View Full Version : What size shaft for 1/2 - 20 TPI manually threaded.

Greg Parent
11-08-2007, 04:36 PM
Hello all,
I am having trouble threading a 1/2 diameter cold rolled steel rod with a 20 tpi NF die. The threads are tearing and look ragged. I am using cutting fluid but still get the problem. Is it because I am going too fast or the OD of the rod is too big?
Doing this by hand makes it difficult to control the speed. The cutting is going on in starts and stops. Should the shaft be tyaken down a wee bit?
Any ideas for the lathe challenged to help solve this problem are greatly appreciated....and yes buying a lathe is impossible at the moment.

11-08-2007, 04:51 PM
I'd suggest getting a better die, better cutting fluid and/or changing material. What is the diameter of the rod, by the way?

11-08-2007, 04:55 PM
I'd say about .490 to .495" for the od.

Lots of thick high sulphur threading oil.

Some mild steels just don't thread well, at least for me.

11-08-2007, 04:57 PM
Cold-rolled steel, especially the stuff from the hardware store metal rack (if that's what you happen to have) doesn't machine very well, the best you can do. So it may just be the nature of the beast.

A top-quality die, if you don't already have one, will help things considerably. You can order a die from www.mscdirect.com or www.travers.com , to named two reliable online suppliers. A shaft diameter of slightly under 1/2", say 0.490", would help too.

Bill Pace
11-08-2007, 05:51 PM
Could very well be the material....the other day I needed some 1/2-13's for a non-critical piece and just grabbed a bar out of the scrap pile and kept getting these really ragged threads, finally got PO'd and found another piece of rod and it threaded beautifully----

Then there is the issue of the die quality... could very well be it also---

11-08-2007, 11:47 PM
Ummm, Is this a trick question?:rolleyes:

11-09-2007, 12:19 AM
who's buried in Grant's tomb :D like others have said, I'd first look to the die - is it quality? is it old and dull?

11-09-2007, 02:09 AM
If my memory and metallurgy are correct, CRS grains are compressed during the rolling and "nice shiny finish" process which will toughen the material so that it is albeit to a lesser extent not dissimilar to a "rolled" thread.

Next, are you using HSS or "other" (Ie "Tool steel" etc.) dies. HSS is far and away the best in most cases.

Why not use "pre-threaded" rod as it is quite tough and strong - good enough for most purposes and its quite inexpensive. I use that as first preference unless there are good and compelling reasons why is shouldn't.

Horses for courses.

It might not "Do" for the purists but "if needs must ................ etc. - "go for it (any one of them).

As regards the "pre-screwed" rod: I keep 1 metre lengths on hand all the time: 6>12 metric; 1/4>1/2 UNC, UNF and BSW.

I hope it helps.

11-09-2007, 10:26 AM
The shaft sizes can be calculated as: OD-P/8
OD= The outside diameter of the shaft
P= The thread pitch calculated as 1/N (1/number of threads per inch)
This formula will give an approximate 75% thread.
That thread is good for all the work I have done in the last 40 years.