Method for controlling a wire-cut electric discharge machine

A method of controlling a wire-cut electric discharge machine to improve machining accuracy in a machining operation for a corner portion of a workpiece to be machined. In a case where the portion to be machined includes a corner, a pressure and/or an amount of machining fluid supplied to a gap between the workpiece and a wire electrode is decreased only in a specified section. The setting of this specified section and adjustment of the machining fluid is carried out on the basis of an allowable discrepancy amount, and shape and size of the corner. Furthermore, in addition to decreasing electric discharge energy supplied between the workpiece and the wire electrode, the machining fluid may be adjusted within the specified region.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for controlling a wire-cut 
electric discharge machine, and more particularly, to a control method for 
machining a corner of the workpiece. 
2. Description of the Related Art 
A wire-cut electric discharge machine generates an electric discharge 
between its wire electrode and a workpiece, and cuts the workpiece into a 
desired shape by shifting the wire electrode with respect to the workpiece 
according to a machining command data. 
As shown in FIG. 1 (PRIOR ART), when the wire electrode 1 moves in a 
predetermined direction (i.e. a right direction in FIG. 1) for forming a 
slit 3 on a workpiece 2, a pressure is usually generated between the wire 
electrode 1 and the workpiece 2 due to electric discharge, thereby causing 
the wire electrode 1 to be bent backward or in a direction of the arrow S, 
i.e. a direction opposite to the direction of movement. For this reason, 
the wire electrode 1 is bent or curved backward with respect to a straight 
line connecting a pair of wire guides 4 and 4. 
In the case of a straight-line machining operation, this deflection will 
not substantially affect the machining accuracy for the worse. However, in 
order to form a corner, the electrode 1 needs to change its direction of 
movement in accordance with a machining command, for example, in a normal 
direction as shown in FIG. 2a (PRIOR ART). In such a corner formation, the 
slit 3 having a profile as indicated by a solid line 3b is not formed. An 
electric discharging portion of the wire electrode 1 is drawn towards an 
inside of the corner because of the above deflection of the wife electrode 
1, to cause a discrepancy from the desired profile, as indicated by a 
dotted line 3a. An amount of such discrepancy is given as .delta.. 
Furthermore, as shown in FIG. 2b (PRIOR ART), a similar discrepancy will 
occur in an arc-shaped corner in the case where a straight-line machining 
operation is followed by an arc machining operation. 
It is known that the problem of discrepancy, such as one occurring when a 
straight-line machining operation is followed by another straight-line 
machining operation differing in the direction, or when a straight-line 
machining operation transfers to an arc machining operation, as is 
discussed in the foregoing, can be solved to a certain extent by changing 
the conditions of an electric discharge at the corner. For example, the 
on-off interval of pulse current supplied can be varied during the corner 
machining operation so that an electric discharge energy supplied between 
the wire electrode and the workpiece can be reduced to reduce the 
deflection of wire and resulting discrepancy of the corner. 
Moreover, in wire-cut electric discharge machining, a highly-pressurized 
machining fluid is usually jetted against the wire electrode in the 
machined slit from upper and lower sides of the workpiece to remove 
machining chips and cool down an electrically heated portion. Hence, this 
machining fluid becomes another cause of the deflection of the wire 
electrode in the corner machining operation, thereby weakening the 
above-described corner discrepancy preventive effect due to the control of 
the discharge energy. 
SUMMARY OF INVENTION 
According to the present invention, a specified section is designated in a 
corner of a portion to be machined when the corner has an arc shape or 
formed with two straight lines intersecting with each other. A pressure 
and/or an amount of the machining fluid is controlled to be lower than 
those of an ordinary straight-line machining operation only within this 
specified section so that deflection amount of the wire electrode can be 
suppressed to or below a predetermined value when the wire electrode is 
moving within the specified section. 
Furthermore, in addition to the pressure and/or amount of the machining 
fluid, an electric discharge energy supplied between the wire electrode 
and the workpiece is controlled to be lower than those in an ordinary 
straight-line machining operation only within this specified section. 
