Electro chemical machining

In electrochemical machining, arcing across workpiece and forming electrode can result in both being scrapped. Efforts have been made to detect the electrical signals which develop on occurrence of the spark which normally terminates in an arc. Ordinary electrical noise however, tends to have similar amplitude to that of the spark and so make it difficult for the detection device to differentiate between that noise and spark noise. Consequently the machine tool was disabled though no spark had occurred. The invention enables differentiation between the two noise sources by virtue of detecting the term over which noise occurs. This is achieved by dividing the noise pulses into two trains and passing one train via a pulse delay device 32 to a gate 34 and passing the other train directly to the gate 34. If pulses from both trains arrive at the gate 34 this is indicative of the generation of a spark and the gate 34 emits a signal which is utilized to disable the machine.

The present invention relates to a machining process wherein an electrolyte 
is caused to bridge a gap between a workpiece anode and a tool cathode 
whilst an electro field is set up in the gap, so as to bring about electro 
chemical machining of the workpiece anode. 
More particularly, the invention relates to a method and apparatus for 
controlling de-activation of the process. 
It is very important to prevent the generation of an electric arc across 
the gap. Such arcs normally start in the form of individual sparks which 
if ignored, propagate into a continuous arc and so damage and possibly 
result in scrapping of the workpiece and/or tool. When the preventitive 
methods fail, the machine must be switched off so as to at least reduce 
the damage which would otherwise occur. 
When a spark occurs, broad band high frequency electrical noise is 
generated. The power supply however, consisting as it usually does of a 
three phase, fully controlled voltage regulator feeding a transformer and 
rectifier, generates a great deal of electrical noise itself, the 
characteristics of which are very similar to those of the spark noise. 
It is known, to provide means whereby occurrance of a machining spark is 
detected. However, the similarity of amplitudes of the noise produced by 
the machining spark and noise produced by the circuity, has resulted in 
switch off being achieved when in fact, no machining spark has been 
generated. 
It is an object of the present invention to provide an improved method of 
detecting generation of machining sparks with consequent disabling of the 
machine. 
It is a further object of the present invention to provide apparatus with 
which to perform the method. 
According to one aspect of the present invention a method of disabling an 
electro chemical machine tool comprising the steps of filtering and 
digitising electrical noise signals which are generated by operation of 
the machine tool and by generation of sparks across the machining gap, 
detecting resulting pulses of a given amplitude and above, dividing the 
detected pulses into two streams and passing both streams towards a common 
input point, one stream being passed at the rate of production of its 
pulses and each pulse of the other stream being passed over a period of 
time corresponding to the period of time over which spark signals are 
produced and if pulses from both streams reach said common input point, 
deriving a signal therefrom with which to disable the machine tool. 
Preferably the method includes passing each pulse of the other stream via 
pulse delaying means towards said common input. 
The method may include the step of preventing pulses of said other stream 
from reaching the common input during the initial operating period of 
machine tool. 
A further aspect of the invention provides apparatus adapted for the 
disabling of an electro chemical machine tool comprising a filter for 
filtering electrical noise signals during operation of the machine tool, 
digitising means whereby filtered signals are pulsed, pulse amplitude 
detecting means connected to sense those of said pulses which have a given 
amplitude and above, a pulse gate and pulse delaying means for passing 
sensed pulses at a reduced frequency to said gate, a pulse delaying means 
by-pass connection to said gate, said gate being adapted to give a useful 
output only in receipt of pulses via both delaying means and by-pass 
connection, and means for receiving said useful output and utilizing it to 
bring about disablement of an associated electro chemical machine tool. 
There may be provided further pulse delay means connected between the 
output of the pulse delay means and the output gate and arranged so as to 
further delay delayed pulses only during initial operation of an 
associated electro chemical machine tool. 
Preferably the pulse delay means comprises dual re-triggerable monostables 
arranged in series. 
Preferably the means for receiving and utilizing the useful signals if any, 
comprises a bistable.

