WO1991007245A1

WO1991007245A1 - Process and device for monitoring the workpiece-gripping force in machine tools

Info

Publication number: WO1991007245A1
Application number: PCT/DE1990/000866
Authority: WO
Inventors: Heiko Noske
Original assignee: Heiko Noske
Priority date: 1989-11-10
Filing date: 1990-11-09
Publication date: 1991-05-30
Also published as: EP0500617A1; DE3937466A1; DE3937466C2

Abstract

In machine tools, in particular turning machines, the workpiece being machined is held in a holder. To ensure safe operation, a certain minimum gripping force must be maintained. If this force is too small, the workpiece can be thrown out of its holder, which represents a considerable accident hazard. It should also be noted that the workpiece-gripping force decreases as the friction surfaces of the chuck become progressively less well lubricated and progressively dirtier. The invention therefore proposes that the chuck jaws act to produce a gripping force, the hydraulic energy producing this force is monitored and, when a given energy threshold is exceeded, a signal is triggered.

Description

Method and device for monitoring the clamping force in machine tools

The invention relates to a method for monitoring the clamping force in machine tools according to the preamble of claim 1.

In the case of machine tools, in particular lathes, the workpiece to be machined is clamped in a workpiece holder. To ensure safe operation; a certain clamping force must be observed. If the tension is too low, devastating consequences can occur. So there is the possibility that the workpiece from the workpiece would be thrown out, which poses a significant risk of accidents for people in the area. The workpiece itself and other objects can also be damaged. If, on the other hand, the clamping force is too great, the workpiece or the workpiece holder is deformed.

For lathes e.g. the required clamping force is applied hydraulically by exerting a force on the inside of the clamping jaws, which are movably mounted in a chuck body. This force is e.g. Introduced via an actuating rod which engages with the clamping jaws via a conical gear. The sliding surfaces formed between the clamping jaws on the one hand and the chuck body and the gear on the other hand require lubrication so that the frictional forces are reduced and material corrosion is avoided.

The manufacturers of such machine tools prescribe cleaning and lubrication of the sliding surfaces in predetermined periods in order to maintain constant, low sliding friction values and to prevent wear of the sliding surfaces. However, it has been shown that regular lubrication and cleaning is common is omitted because the required dismantling of the workpiece holder is time-consuming and cumbersome and disrupts the normal workflow.

However, if regular lubrication and cleaning are not carried out, the properties of the workpiece holder change over time. Due to the insufficient lubrication and the dirt deposited on the sliding surfaces, the force to be overcome when clamping the workpiece holder is greater than with ideal lubrication and cleanliness of the sliding surfaces. The reason for this is that the increase in friction consumes part of the force applied via the actuating rod in order to overcome the friction. Thus only a part of the applied force remains for the clamping force. The result is that with increasing pollution and decreasing lubrication, the clamping force gradually drops to such an extent that the minimum clamping force required for safe operation is no longer achieved.

There are already clamping force meters that are clamped in place of a workpiece and measure the applied clamping force. Practical experience shows, however, that this possibility of monitoring operational safety is also not regular is perceived. With this solution, the point in time at which the critical tension falls below the minimum clamping force can also be missed.

The invention has for its object to improve a method for monitoring the clamping force of machine tools in such a way that an automatic measurement of the clamping force is made possible so that the operating personnel can be warned in time of an excessive decrease in the clamping force.

This object is achieved in a method according to the preamble of claim 1 by the features specified in the characterizing part.

The basic idea of the invention is based on determining the clamping force applied to the workpiece by means of the clamping jaws via the clamping work or the travel pressure or the travel path which is carried out during the hydraulic actuation (it should be expressly mentioned that the term "travel pressure" means both Individual pressures in the cylinder chambers of a double chamber cylinder as well as the differential pressure is included). As a result, it is possible to determine the critical state of friction without great effort and to the operating personnel Signal condition so that cleaning and lubrication can be carried out immediately.

