SI21995A - Procedure of selecting processing parameters in electroerosion processes by determining the share of short-circuit discharges - Google Patents

Abstract

The invention discloses the procedure of selecting processing parameters in electroerosion processes by determining the share of short-circuit discharges D, which take place inside the electroerosion device with a predefined number of processing modes (i = 1 to N), where the attained power P1 is the smallest one and PN the largest one inside the gap (9), where the analysing device (8) acquires the electric voltage within the gap (9), defines the individual discharge types (A, B, C, D) and determines the share of short-circuit discharges D at specific sections of the voltage signal, while on the basis of the calculated minimum share of short-circuit mine (D) discharges and the known critical value of short-circuit discharges Dkr it selects the corresponding processing mode by applying higher or lower power Pi.

Description

The process of selecting the machining parameters of the treatment in the erosion process by determining the percentage of short-circuit discharges

FIELD OF THE INVENTION

The object of the invention is a system for determining the optimum machining parameters of an erosion machining process with respect to a given machining surface without interrupting the machining, or more specifically, a system of on-line selection of such machining parameters of the erosion process that maximize productivity in the machining of a given machining surface.

The system is suitable for submersible and wire erosion as well as micro-erosion treatment.

The state of the art

In the case of rough machining, ie when the achieved quality of the machining surface is not important, it is necessary to provide the optimum electric power in the slot, which ensures the highest degree of removal with even acceptable electrode wear. The electric power in the slot is determined by the selected machining parameters on the machine.

It is well known _by that the optimum electrical power in the gap depends on the size of the projection of the working surface, that is, those surfaces, which is formed by a gap between the electrode and the workpiece, in a plane which is perpendicular to the direction of processing. This is called the eroding surface. The ratio between the average electric power in the slot and the size of the eroding surface is called the power density.

In the prior art there are many known solutions to the current choice of machining parameters of the erosion process for a given machining surface. Many of these solutions are also patented.

U.S. Patent 4,559,434 describes the selection of optimal machining parameters for wire electroerosion for a given machining surface, which is already performed in the NC program, which also specifies the trajectory of the wire, thereby determining the contour of the product. In this case, it is necessary to determine the size of the eroding surface for each section of the workpiece contour already when writing the NC program.

A similarly incomplete solution is described in U.S. Pat. No. 4,361,745 for an immersion erosion process, where the first machining process requires the determination of the optimum machining parameters before machining, and these are stored in the process controller for the following identical products. Since submersible electroerosion is mainly used in individual production, such a solution is useful only in exceptional cases.

According to US Patents 5,276,302 and US 3,369,239, the problem is solved by monitoring the capacitance of the gap between the electrode and the workpiece. However, the eroding surface is not uniquely determined by the gap capacitance. It varies depending on the contamination of the gap with the particles of the removed dielectric and the size of the gap.

Similarly, JP 57138544 A describes a solution based on the principle of slit resistance. However, the measurement of the resistance in the gap compared to the measurement of the gap capacitance has the additional disadvantage because, because of the high sensitivity to the contamination of the gap with the particulate material taken away, it is difficult to conclude on the size of the eroding surface based on the resistance in the gap.

Patents US 4,510,367, US 4,533,811, US 5,243,166 and US 5,362936 are based on empirical models of discharge rate with respect to the discharge energy and calculate the size of the eroding surface with respect to cutting speed (wire travel along the contour of the product) in wire erosion. The parameters of the empirical models are determined on the basis of a large number of experiments. The main disadvantage of such a system is the slow response to changes in workpiece thickness, so the system only works well with continuous changes in thickness. Better results are obtained by an analytical model built by Rajurkar et al (1994, Annals of the CIRP, 43/1, pp. 199-202 and 1997, Annals of the CIRP, 46/1, pp. 147-150), which It is useful for wire electro-erosion machines with isofrequency generators and determines well the size of the eroding surface and reacts quickly enough to resizing the eroding surface. Also, this model is based on known machining parameters and measured cutting speed. Liao et al (2002, Journal of Material Processing Technology, 121/3, pp. 252-258) presented a model for determining the size of an eroding surface in wire-erosion cutting, which was built by non-parametric modeling: the learned neural network captures the processing parameters and performs a prediction of the size of the eroding surface. Submersible electro-erosion was performed by a system that determines the size of the eroding surface from a simple single-charge material removal model and electrode feed rate (Dehmer 1992, WZL Produktionstechnik, 224, Duesseldorf, Volenberg 1996, EDM Technology transfer, pp. 7-13, Schulze 2000, Proceedings on the 2 ^nd International Conference on Machining and

Mesurement of Sculptured Surfaces, p. 295-304, Krakow).

The exact operation of the system requires the same flow of current between the individual charges. All of these models are relatively complex, requiring sophisticated electronics to implement on the machine and a large number of process parameters (eg cutting speed, size of the gap between the wire and workpiece, all machining parameters, etc.) to be able to predict the size of the eroding surface and to choose from optimal machining parameters.

