BETTER SPARK CONTROL - AT NO MORE COST
By Michael Page Reproduced from Engineering News No.297 20 April 1967 with kind permission of the publishers - The Certificated Engineer Nov 1967
An economical alternative to expensive hydraulics usually needed for efficient quill control on pulse-power spark erosion is claimed for an electro-mechanical servo-control developed by a British firm.
Basically, the reason for using hydraulics in the past has been due to the fast response time needed to withdraw the electrode, when products of erosion short the spark gap.
In direct servomotor control, electrode withdrawal involved reversing the servomotor. Response time to quill reversal on receiving a signal from the spark gap condition was sluggish. In turn, only relatively slow feeds were possible with then-existing servo-equipment.
So faster response could only be obtained by the employment of hydraulics in quill control. With servo motor drive, the slow response was the main problem.
The firm has solved this problem by employing a Smiths magnetic particle coupling between the servo motor and quill feed, the motor driving in one direction only, working against a constant-force spring. To achieve instant withdrawal, the clutch slips upon the reception of a signal from the spark gap condition. The resultant response is in the order of 1/400-1/500 sec.
The metal removal rate on the new F 400 RP with pulse generator is six times greater than the relaxation circuits which, however, are incorporated in the machine to give fine finishes in order of 10 microns.
For example, the maximum removal rate using a graphite electrode eroding hardened tool steel is 450 mm3/min.
With the working life of the machine, simplicity in design leaves very little to wear out. Accuracy of reproduction is maintained at 0.0004 in.
The cost of the machine and generator is ￡3 700 compared with about ￡5000 for an equivalent hydraulic quill feed machine.
The firm hopes to interest all tool-makers in this machine. A spokesman for the firm said that the spark-erosion process is still considered relatively new and that the smaller firms tend to shy away from it because of the relatively high initial cost.