Simple threading question

[Brass_Machine]

So here is a simple question(s). I have yet to thread on my lathe or really use a die that much. If I want to create say a 3/8 24 thread, what diameter should the rod be? All the tap & die charts I have found have been tap only.

Eric

[Stilldrillin]

I'd say 3/8", Eric! 

Or, am I missing something? 

This is the second time I've been asked that, this week......

David D

[philf]

Eric,

It should nominally be 3/8".

When you measure a bolt the (outside or major) diameter is usually a little smaller (Say 0.005") than the nominal but never any bigger.

Cheers.

Phil.

[Brass_Machine]

Ya know that is what I thought. Wasn't sure if it was supposed to be slightly under sized or not though. Thought I had heard it should be. Wanted to verify.

Thanks

[AdeV]

Slightly undersize will make a bolt easier to fit into a corresponding nut or threaded hole, the trade-off being the fit will be a bit sloppy. There are 3 classes of fit, I believe, can't remember the names but it's basically a) flops about like cooked spaghetti, b) not difficult to screw in, but doesn't flop about so much, and c) as tight as a duck's bum - hard to do up but a very close fit.

Also, if you slightly undersize the stock, you'll find it easier to thread using a die. If you single-point it, there's no difference between undersize or full size.

[saw]

According to my knowless you should have a diameter 9,3 mm. 

[andyf]

The tips of the crests of the thread won't contribute much to its strength. In most cases, using slightly undersized stock is OK, and it will make a die easier to turn.

Andy

[Pete.]

The problem for us novices is that if you make the OD too tight you can't test-fit the nut to get an acceptable fit. My last thread I turned the OD about .2mm under nominal then consulted my zeus for the infeed, stuck to that and the nut was just a little tight, so I quickly ran a file over the thread tops to make sure it wasn't tight on the OD - and it was!

[jim]

here is all the threading information you could possibly need!


http://mdmetric.com/thddata.htm#idx

[AndyB]

Hi Eric,

I might get shot down for saying this but I have read that as long as you have over 65% of the thread depth you should be ok. As you have been told, 100% will not go on the bolt.

Especially with dies, it is easier to cut less meat off, so it depends on the function of the job in hand; as Ade says, you choose fit for function.

Practice is always going to be best before you bugger up the part that counts. It will also give you enormous insight into the problems. Start loose then tighten up. You can adjust dies somewhat with the screws in the holder...but never not use a holder as your dies will soon become thread chasers! The middle screw inserted to the gap opens the die, the other two close it (first undoing the middle one...).

Andy

[Chuck in E. TN]

Hey Eric, 3/8 x 24 is the thread for several drill chucks, including those keyless ones on cordless drills. I have single pointed many from 3/8 drill rod to make adapters for the smaller chucks on the end of MT2 tapers and straight shank to use in my lathe and mill.
 
Chuck in E. TN

[Tony Wells]

Let's see if this works.

[Andrew_D]

Well, Tony Wells chart shows an external thread having an OD of 0.366".
And further up, saw is showing 9.3mm.

My calc-o-brain says that's pretty darn close!

Andrew

[Tony Wells]

Well, there is a tolerance zone in the chart. Anything between 0.3739 and 0.3667 is fine. When turning before threading, I shoot for the high side normally, so I can run a little paper or Scotchbrite over the major diameter to smooth any rough edges at the crest.