Due to other friction in the transmission, e.g. a force can already be overcome by sealing the piston. Nevertheless, the proportion of the additional frictional force caused by the contamination and decreasing lubrication in the total force applied is still measurable.

In an advantageous development of the invention, the clamping jaws are acted upon by a prestressing force which exerts an increased normal force on the sliding surfaces and thereby causes a significant increase in friction when contamination and reduced lubrication occur. It is thus achieved that the proportion of the frictional force caused by contamination and decreasing lubrication increases compared to the proportion of the other frictional forces. The frictional forces artificially increased by the increased normal force now lead to a clearer influence on the clamping work performed, the travel pressure and the travel path. Therefore, these measurements allow an improved statement about the clamping force. A further development of the invention provides that the clamping jaws are continuously subjected to the prestressing force and the clamping work is also measured continuously.

This ensures that the critical state of friction is reliably recognized without a time delay and that the maintenance intervals for cleaning and lubricating the sliding surfaces can thus be adapted to the actual lubrication and contamination conditions. Accordingly, an unnecessary interruption of the workflow is avoided, but nevertheless it is ensured that the necessary maintenance work can be carried out in good time before the accident and damage consequences which are based on insufficient elasticity occur.

Another alternative provides that the prestressing force is only applied to the clamping jaws at predetermined time intervals and that the clamping work or the travel pressure or the travel path is measured in these time intervals.

In this case, an additional load on the sliding surfaces due to the preload is avoided. But there must be the threshold of tension, below the Maintenance work are carried out, so that the critical threshold of the clamping force, from which there is a risk of an accident, is not reached between two time intervals.

The amount of the preload force with which the clamping jaws are acted upon can expediently be such that it corresponds to the nominal value of the clamping force for clamping a workpiece.

As a result, the measurement can also be carried out without clamping a workpiece, so that e.g. the measurement can be carried out during a changeover without disturbing the workflow. An unnecessary clamping of a workpiece, which would have to be removed from the workpiece holder without being processed without this configuration after the alarm has been triggered, since this has to be maintained primarily, is avoided.

The invention further relates to a device for monitoring the clamping force of machine tools according to the preamble of claim 6.

In this regard, it is the task of improving a device for monitoring the clamping force of machine tools to the effect that a automatic measurement of the clamping force is made possible so that the operating personnel can be warned in time of an excessive decrease in the clamping force.

This object is achieved in a device according to the preamble of claim 6 by the features specified in the characterizing part.

The properties and advantages which are achieved with such a device and the developments according to claims 7-9 correspond to the properties and advantages described in connection with the method according to claims 1 to 4.

In a practical embodiment of the invention, which is used in a machine tool designed as a lathe, the workpiece holder comprises a chuck body with clamping jaws guided therein. The prestressing device consists of spring bodies which are supported on the clamping jaws on the one hand and the chuck body on the other.

It is thereby achieved that the prestressing force acts in exactly the same direction as the prestressing force and also acts at the same point as the prestressing force. Thus, when clamping the workpiece occurring clamping forces are increased and the sliding surfaces are subjected to normal forces in the same ratio as is triggered by the clamping forces.

In one configuration of the device, the hydraulic actuating device consists of an actuating rod which is coupled to the clamping jaws via a gear and a piston which is connected to the actuating rod and is guided in a double-chamber cylinder. The measuring device for the hydraulic clamping work consists of a displacement transducer and a pressure transducer, which are assigned to the double chamber cylinder, and a computer.

With the aid of this measuring device, the clamping work can be calculated in a simple manner, namely via the integral of the pressure difference p on the piston, multiplied by the area a of the piston over the displacement path s. The area of the piston is assumed to be given, so that the signals determined by the displacement transducers and pressure transducers can be evaluated directly in the computer to calculate the clamping work W.

Further developments and advantageous refinements of the invention result from the claims, the others - 10 -

Description and the drawing illustrating an exemplary embodiment.