The specified section is designated on the bases of the shape and size of 
the corner. Furthermore, the degree of adjustment of the machining fluid 
pressure and/or supply amount, as well as the same of electric discharge 
energy supplied, are determined in accordance with the shape and size of 
the corner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A wire-cut electric discharge machine carrying out the method of the 
present invention is designed to perform its machining operation by moving 
the wire electrode, whose feed operation is servo-controlled by an NC 
apparatus, with respect to the workpiece. FIG. 6 is a block diagram 
showing essential components of the NC control apparatus. In FIG. 6, a 
central processing unit (CPU) 10 is connected via a bus 18 to the 
components such as a tape reader 11 for reading machining command data 
stored in NC tape 20, a manual data input device (MDI) 12 for inputting 
machining conditions such as thickness and feed speed of a workpiece, a 
ROM 13 for storing a control program, a RAM 14 for storing various set 
values, and an interface circuit 15. The interface circuit 15 is connected 
to a servo motor 16 for shifting the workpiece mounting base and an 
electromagnetic valve 17 for adjusting jetting or supply rate of machining 
fluid. 
According to the present invention, when the portion to be machined is a 
corner including an arched portion or a corner including two intersecting 
straight lines, supply pressure and supply rate of the machining fluid is 
reduced lower than that applicable to the machining of the straight 
portion for the machining of the portion within specified region. Such 
specified region will be explained with reference to FIGS. 3, 4 and 5. 
Furthermore, according to the present invention, where the corner includes 
an arched portion, the region to be specified and the condition for 
controlling the discharge within the specified region (supply rate and/or 
supply pressure of the machining fluid and the amount of discharge to be 
reduced) is varied depending on whether R.sub.0, the arc radius of the 
locus of the width of the machining, is equal to or smaller than 
.epsilon..sub.0, 1/2 of the width of the machining slit as shown in FIG. 3 
(the case of small arc) or larger than .epsilon..sub.0 as shown in FIG. 4 
(the case of large arc). This is necessary because .delta., the degree of 
discrepancy of the machined workpiece, is substantially independent of 
R.sub.0, the magnitude of arc radius, within the range of 0.ltoreq.R.sub.0 
.ltoreq..epsilon..sub.0 ; however, it varies depending on R.sub.0, the 
magnitude of arc radius, within the range of R.sub.0 &gt;.epsilon..sub.0. 
In the case of a corner including an arc of small radius as shown in FIG. 
3, where the arc as the locus of the center of wire electrode (center of 
arc is given as 0) is given as ab; the intersecting point of a 
perpendicular drawn from an inner edge e of the machining slit and a 
straight line continuously extending from the arc section ab is given as 
c; and the point at a distance of .epsilon..sub.0 from the point c on the 
straight line is given as d, the specific section is specified by the arc 
ab and the straight line bd (=bc+cd). Where an angle formed between two 
straight lines continuously extending from both edges of the arc section 
ab is given as .theta. rad! (hereinafter, this angle is referred to as a 
corner angle .theta.), the central angle aob of the arc is equal to an 
angle obtained by subtracting the corner angle .theta. from .pi. rad!, so 
that the distance bc is expressed by the following equation: 
bc=(.epsilon..sub.0 -R.sub.0) tan {(.pi.-.theta.)/2} 
Thus, in the corner machining operation including a small arced corner as 
shown in FIG. 3, the section to be specified is designated as a section 
including arc ab and point d (point apart by straight distance L from the 
terminating point b of arc ab). The distance L is expressed by the 
following equations. 
##EQU1## 
By the way, as the corner angle .theta. is equal to an angle obtained by 
subtracting the central angle aob of the arc from .pi. rad!, the corner 
angle .theta. can be determined if the central angle aob of the arc is 
fixed. 
If a corner has a small arc-shape, an allowable amount of deflection of the 
wire electrode is calculated on the bases of the corner angle .theta. and 
a predetermined allowable discrepancy amount .delta.'. Then, the electric 
discharge condition is controlled in the specified region in such a manner 
that a curvature of the wire electrode can be suppressed within the 
calculated allowable amount of deflection. 
When a corner has a large arc-shaped portion to be machined as shown in 
FIG. 4, the specified region is designated to be identical with the arc 
section ab. That is, no straight-line section is included in the specified 
region unlike the case of the small arc corner described above. 
