Chucks or sockets – With fluid-pressure actuator – With measuring – indicating or control means
Reexamination Certificate
1999-10-25
2001-06-05
Bishop, Steven C. (Department: 3722)
Chucks or sockets
With fluid-pressure actuator
With measuring, indicating or control means
C279S126000, C092S00500L, C700S174000
Reexamination Certificate
active
06241258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to a method of controlling clamping and unclamping of a hydraulic chuck for use on machine tools on which the workpiece must be rotated in order to machine it.
2. Description of the prior art
Chucks of this type are well known in the art and one such chuck is described by assignees of the applicants in U.S. Pat. No. 5,292,139.
They are generally operated by a double-acting actuator mounted on the rotating shaft of the machine tool, actuating the jaws in a clamping or unclamping direction and coupled to a hydraulic control system controlled by a control loop. A system of this kind enables the clamping force on the workpiece to be modified during machining, for example to change from blank machining to finish machining.
The skilled person knows that blank machining, which is generally required to be completed as quickly as possible, requires a high clamping force at the chuck which cannot be maintained during finish machining, not least to prevent deformation of the workpiece, for example when it is hollow and the clamping is external to the workpiece.
Until now chucks have used an open loop to control the pressure in accordance with a pressure set point specified by the operator. However, an open loop of this kind cannot automatically track changes in the pressure and provide reliable information on correct execution of an instruction to change pressure. This goes against the need to achieve safe operation under all circumstances.
In the control engineering field, correct execution of instructions to change a control magnitude can be achieved using a closed loop comprising a feedback channel that is looped to the loop input, to which the set point signal and a signal measuring the real output are also applied.
However, the idea of applying closed loop control to a chuck of the above kind runs up against the specific problem of the presence of safety valves in the chuck. For obvious safety reasons the chuck is provided with respective safety valves in the feed pipes immediately upstream of the two chambers of the actuator to maintain the pressure therein if the feed pressure drops quickly and by a large amount, or is even lost altogether, because of any kind of operating incident. Without this precaution, the workpiece to be machined might escape from the chuck in the event of an incident.
In the event of a command to reverse the direction of the double-acting clamping actuator to clamp or unclamp the workpiece, the pressure in the chambers of the actuator varies suddenly from a relatively high value to zero, or vice versa. Under such conditions, a closed loop control system cannot establish the new pressure without a transient period, in which the pressure can take values that are difficult to control, before the loop stabilizes on the value corresponding to the new pressure in the corresponding chamber of the actuator. Any such sudden variation during the transient period may cause a high pressure that is maintained by the safety valves. The latter therefore maintain the pressure established beforehand by isolating the actuator from the hydraulic control system, which clamps up the chuck, although in reality there is no condition present necessitating operation of the safety valves.
An object of the invention is to provide a method of controlling a hydraulic chuck for rotary machine tools which does not have the drawbacks mentioned above and enables closed loop control to be used in the control system.
SUMMARY OF THE INVENTION
The invention therefore consists in a method of controlling a hydraulic chuck of a rotary machine tool, the chuck being adapted to clamp a workpiece to be machined and actuated by a double-acting clamping/unclamping actuator, the method consisting in applying to at least one chamber of said actuator a clamping pressure controlled in accordance with a clamping pressure set point and maintaining said clamping pressure in the corresponding chamber if the control pressure upstream of the actuator suddenly falls due to a malfunction, in which method the clamping pressure is controlled by a closed loop and the clamping pressure in the chamber is modified to disable maintaining of the pressure for a predetermined time period in response to an operator request to modify the clamping force applied to the workpiece, the disabling time being at least equal to the time necessary to stabilize the control.
As a result of the above features, the method can discriminate between a situation in which the modification of the clamping pressure results from an action in the context of normal operation of the chuck and an abnormal situation (for example failure of the hydraulic pressure supply) in which safety must be assured. Using the method of the invention, the safety valves can operate only when the clamping pressure is due to a system failure.
According to other features of the method in accordance with the invention:
the disabling time period is in the range from 1.5 seconds to 3 seconds;
when the chuck is adapted to clamp the workpiece internally or externally, if the operator request to modify the clamping force is a request for total clamping/unclamping of a workpiece, the disabling is applied by maintaining the clamping pressure at a value which was present prior to the operator request;
when the operator request to modify the clamping force is a request for partial reduction of the clamping force, the disabling is applied by reducing the clamping pressure in accordance with a predetermined law;
after the partial reduction request, a counter-pressure is applied in the chamber of the actuator opposite the chamber at the clamping pressure for a time period at least equal to the disabling time period;
the counter-pressure is maintained for a time-period in the range from 3 seconds to 5 seconds;
agreement of the clamping pressure in the corresponding chamber of the actuator and the clamping pressure set point is monitored and operation of the machine tool is prevented if the clamping pressure differs from the clamping pressure set point.
Other features and advantages of the invention will become apparent during the following description which is given by way of example only and with reference to the accompanying drawings.
REFERENCES:
patent: 3815929 (1974-06-01), Steinberger et al.
patent: 4443021 (1984-04-01), Buchholz et al.
patent: 4513979 (1985-04-01), Silverman
patent: 5876041 (1999-03-01), Kuckelsberg et al.
patent: 21 50 885 (1973-03-01), None
patent: 24 58 958 (1976-06-01), None
Bishop Steven C.
Gamet Precision
Stevens Davis Miller & Mosher LLP
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