Thread Re-rolling

Here's what I think. I think that even if you have no trouble lining up the threads into the rolls for round two, as soon as the rolls hit the shank to be threaded there will be extensive deflection under load and probably roll breakage. As well as deformation of existing thread. In the usual case, there is no thread engaged and in or behind the rolls when they get to load- the rolls get to load on the start and forces remain normal for the duration of the process. What you propose - This is different.

My opinion.

I'm thinking that the cost of replacement rolls isn't worth the risk.

milo

I thought the 8.8 bolts were heat treated after being rolled and indeed after all the forming processes. So you cannot take the finished bolt which has reduced ductility and re-form it unless you heat it again so that the threads can be rolled.

I should do it with cutting the thread. Did that many times. Use good cutting oil and a bigger cutting tool. If you have a lathe it can go fast too. This works.

This is a common used good practice. Cutting will improve the rolled part and continue the existing thread. If your tools as adjustable, you can quick run over the existing and improve the new thread. GA

Actually this would probably be rejectable as standard doesn't permit both rolled and cut treads. it permits either cut or threaded based on diameter being larger than m20;

I do not know how you envisage cutting improving the prior rolled threads; Why would you cut them if they are already to dimension? What do you mean by IMPROVE?

and the cut threads will not have the cold work grain deformation that makes the rolled threads stronger.

so the cut threads will be weaker than the rolled threads.

I do not know on what basis you can call this a "good practice," and will start preparing expert witness testimony regarding the malfeasance that this bastardization deserves after the bolt fails.

Please provide us with the basis for your characterization of this as a good practice so that we might learn.

OP, DON"T DO IT! milo

Dear Milo and Lyn: As far as I know cut thread is more precise than rolled thread and the grade of finish is tool related.

Dull rolls, dull dyes or good ones. As you can read in the posts here, rolled thread is often, even almost always, performed on hot (soft) bolt bodies.

After this process the bolt will be enhanced to its required strength.

During this process the dimensions and tolerances slightly "adapt" if not fully anticipated.

The body of the bolt here has a strength of 8.8. and this means:

a rupture force of 8 kg/mm2 can be applied on this bolt before breakage.

When rolling a thread, the material doesn't only get vaster but in the valleys it is stretched too. making the bolt a little bit longer

But all needs to be within the 8.8. specs, or it should not be stamped in the head. When you pass over the rolled thread, the dye will cut the over -sized flanks and top, up to the specs of the tool.

Or maybe not, depends on the condition it has been rolled. The thread has nothing to do with the strength.

Because it is on the outside of the valley diameter.

Check the relationship between the rupture force, the net diameter and the outside diameter.

You will also notice that with nuts, there are also differences in tolerance between nuts for cut bolts and nuts for rolled bolts (and even nuts for hot galvanized ones).

Besides a bigger elongation of the bolt, while fastening the nut, the rupture strength should be the same as before, because nothing changed to the active working diameter.

And if or when the standard doesn't permit, one shouldn't do it according the book and buy a longer or a shorter bolt.

What does he do with the original thread anyway.

But then is my first question: how critical has the given bolt been sized?

Maybe you should need a 10.2 bolt or a larger diameter?

BTW 10.2 can also still be cut with a dye. (hope spell this well = filliere)

That is what we have been practicing for 50 years plus. Improve means to me closer to perfection. Less variation from the standard.

I hope this answers your questions. Yours Cherry.

Let me try once again:

Thread is rolled COLD, on hot rolled steel initially at room temperature. The cold working strain moves the grains of the steel to make a stronger thread than the cut thread.

The fact that rolled threads are stronger than cut threads is axiomatic, but google advantages of thread rolling if you would like to double check my thinking.

Properly rolled threads should not need to be adjusted by cutting, they should already be to proper dimensions.

Rupture force is result of quench and temper not cutting of diameter.

I am not certain if the temperature of cutting (result of shear separating chip) will be sufficient ot retemper material locally or not. but it is a possibility.

My professional advice stands, cutting threads on finished q&t bolt after cold rolling is fraudulent, malfeasance, not in accord with specification and creates liability for the "engineer of record" as well as the entity that manufactures them.

milo

"This is a common used good practice. Cutting will improve the rolled part"

Please explain how removing material from the nonthreaded portion of a long rod with a rolled thread, "will improve the rolled part".

I'm missing something.

I see a problem here. I this case, there are already rolled thread. If we do the threading using a cutting tool usig a lathe, we will end up with a notch formation where the rolled threads and machined thread meet. Then this would be a cause for local stress concentration.