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I'm looking to create an (external) thread for M5 nuts at the end of 6063T6 aluminium bar using a hand die of the traditional circular kind from a tap-and-die set. Sadly while the nuts and tools are all metric, the stock only seems conveniently available in imperial sizes, and the closest seems to be 3/16".

I've never cut a thread before (first time for everything). How close do I have to be with the diameter of the stock? Would this metric/imperial mismatch cause an issue?

Additionally, I have been assuming that the 5mm of "M5" for the nuts I have means that I should be seeking 5mm bar (or as near as possible), and using an M5 die. Is it actually this simple or are there tables I need to use?

Re tolerances, the project is a simple handyman type repair: I'm not building rocket engines here!

Dan
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1 Answers1

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In most cases you want a hair (perhaps 0.01mm) under the nominal pitch diameter (5mm in your case) which makes cutting the threads much easier (than "100%" threads) while not losing much strength. If the die only has to cut the flanks and root of the thread, but not the crown, the cutting is less difficult.

If you use 3/16" rod at 4.76 mm you'll be significantly smaller than that, but not smaller than the minor diameter of M5x0.8 (4.13mm) or M5x0.5 (4.46mm) - but you will have reduced thread height, and thus reduced strength.

For the 0.8mm coarse thread, you'll have 0.63mm to put 0.87mm of threads in, (that's "twice nominal thread depth" since threads appear on both sides of the diameter) or about 72% thread depth. That's not great, but may be acceptable for a low-stress application.

For the 0.5mm fine thread, you'll have 0.3mm to put 0.54mm of threads in, or 55% thread depth, which is likely a poor choice for working (but fine threads in metric seem to be less commonly used, as is also true in imperial threading, so this likely does not apply.)

The relevant Wikipedia article on ISO metric threads

Ecnerwal
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