In this case, the allowable deflection amount of the wire electrode is 
calculated on the bases of the predetermined allowable discrepancy amount 
.delta.', the corner angle .theta., and the arc radius R.sub.0. Then, the 
electric discharge condition is controlled in the specified region in such 
manner that, a deflection of the wire electrode can be suppressed within 
the allowable deflection amount. 
When a straight-line portion to be machined has a corner followed by 
another straight line portion to be machined extending in a different 
direction as shown in FIG. 5, this corner is regarded as a small arc 
having an arc radius R.sub.0 =0 at an intersecting point a of two straight 
lines. Thus, the specified region and the electric discharge condition in 
this specified region are determined in the same manner as the case of 
small arc corner as previously described. That is, in FIG. 5, given that 
the corner angle, i.e. an intersecting angle of above two straight lines 
is .theta., a power-down distance is designated to be a distance L' 
between the point a and the point d. 
This distance L' can be obtained by substituting 0 for R.sub.0 in the 
equation L=.epsilon..sub.0 +(.epsilon..sub.0 -R.sub.0) tan 
{(.pi.-.theta.)/2}. That is, 
L'=.epsilon..sub.0 +.epsilon..sub.0 tan {(.pi.-.theta.)/2}. 
In this case, the allowable deflection amount of the wire electrode is 
calculated on the bases of the predetermined allowable discrepancy amount 
.delta.' and the corner angle .theta. so that the electric discharge 
condition can be controlled in the specified region in such a manner that 
a curvature of the wire electrode can be suppressed within the calculated 
allowable deflection amount. 
Next, a control method for the wire-cut electric discharge machine as an 
embodiment of the present invention, executed by the CPU 10 of the NC 
control apparatus of FIG. 6, will be explained with reference to the 
flowcharts of FIGS. 7, 8 and 9. By the way, before starting the procedure, 
the half width .epsilon..sub.0 of the machined slit can be obtained, by 
adding a radius .phi./2 of a wire electrode to be used and an estimated 
machining gap .delta..sub.0 (That is, .epsilon..sub.0 
=.phi./2+.delta..sub.0). These data are stored in the RAM 14 through the 
MDI 12 in advance. Furthermore, flags Fc, Fs and Fd are all initialized to 
"0". 
First, in step S1, the tape reader 11 reads a machining command of the 
present block from the NC tape 20. Then, in step S2, the CPU 10 makes a 
judgement as to whether or not the machining command of the present block 
is a fast feed command. If the machining command is a fast feed command, 
the CPU 10 proceeds to step S3 to store the command value of the present 
block in a register Rs. Then, in step S4, the fast feed processing is 
executed and, subsequently, in step S5, the flag Fs is set to "1". Here, 
flag Fs=1 means that the fast feed command has already been executed. 
Then, the processing returns to the step S1. 
When the read machining command is not a fast feed command, the processing 
proceeds to step S6 to make a judgement as to whether or not the machining 
command of the present block is a straight-line feed command. If the 
machining command is not the straight-line feed command, the processing 
proceeds to step S6 to further make a judgement as to whether or not the 
machining command of the present block is an arc feed command. If the 
machining command is not the arc feed command, the processing proceeds to 
step S8 to execute other processing, and then returns to the step S1. By 
the way, in a case where the above other processing includes a termination 
command, the processing for the present block will be terminated without 
returning to the step S1. 
When the machining command of the present block is an arc feed command, the 
processing proceeds from the step S7 to step S9 wherein the arc radius 
R.sub.0 and the distance .epsilon..sub.0 are compared to judge whether an 
arc shape of the present block is a large arc (R.sub.0 &gt;.epsilon..sub.0) 
or a small arc (R.sub.0 .ltoreq..epsilon..sub.0). 
If the present arc is a large arc (R.sub.0 &gt;.epsilon..sub.0), the 
processing proceeds to step S10 to set the flag Fc to "1", and reset the 
flag Fs to "0". Then, the corner angle .theta. of the large arc is 
determined in step 11. As the corner angle .theta. is equal to an angle 
obtained by subtracting the central angle of arc (the angle aob shown in 
FIG. 4) from .pi. rad!, the corner angle .theta. can be obtained from the 
central angle of arc included in the present machining command data. 
