice and the edge of the grinding wheel 6, this smallest surface being of course that which exists when the axis of the grinding wheel 6 is exactly at the center of the orifice 48.

If this is true at the start it is necessarily so when the grinding wheel 6 then continues to advance.

This condition is quite comparable to that which must be fulfilled when measurements of internal or external diameters are carried out by means of the pins or rings of which we have already spoken. If it were not observed, there would be no possible measurement because the flow rate of the compressed air which would exit through the nozzle 48 and the pressure in the branch 42 would not vary.

The second condition is linked to the invention and to the fact that the horizontal movement of the table 4 and of the head 12 is very rapid. For reasons of clarity the drawing does not take this into account.

This second condition is that the interior volume of the branch 42 and of its branching 54 is as low as possible or at least that it does not exceed a certain limit which is of the order of a few mm³.

In fact, since the diameter of the grinding wheel 6 is large, it can be considered that when its axis passes over the measuring nozzle 48 the area of the surface between its edge and the edge of the the orifice of this nozzle 48 remains constant for a certain time. But this time is very short and for the measurement to be correct it must be sufficient to allow the pressure in the branch 42 and its branch 54 to become uniform.

However, the smaller the volume of the branch and its branching, the faster this standardization.

We will now describe the electronic measuring and control device 14 which is shown in FIG. 1 and which corresponds to the first situation which has been considered previously with regard to the connection of the transducer.

This device firstly comprises an amplifier circuit 64 which receives the output signal from the transducer 58 via two conductors of the cable 16, the other conductors of this cable being connected to a power source, preferably a source current, not shown and a potentiometer 65 which acts on it to adjust the zero of the signal supplied by the transducer 58.

This circuit 64, the gain of which must be positive, can be a simple differential amplifier or a more complicated circuit. It could for example comprise a first operational amplifier on which one could act by means of potentiometer 65 and a second operational amplifier connected following the first on which one could act by means of another potentiometer to adjust the gain of the whole. A circuit of this kind is described in patent US-A-4,538,449.

That said, the apparatus 14 also includes an analog memory 66 for storing the minimum values of the signal supplied by the amplifier circuit 64, which correspond to those of the output signal from the transducer 58, and a switch 68 which makes it possible to connect this memory 66 to a positive terminal of the electronic circuit of which it is a part to reinitialize are contained, more precisely to bring the latter to a determined maximum value.

This switch 68 can for example be a key switch which is actuated manually before the rectification of new parts or a relay controlled by the electrical control of the machine each time it is started for machining a series of coins.

It may also be a transistor which periodically receives, for example every ten seconds, the pulses produced by an electronic oscillator to become conductive.

FIG. 3 shows how memory 66 is produced.

This memory has two inputs 78 and 80 which are respectively connected to the output of the amplifier circuit 64 and to a terminal of the switch 68 and an output 82.

On the other hand, it consists of an operational amplifier 84 whose non-inverting input is connected to terminal 78, a diode 86 whose cathode is connected to the output of amplifier 84 and the anode both at terminals 80 and 82 and at the inverting input of this amplifier 84 and of a capacitor 88, connected between the anode of the diode 86 and the ground, which constitutes the storage element and which can be recharged thanks to switch 68.

Memories of this kind are well known and, as we know, as long as they are not reset they can only memorize successive values which are decreasing.

Consequently, if the grinding wheel 6 passes several times over the measuring head 12 while remaining at the same level and if a minimum value of the signal supplied by the amplifier circuit 64 is slightly higher than that which precedes, this value is not not memorized, unless the capacitor 88 has been recharged in the meantime, which can happen if it is periodically.

Finally, FIG. 1 also shows a display device 70 which is connected to the output of memory 66 and which is responsible for indicating the excess thickness of the pieces relative to their final dimension, and two Schmitt rockers (triggers) 72 and 76 also connected to the output of this memory 66.

