internal form threads M8 in size

[mecoman]

Hello,

We perform many internal thread taps on 304, 316 & 17-4 stainless steels. We have always used a drill, then cut the thread with a tap. We have trouble with breaking the taps. We have been evaluating a change to forming the threads verses cutting. The most frequent problem is with the M8 tap.

Several different options are available for forming the threads resulting in different cost, fit and depth.

In general, is there anything to worry about? Does it make for reduced holding strength? Does it harden the material on the 304SS. Is it easier to strip? Are they more, less or just as likely to gall? Is there something that should be considered if the part is used in a 200F verses a 450F environment? Since there is less thread engagement in a formed thread is it safe to assume there is less strength.

Thanks in advance for some insight.

Cheers,
MECOMAN

 

[dimjim]

What does your tap supplier recommend?
I assume these are M8x1.25 coarse threads since no
pitch is given. I assume then that you are using
6.75mm drills.

 

[mecoman]

Hello DimJim,

Thanks for the question. We have been calling out 17/64, that is practically the same 6.75mm.

Cheers,
MECOMAN

 

[Cockroach]

Yeah, that's an old trick. Your imperial drill is not the problem for the M8 tap.

It could simply be an operational issue. When I tap, usually I go one quarter turn, then back off 1/8 turn, advance again one quarter and back off half that amount, etc. You need a good tapping fluid, Rapid Tap is the most common or some sort of copper coating with anti-sieze compound.

We're essentially talking skinny little noodle of a tap, a guy goes Western on it, you will break the tap. So being aggressive and not paying attention to fatigue on the tap itself would be my concern.

Good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[mecoman]

Thanks for the reply.



Curious if anyone is familiar with form threads. This is where the hole is drilled a little larger and the threads are formed in the hole instead of cut in the hole? Pro/con?

Cheers,
MECOMAN

 

[CoryPad]

In general, there is nothing to worry about.

It does not result in reduced holding strength.

It will harden the material in the threads.

It is not easier to strip.

Galling is complex. Formed threads should have reduced surface roughness, which will help reduce galling. You will still need a proper lubricant between the external and internal threads during assembly to prevent galling.

Your temperatures are not problematic for current materials (metal and coatings).

Why do you say there is less engagement? You can design and produce the engagement you require for your application. There is no reason why the strength should be less than that from a cut thread.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[btrueblood]

We had good success in switching from cut to rolled/formed threads in 347 stainless sheet. Reduced the number of rejected parts due to thread problems.

 

[mecoman]

Cory,

I read that the strength is less with a formed thread. I cannot recall where I read it on my searching of the web.

My mind thought that the material would be work hardened during the process.

My sense on why we have been having problems with the M8 1.25 is that the % engagement is too high. We do many taps of many sizes every day/week. The metric ISO standard are all published with high percentage enagement (90+) verses 75% for SAE.

Thanks for you feedback.

Cheers,
MECOMAN

 

[mrainey]

Working from the standard, my calculations show the mean minor diameter for M8 x 1.25-6H to be 6.78mm, which is a 75% thread.


Software For Metalworking

http://mrainey.freeservers.com

 

[Pud]

Thread "forming" as compared with thread "cutting" involves movement of material. You will find a thread which is formed will have a core diameter less than you drilled it - by virtue of the process, no metal is removed - the material you would usually cut has been displaced to form the upper part of the thread crest.

This action also gives a better grain flow distribution as the grain tends to flow along the threadform flanks.

Most (if not all!) standard external screwforms available are rolled thread - the external version of what you are proposing (nearly).

Imho, a rolled/formed thread is always better than cut...


Cheers


Harry