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| Tolerancing a machined hex key (female hex) |
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| bry1975:
Chaps, I've just measured up a male 12 sided(polygonal) caseback my vernier and micrometer read 38.80-38.83mm across flats what tolerance should I use on the female polygonal key I want to make? Does 39.00 + 0.10mm sound to tight? Should've NEVER sold the machinery's handbook. :( TIA Bry |
| andyf:
Hi Bry, If you call it 39mm across flats, the distance across corners will be 40.4mm. So there isn't much tolerance for oversize before a tendency to round over the corners of the case back, rather than turning it, sets in. My instinct would be to go for 39mm, which with the actual size would give about 0.1mm clearance each side (actually, my instinct would be to try gently closing a vice on opposing sides and turning the case, but I'd probably ruin it doing that). A 12-point hex socket might do the trick: http://www.teng.co.uk/bh12m39-3-or-4-drive-39mm-bi-hex-std_56794_pd.asp (or try your local garage; some Toyotas have 39mm axle nuts). But, being meant for 6-sided nuts, sockets are often a sloppy fit. That being the case, you could try a 1.5" one like this http://www.mra-minis.co.uk/classic-mini-parts/tooling/1-5-16-socket-with-1-2-drive-elora-draper-24525.htm. In theory, it would be about 0.9mm too small, but it might go on. However, you might be thinking of brass, to reduce the risk of marring the case back. Andy |
| Ned Ludd:
Hi Andy, I hate to be a wet blanket, but "Bi-hex" is completely different to the drawing Bry has, which is a dodecagon.. Regrettably no grease monkey tooling will work for caseback removal, where only the right style of tool will work. When the height of the polygon is in the order of 1/2mm ( 20 thou), your average socket-set socket lead-in renders its use pointless. As for tolerance, 38.9 +/- nothing sounds about right, if Bry's measurements are correct. This really is a case for "spot on" sizing, if damage is to be avoided. Or as an old timer was reportedly heard to say "I don't work to tenths(of thous), I do it exactly" :bow: Ned |
| bry1975:
Hi chaps, Thanks for the advice. Forgot to mention the watch caseback to be undone is 316L stainless steel, so I'm hoping to have the tool machined from 6082 Aluminium. You can tell I don't have much experience with tolerancing. :scratch: Probably now go for bang on 38.95 + 0.05 as would prefer the part slightly oversized rather than under. |
| Jonny:
Agree with Ned, aluminium once on will distort, stretch etc and need to make the most of it. Tighter you can keep it the better. |
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