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Surface finish

elfrench

Plastic
Joined
Nov 10, 2011
Location
Western USA
What's the best surface finish achievable with EDM? Can it compare to a surface lapped with 3000 grit(about 6 micron) diamond?
 
If you are expecting shiney , not so much. But check with a profilometer a 6-8 micro inch Is possible with the right condtions and skims or orbit and electrode prep. Are you asking about wire or sinker? What shape and how Large an area?

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Hi elfrench:
The short answer is "NO"

Two things to consider with wire EDM cut form cutters in carbide.
The first is that you can never get as nice a finish as you can with lapping or diamond honing or even grinding with a fine diamond wheel.
The second is that a wire cut tool will break down faster than the equivalent ground tool.
This is because of cobalt depletion at the wire cut surface unless the wire machine runs with oil dielectric instead of deionized water.

So if you need the tool to be the best it can be, I'd have it CNC ground.
It's not a difficult profile, so I'll bet there are a bazillion tool grinding shops out there who can do a nice job for you.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
 
That's one of the simplest profiles. A lot of the profiles have sharp inside angles and/or small radii which EDM would be perfect for. I think we'll just have to try it but keep our expectations low :(

Thanks to both of you for the answers. It was helpful.
 
Hi again elfrench:
Red James made a comment that has merit HOWEVER, if you do this, you have to make a form that can be lapped.
Instead of making the form tapered from top to bottom you can make it constant cross sectional profile and tilted relative to the top surface.
Of course this only works with certain forms...shallow ones with no surfaces close to parallel to the infeed direction.
Your sample form is a good candidate.

Naturally you have to compensate for the form distortion that occurs when you wire cut it on a tilt, but once you have achieved that, it will work just fine.

A simpler way to get better longevity, is to taper cut the form from a thicker blank and then just surface grind the top to final thickness.

This is a bodge in that it does not eliminate all loose carbide grains from the wire cut surface, but it does eliminate a lot of them right at the edge where it matters, and it's a tactic I use routinely and PRETEND I have had better success with than without.

Be aware, this empirical observation could be nothing more than wishful fantasy, but I'm a believer, having made and used many form tools this way.
Not a lot needs to be removed...0.005" is plenty.

Of course when you do it this way, you have to wirecut the form so when you grind it you get the right shape at the finished top edge.
That's easy to lay out in CAD and poses no difficulty for the method.

Form cutters made this way are still inferior to cutters either form ground or wire cut by a tool making house with an oil dielectric wire EDM on their floor (if you plan to farm these out anyway, just find a shop that owns one).
But I still think they are better than wire cut forms that are thicknessed before they are wired.

Knowing all that, I make my own form tools out of 5% cobalt bearing HSS now, and I've had better success using that for the kind of work I do.

To find a shop with an oil dielectric wire, phone the major EDM sellers and ask them who has bought a machine recently.
They always seem to be happy to refer to a customer of theirs, in the hopes they will build enough business for them to order a second machine.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining
 








 
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