I'm in the middle of a build and need to thread a tang, my question is simple by may not have a simple answer. what's the best thread form to use when both sides of the thread are ground off so the guide will fit on, is bigger better? like 3/8... or smaller like 1/4? course thread or fine thread. 3/8 - 16 3/8 - 24 1/4 - 20 1/4 - 28 I know fine thread have more clamping pressure per turn but is it a trade off in possible stripping threads? thanks for thoughts / incite.
That's a good question. One for you. How thick is the tang at the threaded part? If you cut the threaded parts before you heat treat you could make the them part pretty durable. Based on my limited mechanical engineering skills, I would guesstimate two times the thickness of the tang at the threaded part. Say 1/8" tang with a 1/4" - 28 die. This would allow about 33% of the thread area engaged. This is an approximation based on (2 * 0.125")/(0.25" * 3.14) = 0.32. Also you need to take into account the length of the engaged threaded area. As a rule of thumb I use "equal to the diameter." Meaning for a 1/4" bolt you want at least 1/4" (length) of threads engaged. This is why nuts are about the same thickness as the diameter of the bolt. 1/4" of 1/4" thread with 33% engagement can handle a terrific amount of load. Fastenal has a little write up here: https://www.fastenal.com/en/78/screw-thread-design The largest I have done is 10-32 as I use thinner stock mostly 3/32". The force required to squeeze everything together isn't huge if the components are well fitted. The way I make them with epoxy between the components means it wasn't meant to come apart ever. Lots of force also means squeezing lots of epoxy out from between the components and I like to leave a little epoxy in there. If you are making a knife which is meant to be disassembled, then epoxy won't be part of the equation. Later I found a way to add a modified machine screw to the tang after slipping the guard/bolster on. This lets me fit the bolster to the shoulder then attach the screw, then add the arse-end components. With this method I add can be a full round screw that has a larger in diameter than the the thickness of the tang.
The tang is close to 1/8 in that area, i can certainly add more nut length for thread engagement as the nut will be inside the handle. i'll be welding a nut on the Damascus butt and and hiding it in the handle so i'l good there, i can add a longer custom nut. Just have to decide what size to shoot for. probably just go with 1/4 course as it's common and got the gear.
