GB1574568A

GB1574568A - Controlling the feed and retraction of the electrode in a spark-machining unit

Info

Publication number: GB1574568A
Application number: GB10339/78A
Authority: GB
Original assignee: Friedrich Deckel AG
Current assignee: Friedrich Deckel AG
Priority date: 1977-03-17
Filing date: 1978-03-15
Publication date: 1980-09-10
Also published as: DE2711697A1; FR2383749B1; CH627110A5; IT1101980B; IT7848322A0; DE2711697C2; ES467356A1; FR2383749A1

Abstract

The piston rod (8) of a feed drive (7) supported in the machine frame (2) carries an infeed drive (13) guided on the machine frame and having the electrode (16). The feed drive (7), infeed drive (13) and a hydraulic clamp (18) fitted between the piston rod (8) and the infeed drive (13) are connected to the pressure network via control valves (20, 22, 24). To reset during the operation, the feed drive (7) is moved into the desired end position by the admission of pressure to the infeed drive (13) via a valve (22). Jumping of the electrode (16) during starting is avoided here by a flow-regulating valve (32) to which pressure is admitted before starting and by a restrictor (34) in the feed (28). The synchronisation with the control of the control valve (22) is taken care of by its process computer. Since this device only needs one guide, it is simpler and more accurate compared with known devices. <IMAGE>

Description

(54) CONTROLLING THE FEED AND RETRACTION OF THE ELECTRODE IN A SPARK-MACHINING UNIT (71) We, FRIEDRICH DECKEL AKTIEN GEsELLscHAFr, a German Company of Plinganserstrasse 150, D-8000 München 70, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to apparatus in a spark-machining unit for controlling the feed and retraction of the electrode, and comprising a drive for automatically controlling the operational feed, as well as means for resetting said drive after a predetermined feed distance has been covered.

To enable adequate rigidity and precision to be obtained in the feed drive of sparkmachining units it is necessary to keep the distance covered by the feed drive as short as possible. This problem arises particularly in the case of apparatus comprising a hydraulic feed drive, the precision of which diminishes considerably with increasing length of the oil-pressure column. However, shortening of the feed distance also limits the possible working range. Means are therefore known for resetting the entire feed drive after a short predetermined feed distance has been covered, and these means enable a total feed distance which is the sum of several such predetermined distances to be obtained.These means are generally so constructed that, after the electrode holder has reached the end of its feed movement over one predetermined distance or increment, the housing which accommodates the electrode holder is advanced in the direction of feed by an amount corresponding to the distance of feed, while the electrode holder is retracted relative to the housing to the same extent and is moved back to the starting point of the predetermined feed distance so that it is ready for a further feed cycle.

In a known apparatus the entire feed drive is contained in a slide which in turn is supported on the machine stand and is displaceable in the direction of feed (German Patent Specification OS 2455 544). The slide is displaced by means of an electrically driven screwed spindle arranged in the stand.

The forces acting on the electrode are first transmitted through the feed drive and guides of the electrode spindle into the slide and from this on to the slide guide and the drive for moving the slide. Thus, it is necessary to provide two guides, each taking up a considerable portion of the operating forces, so that the construction is rendered complicated and expensive. Furthermore, increase in the number of guides is accompanied by an increase in the risk of inaccuracies due, for example, to play in the guides or inadequate parallelism.

The object of the present invention is to provide an apparatus of the initially stated kind which is of simple construction and possesses high precision and which, in particular, uses only one guide.

According to the invention, there is provided an apparatus in a spark-machining unit for controlling the feed and retraction of the electrode relative to the machine stand of the unit, said apparatus comprising a drive for automatically controlling the operational feed of the electrode through a predetermined distance and an adjustment.

drive for resetting said feed drive after the said predetermined feed distance has been covered, wherein the feed drive is directly supported by the machine stand and comprises a movable member which in turn carries a member of the adjustment drive means that is movable in the direction of feed, that carries the electrode, is directly guided on the stand and can be fixed in relation to the movable member of the feed drive.

The feed drive is arranged to be directly supported by the machine stand which can be designed to provide the required stability.

During the working phase, the movable part of the feed drive tows the movable part of the adjustment drive-which latter part is movably guided on the stand-and is firmW connected to the latter part in the direction of feed. Since that component of the adjust mcnt drive that carries the electrode is directly guided on the stand, the adjustment drive and the feed drive are loaded only in the direction of feed. All the other machining forces acting on the electrode are directly absorbed by the guide of the adjustment drive. A second guide, in the zone of the feed drive for example, is not necessary.

