What decrease in torque can be expected with a 3mm hex compared to a 4mm hex?

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sbbiomed
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I have done mechanical testing on a set screw with a 4mm hex and its driver. I now have a set screw with a 3mm hex. Without the time and expense of making parts and testing, I want to know the percentage decrease in maximum torque that the set screw and/or driver will be able to withstand on the 3mm hex. I am getting a torque limiting device and I need it to be used for both set screws and I don't want it to strip the hex on the 3mm driver by being too large of a value.

If the materials and hex depth do not change (only change is 4mm to 3mm hex), what decrease do I expect to see in the 3mm hex?

Thanks,
sbbiomed
 

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If I understand your question correctly, you should use about 40% of the 4 mm torque when tightening the 3 mm screw.
 
How did you get 40%

How did you get to 40%?