In FIG. 1 a workpiece 10 has a shape 12 electro chemically machined in a 
surface by an appropriately shaped machine tool electrode 14, in known 
manner. A power supply unit 16 provides the electrical power necessary to 
achieve the machining action. 
A machine tool disabling device 18 is connected across the circuit, such 
that in operation of the machine tool the device 18 receives electrical 
signals which result from the electrical circuitry of the machine tool and 
electrical discharges if any, in the form of sparks across the gap 20 
between the machining electrode 14 and workpiece 10. 
The device 18 is connected via line 22 to the power supply unit 16 for the 
purpose of disabling the machine tool in a manner to be described 
hereinafter. 
Referring now to FIG. 2. The device 18 comprises a filter 24 connected via 
a line to the machining circuit, so as to receive the noise signals 
generated by operation of the machining circuitry and by generation of 
sparks, i.e. undesired electrical discharges across the gap 20 (FIG. 1). 
The filter passes the signals in the form of voltage pulses, to an 
amplifier 28 which after amplifying the signals, passes them still in the 
pulseform, to a pulse height detector 30. The pulse height detector 30 is 
so arranged as to only pass on those pulses which have a pre-selected 
minimum amplitude. 
Pulses detected by the detector 30 derive from machine circuitry i.e. the 
switching action of diodes and thyristors and from intermittant electrical 
arcing across the machining electrode 14 and the workpiece 10 (FIG. 1). 
Observation has shown, that the former tends to generate short term, high 
frequency electrical noise and the latter to generate long term high 
frequency electrical noise. In order to avoid the machining tool being 
disabled by a wrong pulse, a pulse delay device 32 is inserted between the 
output of pulse height detector 30 and a final output gate 34 which is a 
single, re-triggerable monostable. The function of gate 34 is to gate that 
pulse, if any, which will disable the machine tool in mid operation. 
The pulse delay device consists of two pairs of dual re-triggerable 
monostables 32a and 32b arranged in line, the second in each pair emits a 
pulse after the duration of the time delay output from the first in each 
pair. 
The second pair of dual re-triggerable monostables 32b will only operate on 
receipt of signals of direct pulses from the pulse height detector and 
delayed pulse from the first pair of dual retriggerable monostables 32a, 
hence the delaying effect. 
During operation, the pulsed output from pulse height detector 30 is 
effectively divided, each pulse being taken via line 40 directly to output 
gate 34, via line 38 directly to monostable 32b and via line 36 to 
monostable 32a. 
The components of the monostables 32a, 32b are chosen so that the time 
which elapses from a pulse entering monostable 32a to a consequent pulse 
being emitted by monostable 32b, corresponds to the long term over which 
machining sparks are generated. It follows that, if on the consequent 
pulse reaching gate 34, gate 34 is still receiving pulses directly from 
the pulse height detector 30 via line 40, these latter pulses must be 
spark generated pulses. Gate 34 is thus switched and its resulting output 
activates a `D` type latch 42 which consequently switches on a lamp 44 in 
the form of a light emitting diode. This in turn trips a relay 48 (FIG. 3) 
which disables the machine tool. The circuit could easily be arranged such 
that activation of the `D` type latch 42 would directly trip the relay 46. 
The lamp however, provides a visual indication of machine tool 
disablement. 
A "delay on start" device 46 is provided between the monostable 32b and the 
output gate 34. When the machine tool is initially activated, its 
electrical circuitry generates more high frequency noise of long duration 
than when the machine tool has been operating for a short time. The device 
46, which is a timer, delays the passing of the output of monostable 32b 
to the gate 34, for a time which is sufficient to allow the machine tool 
to stabilize. The pulses are thus obviated by the time device 46 is cut 
out. 
Brief reference will now be made to FIG. 3, which is the circuit diagram 
corresponding to the block diagram of FIG. 2. Those components contained 
by inference in the respective blocks of FIG. 2, are bounded by 
corresponding blocks in FIG. 3 and in consequence have like reference 
numerals. The components themselves will not be described in as much as 
they are easily recognizable by the man skilled in the art.