Will EDM heat distort part?

[widgitmaster]

Last week I made a nice set of tompson linear round rails into supported rails, by drilling and c-boring holes in the case hardened surfaces. The carbide endmills went through the hard case with little difficulty; however, the heat generated caused severe distortion to the centerless ground shafts!

My question is this: will using an EDM machine to burn through the hard case generate the same amount of heat?

Also, was stress created by puting a .15625 dia c-bore and .09375 hole through a .2495 dia thompson rail? There was a full blast of spray-mist on the carbide endmills during the opperation!

I would like to make more of these little units, but the aggrivation level is too high, along with the manual labor to straighten and debur the hardened shafts. So anything that would simplify the process would be appreciated!

Eric

[fkaCarel]

Just my humble input. A thompson rail as you name it, is soft steel carburized bar, which is hardened and then ground. So in a certain sense it's a tube and it's stressed. By asymmetric drilling it you changed the stress model. A suggestion would be to drill on both sides just through the hardened layer and tap it, and let the screws come from the other side.

I think with all the sizes the thickness of the hardened outside will be equal(ish). So the smaller the rail, the ratio hardened layer/core goes up. So maybe a little up on size will move the problems more into the gray area.

Carel

[widgitmaster]

When I designed this gizmo, I choose to make the rails in a vise. This way I could do the entire opperation in one setup. To flip or rotate the part, and mantain regidity for cutting with carbide would only allow one hole at a time. Plus the extra setups for picking up centerline of the previous hole, that is also aggrivating!

Therefor it is best done in a vise!
I had considered drilling one side, and tapping the soft center, but tapping #2-56 blind holes in such a situation is also aggrivating!

Eric

[fkaCarel]

What I described is the same operation, with an extra and maybe unnessary removal of the hardened layer on the other side, so you can tap through and maintain symmetry. I don't see why this would completely change your operation with the vise.

The only thing is that sometimes you get with a less aggravating production method an aggravating product, which was your initial problem.

Carel

[DareBee]

I find it hard to believe that you would make any excessive heat with a properly operated CBide endmill, I believe (similarly to Carel) that it is the removal of material from the small Thomson shaft that is the issue - not the heat.
Both Thomson and Misumi will make these shafts for you as a special with little extra cost.
My other suggestion is making these from through hardened tool steel shafting.

To answer the original question - EDM work causes very little heat to transfer to transfer into the part.

[widgitmaster]

Maybe I mis-understood Carel, but plunge drilling the .093 dia thru one case with a carbide ball endmill iat 1800 RPM n my bridgeport mill caused excessive heat on the exit side! The drilling required excessive pressure to get through the case!
Could it be the Enco endmills?

when I removed the part, it curled right up!

Eric

[fkaCarel]

The carbide endmills went through the hard case with little difficulty; however, the heat generated caused severe distortion to the centerless ground shaft There was a full blast of spray-mist on the carbide endmills during the opperation!

Eric, I don't read a red glowing exit here, only suspected heat under a full blast and went through the hard case with little difficulty, this means in and out. However, it also fits the disturbed stress model.

Edit: first time I discovered how to make the grey box around the quote! Much more important than shooting Thompson bars!

Carel

[widgitmaster]

Pushing a 3/32 endmill through a 1/4 shaft did require little effort, but I can bench press 283lbs too!

Because I have little knowledge of EDM, except that it is a slow process!
How long would it take to disintegrate 5 holes through a 1/4 diameter shaft?
What would be an expected cost ?

Eric

[fkaCarel]

I share your knowledge about EDM. Where I worked we had one for broken taps. The "Shame Corner". Once a collegue broke a tap in a very expensive machined piece of Nylatron. My boss put it under it to find out plastic didn't conduct electricity! What a laugh!

Carel

[DareBee]

$20 - $50 per hole.
You could just get it EDM tapped as well, 2-56 is too small for me.
I would EDM tap those parts in 8-32 for $25/hole.

[widgitmaster]

Originally Posted by DareBee $20 - $50 per hole. You could just get it EDM tapped as well, 2-56 is too small for me. I would EDM tap those parts in 8-32 for $25/hole.

But I thought EDM could do all 5 holes with c-bores at one time?

But EDM tapping is news to me!

[DareBee]

To make an electrode to do all 5 holes at once will cost you another $520 on top.
To use a piece of off-the-shelf electrode rod and plunge it through while orbiting and then circular interpolate the c/bore will be $30/hole (done in 1 setup).
EDM tapping can be done be circular interpolation or the inexpensive way (that I like) is with a tapping fixture using threaded-rod electrodes.