U.S. Patent 4,504,722 describes a solution for selecting the optimum machining parameters, i.e., those that achieve stable machining. In general, the stability of machining is measured by the proportion of predefined discharge types: free A, working B, arc C, and short-circuited D (Figure 1). A higher proportion of B discharges indicates more stable processing. According to the invention, the solution to the problem of determining the appropriate electrical power in the slot during machining is in the form of a search algorithm that searches for predefined ranges of machining parameters and verifies that the machining parameters selected achieve satisfactory process stability: a sufficiently large proportion of operating discharges B. at least two mutually independent causes of an unstable machining process, that is, a contaminated slot and an excessive power density in the slot, such a search is not the best solution, since both types of unstable processing are evaluated by the same process parameter, that is, by the share of work discharges. In addition, said invention selects values for four processing parameters at a number of levels, which requires a relatively long search time for the correct combination of values of all processing parameters. Also, the algorithm for finding the optimal machining mode is quite complex, and so is the device itself.

Description of a technical problem

The technical problem that the present invention solves is the current selection of the machining parameters of the erosion process, which determine such average electrical power, which reaches the highest rate of removal, where said selection of the parameters takes place on a regular basis, without interruption of the processing process.

The optimum average electrical power that achieves the highest withdrawal rate depends on the machining surface formed by the electrode, which is preferably the anode, and the workpiece, which is preferably the cathode. In general, the machining surface changes during machining as it penetrates the workpiece during machining. It is therefore necessary to vary the machining parameters during the processing in order to obtain the highest possible withdrawal rate.

During the erosion treatment, electrical discharges occur in the gap between the electrode and the workpiece, which are the result of electrical impulses generated in the impulse generator. Each discharge accumulates and evaporates some of the workpiece material and a crater forms on the workpiece surface. By loading craters on the workpiece surface, material is removed. The size of the craters depends mainly on the discharge energy, ie on the discharge current, the discharge voltage and the duration of the discharge. The withdrawal rate depends mainly on the average electrical power in the slot, so in addition to the above processing parameters, it also depends on the break time between charges. In the case of excessive average electrical power in the slot, the machining process becomes unstable, which is reflected in a lower rate of removal and greater wear on the electrode. Therefore, in the case of a changing work surface, these work parameters need to be directly or indirectly monitored or adjusted to the work surface in order to achieve maximum removal rates.

Description of the invention

According to the present invention, the problem of determining the optimum machining parameters is solved by determining the proportion of short-circuit discharges D. In the case of excessive power density in the gap, which causes an unstable machining process and thus a lower rate of withdrawal and greater wear on the electrode, the proportion of short-circuit discharges is noticeably increased. A higher power density in the gap causes more resistance in the gap, which results in a reduction of the gap to such an extent that short-circuit discharges occur more frequently. In this way, the power density in the gap is directly reflected in the fraction of short-circuit discharges D.

Research confirms that the fraction of short-circuit discharges D is a sufficient parameter to determine the optimum machining parameters for all electro-erosion processes.

Short description of the pictures

The invention will now be described in the following example, where reference is made to the accompanying drawings

Figure 1: Four typical charges shown on a voltage signal in the slot

Figure 2 schematic diagram of an electro-erosion machine with a pulse analyzer that transmits information on the progress of the machining process to the machine controller

Figure 3: algorithm for determining the optimum machining mode

DETAILED DESCRIPTION OF THE INVENTION

As shown in Figure 2, the electrode 2 and the workpiece 1 are spaced apart by a small gap 9 (0.01 to 0.1 mm), which is taken care of by the feed system 4. The gap 9 is filled by an electrically conductive fluid, a dielectric. In the gap 9, discharge occurs between the electrode 2 and the workpiece 1 as a result of electrical impulses from the generator 3. A plasma channel is locally created in the gap 9 in the workpiece B and arc C discharge, which causes the workpiece material 1 to heat, deposit and evaporate in place. where it appears. As a result, a crater forms at this point. In this way the discharge of the workpiece material is carried out by periodic discharge. Discharge parameters and slot size 9 are transmitted by controller 6 to generator 3 or feeder 4.

The machine operator communicates with the controller via the user interface 7. During the processing, the dielectric is contaminated with the particulate matter and the dielectric pyrolysis products; the flow of fresh dielectric into the slot 9 is provided by the dielectric flow system 5.

In cases where the machining surface changes during processing (conical electrode, change of thickness of the workpiece during wire erosion, etc.), the machining parameters or discharge parameters must be varied or optimized during machining. This is directly handled by the machine operator, and preferably by the machine controller using the appropriate algorithm.

