Apparatus and method for EDM polishing

A method and apparatus for polishing a workpiece by electrical discharges by way of an electrode tool, machining pulses of relatively low power being obtained from a machining pulse generator. A cross-slide table provides the workpiece with a translation cyclical motion during polishing. Input control circuits enable the machine operator to program a chosen speed of translation for the cyclical motion, the radius of circular translation and the time of duration of machining. A counter counts the number of cycles being effected and a discriminator stops the pulse generator as soon as the chosen number of desirable cycles is obtained, such number being inversely proportional to the amplitude chosen for the translation motion.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for polishing a 
workpiece by electrical discharges by means of an electrode tool, for 
achieving a smooth surface finish on the workpiece. 
It is known, in electrical discharge machining, to effectuate consecutive 
finishing passes by displacing the workpiece and electrode tool according 
to a relative cyclical translation motion in a plane perpendicular to the 
direction of feed of the electrode tool into the workpiece, the finishing 
passes being effected with electrical discharges of progressively 
decreasing intensity. The quality of the finished surface thus obtained 
has a limit which does not always correspond to industrial requirements, 
more particularly for obtaining a surface having craters of less than 0.1 
microns in average value. 
A process is known for polishing, by electrical discharge machining, a 
workpiece electrode by means of an electrode tool, wherein the electrodes 
are displaced according to a relative cyclical motion of translation at a 
predetermined amplitude, while supplying across the electrodes consecutive 
electrical discharges of a predetermined intensity, the amplitude of the 
translation motion being chosen as a function of the decrease of 
dimensions of the electrode tool relative to that of the surfaces to be 
polished. 
SUMMARY OF THE INVENTION 
The present invention has for principal object to achieve the best 
available surface finish under predetermined starting conditions. The 
duration of the polishing operation has a positive effect upon the surface 
finish which is obtained. It is evident that if the polishing operation is 
too short it is not possible to obtain an optimum surface finish. However, 
if the polishing operation is continued until the best surface finish has 
been obtained, a progressive deterioration of the machined surface is 
observed beyond that point. 
The present invention permits to automatically obtain the best surface 
finish which is possible to achieve, as a result of adjusting the speed of 
translation to a predetermined value and effecting a number of translation 
cycles inversely proportional to the amplitude of motion chosen. 
The invention also provides an apparatus for practicing the method of the 
invention. 
A better understanding of the invention will be obtained from a reading of 
the following description of the best mode contemplated for practicing the 
invention, when read in conjunction with the accompanying drawing given 
for illustrative purpose only, and wherein:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is illustrated a curve of surface roughness CH 
as a function of the tangential velocity V of circular translation. The 
latter is equal to the product of the angular velocity .omega. by the 
amplitude R of translation. The curve shows that the minimum roughness is 
obtained at a predetermined tangential velocity Vo. 
At FIG. 2, there is illustrated a curve of the surface roughness CH as a 
function of the duration to of machining for different qualitative values 
of the machined surface at diverse selected values Vo of tangential 
velocity. The curve S1 relates to a small surface area, while the curves 
S2 and S3 relate to surface areas which are comparatively larger. It is 
surprising to observe that each of the curves provide a minimum roughness 
for a machining duration to which is independent of the machined surface 
area. Practically, the time duration to has an optimum value of the order 
of 60 minutes, but it is clear that the optimum duration could vary more 
or less from the optimum value according to the machining conditions, more 
particularly according to the materials of which the two electrodes, 
workpiece and tool, are made. 
According to a practical example, the tangential velocity Vo in the curve 
according to FIG. 1 is 1,500 microns/min. and, for a machining duration to 
of 60 minutes, the product Voto is equal to 90,000. As the velocity Vo 
must remain independent from the radius of circular translation motion, it 
is clear that the number of cycles by minutes of the translation motion 
must be adjusted according to an inverse function of the chosen amplitude. 
Similarly, the total number of cycles to be effected in order to obtain 
the machining duration to is similarly an inverse function of the chosen 
amplitude. 
In the example hereinbefore given wherein the product of the number of 
cycles by the translation amplitude is equal to 90,000, 900 translation 
cycles must be effected if an amplitude of 100 microns has been chosen in 
order to achieve a velocity Vo of 1,500 microns/min. simultaneously with a 
machining duration of 60 minutes. 
The curve illustrated at FIG. 3 shows the relationship that must be 
maintained between the number of machining cycles N and the amplitude R of 
the translation motion in order to achieve a machining duration to at a 
velocity Vo, i.e. under the machining conditions permitting to obtain the 
best possible surface finish. 
As the product Voto defines the total distance travelled by a single point 
of the electrode in the course of a machining operation, it is clear that 
the product Voto is also equal to 2.pi.RN, wherein R is the radius of the 
circular translation motion and N is the total number of cycles. Thus, in 
order to achieve optimum conditions, R and N must vary according to the 
inverse function illustrated at FIG. 3. Therefore, after having chosen the 
value of R, the function permits to determine the number N of translation 
cycles to be effected in order to achieve the best possible surface 
finish. 
The apparatus illustrated at FIG. 4, for machining by electrical 
discharges, obtained from a pulse generator G, a workpiece 1 by means of 
an electrode tool 2, comprises a cross-slide table 3 driven by two 
stepping motors 4 and 5 which are controlled by a circuit 6 responding to 
two signals corresponding to the preset values of .omega. and R, 
representing respectively the angular velocity and the radius of 
translation motion. 
