Hollow Ball Bearings, EDM And Injection Moldmaking?

Anyone who has done research on the history of electrical discharge machining, a.k.a. EDM, knows that  two Russian scientists, B.R.Lazarenko and N.I. Lazarenko are credited with developing the first somewhat usable EDM machine. It is interesting to see how the process developed into a major part of plastic injection mold making. 

I don’t know if it is true, but somewhere along the line I was taught that they were attempting to make hollow ball bearings using electrical discharge machining. Apparently the “chips” from EDMing resemble hollow ball bearings, except that they are only mostly round, and one part is very jagged. It does sound like a Russian type of experiment, who knows?

Like a lot of inventions, there were simultaneous developments in the USA, which led to a machine that was made to remove broken taps and drills. Soon the Swiss were in on the race to develop a usable EDM machine, as were the Russians.

Toolmakers begin using EDM

Many years ago I worked with a German toolmaker  who had an injection moldmaking shop without an EDM machine. When he told me this I could hardly imagine what he had to go through to produce the shapes required for even a basic mold.

Yet, up until the Swiss perfected the electrical discharge machining process, there simply was no EDM. Toolmakers had to use hand engraving, files, single lip cutters and all manner of creative methods to produce the shapes designed for the mold.

My friend, who had also made his own tool-and-cutter grinder, told me that he had to use a lot of inserts and do a great deal of insert manufacturing and hand fitting. Many of the skills he possessed, such as hand engraving, are long gone, though the work deserves to be in a museum.

Manual EDM’s are long gone, mostly

The old Eltee Pulsitron’s are nearly all gone now, as are the X-Cello and old Charmilles manual EDM machines. Quite a few shops keep a manual on around, just for the odd repair job that works out better in a manual machine. Otherwise, it is all CNC, robots, CMM’s and CAD/CAM.

Enter the modern age of plastic injection moldmaking

The modern moldmaking shop depends entirely on EDM and usually these machines run 24/7. With the addition of pallets and robotics this is quite possible and happens on an everyday basis the world over.

EDM machines are married to High Speed Machining Centers and Coordinate Measuring Machines by robotics and software. With careful planning and execution the electrodes are machined, inspected, and loaded in the EDM’s.

EDM starts with the designer

Ideally, the mold designer would use software made specifically for EDM, such as Cimatron, the mold wizard in Unigraphics. This makes everything easier for the entire shop because the software is integrated, meaning that any changes made are carried out throughout the design, which naturally helps eliminate errors and aids in communication.

Consistency in the electrode design process adds predictability to the manufacturing, thus making the process more reliable. The designer who considers manufacturability into his electrodes is a tremendous asset to the entire moldmaking process.

Electrode manufacturing

Anyone who cut their teeth on grinding graphite electrodes well remembers holding their breath during the machining process on a surface grinder or Bridgeport milling machine. Soon the conscientious shop owner installed shop vacs, which did nothing whatsoever to solve the dust problem. You see, the graphite particles were smaller than the filter could handle, so most of the dust was blown back into the room. No wonder everyone had on a particle mask! Thankfully this scene is one from the past.

Ideally, this work is done on a high speed milling machine equipped with a negative pressure chamber, such as a Makino or Mikron. These amazing machines can cut an electrode to a high degree of accuracy, while keeping the work area and surrounding area free from annoying graphite dust.

The same holds true for copper electrode manufacture; the high speed milling machine quickly and efficiently produces highly accurate electrodes that also have a great surface finish. Unfortunately, copper still leaves burrs that must be removed by hand, unlike graphite, which is burr-free.

Electrode inspection

Ideally, the trodes leave the high speed machining center and are subsequently inspected on the CMM automatically, via the robot. The inspection data is fed directly into the EDM program to compensate for any offsets or errors.

The next alternative is to manually inspect the electrodes, and enter the data manually during the setup process. This obviously works, yet the potential for error increases with every manual data input operation.

A great deal of time can be wasted by having multiple inspections of each electrode. For example: the high speed machining center operator inspects the electrode, the EDM operator does the same and possibly an inspector also checks them.

This triple checking might help insure accuracy, but it also takes a great deal of time. Having a specific process in place with individuals responsible can save time and still ensure the accuracy of the work.

Robots vs. tool changers

At first glance it seems that using robots to change electrodes is superfluous, after all, the tool changer worked well for years. There are many advantages to using robots to store and mount electrodes, however.

Overall, it is easier to organize and manage both workpieces and electrodes by using a robotic system together with pallets. Both pieces can be easily stored and retrieved at any point in time in the library and it is much more realistic to keep the EDM’s running 24/7 with minimal operator intervention.

Ram type EDM, or sinker EDM, performance

EDM long ago left the realm of “black art” and is now entirely predictable and reliable. The settings provided by the manufacturer work just as promised, all the time. This facilitates accurate machining, even within “tenths”.

Today there is much less “cut and check” in the process, though this certainly still occurs. In the case of super high precision, such as for a silicon mold, there is no way around the “cut and check” method. Yet even this can be made predictable over time and with experience.

Another interesting role EDM plays in the plastic injection moldmaking process is that of mold polishing. If you have enough time and can afford to allow the EDM to run for days on end, you can achieve very highly polished surfaces. Or you can compromise and semi-finish the polishing and let the mold polisher finish it manually.

This is a great option for highly intricate details that are difficult to reach with a polishing stone or where the risk is too high due to damaging the geometry by manually polishing.

Conclusion

Electrical discharge machining, or EDM, plays a central role in the production of plastic injection molds, as well as many other industrial processes. Aerospace, computer manufacturing, tool-and-die making and precision machining are some other industries that rely on EDM.

Automation has greatly influenced EDM, with robotics, specialized software, palletizing of workpieces, automatic CMM inspection and data entry, and unattended mold polishing pushing the technology forward. You can find us listed here as an educational resource.

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