After determining the corner angle .theta. of the large arc, the processing 
proceeds to step S12 to obtain the allowable deflection amount of the wire 
electrode corresponding to the allowable discrepancy amount .delta.' on 
the bases of the corner angle .theta. and the arc radius R.sub.0. In this 
case, the allowable discrepancy amount .delta.' is predetermined taking 
account of the finishing conditions required to be met by the machine. 
Subsequently, in step S13, the degrees to which supply amount and/or 
supply pressure of the machining fluid are controlled on the bases of the 
allowable deflection amount obtained in the step S12 and the power 
reduction amount on the bases of the allowable deflection amount obtained 
in the step S12, are controlled. Then, the machining operation is carried 
out conforming to these conditions. 
That is, during the machining operation covering the whole of the arc 
section between the point a and the point b shown in FIG. 4, not only in 
the electric power of the wire electrode reduced corresponding to the 
allowable deflection, but also the supply amount and/or supply pressure of 
machining fluid is controlled so that the wire electrode will not deflect 
exceeding the allowable deflection. Then, the processing proceeds to step 
S14 to make a judgement as to whether or not the present block has 
terminated. If the judgement in the step S14 is YES, the processing 
returns to the step S1. 
In the step S9, if the present arc is judged to be a small arc (R.sub.0 
.ltoreq..epsilon..sub.0), the processing proceeds to step S15 to set the 
flag Fd to "1"; after resetting the flag Fs to "0", the processing 
proceeds to step S16 to calculate the corner angle .theta. of this small 
arc in the same manner as the large arc. That is, the corner angle .theta. 
is obtained after obtaining the central angle of the arc included in the 
present machining command data. 
After calculating the corner angle .theta. of the small arc, the processing 
proceeds to step S17 to store the value of the corner angle .theta. in a 
register R.sub..theta.. Then, the processing proceeds to step S18 to 
obtain the allowable deflection amount of the wire electrode corresponding 
to the allowable discrepancy amount .delta.' on the bases of the corner 
angle .theta.. That is, in the above calculation of the allowable 
deflection, the arc radius R.sub.0 of the small arc is not used. 
In step S19, the degrees to which supply amount and/or supply pressure of 
the machining fluid are to be controlled are determined and the amount of 
power reduction is determined on the bases of the allowable deflection 
obtained in the step S18. Then, the arc machining operation is carried out 
conforming to these conditions. 
That is, the electric power of the wire electrode is reduced during the 
machining covering the whole of the arc section between the point a and 
the point b in FIG. 3, and also the supply amount and/or supply pressure 
of the machining fluid are adjusted in accordance with the allowable 
deflection amount so that the deflection of the wire electrode will not 
exceed the allowable deflection amount. When the present block is judged 
to have been finished in step S20, the processing returns to the step S1. 
In the step S6, if the machining command of the present block is judged to 
be a straight-line command, the processing proceeds to step S21 to judge 
the value of the flag Fs. If the value of the flag Fs is "1", it is 
concluded that the machining command in the preceding processing was a 
fast feed command, and the present straight-line machining processing 
follows this fast feed command. Thus, the processing proceeds to step S22 
to reset the flag Fs to "0", and proceeds to step S32 to execute an 
ordinary straight-line machining processing. That is, the processing for 
power reduction and processing for adjustment of the machining fluid will 
not take place. When the present block is judged to have been terminated, 
the processing proceeds to step S34 to store the command of the present 
block in the register Rs, and returns to the step S1. 
On the other hand, when the flag Fs is "0" in the step S21 (i.e. when at 
least one arc or straight-line machining processing has already been 
performed), the processing proceeds to step S23 to make a judgement as to 
whether or not the value of the flag Fc is "1". If the value of the flag 
Fc is "1", it is concluded that the straight-line portion to be machined 
of the present block follows the large arc-shaped section of the previous 
block. Then, after resetting the flag Fc to "0" first, the processing 
proceeds to the step S32 to execute the ordinary straight-line machining 
processing. Then, in the step S33, when the present block is judged to 
have been terminated, the processing proceeds to the step S34 to store the 
command of the present block in the register Rs, and returns to the step 
S1. 
