Tapping 10x Dia at 75% Thread Engagement, How to explain it doesn't work

[Helmut]

I have to tap an 11/16-16 UN thread into a 316L SST tube that's over 10x the tap dia. The customer "wants a class 2b fit" i.e. about 75% of thread engagement.

The logic in Engineering department is that the tap is only cutting a small amount at a time (1" of thread length engagement) and "if I was only tapping 1 inch deep it would work, how come you can't tap it 10" deep using the same tap?"

I'm not smart enough to answer that, but my decades of experience cutting threads tells me it's a no-brainer I just can't explain it in a way they will understand.

Any suggestions on what I can tell them to open their eyes?

[Colin Gilchrist]

Show them the actual tool, in a holder, and say "ok, genius, tell me again how I'm supposed to go 10X Deep, with a tool that is only designed to cut 2X-3X Deep, MAX? THE HOLDER IS GOING TO CRASH INTO THE PART!"

Is this on a Lathe? (you did say 316L SS TUBE) If so, what about the possibility of using an I.D. bar, with a threading insert?

If not, then how about a custom Thread Mill, with an extended shank? You might even be able to get a small extension holder, that would still clear the threads during cutting...

[crazy^millman]

You can do it with the right tap, but it would need to be a specifically made tap with a skipped tooth design.

Here is an example of a skipped tooth insert Advent makes. I have order taps like this for doing what you are being asked to do. What I have done is Thread with a threading tool then come back and chase with a tap using cutting oil. Coolant will not work in this situation with the tap with the ID Threading tool you are okay with the coolant. Then use a tailstock with a center and then chase the threads with the tap. Not fun and a job I really hated, but I have got these stupid things done.

[Helmut]

28 minutes ago, C^Millman said:

You can do it with the right tap, but it would need to be a specifically made tap with a skipped tooth design.

Here is an example of a skipped tooth insert Advent makes. I have order taps like this for doing what you are being asked to do. What I have done is Thread with a threading tool then come back and chase with a tap using cutting oil. Coolant will not work in this situation with the tap with the ID Threading tool you are okay with the coolant. Then use a tailstock with a center and then chase the threads with the tap. Not fun and a job I really hated, but I have got these stupid things done.

I'll get on the phone with Advent, thanks. I heard one other guy talk about offset / skipped tooth but I've never done it. 

Colin, We've got pulley taps so clearance is not the issue. The issue is the thread engagement; the customer wants 75% and that's crazy for steel that deep. I'm guess it should be closer to 50-55%.

 

 

[crazy^millman]

Tap Example

Here is what I have used before to tap some deep Acme Threads before. Need to talk to a tap manufacture and see if they can make you something like this for your application. We had some with a 24" tapping depth capability. 2 or 3 hours tapping a hole by hand was not fun, but you did what you needed to get the job done.

[Helmut]

22 minutes ago, C^Millman said:

Tap Example

Here is what I have used before to tap some deep Acme Threads before. Need to talk to a tap manufacture and see if they can make you something like this for your application. We had some with a 24" tapping depth capability. 2 or 3 hours tapping a hole by hand was not fun, but you did what you needed to get the job done.

Looks like that taps a standard thread shape then chases it with the squared off ACME shape. Seems that would be a single-tap way of doing a step-tap operation with multiple taps. How's the strength on that sucker?

[MrFish]

Roll form ?

[Matthew Hajicek - Singularity]

Print it.

[crazy^millman]

1 hour ago, Helmut said:

Looks like that taps a standard thread shape then chases it with the squared off ACME shape. Seems that would be a single-tap way of doing a step-tap operation with multiple taps. How's the strength on that sucker?

Tapped many holes using those kind of taps many years ago and they never gave us an issue. Cutting Oil was the key when using these kind of taps. Yes it was a 2 stage tap for Acme, but was thinking of suggestions to throw you way. We can trap ourselves into not looking outside of our comfort zone. I have done it so just trying to offer food for thought in the conversation that may or may not work., but nothing ventured nothing gained.

[Gary]

Go as deep as you can with regular tap to pilot the long tap. It is not going to tap any harder once the tap is buried in the material. The front half the tap is doing all the work anyway.  Reversing without blowing out the chips first is whats gonna get you.  We get stuck doing dumb stuff like this quite often on the engine lathes.