JP2839628B2 - Electric discharge machine - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric discharge machine capable of automatically controlling machining conditions in electric discharge machining according to a change in a machining area.

[Prior Art] In electric discharge machining, if machining is performed with an excessively large discharge energy with respect to a machining area, electrode consumption increases and machining becomes unstable. In addition, when processing is performed with energy lower than appropriate processing conditions, processing efficiency is reduced. That is, it has long been known that there is an appropriate condition range for the magnitude of the discharge energy supplied per unit area, and it is necessary to select an appropriate processing condition range according to the processing area.

Appropriate processing conditions can be determined by calculating the electrode area in the processing depth direction on a desk. However, in the case of a complicated shape, a large amount of calculation time is required. Therefore, a method has been proposed in which a value corresponding to a processing area during processing is obtained, and appropriate processing conditions are set based on the result.

That is, in the case of machining using only the Z-axis, the area occupied by the machining gap on the side surface is generally very small with respect to the machining area, so the machining gap between the electrode and the workpiece is ignored. Since the consumption speed (machining speed) of the workpiece is very small, the relative feed speed V between the electrode and the workpiece is regarded as the electrode feed speed.

Then, between the processing area S projected in the processing feed direction, the relative feed speed V of the electrode and the workpiece, and the volume processing speed Vw,
Since the relationship represented by the general formula (1) holds, the machining area may be calculated by measuring the relative feed speed V and the volume machining speed Vw.

S = Vw / V (1) However, in the above equation (1), the electrode feed speed during machining can be relatively easily detected, but the volume machining speed Vw
Cannot be measured directly.

Therefore, for example, in Japanese Patent Publication No. 51-35273, the electrode feed amount within a certain time and the number of times of normal discharge are detected, and the number of times of normal discharge is associated with the volume machining speed.

In Japanese Unexamined Patent Application Publication No. Sho 59-1123, the volume machining speed Vw is rewritten to equation (2) by using a pulse (rectangular) current waveform.

_{Vw = η · Ir · V G} (2) Ir = Ip · r · f (3) where, eta efficiency, Ir true machining current, V _G is the gap voltage represented by the formula (3), Ip is The peak current, r is the actual current pulse width, and f is the discharge frequency.

Then, the material efficiency η and the gap voltage V _G to be changed in the electrode and the workpiece during processing is a good regarded as substantially constant.

That is, to determine the volume machining speed Vw, the former refers to the number of occurrences of normal discharge, and the latter refers to the efficiency η and the gap voltage V _G
Is a constant determined by the material of the electrode and the workpiece, and the true machining current is used.

[Problems to be Solved by the Invention] However, as in the former case, the number of times of normal discharge can be related to the volume machining speed only when the single-shot discharge energy is constant. In other words, no consideration is given to controlling the single discharge energy according to the machining area.

In the latter case, the processing speed is estimated by the equation (2) assuming that the efficiency η is a constant determined by the material of the electrode and the workpiece. However, the efficiency η is not determined only by the material of the electrode and the workpiece, and the pulse generation method , Depending on the pulse width of the pulse current and the peak current.

Furthermore, although the true machining current is a current that contributes to machining, the true machining current detects a current in a state where effective discharge that substantially contributes to machining and sustained arc discharge that does not contribute to machining are mixed. The processing speed cannot be estimated accurately.

An object of the present invention is to solve the above-described problems and to provide an automatic machining condition control device for electric discharge machining that automatically selects an appropriate machining condition range according to a machining area.

[Means for Solving the Problems] The above-mentioned problem is achieved by a means for detecting a relative speed between a workpiece and an electrode during machining, and an effective discharge time rate (a ratio of a discharge time of an effective discharge pulse per unit time). Means for detecting, means for obtaining current efficiency, means for calculating a value corresponding to the processing area from the above values, and means for changing the processing conditions according to the calculation result are provided, and the current efficiency described above is provided. The problem can be solved by having a relationship in advance as any one or a combination of the material of the electrode and the workpiece, the peak current, the pulse width, and the output method of the power supply.

