Whatâ€™s the formula to find torque required for screwing machine screw to plastic boss & also to metal insert

T=KdF T==Torque K==Nut Factor d==nominal bolt diameter F==Force Nut Factor http://machinedesign.com/article/un...ded-fastener-torque-tension-relationship-0818 I got this information from Handbook of Bolts and Bolted Joints by Bickford and Nassar. The link above has similar descriptions. You can do a more thorough web search for tables of nut factors or just calculate it yourself.

All the formulas are approximate, as there is considerable variation in materials, thread form facters, pilot hole sizes, etc. Your best bet is to get a torque wrench (borrow from any engine builder or get one on-line for ~$20 or less) and do some actual measurements. You will note that there will be considerable stick-slip exibited, so turning speed will have an effect.