This results in a simple low-cost and precise construction. The resetting operation is carried out by displacing the movable part of the adjustment drive in the direction of feed, while at the same time the movable part of the feed drive is displaced in the opposite direction as will be explained in greater detail below in relation to a preferred embodiment. The result of the two superposed movements is that the feed drive is returned to its initial position while the electrode remains in working contact with the workpiece, so that the machining operation is not interrupted.

A- further disadvantage regarded as existing in the known apparatus is that the mass that has to be moved by the adjustment drive for the slide is relatively large and, furthermore, is disposed eccentrically of the drive so that during high-speed movements considerable forces and moments act on the guide and the drive system. With the apparatus of the present invention, however, this disadvantage can be avoided in that the feed drive and the adjustment drive can be arranged coaxially with each other and so that the lines along which they act extend approximately through the centre of gravity of the member carrying the electrode. Thus, mass acceleration does not produce forces or moments that load the guide.

An apparatus of the initially stated kind is known which comprises an electrode spindle arranged and guided in a slide displaceably mounted on the machine stand and in which the drive for displacing the slide likewise acts approximately through the centre of gravity thereof so that to that extent no moments that load the guide for the slide result from mass acceleration (German Patent Specification AS 1 565 393). In this known apparatus the feed drive and the drive for displacing the slide are combined.

Either the slide or the spindle can be clamped against the machine stand as re quired, and the free component can be displaced hydraulically.

However, in this apparatus the sparkmachining operation has to be interrupted during resetting since the spindle is fixed relatively to the machine stand and therefore to the workpiece bench. Furthermore, a long distance is not available for the rapid return movement. Moreover, the return mpvement can be carried out only by alternately moving the spindle and the slide.

Finally, this known apparatus has two guides each of which has to absorb a considerable portion of the forces and moments occurring during machining, these guides being that for the spindle in the slide and that for the slide on the stand.

In a preferred form of the apparatus in accordance with the present invention, the feed drive and the adjustment drive are formed by linear hydraulic drives wherein, for example, the piston rod of the feed drive carries the adjustment piston which cooperates with the adjustment cylinder which is guided on the stand and carries the electrode. Preferably also, a hydraulic clamp is incorporated and acts between said piston rod and the adjustment cylinder.

Preferably the feed drive, the adjustment drive and the clamp are each supplied with pressure medium through separate circuits, and each of these circuits is connected through a respective control valve to a common source of pressure medium. To prevent a sudden variation, in particular a reduction in the size of the working gap upon commencement of the resetting operation, a number of steps may be taken. A quantityregulating valve for limiting the quantity of pressure medium may be provided in the circuit to the adjustment drive upstream of the control valve, The regulating valve is connected so that there is a constant flow of pressure medium through it when the control valve is in its neutral position, so that upon commencement of the resetting operation in the sudden start-up movement characteristic of such valves does not occur.

Furthermore, a flow-restricting means may be provided in the circuit between the control valve and the adjusting drive, and this means prevents the sudden application of pressure and instead causes pressure to build up over a specific period. A counter-hold valve in the return line of the circuit from the adjustment drive performs the function of building up a counter-pressure in the return line and therefore of hydraulically clamping the adjustment cylinder. This measure likewise contributes to preventing sudden movement of the adjustment drive.

The actual control of the machine and therefore also the actuation of the control valves, can be carried out electrically in the known manner by means of a process computer. Adjustment of the working gap as well as the return movements for the purpose of additional flushing, interruption of a by-pass etc., are co-ordinated with the feed drive in the known manner.

Rapid retraction for the purpose of interrupting machining is performed using both the adjustment drive and the feed drive.

Thus, the sum of the previously covered adjustment distance and feed distance is available for each such movement.

The distance travelled by the electrode can be measured directly between the adjustment cylinder carrying the electrode and the machine stand.

The movement of the adjustment drive can be controlled by hand or automatically, for example, when the feed distance has been covered i.e. when the machining depth has been reached.

A preferred embodiment of the invention is illustrated diagrammatically in the accompanying drawing and will now be described in greater detail. The drawings shows a feed drive and an adjustment drive with the associated hydraulic circuits for controlling these drives.

