Threading Rod

[John]

I can find many tables which suggest the appropriate drill size to use when tapping a thread into a hole, but I cannot find any information on what diameter I should turn a rod down to when I wish to use a die to run a thread onto the rod.
For example, I wish to thread a rod 3/8" x 24 UNF. What diameter should the rod be to ensure a good thread?
Thanks
John

 

[tomrux]

somewhere just under 3/8 would have to be pretty close. I normally aim for bang on size.

Tom R

 

[xpylonracer]

Hi John
Like Tom says .375". (3/8")
Most charts will list the thread diameter as OD (Outside diameter) and this can be applied to all outside threads, inch and metric sizes.
If you are cutting with a split die you can adjust the finish diameter of the thread a small amount by using the centre tapered screw to open the die, this may be the best place to start, adjust until the required fit is achieved.
Emgee

 

[John]

Thanks guys,
Will start with 3/8" diameter on some scrap and see how it goes.

John

 

[goldstar31]

If you are wanting a long thread, you'd be wise to screwcut first.

You could otherwise end up with bad threads
Regards

Norman

 

[Toolguy]

Any time I use a thread die, I will single point to a little oversize, then chase it to size with the die. This way the threads are straight and true. This avoids what is called "drunken threads" where the die goes crooked due to not getting started straight. Also, this method makes it easy to make threads the same size, even over a long period of time. Just leave the die set at the desired diameter and they will always be the same. I use a tap of that size for a plug gage, then the screw will always fit a standard size.