The drawing shows:

1 shows a longitudinal section through a hydraulically actuated workpiece holder of a lathe,

FIG. 1 shows a model of a hydraulically operated chuck,

3 shows a cross section through a chuck body of a lathe with a pretensioning device according to the invention, and

4 is a graphical representation of the relationship between the clamping force and the clamping work.

In Fig. 1 is a longitudinal section through a hydraulic actuated workpiece holder 10 of a lathe shown. A spindle 34 of the lathe is rotatably supported in spindle bearings 36. At the right end of the spindle 34 there is a workpiece holder 10, consisting of a chuck body 18 firmly connected to the spindle 34 and clamping jaws 12 guided therein. The clamping jaws 12 are mounted in, for example, double-T grooves of the chuck body 18 and are radially adjustable. The clamping jaws 12 are adjusted by means of a gear 22 which is conical and also engages with the inside of the clamping jaws 12 via double-T grooves.

A longitudinal movement of the gear 22 to the right in the plane of the drawing expands the jaws 12 while they are compressed in the opposite direction. The transmission 22 is connected via an actuating rod 24 to a piston 28 which is guided in a double cylinder 26. This double chamber cylinder 24 is supplied with hydraulic fluid from a hydraulic source (not shown) via hydraulic lines 38.

The hydraulic lines 38 are connected to a pressure sensor 32, which has the differential pressure p between one and the other chamber of the double-chamber cylinder 26 recorded. The actuating rod 24 is still led out of the double chamber cylinder 26 in the drawing. There is a displacement transducer 30 which determines the travel path s. The signals from the displacement transducers 30 and pressure transducers 32 arrive at a computer, not shown here, which determines the clamping work W from the values supplied for the travel path s and the pressure difference p. If we also take into account the area a of the piston 28, so that the following calculation formula results:

W = ΔP a * ds

; ^■

Fig. 2 shows a model of a hydraulically operated chuck, in which the representation of the workpiece holder is limited to the parts that are involved in the creation of frictional forces. Arrows 1 to 5 point to the surfaces between which frictional forces occur.

With good lubrication, the frictional forces between surfaces 1, 2 and 3 are relatively small compared to those between surfaces 4 and 5. The latter surfaces result from that for hydraulic operation necessary piston seal and the cylinder seal. Nevertheless, if the frictional forces between surfaces 1, 2 or 3 increase, changes due to the higher frictional forces between surfaces 4 and 5 can still be measured.

The pretensioning device according to the invention can be used to artificially exert an increased normal force on the surfaces 1, 2 and 3. This normal force multiplied by the friction factor increases the friction force if the friction factor increases due to increasing wear, reduced lubrication or increasing contamination.

The desired consequence is that the proportion of the frictional forces between the surfaces 1, 2 and 3 increases compared to the proportion of the frictional forces between the surfaces 4 and 5 and can thus be evaluated even better in terms of measurement technology. Thus, the relationship between the clamping force and the clamping work can be determined even better with increasing friction forces on surfaces 1, 2 and 3.

Two alternatives are shown in FIG. 2 as pretensioning device 14. The one alternative shows a spring element 40 that between the mass m2 and an abutment is attached. With this spring element 40 a biasing force is constantly generated, so that a continuous monitoring of the tensioning work is possible.

The other alternative shows a spring element 42, which is attached between the mass m2 and an abutment, but in contrast to the spring element 40, is not constantly in contact with the mass m2. A prestressing force is generated with the spring element 42 at certain time intervals by either moving the mass m2 towards the spring element 42 or, conversely, moving the spring element 42 towards the mass m2.

Fig. 3 shows a cross section through a chuck body 18, e.g. is shown in Fig. 1. In the drawing, the three jaws 12 and a chuck heart 44 can also be seen. Between the clamping jaws 12 and the chuck body 18 there is the pretensioning device 14 in the form of spring bodies 40 or 42. These can be supported continuously or if necessary between the clamping jaws 12 and the chuck body 18. Instead of the spring body 42, an elastic deformation ring can also be used.