Formed in a machine stand 2 is a feed cylinder 4 in which a feed piston 6 is displaceable. The feed cylinder 4 and the feed piston 6 form a feed drive 7. The piston rod 8 of the feed piston carries at its free end an adjusting piston 10 which co-operates with an adjusting cylinder 12 and, with this, forms an adjustment drive 13. The adjustment cylinder is arranged in guides 14 on the machine stand 2 in such a way that it can be displaced in the direction of feed.

The adjustment cylinder also carries an electrode 16 which is supplied with operating current in a known manner which is therefore not described here. Co-acting between the adjusting cylinder 12 and the piston rod 8 is a hydraulic clamp 18 which is adapted to connect the two components firmly to each other in the direction of feed.

The feed drive 7, the adjustment drive 13 and the hydraulic clamp 18 are each supplied with pressure medium through separate circuits, these circuits being connected through control valves 20, 22 and 24 respectively to a common source of pressure medium, here illustrated by a pressure line P and a return line T.

The feed drive 7 is controlled through the control valve 20 which takes the form of an electro-hydraulic servo-valve. In the neutral position as illustrated, control pressure has not been admitted to either of the two chambers of the feed cylinder 4, so that the feed piston 6 is stationary. The servovalve 20 is controlled by a process computer in a known manner, which is therefore not described in detail, the control being such that the working gap between the electrode 16 and the workpiece 26 always remains the same.

When the feed piston has reached a lower working position, for example in a downward spark-machining operation, then the resetting movement is started by way of an electric contact for example. For this purpose, first the clamp 18 is opened. At the same time the control valve 22 connects the pressure line P to the supply portion 28 of the control circuit for the adjustment drive 13, so that pressure is admitted to the lower chamber 30 of the adjustment cylinder 12. The cylinder 12 executes a slight downward movement so that the working gap is reduced in size. The automatic feed control then retracts the feed piston 6 and therefore the adjustment piston 10 since said control is aimed at keeping the working gap between the electrode 16 and the workpiece 26 constant.In this way the feed piston is moved into an upper position without interruption of the machining operation, and in this position the control valve 22 and the clamp 18 are closed again by a cut-out switch. The feed piston is then once more in its initial position from which it can carry out machining to a depth corresponding to its feed stroke.

In order to prevent the adjustment cylinder 12 from suddenly moving downwards upon commencement of the resetting operation before the superposed upward movement of the feed drive 7 begins, a quantity-regulating valve 32 is provided in the pressure line P upstream of the control valve 22. Also, in the illustrated neutral position, the control valve connects the pressure line P to the return line T thus effecting a by-pass. Pressure medium therefore continuously flows through the quantity-regulating valve 32, so that the disadvantageous sudden start-up, characteristic of such valves, does not take place. A flow-restricting means 34 in the supply portion 28 prevents pressure from building up too rapidly, so that the feed control 7 is always able to compensate the movement of the adjustment cylinder 12 as described above.In the return portion 36 there is provided a counter-hold valve 38 whereby a counter-pressure can be maintained in the upper chamber 40 of the adjustment cylinder, so that the adjustment piston 10 is clamped in the chambers 30 and 40 by specific pressures.

The hydraulic clamp is opened by the control valve 24 at the moment at which the resetting operation begins, by cutting off the supply portion 42 from the pressure line P and connecting it to the return portion (illustrated position). The precise timed co-ordi- nation with the operation of the control valve 22 is provided by means of a process computer, not illustrated.

The other components illustrated are of the known kind and will therefore only be described briefly. A control valve 44 enables the quantity-regulator 32 to be by-passed to carry out high-speed movements of the adjustment drive 13. The direction of these high-speed movements is determined by the position of the control valve 22 in a known manner which will be readily understood from the circuit symbols.

The flow-restricting means 46 provided in the line 36 performs the same function as the flow-restricting means 34 in the case where the line 36 becomes the supply portion when resetting takes place in the reverse direction i.e. the upward direction.

The non-return valves 48 and 50, connected in parallel with the flow-restricting means cause each of these means to be effective only in one direction of flow; likewise, the non-return valve 52 is used for by-passing the counter-hold valve 38 when resetting is carried out in the upward direction, since in this case the weight of the adjustment cylinder 12 together with the electrode 16 already counteracts a sudden movement, so that a counter-pressure is not necessary.

