I sent my drawing to a machine shop. It is a pinion gear with a m3x3 set screw locking on the motor shaft. The machined part comes with a m3x3 set screw hole but it is too lose. I need it more tight fit. How can I represent it to the machine shop? They are asking me the number. I understand it is impossible to explain how tight I need here. But let me try here I want to screw in the M3x3 set screw with a little force. So the set screw is not easy to come off when spinning. Please teach me how to represent this to machine shop. thanks
First idea that comes to mind is that taps are tapered, so if you run the tap in a predetermined depth the last few threads wouldn't be fully cut and would be tight just the last turn or so...
you might want to have them cut the threads most of the way and finish it yourself with a tap and some sort of locking collar/ depth stop.
But as above, best bet is to prevent it from coming loose in the first place.
Pandinus: Yes, it is tapered. Your solution works and I see if I can make it better. What I can ask them to custom make a thread cutter with -0.5mm. Would it be prefect solution? Because there is another problem I didn't mention before is the alu 6061 of the gear they are using is just a bit too soft so if I keep on turn in and out the set screw hardly. It will strip the gear thread. And if I have a tighter thread then this issue can be slove too and the set screw is more secure too. If my idea is correct, what would be the prefect size of the thread cutter for the M3 set screw? thanks
http://www.boltscience.com/pages/screw8.htm will help with your fit issues, also if this screw is in Alu, and is going to be removed or adjusted often then consider using a Heli-Coil Insert,