Tolerancing a machined hex key (female hex)

[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" 
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.  

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.

[David Jupp]

A female duplicate of the shape to act as a driver will tend to try to drive 'on the corners', leading to damage.  It is also difficult to machine the corners 'sharp' - probably broached when making sockets commercially.

Taking a leaf out of 'flank drive' sockets and spanners, and perhaps also to minimise fit problems, you might want to think about relieving the corners of the driver slightly, a slight recess at each corner.

[bry1975]

Thanks Chaps,

I've sent a rough drawing to the NEW CNC shop I've found and will take it from there, so do you reckon having the corners rounded with a tiny slot drill would be oks?

Also I know when I wanted to design some bicycle crank levers I always thought the squared taper was broached as would be difficult to make any other way.


Bry

[bry1975]

Thanks chaps,

And from the drawing above the 12 sided polygon is practically round so the tool really is going to want to try and round the edges! 

[andyf]

Bry, can't you specify that a small hole be drilled centred on each corner before the dodecagon is milled out, to make sure the corners aren't left rounded so much that they interfere with the corners on the case back. .

Ned, thanks; I'm sure you are right to correct me.

Andy

[bry1975]

I will Andy I'll have a good word with the CNC machinists.

Thanks.

[jim]

Bry, can't you specify that a small hole be drilled centred on each corner before the dodecagon is milled out, to make sure the corners aren't left rounded so much that they interfere with the corners on the case back. .

Ned, thanks; I'm sure you are right to correct me.

Andy

this is the way i'd program/machine it

[Jasonb]

Or get it cut on an EDM machine and have nice crisp corners.

I'd spec a tight tollerance as you can always file a bit off rather than try to put it back on.

J

[bry1975]

EDM

Always thought EDM cost many thousands well it does with punch n shearing dies!?

An idea what about having it lasered? so a two part assembly.?


Bry

[jim]

EDM is not that bad, if you can find somewhere to do it.............

[bry1975]

Chilwell Toolmakers Beeston Notts use to do it!

[loply]

Are you possibly better off using a mouldable material here, rather than machining a metal socket?

Look at videos of Tormach's Super Soft Jaws on YouTube for inspiration.

Maybe even hot glue could be of some use here for a one-off use. Make a shallow socket oversized, fill it with hot glue, pop it on the back of the watch, wait for it to set and then unscrew. Pop out the mould and replace if/when required?

[bry1975]

Hi there,

The idea is I'll use the tool myself and sell the odd one.