WHAT WE CLAIM IS:- 1. An apparatus in a spark-machining unit for controlling the feed and retraction of the electrode relative to the machine stand of the unit, said apparatus comprising a drive for automatically controlling the operational feed of the electrode through a predetermined distance and an adjustment drive for resetting said feed drive after the said predetermined feed distance has been covered, wherein the feed drive is directly supported by the machine stand and comprises a movable member which in turn carries a member of the adjustment drive means that is movable in the direction of feed, that carries the electrode, is directly guided on the stand and can be fixed in relation to the movable member of the feed drive.

2. Apparatus according to Claim 1, wherein the feed drive and the adjustment drive are arranged co-axially with each other.

3. Apparatus according to Claim 1 or Claim 2, wherein the lines along which the feed drive and the adjustment drive act extend approximately through the centre of gravity of the member carrying the electrode.

4. Apparatus according to any one of Claims 1 to 3, wherein the feed drive comprises a hydraulic feed cylinder and a feed piston which is displaceable in said cylinder and the piston rod of which carries the adjustment drive.

5. Apparatus according to any one of Claims 1 to 4, wherein the adjustment drive comprises an adjustment piston arranged on the movable member of the feed drive, which piston co-operates with an adjustment cylinder which is displaceable relative to said piston and is arranged to be guided on the machine stand.

6. Apparatus according to any one of Claims 1 to 5, including a hydraulic clamp for fixing the movable member of the adjustment drive in relation to the movable member of the feed drive.

7. Apparatus according to Claims 4, 5 and 6, including separate circuits for supply- ing pressure medium respectively to the feed drive, the adjustment drive and the hydrau lic clamp from a common source, said circuits each having a respective control valve.

8. Apparatus according to Claim 7, wherein the supply circuit for the adjustment drive includes a quantity-regulating valve connected between the respective control valve and the source, and the control valve has a neutral position which permits a continuous flow of pressure medium from the source through the quantity-regulating valve.

9. Apparatus according to Claim 7 or Claim 8, wherein the supply circuit for the adjustment drive includes a flow-restricting means connected between the respective control valve and the adjustment drive for restricting flow of pressure medium to the adjustment drive during the resetting movement.

10. Apparatus according to any one of Claims 7 to 9 wherein the circuit for the adjustment drive includes a counter-hold valve connected between the drive and the control valve in a line which serves as a return line for pressure medium during the resetting movement.

11. Apparatus according to any one of Claims 7 to 10, wherein the control valves are actuated electrically by way of a computer.

12. Apparatus according to any one of Claims 1 to 10, wherein the control of the operational feed and the return control for the purpose of flushing, interruption of the bypass etc., are correlated in known manner with the feed drive.

13. Apparatus according to any one of Claims 1 to 12, wherein a return movement for the purpose of interrupting machining is correlated both with the feed drive and the adjustment drive.

14. Apparatus according to any one of Claims 1 to 13, including means for measuring the distance travelled by the electrode by direct measurement of the distance between the movable member of the adjustment drive that carries the electrode, and the stand.

15. Apparatus according to any one of Claims 1 to 14, including means for automatically resetting the adjustment drive when the predetermined feed distance has been covered.

16. Apparatus according to any one of Claims 1 to 15, wherein the movement of the adjustment drive can be controlled by hand.

17. Apparatus in a spark-machining unit

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The flow-restricting means 46 provided in the line 36 performs the same function as the flow-restricting means 34 in the case where the line 36 becomes the supply portion when resetting takes place in the reverse direction i.e. the upward direction. The non-return valves 48 and 50, connected in parallel with the flow-restricting means cause each of these means to be effective only in one direction of flow; likewise, the non-return valve 52 is used for by-passing the counter-hold valve 38 when resetting is carried out in the upward direction, since in this case the weight of the adjustment cylinder 12 together with the electrode 16 already counteracts a sudden movement, so that a counter-pressure is not necessary. WHAT WE CLAIM IS:-

1. An apparatus in a spark-machining unit for controlling the feed and retraction of the electrode relative to the machine stand of the unit, said apparatus comprising a drive for automatically controlling the operational feed of the electrode through a predetermined distance and an adjustment drive for resetting said feed drive after the said predetermined feed distance has been covered, wherein the feed drive is directly supported by the machine stand and comprises a movable member which in turn carries a member of the adjustment drive means that is movable in the direction of feed, that carries the electrode, is directly guided on the stand and can be fixed in relation to the movable member of the feed drive.

17. Apparatus in a spark-machining unit

for controlling the feed and retraction of the electrode constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.