Small EDM job

[gbritnell]

Hopefully a couple of the EDM guys on the forum will catch my thread. On the transmission I'm making there is a small bolt (10-32) with about a 5/16 round head. It is the pivot bolt for the reverse linkage. On the full sized trans this bolt has a Torx drive in it. I would like to know if somebody could burn the Torx drive into my little bolt (smaller naturally)? If so contact me and we can work out the details.
Thanks,
gbritnell

 

[keith5700]

George, just as a back up plan, could you screw something like an m2.5 bolt with a T8 torx head into your bolt and flush it off?

 

[gbritnell]

Just bumping this back to the top to see if anyone can help me with this job.
gbritnell

 

[Ken I]

George - the hard part is making the electrode - which should be in copper or preferably chrome zirconium copper.

This should be the same shape as the Torx size you are selecting (you obviously can't just scale it - but choose the nearest to scale) allowing about 0.0015 spark gap all round - check with whoever is going to do the EDM work for you for recomended clearance.

Pre drill the holes - core size.

Blind holes are normally very hard on the electrodes as the electrolyte can't "flow" so normally a roughing and finishing electrode - make both the same but the one that wears out first becomes your "rougher".

There are specialist tooling suppliers that supply ready made EDM electrodes - such as Saturn - Google EDM electrode suppliers.

Apart from the electrode - which I'm sure you can make - your local friendly EDM shop shouldn't charge too much - in fact if you show them what you are doing they would probably do it for nothing.

Having said all that - have you tried broaching the Torx hole - use a hardened and sharpened Torx key and press it into the core drilled hole - I've done this for Allen key holes and it works fine.

You must use a good quality key - not those cheap sintered points.

Another method is a "wobbly broach" in a lathe - here the broach runs in ballbearings in a floating holder at one degree skew to the machine axis - you float it into the predrilled hole - lock it and force it into the rotating part with the tailstock - as the broach is only cutting on one corner at a time this vastly reduces the force required.

The drawback is you need to make a hardened broach with 1+° clearance (plus of course the wobbly broach tool).

Hope this helps.

Ken

 

[stevehuckss396]

Just bumping this back to the top to see if anyone can help me with this job.
gbritnell

As a backup plan. If you provide a drawing I could try to machine a punch on the end of some O1 drill rod. You could harden then stamp the head. I have CNC and endmills down to .020 If you predrill the head of the bolt it might stamp right in there.

Let me know and ill try it. I also have the furnace so if you tell me what temp for how long I could harden and temper it for you before sending.

 

[joeby]

I've been thinking about this little project. The hardest part of the job is going to be the electrodes.

Fine grade graphite would be fine for an electrode, even a commercial torx bit could be used for an electrode but the hole will be oversize because of the spark gap. That may or may not be an issue for you.

The actual EDM machine time won't be much. The time can be cut down further if the electrode is drilled thru the center to allow for flushing.

I don't have the time available to make the electrodes at work, without looking at the profile of the torx socket I think it would need to be ground to get the profile correct.

I would make the electrode long enough to be able to cut the end off several times to redress it, saving the fabrication of multiple electrodes. Drill a small hole through for flush, the small tit left at the bottom of the socket can be broken out or burned out with a plain round electrode. The profile should be from .0015 to .002 per side undersize for the overburn. It should be made on a standard size piece of material not over 1/2" diameter to allow it to be held in a standard collet.

Keep us informed.

Kevin

 

[Ken I]

In case anyone is curious about a "wobbly broach"

The name is a misnomer - it doesn't wobble but runs at 1° to the axis of rotation (so it sort of wobbles relative to the work).

I believe this was originally a Westinghouse patent.

I used this to make hundreds of thousands of 7mm AF hex key holes in the top of the McPhearson strut rods used on the VW Golf (Rabbit etc.) on N.C. lathe production - I was frequently asked how did I "turn" an internal hexagon.

Photo attached.

Attatched ACAD.dwg file attached - zipped.

Ken

View attachment WOBBLY.zip

 

[Tin Falcon]

gearge may be sa stupid question but why not by a 10-32 torx head bolt
http://www.aaronstorxscrews.com/cfusion/DisplayProduct.cfm?ITEM=7041TF4154
Tin

 

[gbritnell]

Thanks for the suggestion Tin. The part I need is more than just a bolt. It has a length of shoulder that the lever rides on and a reduced diameter at the tip for a small circlip.
George

 

[Lakc]

Thanks for the suggestion Tin. The part I need is more than just a bolt. It has a length of shoulder that the lever rides on and a reduced diameter at the tip for a small circlip.
George

If you cant cut down and rethread a shouldered bolt, Ken's suggestion is called a rotabroach here in the states.

 

[techonehundred]

Lms actually sells some rotary Broaching bits and holders. Unfortunately they are out of stock right now. Here is the link. http://www.littlemachineshop.com/products/product_view.php?ProductID=3772&category=