EDM of NiHard prts

Question

EDM of NiHard prts

Tmoose (Mechanical)

(OP)

10 Apr 14 09:15

A shop has declined to machine the bore of an 82 inch diameter part made of A532 Class II Type D abrasion resistant cast iron. Not quite the same as any of the "nihards" but Rc 56.
Their suggestion was we would need to have the parts EDMed.

My experience with NiHard and similar alloys is it takes very little localized heating (and especially subsequent cooling) to cause spontaneous cracking. I expect that the EDM process may pre-crack the surfaces it creates.

There was a time when EDM was related with unavoidable surface damage and reduced materila propeties which for some applications meant the EDMed surface to some depth had to be removed if the part was to perform well. Knife blades, forging dies and suspension parts come to mind.

These parts will be installed on a hub with 0.006" - 0.014" interference, so the material of the bore may already be in tension.

Is there now a gentle EDM that will not compromise or pre-damage sensitive ferrous materials?

arunmrao (Materials) · Answer 1 · 2014-04-10 10:35:06

Tmoose, I would suggest you look for another machine shop. EDM will not only increase your cost (my estimate 10 fold) ,but also risk cracking. RC56, is not a great hardness for machining bore ( impeller?)

BillPSU (Industrial) · Answer 2 · 2014-04-10 13:01:47

I've seen NiHard being machined many years ago and know there are ceramics which can machine this material now. They were using carbide at the time and basicly pushing the material off of the casting. The machine and the setup had to be extremely rigid. They were making NiHard piping for coal slurry.

You need to locate a shop with NiHard experience. The place I witnessed the machining is out of business.

Bill

CastMetal (Mechanical) · Answer 3 · 2014-04-10 13:17:32

Any EDM operation will have a HAZ. The lower the Amp/area the thinner the heat affected zone. You may be able to find someone to burn low and slow but it may be difficult to find any information on just how large a HAZ your material can tolerate before you start to be concerned with micro cracking. Just for reference for diecasting dies, which have a similar hardness, Amperage is set at a level that gives a ~.0005" thick HAZ and then a couple tenths is taken off the surface to remove the heavily effected material.
I would agree with the posts above, finding someone with a boring mill would be preferable.

Comprehension is not understanding. Understanding is not wisdom. And it is wisdom that gives us the ability to apply what we know, to our real world situations

arunmrao (Materials) · Answer 4 · 2014-04-10 13:47:57

Good machine shops ,with new machines refuse to machine hard castings. They always find excuses.

Old and rigid machines with slow speed attachment and Cobalt based tool bits are preferred. At times, if the casting is not properly supported or clamped, it can crack.

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EDM of NiHard prts