We know that EDM is process of electrolytically etching off workpiece, while electrochemistry is dissolution of workpiece by electrolyte. Relatively speaking, EDM is now very popular.

 

For example, in the manufacture of molds, EDM is an important process. However, some technical misunderstandings in mold factory will cause numerical control EDM to fail to meet expected accuracy, surface, efficiency and other requirements. Let us analyze these common problems.

Method of directly touching workpiece with electrode belongs to surface contact. There are inevitably more or less fines between contact surfaces, contact faces also have clamping accuracy errors, which will directly affect accuracy of edges and points. With this method, contact surface must be cleaned strictly, but accuracy may be unstable due to human factors.

 

For CNC discharge machines, it is recommended to use reference ball to divide workpiece, which is a necessary method for discharge of mold factory. Usual practice is:

 

Specific steps:
Clamping workpiece; placing a reference ball on workbench; mounting probe on spindle head; using probe to divide workpiece; using probe to divide reference ball; removing probe and mounting electrode; subsequent electrodes are all centered on reference ball.
Since centering process is a point-to-point sensing contact, high-precision positioning accuracy of μm level can be achieved. In addition, process movement distance of electrode sub-reference ball becomes smaller, stroke of machine tool can be fully utilized, and efficiency is also improved.

Most mold company in China use copper as an electrode material. In pursuit of high-efficiency processing today, have you examined processing advantages of graphite electrode? Perhaps you will simply think that graphite electrode are only suitable for large mold manufacturing or roughing. In fact, this kind of understanding is one-sided or still stuck in traditional modelling concept.

 

More and more mold company are now using graphite electrodes to significantly shorten mold manufacturing cycle. Because it can save about 50% of time whether it is milling electrodes or electrical discharge machining, this is a significant advantage of graphite electrodes. In addition, graphite is a large electrode with light weight, narrow slit processing is not easy to deform, CNC milling has no burrs, and overall electrode can be designed to reduce number of electrodes, etc., which fully demonstrates advantages of graphite materials. Of course, graphite processing is not suitable for fine surface processing requiring Ra below 0.4 μm.
For micro-class processing, extremely low electrode losses are required. At this time, it is necessary to use high-quality copper electrodes. For EDM machining of high value-added parts, use of more expensive copper-tungsten alloys results in smaller electrode losses, especially in machining of cemented carbide workpieces.

Many mold company upgrade from traditional discharge machines to digitally controlled discharge machines. When many mold factory use digitally controlled discharge machines, electrode spark position process still refers to conventional discharge machines. For example, roughing electrode has a single side 0.15mm and finished electrode has a single side of 0.06 mm.

 

Small electrode spark position greatly limits ability of digitally controlled discharge machine to use higher current for high speed machining. In fact, after high-speed cutting process, side of cavity can be quickly repaired by translation process, which is a perfect method for achieving perfect effect of discharge surface, efficiency and accuracy index. Here is a reference, spark electrode of roughing electrode of numerical control discharge machine takes a single side of 0.3~0.15mm, and finished electrode takes a single side of 0.15~0.1mm. It is necessary to refer to discharge area and processing amount, and if area permits, spark position is made as large as possible, and even a processing efficiency of several times can be obtained.

Enterprises use traditional manual chucks to install and adjust electrodes for strength or cost considerations. This method is simple and practical, it is widely used. However, some companies have purchased hundreds of thousands of digitally controlled discharge machines and are still using traditional chucks.

 

With traditional manual chuck, actual utilization rate of machine tool is not high, and it can only invest more money to increase discharge machine if production efficiency cannot be met. In fact, good horse needs to be equipped with a saddle. CNC machine should be equipped with a 3R quick-clamping positioning fixture, which can save manual process of watch, reduce frequent standby of machine and improve production efficiency.

 

Digitally controlled discharge machine can realize side-to-side and multi-axis machining. Illustration shows a molded insert of an injection mold with a thin, deep glue position around it, which is suitable for side impact.

 

Discharge clearing R angle of remaining tool after cutting is a common type of machining. If three-axis linkage method of X, Y, and Z is used, that is oblique processing, it is possible to avoid phenomenon that discharge is unstable and electrode is locally lost due to small area of processing portion.

 

If die discharge machining is of a large area (30 square centimeters or more) and surface is required to be below VDI18, a uniform spark texture is required, like a TV remote type cavity. Then electric discharge machining is a headache problem, and it is often repeated for texture, and processing efficiency is also very low.

 

If it is a large-area, large-cavity mold for mass discharge machining, consider using mixed powder processing technology, which can greatly improve processing efficiency and make it easier to obtain a large area of fine texture or mirror.

 

For some mold company, requirements for mold manufacturing are not very high, and discharge parts are basically polished later. In this case, die electrical discharge machining is pursuing requirements of VDI18 (Ra 0.8 μm) or even mirror processing, while at the same time complaining that discharge speed is too slow and delivery time is too late.
Quality of discharge surface should be properly controlled according to different requirements of mold, and it is clear whether priority of discharge is efficiency or quality. For most of subsequent processing parts to be polished, electric discharge machining can reach VDI22 (Ra1.25 μm) or more. For fine parts, it can be processed to avoid polishing deformation. It should be emphasized here that in the pursuit of high-quality matt surface requirements below VDI22, discharge time will increase significantly and electrode loss will increase.

For mold company that have not been exposed to mirror discharge machining, they will be interested in this technology. But unfortunately, due to lack of practical experience, some of their incorrect perceptions lead to cases of processing failure.

 

In fact, for CNC discharge machines, mirror processing is not difficult, sub-mirrors such as VDI7 (Ra0.2μm) are extremely difficult to machine. Whether it can achieve high-quality mirror effect, in addition to selected processing parameters, it depends largely on workpiece material. Some materials such as SKD11, DC53, and counterfeit S136 do not achieve good mirror effect anyway, so it is necessary to judge material and then making a mirror discharge, otherwise it may waste time and fail to meet requirements. For electrodes, there are not so strict requirements, and it is not required that electrodes must be mirror polished.
Main experience of mirror processing is control of time. How much time should be set, and experienced masters can flexibly achieve high-efficiency mirror production. Without experience, they can only rely on expert system of CNC machine tools, and slowly perform electrical discharge machining. Anyway, quality is guaranteed...
Above is 8 major misunderstandings in mold processing. It must be said that many of them are derived from traditional processing concept. This also means that processing technology is improved, equipment level is one aspect, and concept innovation must also keep up to effectively achieve today's precision machining requirements.