Coil repair kit

J

joefroooo

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Working on a part and broke of a 5/16 18 tap in 2024 aluminum. Don't have a coil repair kit on hand. Does anyone know what drill size for a Master Coil repair insert? I'm trying to figure out what carbide end mill to cut the tap out. Cant get to Grainger tonight but would like to get the part out of the vice so I can work on some other parts. Have a 1/4 and 5/16 cutter in the box. I dont want to go to big on the hole so the coil wont fit. Let me know if have any intell on this.
Thanks in advance
Joe
 
I used to remove taps regularly in an Inconel part that ate 1/4-20 taps. If I were facing what you have, I'd take the butt end of a used 1/4" carbide end mill, and grind a 4 sided pyramid point on it @ ~45 deg. If the tap is protruding at all, and is crooked, try to grind it a bit to get a flat. If it's below the surface, forget that step. Run the mill as fast as possible. Faster the better. Use blasts of compressed air to keep the tiny chips cleared. Believe it or not, it will cut the core out of the tap and the flutes will fall out of the threads, or leave them so you can fleck them out with a scribe.This will get hot enough to anneal the tap at the point of cut, so be careful with the compressed air, seeing that you are in aluminum. Use moderate pressure, pecking every second or two.

You should be able to go back in with a 5/16-18 tap after that.
 
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Thanks for the response Tony. I tried the 1/4 end mill but had speed at 1000. cut into it about .250 before shattering. What happened as the cutter cut out the tap i guess the tap cutting flutes would come out of the aluminum and chip the end mill causing it to damage the cutter (quickly may I add). I wish I could grind the end of the cutter to a point like you said but dont have the grinder to do it. Mill speed goes up to 4200 . Would really like to get this damn thing out as I have many hours in it. Ill try to up the speed and try again.
The tap had tapped about 50 holes before letting go. Should have possibly retracted the tap to clean out the hole before turning the last 1/4 turn but getting tired and anxious to get it done. Ive learned a lesson here. Pull out and clean often before reinserting. Sick aint I...............
Back to the shop. Fingers crossed
Thanks Joe
 
The main part of the process I described is the extreme negative cutting edges. They are very tough, no matter what grade the carbide. You should check into a green wheel for carbide. Lots of thongs you can use dulled tools for if you can grind them.

Try the end mill again, faster, light touch since it has sharp, fragile edges. Should be able to peck it through. Still use air blasts.
 
:thumbzup:SUCCESS

I cranked up the speed and pecked using a even pressure on the quill. Cleaned out using Denatured alcohol as I went. I now have roughly .650 thread depth after cleaning out. This will work as there are other fasteners along the same line. Killed a few cutters though. But don't have to go purchase a coil kit now. Thanks for the help Tony. I didn't think to crank up the speed.
Joe
 
Good deal. The high speed is the key. The HSS softens with the heat generated by the carbide. It actually works better with that pyramid point about 0.005 off center. I did some on a tool grinder as precise as I could once, and they didn't do as well as the hand ground. An old timer, now dead, taught me that trick. If you're careful, a normal end mill works, but I like the "free" angle.
 
Good idea Tony about the pyramid point on the carbide end mill, I have a box of old ones looking for an application since I can never bear to throw them out.

I was going to suggest (post-mortem now though) that the OP try a tap extractor first. They don't always work that well, especially on smaller sizes but 5/16 is large enough to be worth a try.

Tom
 
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