Know-how of CNC slow feeding wire-cut machining

      Slow feeding wire-cut machine is mainly used for processing high-precision parts. The work-piece surface roughness, roundness error, linear error and size error from slow wire cutting is much better than fast wire cutting. We need pay more attention to processing accuracy and surface quality for high quality parts' needs.

How to obtain the satisfactory processing quality? There are five things that need to be considered,

1. Process to prepare

(1) Technical performance analysis of work-piece materials:

Different work-piece materials, the melting point, gasification point, thermal conductivity and other performance indicators are not the same, even according to the same way of processing; the quality of the work-piece is not the same. Therefore, it is necessary to choose the work-piece material according to the actual surface quality. For example, to achieve high precision, we must choose cemented carbide materials, and should not choose stainless steel or unquenched high carbon steel, otherwise it is difficult to achieve the required requirements. This is because the internal residual stress of cemented carbide materials has little influence on the processing, and the processing accuracy and surface quality are better.

(2) Selection of working fluid

Spark discharge must be in a certain insulation performance of the liquid medium, the insulation performance of working liquid can make after the breakdown of discharge channel compression, thus limited spark discharge within the radius of smaller channels, form the instantaneous and local high temperature to melt and gasification metal, quickly recover after the discharge and discharge gap become insulation condition. Insulation performance is too low, will produce electrolysis and form a breakdown spark discharge; Insulation performance is too high, the discharge gap is small, chip removal is difficult, cutting speed is reduced.

Tap water has the advantages of good fluidity, non-flammable and fast cooling. But when directly using tap water as working fluid, due to the effect of conduction of the ions in water, its resistivity is low, about 5 k Ω cm, not only affect the discharge gap deionization, prolong the recovery time of the insulation, but also produces electrolytic action. Therefore, the working fluid of slow wire EDM is generally de-ionized water. General resistivity should be in 10 ~ 100 k Ω cm, the concrete numerical depending on the work-piece material, thickness and machining precision. Such as copper wire processing steel, the resistivity of the working fluid should be low, can improve the cutting speed, but the reverse is true when processing cemented carbide.

The resistivity meter must be observed before processing, especially when the machine tool is just started, it is often found that the resistivity is not in this range, then do not rush to process, let the machine tool run for a period of time to achieve the desired resistivity before the formal processing. In order to ensure the processing accuracy, it is necessary to improve the resistivity of the processing fluid, when it is found that the resistivity of water is no longer increased, the ion exchange resin should be replaced.

In addition, the conditions associated with the coolant must be checked. In slow wire EDM processing, the working liquid sent to the processing area is usually poured liquid supply mode, or the work-piece can be fully immersed liquid supply mode. So to check the processing fluid volume and filter pressure gauge. When the processing liquid flows from the dirty tank to the cleaning tank in reverse, the filter needs to be replaced to ensure that the insulation performance, washing performance and cooling performance of the processing liquid meet the requirements.

Kerosene with high insulating property can also be used as working fluid when special finishing is carried out with slow wire EDM machine.

(3) Selection and correction of electrode wire

Electrode wire of slow wire cutting is multi-purpose copper wire, brass wire, brass plus aluminum, brass plus zinc, brass galvanized, etc. For precision wire cutting, the tension of electrode wire should be increased as much as possible under the premise of continuous wire, molybdenum wire or tungsten wire can also be used.

At present, the diameter specifications of domestic electrode wire are 0.10 mm, 0.15 mm, 0.20 mm, 0.25 mm, 0.30 mm, 0.33 mm, 0.35 mm, etc., and the diameter error is generally less than 2 gm. The diameter of the electrode wire produced abroad can be up to 0.03 mm, or even 0.01-0.003 mm, which is used to finish the precision micro EDM wire cutting for corner clearing and narrow slit. The surface of electrode wire exposed to air for a long time is easily oxidized because it contacts with air and cannot be used for machining high-precision parts. Therefore, care should be taken not to damage the packaging film of the electrode wire when keeping the electrode wire. The quality of the electrode wire must be checked before processing. In one of the following circumstances, the verticality correction of the electrode wire must be re-carried out: the wire cutting machine generally in the processing of 50 ~ 100h after the replacement of the guide wheel or its bearing must be considered; Change the cutting position of the conductive block or replace the conductive block; If there is dirt, it needs to be cleaned with washing liquid.It must be noted that when changing the position of the conductive block or replacing the conductive block, the verticality of the wire electrode must be recalibrated to ensure the precision and surface quality of the processed work-piece.