The first, 72, of these flip-flops is responsible for constantly comparing the value contained in the memory 66 with a positive reference value which can be adjusted by means of a potentiometer 74 and for generating a control signal which makes it possible to pass the grinding machine from the roughing phase to the finishing phase when the parts have reached a certain dimension which corresponds to this set value.

As for the second flip-flop 76, it is used to compare the stored values to the zero value and to produce a signal which makes it possible to control the recoil of the grinding wheel 6 when the parts have reached their final dimension.

Of course, in reality the measuring and controlling apparatus of a grinding machine is not as simple as that of FIG. 1, but it would be useless to speak here of all the additional means of switching, adjustment and indications which are usually part of an apparatus of this kind and which the skilled person knows well.

Let’s see how we can modify this device which we have described in part to adapt it to the case where the transducer 58 of the measuring head 12 is connected to provide a signal which represents the difference between the pressure in branch 44 and that in branch 42 (see FIG. 2).

A first solution consists quite simply in replacing the amplifier circuit 64 with positive gain by an amplifier circuit with negative gain, which makes it possible to conserve the same memory 66 and the same switch 68.

Another solution is to keep on the contrary the same amplifier circuit 64, to replace memory 66 by an analog memory capable of memorizing maximum values and switch 68 by a switch which makes it possible to connect this memory to ground and to predict in addition an operational amplifier, for example of gain -1, for transforming the stored values into corresponding minimum values.

As is known, it is very easy to obtain a memory for maximum values from the memory which is represented in FIG. 3 by only reversing the direction of connection of the diode 86.

Furthermore, it is clear that the invention is not limited to the embodiment which has been described and to those which have already been envisaged.

Indeed, one could very well imagine for example another embodiment in which the reference nozzle would not be adjustable and in which the zero of the output signal of the measuring head could only be adjusted by one or two potentiometers provided in the measuring and control device.

In addition, the invention can also be applied to flat grinding machines with a sweeping table, the grinding wheel of which is not a horizontal but a vertical axis.

It can even be extended to flat grinders with a rotary table but it is then necessary to provide for example rotary couplings and a slip ring to allow respectively the compressed air supply lines and the cable which connects the measuring head to the electronic measuring and control device to follow the movement of this table.

Claims (3)

1. Self-calibrating equipment for a flat grinding machine which comprises a horizontal table (4) for carrying workpieces (10) and a grinding wheel (6) disposed above this table, which has a slice by which it machines the said parts and which, by a movement of forward and backward, can respectively come into contact with and deviate from them, equipment which comprises a measuring head (12) fixed to the table of the grinding machine to produce, during the machining of the parts, an electrical output signal which passes through an extreme value each time the grinding wheel passes over it, this extreme value being a function of the distance which then separates the grinding wheel from the table, means (66 ) to store values which correspond to at least some of the successive values taken by the output signal from the measuring head and from the means (72 - 76) for producing signals for controlling the movement of advance and retraction of the grinding wheel when the stored values correspond ripple at setpoints, characterized in that the measuring head comprises a pneumatic circuit with a first branch (42, 54) of very small internal volume, provided with an inlet nozzle (46) and a measurement output nozzle (48) oriented so as to blow towards the edge of the grinding wheel, a second branch (44, 56) provided with an input nozzle (50) and a reference output nozzle (52 ) and means (40) for connecting the inlet nozzles of these branches to an installation (18) supplying compressed air at a regulated pressure, and a differential pressure transducer (58) with semiconductor element for detecting the difference between the pressures prevailing in said branches and producing said output signal. 2. Self-calibrating equipment according to claim 1, characterized in that the extreme values of the output signal produced by the transducer (58) of the measurement head (12) and the corresponding values stored are minimum values. 3. Self-calibrating equipment according to claim 1 or 2, characterized in that the measuring head (12) comprises at least one additional cleaning nozzle (62) also oriented so as to blow towards the edge of the grinding wheel (6 ), to remove the coolant used for machining the parts (10) from the place to which the measurement outlet nozzle (48) blows and means (60) for supplying compressed air to this cleaning nozzle.

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