Screw Threads with Groove

Dear master of failure. welcome to CR4.

The cavities at the crests of the threads are not deliberately cut in features. they are forensic evidence that the thread was roll formed rather than cut. The irregular form and dimension of these blemishes/cavities on the crests are undeniable proof that they were not formed by a deliberate machining process. if you were to etch the part that you have sectioned, you will see flow lines of the grain that correspond to the flow that created these cavities. This is particularly common on "male " threads, which are often roll threaded, but are not unexpected on female threads formed by rolling rather than cutting. If you do not believe this explanation, then compare the geometry created in the valleys very smooth and repeatable, vs the random and highly variable shape and dimension of these cavities.

Crests not filled out is a common problem for rolled threads in machine shops making threaded partsfrom bars. Industry leader CJ winter makes thread rolls and offers the following advice on thread crests not filled out:

SITUATION PROBLEM SOLUTION

Crests not filled out

1. Blank too small. 1. Increase blank diameter. 2. Thread on roll too deep. 2. Replace rolls with rolls of correct depth for job. NOTE: Special truncated rolls are available from Winter.
(Many users do not consider the above situation with serious objection, and by allowing their threads to pass with crests not filled out, overloading of rolls is avoided and roll life is prolonged.)

http://www.cjwinter.com/trouble.html#crests

Agreed that this company makes rolls to generate male thread forms, but the plastic deformation process is the same.

By the way, nice job posting photos allowing us to clearly see the condition. That probably saved about three iterations of question and answer, call and response to figure out if your descriptions were aout this condition.

milo

Since this is out of my realm of experiece I was reading for fun, but I must support Milo's compliment. You did a very good job posting a well formed question. I wish your level of detail and thought was put into more questions posted here, it would make for a much better forum. Thank you.

Thanks Milo. Your groovy answer makes perfect sense now. I never would have thought that female threads could be formed by anything other than a tap.

Also, thank you Milo and Juba-Jabba for the compliment.

"...Your groovy answer..."

nice pun!

Yay Team!

Milo

Is it possible that these holes were tapped by forming taps rather than cutting taps? This is a process (which I've never done nor seen) similar in principle to roll-forming external threads. If a rod being roll-threaded is a bit undersize, the two sides of the thread don't quite fill to the tip, leaving a similar groove.

Just a guess!

Dick

...groove collapses onto itself...

This ought to be the giveaway you're looking for.

It collapses onto itself to avoid friction, with it's usual contact point, at the bottom of the V.

Note the cutaway is not symmetrical, only done on the top of the V, not at the bottom, and it's probably on purpose, having such relatively soft alloy.

One of the comments is right.

The trend, for soft materials, is to form the thread without cutting, using a forming tool. Many investigations have been done. Such a process presents several advantages and there are special tools already available from tool manufacturers. There is a time reduction and a elasto-plastic deformation of the wall which leads to a higher resistance, the wall is stronger due to it. The problem is only to use the right lubricant at the right speed since the high friction generates heat and destroys the surfaces. Some experiments concern as well steel but only low grades.

Do you mean the cutaway is to hold a lubricant bleeding off to the groove ?

No the extrusion toward inside of the wall material forms those groves if you want I can send you photos from such results obtained in the lab.

Please do.

A silly question: Doesn't the originator mention what it's for?

Thank you all for great insight into roll-formed threads. You have made my first post to CR4 a good experience.