The circuit for developing the two control signals and for obtaining a 
command signal stopping the machining operation at the moment the best 
possible surface finish is obtained comprises three programmable 
instruction input elements 7, 8 and 9 illustrated at the bottom of FIG. 4. 
The input element 7, providing the instruction relating to the tangential 
velocity Vo, consists of a pulse generator supplying pulses FV at a 
frequency proportional to the chosen tangential velocity. The input 
element 8 is also a pulse generator providing pulses FR at a frequency 
which is proportional to the chosen amplitude R of translation. Finally, 
the input element 9, accepting the parameter to, is a frequency multiplier 
which supplies pulses at a frequency proportional to the frequency FV 
supplied by the pulse generator 7 multiplied by the time to chosen for the 
duration of machining. The frequency multiplier 9 thus supplies at its 
output pulses at a frequency toFV. 
The pulse signal FV at the output of the pulse generator 7 is applied to an 
input of an up-down counter 10 whose content is indicated by a digital 
signal W applied to an input of the control circuit 6 for the stepping 
motors 4 and 5, the other input of the control circuit 6 receiving the 
output signal of the pulse generator 8 via a converter unit 8a. The same 
signal W is also applied to a binary rate multiplier BRM1, whose 
multiplication rate is determined by the signal W. 
Another input of the binary rate multiplier BRM1 receives the pulses FR 
supplied by the pulse generator 8 at a frequency defining the desired 
amplitude of translation. The signal at the output of the binary rate 
multiplier BRM1 is therefore equal to the product of the frequency FR by 
the multiplication rate .omega. and the signal .omega. FR is applied to 
the down-counting input of the counter 10. The pulses of frequency FV 
applied to the up-counting input of the counter 10 increase the value of 
the digital signal at its output until the pulses at the output of the 
binary rate multiplier BRM1 reach the same frequency as FV and, 
consequently, the output signal is stabilized. At this time, the digital 
signal defines .omega., because FV=.omega.FR, which is the same as the 
relationship Vo=.omega.R. 
An arrangement similar to that determining the digital representation of 
the value of .omega. is included in the circuit of FIG. 4 for providing a 
digital signal representative of the number of translation cycles to be 
effected. Such an arrangement comprises an up-down counter 11 having an 
input receiving the signal toVF from the frequency multiplier 9 and 
another input receiving the signal from the output of a second binary rate 
multiplier BRM2 whose multiplication rate is provided by the digital 
signal at the output of the up-down counter 11. 
Under such conditions, the signals at the output of the up-down counter 11 
represent a value proportional to the total length of travel of any point 
of the electrode tool during the total machining time. In effect, the 
input connected to the frequency multiplier 9 receives a signal 
representing the multiplication of to by the tangential velocity of 
translation, such that in order to obtain an equilibrium of the up-down 
counter 11, the output signal from the binary rate multiplier BRM2 must 
correspond to the length of one translation cycle, 2.pi.R, multiplied by 
the total number N of translation cycles. In the equlibrium state of the 
up-down counter 11, the digital signal at its output thus defines 2.pi.N. 
Such output signal is applied to an input of a discriminator 12 having 
another input receiving from a counter 13 a signal proportional to N, 
obtained by counting the cycles of translation at the output of the 
stepping motor control circuit 6. When the number of cycles reaches the 
number of cycles set by the counter 11, the discriminator 12 provides a 
signal at its output which is applied to the pulse generator G to stop 
machining, and to the counter 13 to reset it to zero. 
The apparatus hereinbefore described thus permits to automatically 
terminate the polishing operation at the end of the optimum time duration 
to. In addition, the optimum velocity Vo being provided by the 
programmable input element 7, the translation angular velocity is 
calculated by taking into consideration the amplitude R which has been 
chosen for the translation motion. 
It will be readily apparent to those skilled in the art that, in FIG. 4, 
the elements 9, 11, 12, 13 and the second binary rate multiplier BRM2 
could be replaced by an elapsed time counter capable of cutting off the 
machining pulse generator G after passage of the predetermined timer 
period to, which is the equivalent of setting the total number of 
translation cycles. 
The choice of the translation amplitude is effected by the machine operator 
as a function of the work to be accomplished. As an example, for polishing 
a medal, a translation radius of 20 microns may be chosen, while, for 
polishing a large workpiece having a uniform surface, a translation radius 
of the order of 800 microns may advantageously be chosen. 
It will be further appreciated by those skilled in the art that the 
translation motion may not necessarily be circular, and that the same 
principle of operation may be applied to different types of translation 
paths, for example to rectangular or eliptical paths. The example of 
operation described herein relates to polishing the surface of the 
workpiece 1 by means of the frontal surface of the electrode tool 2, but 
it will be understood that the same principles are used for polishing the 
lateral walls of the workpiece 1. It is to be noted that it is preferable 
to apply machining pulses of relatively low power across the workpiece and 
electrode tool, and to maintain the machining fluid in the machining zone 
without supplying fresh machining fluid to the machining zone, in order to 
obtain the best surface finishes. 
Having thus described the present invention by way of a typical example of 
structure well designed for practicing the method of the invention, 
modification whereof will be apparent to those skilled in the art, what is 
claimed as new is as follows: