OD of small metric threads

I purchased some SS socket grub screws described as M1.6 DIN916 which were to replace some slotted head screws in some photographic equipment

The OD of these new screws measures 1.5mm presumably because the tops of threads are radiused. I measured the old screws at 1.6mm so I now realise they must be M1.7

In these small sizes how does one correctly identify the thread size?

On this bit of equipment (its a lens) the grubscrews hold a knob on to a shaft but weirdly the shaft is exactly 3.270mm diameter. It is a precision item manufactured to metric dimensions and the shaft to bore fit is very close. It seems odd not to use 3mm or 3.5mm (or even 1/8" as a non standard diameter would need special reamers etc.

Ian P

Look here for guidance, not an unusual occurance for what to us are lesser used sizes.

I had a problem with a vernier caliper knurled locking screws, thought they were M2.5 but actually M2.6 so had to remake them!

http://mdmetric.com/tech/M-thead%20600.htm

It's clearly an imperial shaft: 33/256"

Neil

I've had the size problem with screws in microscopes. It's best to check with screw pitch gauges as well. The other problem can be that manufacturers may have access to more precisely sized screws than us mere mortals can obtain easily. Down at the small end the tolerances have to be a bit tight but it is best to be wary of bought screws.

John

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Kwil, It was the Maryland site I was using but looking again after you linked to it I realised I had missed the column that shows min and max thread OD.

John, I like your 'be wary of bought screws' advice. However,,,, I dont fancy screwcutting 1.6mm threads in stainless, let alone forming the 0.7mm AF hex recess!!!

Ian P

Ian - try 10BA, 1.7mm od and 0.35mm pitch.

LOL. At some point I need to make one around 1.5mm dia with a slotted end that is also shorter than it's diameter.

I'm hoping the pitch and dia fit in with some dies I bought a couple of weeks ago.

All because I dropped one. I suspect the dog ate it. She's very quick if she thinks something looks tasty.

John

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