Finally, FIG. 4 shows a graphical representation Position of the temporal relationship between the clamping force Fsp and the clamping work W. In addition, the travel pressure p is also specified, which corresponds to the clamping work W here. As the drawing shows, the clamping work W and the travel pressure increase with time due to decreasing lubrication and increasing contamination. The tension force Fsp, however, decreases. One can now, as indicated in the drawing, define a threshold which, when exceeded, causes a signal to be triggered by the clamping work W. The threshold then corresponds to a value of the clamping force Fsp at which a secure clamping of the workpieces is no longer guaranteed.

After maintenance of the workpiece holder, i.e. cleaning and lubrication of the sliding surfaces, a lower clamping work W has to be applied, so that a condition which corresponds to the left curve branches is practically reached again.

In the exemplary embodiment of the invention described so far, it was assumed to use the clamping work as a criterion for triggering a signal. However, it is also possible to use only the travel pressure p and / or the travel path as parameters in order to trigger a maintenance signal in the manner desired according to the invention when a threshold value is exceeded.

Claims

Patent claims

1. Method for monitoring the clamping force of machine tools, in particular lathes, which comprise a hydraulically actuated workpiece holder with movable clamping jaws, characterized in that the hydraulic clamping work is determined or the travel pressure or the travel path is measured and that when a predetermined threshold value of the clamping work is exceeded or a signal of the printing or the travel path is triggered.

2. The method according to claim 1, characterized gekenn¬ characterized in that the clamping jaws are subjected to a biasing force.

3. The method according to claim 2, characterized gekenn¬ characterized in that the clamping jaws are continuously subjected to the biasing force and that the clamping work or the travel pressure or the travel path are also measured continuously.

4. The method according to claim 2, characterized marked. that the clamping jaws are only subjected to the prestressing force at predetermined intervals and that the clamping work or the travel pressure or the travel path are determined or measured in these time intervals.

5. The method according to claim 4, characterized gekenn¬ characterized in that the amount of the biasing force with which the clamping jaws is applied is dimensioned such that it corresponds to the nominal value of the clamping force for clamping a workpiece.

6. Device for monitoring the clamping force of machine tools, in particular lathes, which comprise a hydraulically actuated workpiece holder (10) with movable clamping jaws (12), characterized in that a measuring device (16) for the hydraulic clamping work and / or a measuring device ( 32) for the travel pressure and / or a Measuring devices (30) are provided for the travel path, with which a signal transmitter is connected, which triggers a signal when a predetermined threshold value is exceeded.

7. The device according to claim 6, characterized gekenn¬ characterized in that a biasing device (14) with the jaws (12) is gekoppe11.

8. The device according to claim 7, characterized gekenn¬ characterized in that the biasing device (14) is constantly coupled to the jaws (12) and that the measuring device (16) for the hydraulic clamping work and / or the travel pressure and / or the travel path is constantly active are.

9. The device according to claim 7, characterized gekenn¬ characterized in that the biasing device (14) is coupled at predetermined intervals with the clamping jaws (12) and that the measuring devices (3o, 32) for the hydraulic clamping work during the coupling of the biasing device ( 14) with the jaws (12) is active.

10. The device according to one or more of claims 7 to 9, wherein the machine tool as A lathe is formed in which the workpiece holder (10) comprises a chuck body (18) with clamping jaws (12) guided therein, characterized in that the pretensioning device consists of spring bodies (14) supporting the clamping jaws (12) on the one hand and the chuck body (18) on the other hand ) consists.

11. The device according to claim 10, wherein a hydraulic actuating device (20) via a gear (22) with the clamping jaws (12) coupled actuating rod (24) and with the actuating rod (24) connected and in a Doppelkammer¬ cylinder ( 26) guided piston (28), characterized in that the measuring device (16) for the hydraulic clamping work consists of the double chamber cylinder (26) associated displacement transducers (30) and a differential pressure transducer (threshold value transducer) (32) and a computer.