The process of determining the optimum processing parameters is carried out in such a way that the analyzer 8 captures the electrical voltage in the slot 9, determines the types of single charges and calculates the proportion of individual types of charges on a given signal section. It makes sense to deal with 100 to 2000 discharges at a time. How a single type of discharge shows up on a voltage signal in slot 9 is shown in Figure 1. Type A charges are free charges where the circuit does not contract during an electrical impulse, so the voltage in slot 9 remains the same as that generated by the generator. Type B discharge is a working discharge where a plasma channel in slot 9 is established shortly after the start of the pulse. At that time, the voltage in slot 9 is reduced. Type C discharges are arc discharges where the plasma channel is established early in the pulse. Type D charges, however, are short-circuit discharges. These do not establish a plasma channel and indicate that electrode 2 and workpiece 1 are in electrical contact. In this case, the voltage in slot 9 during the whole pulse is almost equal to U = 0 V.

According to the present invention, only the fraction of short-circuit discharges D is relevant for determining the suitability of electrical power in slot 9. In the case of excessive power density in slot 9, the proportion of short-circuit discharge D is significantly increased. This fact is the basis for the process of determining the appropriate machining parameters for a given machining surface, which is given in the form of the algorithm in Figure 3.

The machine operator selects the initial machining parameters via the user interface and starts the machining. The processing parameters are grouped into modes. The modes are ranked from mode 1, which reaches the minimum power Pi in slot 9 (the index of such mode has a value of 1: z = l, where i is a liquid index, which runs from 1 to N) to mode N, which reaches the maximum power Pn in slot 9 (the index of such mode has the value N: z = N, where N is equal to the number of modes that can be set or selected on a given machine).

If the conditions for completion of the workpiece are fulfilled (the required depth of the workpiece is reached, the end of the prescribed trajectory of the cutting wire is reached, etc.), the workout is completed, otherwise the work-up continues. During processing, based on the voltage signal in slot 9, analyzer 8 determines the presence of short-circuit discharges D. Due to the randomness of the machining process, it is necessary to determine the proportion of short-circuit discharges D on several sections, preferably 2 to 10 sections, of the voltage signal and determine the smallest of the measured short-circuit proportions. discharge: min (D). If the minimum fraction of short-circuit discharges min (D) exceeds the critical Dkr value, which depends on the length of the voltage signal section under consideration or the number of charges on the voltage signal section under consideration, the processing parameters are selected according to the method of the present invention, which determine the lower power in slot 9 (z = zl). Dkr values range from 0 to 5%.

In the event that the minimum fraction of short-circuit discharges min (D) does not exceed the critical Dkr value, the power in slot 9 is optimal or even too small to achieve the maximum withdrawal rate. In order to determine the optimum power in slot 9 in this case, however, the method of the present invention provides that, if the minimum proportion of short-circuit discharges min (D) on several consecutive sections of the signal is less than the critical Dkr value, then select machining parameters that determine a larger power in slot 9 (z '= z' + l).

When the optimum machining parameters or optimal mode are already selected on the machine, switching to a mode that achieves greater power in the slot would immediately be followed by a switch to the previous, optimal mode. If this were repeated several times in a short time, it would be a disruption to the processing process; the controller would be incapable of maintaining the optimum machining mode.

Therefore, switching to a mode that achieves higher power P is only possible after a few consecutive measurements (K) that give the result: min (D) <Dkr. This is achieved by implementing the counter k. The counter k increases (k = k + l) when the condition min (D) <Dkr. When the counter k exceeds the value of K, the mode that determines the higher power in the slot (z = z + l) is selected and the counter k is reset (fc = 0). The number of consecutive measurements (K) giving the result: min (D) <Dkr is between 2 and 50.

Claims (4)

PATENT APPLICATIONS 1. A process for selecting machining parameters in an erosion process, determining the fraction of short-circuited discharge D that takes place on an erosion device with a preset number of machining regimes (i = 1 to N), where the power Pi is the minimum power and Pn the maximum power in the gap (9), characterized in that the analyzer (8) captures the electrical voltage in the slot (9), determines the types (A, B, C, D) of the individual charges and determines the proportion of short-circuited charges D on certain sections of the voltage signal, and based on the calculated minimum fraction of short-circuit discharges min (D) and the known critical value of short-circuit discharge Dkr, if necessary, select the appropriate treatment regime, namely: - in the case of min (D)> Dkr, select the mode with lower power in the gap (9) (i = i - 1), - if min (D) = Dkr, it does not change the processing mode, and if min (D) <Dkr, it selects the higher power mode in slot (9) (i = i + 1). Method according to claim 1, characterized in that the determination of the proportion of short-circuit discharges D is carried out on several sections, preferably in 2 to 10 sections, and the voltage signal, and the smallest of the so-called short-circuit discharge ratios is said min (D). A method according to any preceding claim, characterized in that said known Dkr value depends on the length of the voltage signal section considered, or the number of discharges on the voltage signal section being considered, and that its Dkr value ranges from 0 - 5%. Method according to any one of the preceding claims, characterized in that switching to a mode with higher power Po + d is only possible after a certain number of K consecutive measurements giving the result: min (D) <Dkr, where said number of consecutive measurements K is between 2 and 50, which is done by introducing a counter k, which under the condition min (D) <Dkr is increased by 1 (k = k + l) and when the counter k exceeds a certain value K, a mode is selected which determines more power in the slot (9), and the counter k is placed at the starting position (k = 0).