Is it possible to pick up an existing thread?

[tony1001]

I have a piece of heat treated Elmax (similar to 440C) that needs an existing 1/4-20 screw hole to go deeper. It was originally tapped in its soft state, but a modification has been made so now the hole needs to go .250 deeper. It is too hard to use a carbide tap on it, so I would like to thread mill it instead. I have thread milled stuff like this before, but the problem I'm having is how to pick up the same lead on the thread that is already there. Is this possible?

[Jobnt]

That's a big ask. I can't really think of a bullet-proof way to do it unless you can physically see where the start thread is. Even that will be sketchy at best. 

Not sure the application but if it were my money I'd reference the Bible where it says harder metals don't "need" 75% thread engagement and drill it out for about 60% thread engagement then hand tap it with Molly D.

[cncappsjames]

The ONLY way I see this going as planned is if you thread-milled it in the first place AND have a clocking feature so you know the angle of the start AND you use the same thread-mill. If any of the 3 are not there, it'll probably be a scrapped part when you're done.

Quote

Not sure the application but if it were my money I'd reference the Bible where it says harder metals don't "need" 75% thread engagement and drill it out for about 60% thread engagement then hand tap it with Molly D.

This right here... ^^^^^^^^

[Aaron Eberhard]

Can you over-drill it and just do a thread for an STI?   Helicoils and their friends solve a lot of problems...

[Newbeeee™]

If the thread is "THAT" hard, I would expect it to fail in use....brittle, AF?

Aaron's idea of wire insert is best IMHO.

Otherwise....could you 'gram an external thread (1/2-20) above your part. Have the Z start position come down to a known clear position (say +0.100).

Then feed hold and stop spindle and zero your control. Then wind in a piece of studding, and in handwheel mode move and eye up the threadmill to the stud.

Then alter your TLO to be the new Z position.

Now 'gram your 1/4-20 thread with the same parameters you just used (without removing part, same start position and TLO etc). I think that's the best you could hope for?

[tony1001]

Thanks for all of the responses. It has been decided after all that we are going to make an electrode to clear the material below the existing thread.

[kyle528]

Use a shorter bolt?

[Leon82]

tslot it below. harvey makes thread back chamfer cutters

[Leon82]

ive done this on a large thread.

 

program a threadmill for the finish pass. single block the machine without the tool to where it starts the full pass. set relative to zero and move the axis off. put the tool in loose and walk it back to zero while guiding it into the thread with an eye loop and tighten it.

 

[amw]

It can be done. Go really slow, start from above part feeding down and your just going to have to get in there and look at the entry as its about to contact the part. Hit feedhold and reset program of course before it actually cuts anything. Keep adjusting Z and trying again until its syncronized. You should be able to blue or sharpie the top of thread and get to point where bottom of cutter is just cleaning of the part. then run it to depth.