UNC threads outer diameter...???

[Henning]

Hi all!

In an attempt to make a very simple threaded in both ends stud i have encountered a problem which has me somewhat confused.
My question is; What diameter does the portion of the stud that is going to be threaded in 1/4"- 20 need to be? My engineering book says that for regular threads (this is no critical part) that the absolutely smallest diameter is 6,22mm. I have the portion of screw down to that but i still can't make the die start threading. Have i got the size wrong? Or is it a problem with material or die?
 
Any help appreciated, it's been a "few" years since i had to make a UN- screw last time...

[steamman]

Seems as though you may have a problem with your die, if you are turning the dia to6.22mm as you say you are slightly smaller than 1/4 which is 6.3mm dia.I regularly use and make 1/4 unf studs for my classic cars ,I sometimes use Ground Silver Steel this is very good size wise and I have never turned it down and never had any problems threading it even up to 5/8" dia .I would check the Start of your die.

[AndyB]

Hi Henning,

Forgive me for asking the obvious but are you using the die the right way round?
The side with the writing on also has a tapered infeed that begins the cut.

Andy

[David Jupp]

A slight chamfer on the end of the rod is useful to get the die started.

[Lew_Merrick_PE]

Hi Henning,

Generally speaking, the actual major diameter of Unified National screw in sizes smaller than 1/2 inch is 0.5% to 1.25% smaller than the nominal size.  Tables of it are available in Machinery's Handbook or in (you can download it from https://assist.daps.dla.mil/quicksearch/) FED-STD-H28 Screw Thread Standards for Federal Service (more information on screw threads than anybody in their right mind would ever want to know).

Cheaply made taps and dies can be a real PITA!  I have become quite a snob about such things over the years.  The really cheap Chinese-made top & die sets (generally) require a lot of work before they become useful in any way.  Spending the extra money for good taps and dies is a real savings.  I (almost exclusively) buy OSG taps and dies these days.  The extra cost pays back in less time spent on this type of threading, fewer broken taps and dies, better project completion, and a massive reduction in swearing!

[Dean W]

Henning, follow the advice about putting a bevel or chamfer on the end of your metal stock, and about starting with the proper side of the die.
If you are using a good die, it should have a split in the circumference.  It may help to leave the die in your die-stock slightly loose, so that split can spread a bit.  Once it is started, you can back off the die and tighten the die-stock screws to get to the proper thread depth on your work piece.
If you are using a die nut (thread chaser) instead of a normal die, they can be hard to start, and they are also usually made of carbon steel, and often not very sharp.  Getting a proper adjustable die is the only way to help that situation.

At the diameter you have your piece right now, (6.22mm) it should be quite easy to thread.  However, what Lew said about quality dies could be your problem.  I've come across cheap far east dies that would not put a thread on undersized brass, much less steel of the correct diameter.  A good die will thread that piece at full proper diameter with little trouble.

Dean

[Henning]

Wow!

That was a lot of good advice very quickly!

I'll try and sum up:

1. Die is the right way around 
2. Ends chamfered
3. Die is top-of-the-line (I'm a toolsnob extraordinaire, ESPECIALLY on cutting tools) Gühring 182 european style closed threading die. (Link: http://www.guhring.co.uk/pdfs/threading-tools/dies.pdf#page=2&zoom=100 )
4. Part to be threaded has been measured and control measured several times! (Funnily enough  )

Which leads me to believe it may well be a problem either with the Die or the material. I will measure the die and also try another piece to see if it's something to do with the material.

Thanks all for your comments!

 

[tom osselton]

Not trying to be smart but are you turning clockwise or counterclockwise / lefthand thread?

[bigmini]

Hi all!

In an attempt to make a very simple threaded in both ends stud i have encountered a problem which has me somewhat confused.
My question is; What diameter does the portion of the stud that is going to be threaded in 1/4"- 20 need to be? My engineering book says that for regular threads (this is no critical part) that the absolutely smallest diameter is 6,22mm. I have the portion of screw down to that but i still can't make the die start threading. Have i got the size wrong? Or is it a problem with material or die?
 
Any help appreciated, it's been a "few" years since i had to make a UN- screw last time...

I had this exact problem with a cheap tap and die set. The taps were good for about one use. Never could get the dies to start no matter how much taper I put on the end. I gave up  and they're all in my scrap steel box now, or have been used to make other tools. Bought a quality tapping set including the correct drills and have been happy (for internal threads at least) ever since.