[Operation] The volume machining speed can be accurately obtained by calculating using the effective discharge time rate and the current efficiency, and the machining area can be estimated with high accuracy from the obtained volume machining speed and the electrode feed speed. . Then, since appropriate processing conditions are set according to the obtained processing area, efficient processing can be performed.

[Embodiment] Hereinafter, an example in which a current waveform is rectangular and a pulse width is constant as a power supply output, that is, a power supply output method is a constant current pulse width method will be described with reference to the drawings.

 FIG. 1 is a connection diagram showing a first embodiment of the present invention.

In FIG. 1, an electrode 1 and a workpiece 2 face each other via a narrow gap, and the gap is filled with a working fluid (not shown). The electrode 1 is mounted on an electrode feed driving device (Z axis) 3a. The workpiece is also attached to the X axis 3b and the Y axis drive 3c. A machining power supply 4, a discharge state detection circuit 5, and a low-pass filter 6 are connected to the electrode 1 and the workpiece 2. The discharge state detection circuit 5 outputs a discharge time signal C of an effective discharge pulse. And signal C is a counter
The output D is input to the CNC device 9 where it is processed as an effective discharge time ratio Φ.

The output of the low-pass filter 6 and the servo reference voltage e ref are input to the comparator 7, and the output Δe is input to the CNC device 9 via the A / D converter 8, and is converted into an electrode feed signal inside the CNC device 9. It is converted and output to the electrode feed driving device 3. As the servo reference voltage e ref, either a programmed value or a value of the input device 10 is used.

FIG. 2 shows an example of the discharge state detecting circuit 5, and FIG. 3 shows an operation explanatory diagram thereof. The voltages of the electrode 1 and the workpiece 2 are input to the differential amplifier 16, and the output is input to the comparator 12.

The other input of the comparator 12 receives the reference voltage eo. The reference voltage eo is 60 to 80 of the no-load voltage E.
% Value. When the output signal A of the comparator 12 is an effective discharge pulse as shown in FIG. Here, the pulse width Te of the signal C corresponds to the discharge time of the effective discharge pulse. On the other hand, in the case of the sustained arc discharge shown in FIG. 3B or the short circuit shown in FIG. 3C, the signal C is not output.

Reference numeral 13 denotes a flip-flop. As a reset signal of the flip-flop 13, a control signal B for driving a power transistor in a processing power supply is input. flip flop
The 13 output signal Q1 is input to the AND gate circuit 14. The signal B is input to the other input of the AND gate circuit 14.

The signal C and the clock pulse CK of about 20 MHz are input to the counter 11, and the signal D is obtained. If the clock pulse cK is 20 MHz, the detection resolution of the discharge time is 50 ns.
However, the pulse width used for normal processing is several tens to several tens
Since it is 00 μs, a sufficient resolution can be obtained with such a clock.

The signal D is input to the CNC device 9 and the discharge time Te of the effective discharge pulse is counted.

Next, the contents of data processing in the CNC device will be described first with reference to FIG.

When the execution of the program of the CNC device is started, the machining condition data table shown in FIG. 5 described in detail later is read from the internal memory of the CNC device or an external memory such as a floppy disk and displayed on the display of the CNC device. .

Note that the processing condition data table shown in FIG. 5 includes a power supply output method (for example, a constant voltage pulse width method and a constant current pulse width method), a peak current I, a pulse width T, and information necessary for actual processing. Processing parameters (for example, pulse duty factor τ, servo reference voltage e ref), current efficiency Ko described later, and the like are determined for each processing condition.
Note that the processing conditions are arranged in the order of fine surface roughness to rough surface roughness. In addition, the machining condition data table shown in FIG. 5 is prepared not only for the combination of the material of the electrode and the workpiece, but also for each region of the electrode consumption, so that whether the processing efficiency is important or the electrode consumption is important. You can choose.

As described above, since the machining condition data table is used in accordance with the combination of the electrode and the material of the workpiece and the target electrode wear value, the operator selects the electrode and the material of the workpiece from the displayed data table, Select the processing conditions suitable for the processing specifications such as the target processing depth and electrode consumption rate, and input the initial processing conditions M1 and the final processing conditions Mr.

