Thread on a 12mm rod

[robgraham]

I want to put a bit of thread onto a rod that is measuring 12.0mm - will it take an M12 x 1.25 thread or does the OD require to be bigger than the thread ?

Thanks Rob

[dennis]

It should be the same as the thread, although it is slightly easier to cut a thread with a die if the bar is a few thos smaller. So yes you can cut a 12 mm thread into it.

[Cerberus .]

M stands for metric size, the number is the outer (bolt) thread diameter in mm.

So the answer is yes.

Don.

[Andy Dingley]

Although a "tapping drill" is undersize, rod stock for external threads is the same as the nominal size. Dies are adjustable, so start by opening it up to cut an oversize thread, then gradually close it down until the finished thread fits the nuts well. Make sure to use a thread cutting lubricant.

M12 is too big to cut by hand though. You'll have to heave on it to cut anything, which isn't conducive to good results. Best bet is usually to start with ready-cut studding instead.

[robgraham]

Thanks all. Andy, that's the cruncher then; I was aiming at modifying something to bring it up to a current design, but as the new one is only =A318 I think I will just buy it.

Rob

[Andy Wade]

I disagree. The difficulty does depend on the material of course and that seems to be unstated so far in this thread. From experience I'd say that hand-cutting M12 on mild steel would be perfectly feasible. On brass or aluminium alloy it would be quite easy. The going does get tough on higher carbon steels though, or on stainless.

Electricians, plumbers and gas-fitters routinely hand-thread mild steel tubes up to an inch or so o/d, using only pipe-vice, stock and die (and oil).

[Andy Dingley]

It's certainly cuttable, but what's the quality going to be like?

I'm no fan of die-cut threads. Even if they're supported in a lathe's tailstock, the quality isn't generally too good. Freehanding the diestock is even worse. If they're a large diameter then the length of the thread (in thread turns) for a standard thickness of die becomes less and less, so the problem of "drunken" threads becomes worse. I don't think I can cut an M12 of any more than "acceptable" quality on freehand bar in a vice. If I'm working at that low level of precision, I'm likely to find it better and cheaper (Decent M12 dies aren't cheap!) to construct it from studding or re-manufactured bolts with rolled threads instead.

If I did cut it by hand, I'd be taking repeated shallow cuts with the die, onto a bar that was lubricated and well-tapered at the end beforehand.

If I needed a high quality thread, I'd cut it in a lathe with a single- point tool.