External thread

[ptjw7uk]

Help, Having some problem in cutting a simple 3/8 16 tpi thread on a rod. I have turned the rod to 3/8 diameter but trying to cut the thread using a die the amount of force I think is excessive. So I have searched the web to see if there are any dimensions as to the external diameter for cutting external threads but can find none, lots of tapping drill sizes but no external sizes.
When I have cut threads before I do not remember the force required being so high or am I just getting weaker!!
Peter

[Ken_Shea]

You are just getting weaker Peter.

.375 MD is correct.
Couple of thoughts.
1) This may be harder steel then previously threaded
2) Use threading lubricant
3) If specific tolerances and/or strength are not required undersize the major diameter .003 - .005.
4) Die may be dull


Ken

[ptjw7uk]

Possible that the die is dull as I have borrowed it!! Will order a new one asap
Thanks for your input
Peter

[GRH]

Make sure you have the die facing the right way, it should be marked
"Start from this side"

Regards Graeme

[mrainey]

Remember that .375 is the max diameter - you can go down as far as .365 and still be in tolerance. Smaller major makes for easier threading.

[loose nut]

I had the exact same problem with a brand new 1/2" die, it was just no good, replaced it and everything is fine.

[Fasttrack]

Speaking of tolerances - i found that cutting external threads at the exact nominal dimension and then cutting the internal threads to match exactly works well for pieces you don't want to come un-done. It galls pretty bad if you try to take it off later, but if its a one time deal it works great! Not sure about structural integrity though...

[DBW]

If you are using an ajustable die rather than a solid one, try screwing in the adj. screw to open the die up.005" or so. Also if the screw is missing be sure not to squeze the die tight with the 3 holding screws in the die holder.

[RobDee]

When working with dull dies you can reduce your diameter quite a lot. If I remember 75% reduction in diameter from max only weakens the thread 5%. If you look at the thread contour you'll see why. There is very little metal at the peak of the thread.

With poorly made dies or metals like brass and SS you in fact have to do this to prevent galling and seizing. Also use a wax lubricant made for tapping like Tap Ease.