Here, when the processing condition number (No.) is not particularly input, for example, the initial processing condition M1 = No. 1, the final processing condition Mr =
A default value may be determined so as to be No. n. Note that n is a value determined by the power supply capacity of the processing power supply to be used.

 Next, processing is started.

When machining starts, the automatic setting routine starts,
When the automatic setting routine ends, a check of the machining depth is performed. Then, the automatic setting routine and the check of the machining depth are alternately and repeatedly executed until a predetermined machining depth is reached.

The above-described automatic setting routine will be described with reference to FIG.

When the automatic setting routine starts, first the processing conditions Mt
Is not the final processing condition Mr. Mt
If ≧ Mr, this routine is skipped and the machining condition Mr
Continue processing with.

If Mt <Mr, this routine is executed, and the timer T1 starts. When the timer expires, the next detection routine is executed. In this detection routine, the electrode feed amount L and the effective discharge time Te within a certain time Ts are counted. These are converted into an electrode feed speed V and an effective discharge time ratio Φ by Expressions (4) and (5). The above-mentioned certain time Ts must be a value sufficiently larger than the pulse width, and is 0.1 to several tens of seconds.

V = L / Ts (4) Φ = Te / Ts (5) By the way, since a pulse-like current waveform is used in the present embodiment, the volume machining speed Vw can be expressed by Expression (6).

Vw = Ka · Φ · I · Ea (6) where Ka is discharge energy efficiency (processing speed per unit discharge energy), Φ is effective discharge time rate, I is average peak current, and Ea is average arc voltage. . In addition, since the average arc voltage Ea is almost constant, it may be regarded as a constant. Further, if Ko = Ka · Ea (7), Φ · I corresponds to the effective discharge current, and Ko is the current efficiency (the machining speed per unit effective discharge current).
Equation (6) can be rewritten into equation (8).

Vw = Ko · Φ · I (8) Then, the values obtained by Equations (4) and (5) are substituted into Equations (1) and (8) to calculate the machining speed Vw and the machining area S. , The operation values Vw and Φ are input to the memories Vwo and Φo.

Next, a trial condition Mt is determined based on the obtained processing area S. The trial condition Mt is a processing condition of the maximum number that satisfies S ≧ SL and is equal to or less than the final processing condition Mr using FIG.

However, if the selected condition is considerably larger than the currently set condition, the discharge energy is rapidly increased in a short time, and the machining state is likely to be unstable. Therefore, in the present embodiment, the number of steps that can be increased by one condition change is limited. That is, assuming that the machining condition currently being machined is Mi-1, the trial condition of the calculation result is Mi, and the limiting step width is ΔM, Mi−Mi
The trial condition to be executed when −1 ≦ ΔM is Mt = Mi, and the trial condition to be executed when Mi−Mi−1> ΔM is Mt = Mi
Mi−1 + ΔM.

The machining conditions are the trial conditions selected as described above.
Switch to Mt and start timer T2. Timer T2
, The detection routine starts again. The effective discharge time rate Φi detected here
Is compared with Φi-1 (the value of the memory Φo) obtained under the above condition, and is compared with a certain ratio α (0 <α <1) or more.

If Φi ≧ αΦi−1, i-th condition (trial condition)
Is determined to be good and processing is continued.

If Φi <αΦi−1, it is determined that machining has become unstable, and machining is performed by returning to the machining condition of the immediately preceding stage. Α is 0.4 to 0.6 in general processing. Also, the calculated value of the processing speed (the value of Vw detected here and the value of the memory Vwo) is compared, and if the value becomes lower than the processing speed of the previous stage, the processing is returned to the previous processing condition and the processing is performed once. A determination to continue may be made.

In the present embodiment, the current efficiency Ko is selected from the processing condition data table shown in FIG. 5, but the current efficiency Ko may be used as a mathematical expression based on certain preconditions.
For example, when processing a steel material with graphite, the current efficiency can be expressed by equation (9). Where the constants a, b,
c is a coefficient related to the material of the electrode and the workpiece, and the power supply method. For example, in a constant current pulse width method, when processing a steel material with a graphite electrode, a = 1.1 to 1.4 × 10−
6, b = 1.2 to 1.4, c = −0.9 to 1.1. However, the unit of the peak current is A, and the unit of the pulse width is μs.

