Is a 12 x 1.8mm bolt cheating?

[jack620]

I was practicing thread cutting again today on my Hercus 260. I thought I would make a M12x1.75 bolt. I soon realised I would have to swap to the 45T gear to get 1.75mm pitch. If I left the 18T gear on the closest I could get was 1.8mm. I couldn't be bothered swapping the gears and I got to thinking whether a 1.75mm pitch nut would fit on a 1.80mm pitch bolt. You can see from the photo that it does. Admittedly the nut is construction grade and is a fairly loose fit. There are roughly 4 threads in the nut, so the total error along the length of the nut is about 0.2mm.

Have I committed a grave sin, or is this acceptable in a non-critical situation?

Chris

P.S. don't ask me to define 'non-critical'. :)

[Michael G]

Chris, it's bad practice because the load on the bolt is only being taken by (1 turn of) 1 thread. Nice thread cutting though.

Michael

[jack620]

OK, that makes sense. Thanks Michael.

[scottyd]

If you had two bolts manufactured using the same means, but with one correctly pitched and the other out by a small amount like yours, the incorrect one will strip with a lot less torque / load. As Michael said, there is much much less thread face contact throughout the nut, so the load cannot be distributed throughout as much metal.

So you could get away with it in a really really non-critical, low load application...but given that youre just one gear swap away from getting it right, you really wouldnt bother!

[jack620]

Thanks scotty, I haven't been game to try swapping gears yet. I guess I'll have to bite the bullet and give it a go.

[scottyd]

It aint rocket surgery. Well, thats what I tell the kids at school anyway (and they simply reply with a blank stare...). There really isnt a lot that can go wrong, just turn the chuck by hand after youve fiddled with anything. If you can spin it, then the chances of anything going particularly wrong are pretty slim.