When the flags Fs and Fc are "0", while the flag Fd is "1", i.e. when the 
straight-line portion to be machined of the present block is a machining 
block following another straight-line portion to be machined, the 
processing proceeds from step S25 to step S26 to calculate the corner 
angle .theta. with reference to the command position of the preceding 
block stored in the register Rs. For instance, the corner angle .theta. is 
obtained on the bases of the advancing directions of two straight lines of 
the preceding and present blocks. After obtaining the corner angle 
.theta., the processing proceeds to step 27 to further obtain an allowable 
deflection amount of the wire electrode corresponding to the allowable 
discrepancy amount .delta.' on the bases of the corner angle .theta.. 
Next, in step S30, not only the degrees to which supply amount and/or 
supply pressure of the machining fluid are determined but also the amount 
of power reduction is determined on the bases of the allowable deflection 
amount obtained in the step S27. Then, the machining operation is carried 
out conforming to these conditions. 
The electric power of the wire electrode is reduced during the 
straight-line section between an initiating point of the present straight 
line machining and a point at a distance L' (L' is obtained by applying 
R.sub.0 =0 in obtaining L). More particularly, during the machining of the 
section between the points a and d of FIG. 5, not only the amount of the 
power reduction is adjusted corresponding to the allowable deflection 
amount but also the supply amount and/or supply pressure of the machining 
fluid is adjusted in accordance with the allowable deflection amount so 
that the deflection of the wire electrode will not exceed the allowable 
deflection amount. When the progress of the machining is judged to have 
passed the specified section in step S31, the processing proceeds to step 
S32 for resuming the ordinary (straight-line) machining operation. Then, 
when the present block is judged to have been terminated in step S33, the 
processing proceeds to the step S34 to store the command of the present 
block in the register Rs, and returns to the step S1. 
When the flags Fs and Fc are "0", and the flag Fd is "1", i.e. when the 
straight-line portion to be machined of the present block is a 
straight-line machining block following a small arc-shaped portion to be 
machined, the CPU 10 proceeds from step S25 to step S28 to obtain the 
allowable deflection amount in reference to the content (the corner angle 
.theta.) of the register R.sub..theta. which has been obtained and stored 
during the processing of small-arced portion in the preceding block. Then, 
the processing proceeds to step S29 to reset the flag Fd to "0". In step 
S30, not only the degrees of control for supply amount and/or supply 
pressure of the machining fluid are determined on the bases of the 
allowable deflection but also the amount of power reduction is determined. 
Then, the machining operation for the present block is carried out 
conforming to these conditions. 
The electric power of the wire electrode and the machining fluid are 
reduced during the straight-line portion to be machined between an 
initiating point of the present straight line and a point apart a distance 
L=.epsilon..sub.0 +(.epsilon..sub.0 -R.sub.0 ) tan {(.pi.-.theta.)/2}. To 
make a judgement when the wire electrode is judged to have passed the 
predetermined distance corresponding to the designated power-down section. 
If the judgement in the step S31, the processing proceeds to the step S32 
to execute the ordinary straight-line machining operation. Then, in the 
step S33, when the present block is judged to have been terminated, 
processing proceeds to the step S34 to store the command of the present 
block in the register Rs, and returns to the step S1. 
By controlling the machine in the above-described manner, the deflection of 
the wire electrode occurring due to the supply of the machining fluid at 
the corner section and the resulting corner discrepancy can be prevented 
to improve the machining accuracy. 
In the above embodiment, both the power reduction processing and the 
processing for adjusting the machining fluid supply are executed in 
parallel. However, greater emphasis may be placed on the adjustment of the 
machining fluid supply than the power reduction to reduce the deflection 
the wire electrode during the machining of the corner portion. Further, 
the power down processing may be omitted so that the deflection of the 
wire electrode is reduced only by adjusting the supply of the machining 
fluid. The supply of the machining fluid can be adjusted by adjusting 
either the supply pressure or supply rate, or by adjusting both. 
Furthermore, in the above embodiment, a section between a machining 
initiation position and a position at a distance of L=.epsilon..sub.0 
+(.epsilon..sub.0 -R.sub.0 ) tan {(.pi.-.theta.)/2) is designated as a 
machining fluid adjusting section on the straight-line portion to be 
machined following the small-arc portion to be machined (FIG. 3) or on the 
straight-line portion to be machined following another straight-line 
portion to be machined (FIG. 5). This machining fluid adjusting section 
is, however, not limited to the above-described region. For example, the 
terminal point of this machining fluid adjusting section can be further 
extended.