Ko = a ・^Ib・^Tc (9) When this is substituted into equation (8), equation (10) is obtained, and the machining area may be estimated using this equation.

Vw = a ・ I ^{(b + 1)}・^Tc・ Φ (10) In the above-described embodiment, the comparator 7 is arranged outside the CNC device 9. However, the output of the low-pass filter 6 is subjected to A / D conversion. Alternatively, the output may be input to the CNC device 9, the output thereof may be input to the CNC device 9, a comparison operation is performed inside the CNC device 9, an error voltage is obtained, and the electrode feed control may be performed.

As described above, the current efficiency Ko is affected by the discharge time Te. However, when the power output method is a constant voltage pulse width, the discharge time Te varies, and the current efficiency Ko varies. Therefore, if an average value when standard machining is performed under the machining conditions in FIG. 5 is entered as the current efficiency Ko in the machining condition data table in FIG. 5, a large error does not actually occur. .

The method described above uses the current efficiency for estimating the machining speed.
A second embodiment using Ka will be described with reference to FIG.
In this figure, the same components as those in FIG. 1 are denoted by the same symbols.

An arc voltage detection circuit 17 detects an arc voltage at the time of machining, A / D converts the detection result, and inputs the result to the CNC device 9. 18
Is a current detector which directly detects the current of the pulse being processed. Reference numeral 19 denotes a peak current detection circuit, which performs A / D conversion of the peak current and inputs the result to a CNC device. In addition, in the machining condition data table of FIG. 5, the discharge energy efficiency Ka is used instead of the current efficiency Ko.
Is described.

Then, the detected value is substituted into the equation (6) to estimate the volume machining speed Vw. By detecting the arc voltage and the peak current in this manner, the processing speed can be more accurately estimated.

In the above embodiment, the arc voltage at the time of machining is detected, but this value may also be described in a machining condition data table, and the value may be used for the calculation.

[Effect of the Invention] Since the effective discharge time rate is detected, the machining speed can be accurately estimated. Therefore, since an accurate machining area can be estimated from the estimated machining speed and the detected electrode feed speed, machining can be performed under appropriate machining conditions according to the machining area, and highly efficient machining can be performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention. Second
The figure shows an embodiment of a machining state detection circuit. FIG.
FIG. 3 is an operation explanatory diagram of FIG. 2. FIGS. 4 and 6 are flowcharts for explaining the operation of FIG. FIG. 5 is an example of a processing condition data table. FIG. 7 is a configuration diagram of the second embodiment. 1: Electrode, 2: Workpiece, 3: Electrode drive, 4: Machining power supply, 5: Discharge state detection circuit, 6: Low pass filter, 7: Comparator 8: A / D converter, 9: CNC device

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-146420 (JP, A) JP-A-62-19329 (JP, A) JP-A-59-1123 (JP, A) JP-A-1- 121123 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) B23H 1/02

Claims (5)

(57) [Claims] 1. Means for detecting a relative speed between a workpiece and an electrode at the time of machining, means for detecting an effective discharge time rate, means for determining a current efficiency, a method for detecting the relative speed, an effective discharge time rate and a current efficiency. An electric discharge machining apparatus comprising: means for calculating a value corresponding to a machining area from the above; and means for changing a machining condition according to a result of the calculation. 2. The method according to claim 1, wherein the means for obtaining the current efficiency is a selection from a table having a relationship in advance as one or a combination of the material of the electrode and the workpiece, the peak current, the pulse width, and the output method of the power supply. The electric discharge machining device according to claim 1, wherein: 3. The method according to claim 1, wherein the means for determining the current efficiency is a calculation from an equation having a relationship in advance as one or a combination of the material of the electrode and the workpiece, the peak current, the pulse width, and the output method of the power supply. The electric discharge machining device according to claim 1, wherein: 4. The electric discharge machining apparatus according to claim 2, wherein an output system of the power supply has a constant current pulse width. 5. An electric discharge machining apparatus according to claim 2, wherein an output system of the power supply has a constant voltage pulse width.