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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 Sheppard
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    Why not buy the imperial threaded rod and corresponding nuts? – UnhandledExcepSean Apr 17 '23 at 01:02
  • @UnhandledExcepSean I need to thread the rod myself, so that would mean buying an imperial tap-and-die set, too. Having imperial this-and-that hanging around in my kit to take up space and confuse me from time to time for repairs would be irritating. The rods only need threads for a small proportion of the length. Almost everything over here is metric except, it seems, to-consumer-suppliers of aluminium rods. – Dan Sheppard Apr 17 '23 at 01:35
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    @UnhandledExcepSean I've added the UK flag to the question now, to reflect this country's annoying fence-sitting on the imperial/metric issue. – Dan Sheppard Apr 17 '23 at 01:36
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    What have you done to your google such that it can't find lots of results for "aluminium rod 5mm" in the U.K.? – Andrew Morton Apr 17 '23 at 15:02
  • @AndrewMorton The nearest place here, in Gateshead, where I can pick stuff up rather than have it delivered (delivery is complicated, atm) and does stuff in small quantities and deals with consumers, and where 5m is less than £15, does everything in imperial. If you know of another supplier who meets those criteria, I'll happily use them! I'm still interested in knowing how this stuff works in general, though. – Dan Sheppard Apr 17 '23 at 15:16
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    @AndrewMorton to be fair I did hide all that under the word "conveniently". The reason I want the metal cheap despite also shelling out for a (more expensive) tap-and-die set is that I'm making a substitute for something I can buy finished for £25. My main motivation is learning and tooling up, but I'd feel daft paying many more times in material than finished product commercial cost (at least I get to keep the tools). Sadly my other criteria I can't really go into without giving away too much personal information. – Dan Sheppard Apr 17 '23 at 15:22
  • @DanSheppard That makes sense. Don't smithmetal.com at Gateshead cater to DIY enquiries? I guess they may not have the low prices of your current supplier. How strong does the bar<->nut connection need to be? – Andrew Morton Apr 17 '23 at 15:31
  • @AndrewMorton Not strong, it's only for drying clothes. There will be somewhat of a cantilever at the connection (the mounting is U-shaped, secured at both terminals by these threads passing through existing, fixed eyes, the bar secured with nut and washer on both sides), but the maximum moment arm is less than 50cm total mass hard to calculate (+water, which items, etc) but I think the answer below has reassured me that it's at least worth trying with the imperial bar, at least I get the experience if it all comes crashing down around me and I have to buy something ready-made! – Dan Sheppard Apr 17 '23 at 17:33

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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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    [The major external dimension range for M5x0.8 thread](https://www.engineersedge.com/hardware/metric-external-thread-sizes1.htm) is 4.826 - 4.976 mm, so the 4.76 mm is only very slightly under the spec. It may even expand that much when threading. – jpa Apr 17 '23 at 15:01
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    An excellent resource for more details on metric thread spec: https://amesweb.info/Screws/metric-thread-dimensions-calculator.aspx – R.. GitHub STOP HELPING ICE Apr 17 '23 at 15:02