Thread Strength ?

Hi all.

Seeing the recent thread on metric fine grub screws sets me thinking there must be as relationship between thread pitch and 'strippability' for want of a better word of what it's going into..

I'm currently making a tail stock die holder as I stripped the threads of a commercial one (not 100% totally my fault as they weren't at the right spacing so didn't line up with the dimples on the dies).

So do I go for a fine -ish metric or BA, or a corser whitworth form ?

thanks

WALLACE

Wallace,

There is quite a lot more to it than just thread pitch. I assume you are using steel to make your die holder. What thickness will the metal be where the adjusting screw fits? You see it is necessary to have several complete threads to engage the set screw so if you use a too course thread in a thin area you may only end up with only one or two threads that would easily strip. Fine threads would probably best in thin sections. It also depends on the tapping size drill used which must be a bit larger than the core diameter of the chosen thread. I suggest you buy a copy of "Drills, Taps and Dies" by Tubal Cain which explains thing very well, is inexpensive and is a mine of information.

By the way it should never be necessary to tighten the adjusting screw on a die holder so hard that the thread strips. I understand your commercial one may have been faulty (most likely a Far Eastern product). Split dies are only intended to have a very limited range of adjustment and is bad practise to try to get a looser thread by over tightening them so try a slightly reduced outside diameter work piece or most likely the die is blunt.

Wallace, you need one and half thread diameters for an effective thread.

If you use a 3mm thread for the adjusting screws then the wall thickness will be 4.5mm.

Clive

Take my advice, make the walls of your die holder thicker than the minimum - otherwise your screws will keep falling out because you have to back them right out for the tip to clear the holes in the die and you will have to keep crawling around on the floor looking for them.

Don't ask how I know...

Neil