(4) Processing of thread hole

In the actual production and processing, in order to prevent the residual stress deformation and thermal stress deformation caused by discharge inside the work-piece blank, whether it is the processing of concave die type closed work-piece or punch type work-piece, it is necessary to first process a certain diameter of wire hole in the appropriate position to carry out closed cutting, to avoid open cutting. If the work-piece has been coarse-cut on the fast wire-EDM machine, the wire holes will not be punched in the further machining on the slow wire-EDM machine.

(5) work-piece clamping and alignment

The work-piece to be machined by the slow wire EDM machine should be machined to an accurate datum in the previous procedure for clamping and aligning on the slow wire EDM machine. Make full use of machine tool accessories to clamp the work-piece. For some complex structure shape, easy to deformation of the work-piece clamping, if necessary, can design and manufacture special fixture.

After the work-piece is clamped on the machine tool, the accurate position of the work-piece can be determined by the contact perception of the machine tool, the automatic alignment of the center of the circle and other functions or by the use of the dial table, so as to set the origin of the coordinate system and determine the starting point of programming. When alignment, attention should be paid to the operation several times, and strive for accurate position, the error control to the minimum.

When the work-piece will be cut, the strength of the connection with the parent material is bound to decrease, at this time, we should pay attention to the fixed work-piece, prevent the impact of the working fluid to make the work-piece deflection, thereby changing the cutting gap, light affect the surface quality of the work-piece, the weight of the work-piece scrap.

2.Perform a small number of multiple cuts

Small, multiple cutting way is refers to the use of the same diameter of electrode wire on the same surface has two or more cutting, for the first time set aside in advance before cutting machining allowance for finish, and then to leave allowance for finish, switch to finish machining conditions, using the same trajectory program to offset by stages, then cutting processing. Generally can be divided into 1 ~ 5 cutting, in addition to the first processing, the processing quantity is generally from dozens of microns gradually decreased to a few microns, especially the last processing times more, the processing quantity should be small, that is, a few microns. A small amount of, multiple cutting can make the work-piece with a single cut incomparable surface quality, and the more processing times of the work-piece surface quality is better.  Specific values are generally determined by the machining parameters of the machine tool.

Using a small amount, cutting way for many times, can reduce the cutting machining deformation of work-piece material, improve the machining precision and improve surface quality of work-piece, when rough machining or semi-finishing can keep certain allowance, to compensate for material due to the stress equilibrium deformation and destruction of the last time when finishing the required machining allowance, finally finishing can obtain more satisfactory processing effect. It is a simple and easy method and measure to control and improve the quality of processed surface, but the productivity is reduced.

3.Reasonable arrangement of cutting route

The guiding idea of this measure is to avoid destroying the original internal stress balance of the work-piece material as far as possible, to prevent the work-piece material in the cutting process due to the role of the jig and so on, because the cutting route arrangement is not reasonable and produce significant deformation, resulting in the cutting surface quality and precision decline. Under normal circumstances, a reasonable cutting route should be separated from the work-piece and the clamping part of the cutting section arranged at the end of the total cutting program, the pause point is located near the blank clamping end.

 

4.Choose cutting parameters correctly

The wire parameters, water parameters and electric parameters should be controlled and allocated reasonably in slow wire cutting.

When the wire tension is large, the vibration amplitude decreases and the discharge efficiency increases, which can improve the cutting speed. High wire speed can reduce the chance of wire breaking and short circuit, and improve the cutting speed, but too high will increase the vibration of the electrode wire, and affect the cutting speed. In order to ensure that the work-piece has higher processing accuracy and surface quality, the parameters of wire speed and wire tension provided by machine tool manufacturers can be adjusted appropriately.

Increasing the pressure and flow rate of the working fluid can easily discharge the corrosion and improve the cutting speed, but too high will cause the vibration of the electrode wire, affecting the cutting speed, and can maintain the laminar flow limit.

Short pulse width, peak current and positive polarity is widely used in rough machining. Very short pulse width (100 nanoseconds) and single pulse energy (several micro-joules) can be used to improve the surface quality.

In addition, a stable supply voltage should be maintained. The instability of the power supply voltage will cause the instability of the electrode and the work-piece, which will lead to the instability of the breakdown discharge process and affect the quality of the work-piece.

 

5. Control the distance between the upper guide and the work-piece

Distance dense processing can be used in slow wire EDM machining, that is, the distance between the upper guide and the work-piece is as close as possible (about 0.05 ~ 0.10 mm), to avoid the electrode wire amplitude is too large to affect the processing quality of the work-piece due to the distance is too far.

  At Accu, we’re providing various CNC machining process, including CNC slow wire cutting,  services. If you have any prototypes or CNC machining parts needs, you’re greatly welcomed to contact with our product manager Flemming at This email address is being protected from spambots. You need JavaScript enabled to view it.. Thank you!