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| UNC threads outer diameter...??? |
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| 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! |
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