EDM or Electrical Discharge Machining is the burning/melting of the steel to form a tool or part. Proper heat treat of the steel is an ABSOLUTE MUST in preparing for the EDM process; 3 tempers minimum is required. When sinker EDM was introduced, the part/tool was made 1st and then heat treated after the EDM process. As wire technology was introduced, the tool or part tolerances became tighter (tenths .0001 are now common) and the steel has to be heat treated prior to the EDM process.
We will cover the most common issues related with EDM below:
Wire/Electrode Breakage: due to a non-metallic element in the steel; will not conduct electric current.
Wire Bounce: due to improper heat treating leaving “Hard Carbides” in the steel that did not go into solution. These carbides can exceed 80+RC.
Movement: Excessive material movement or distortion occurring during the EDM process is typically due to improper heat treating procedures.
Burn Area: the depth of heat during EDM that has changed the microstructure of the steel; “HEAT AFFECTED ZONE”. Remember, you are melting the steel! Do not confuse this with the cast residue left from EDM.
Cast Residue or Recast layer: This is the “White Layer” and is extremely hard & brittle and can cause micro-cracking of the tool. Removal of this area is vital to tool life and the only way to remove this layer is to either grind, stone or polish it off. Multiple skim passes can help reduce the “white area”, but does not eliminate it.
Stress Temper: All tooling/parts should be stress tempered after EDM. This should be done at 50 degrees “F” below the last tempering temperature. This is critical as this tempers the layer of un-tempered microstructure next to the un-affected base area caused by the EDM process and will not affect the Rockwell hardness. A cryogenic (sub-zero freeze) process will also have the same effect.
Good quality tool steel & heat treating practices equal a great tool!
At Hudson, our goal is to “Make Tool Steel Easy”. We hope the discussion in future posts will help you gain a better understanding of materials and processing through asking the right questions. Have a question for us? Call us at 800-996-0411 or send an inquiry through our “Have a Question?” form. We look forward to helping you with any of your tool steel questions.