In machining, you drill holes and press in a spring pin, only to have the pin fall out because hole size is an afterthought. You push the pin in and it seems like it’s going into place; but then later it come out. It has to be able to move a little to compress, but if there is too much space, it slips out.
This chart plots out the give-and-take between those two forces (for both imperial and metric). So next time, you’ll know what drill bit to use to secure the piece in place with pressure.
How to Use Spring Pins Correctly
Class H13 is basic tolerancing rule (tolerance for hole). So it requires the hole to be drilled ever so slightly larger than nominal pin diameter. That sounds backwards to someone who’s accustomed to snug fits, but that’s how spring pins operate: through elastic deformation. By having just a hair of play in there, they can fits down inside the bore, then stretch outwards as they rub up against sides. If you drill your hole exactly on pin size, chances are good you’ll either crack your workpiece or strip the pin when installing it. Select a drill size from the chart within the acceptable range. The pin should stay in place without popping loose, but it shouldn’t require too much force to press into place.
The depth is where things go wrong with most amateur endeavors. The minimum length of the hole should of be half the diameter of the pin (the part that goes in) plus the nominal length of the pin. This gives the pin enough room so it do not bottom out on the sharp corner of the drill point. This ensures the pin can expand fully while assembly remains tight. It will also give it some room when you want to take it apart later if necessary. It gives the spring something to breathe at the rear-end.
The other silent player regarding retention is surface finish. Too smooth and the pin simply slides out on vibration. Too rough and it galls or seizes. Typically, just a regular drill finish work best. There should be enough roughness to grab the metal to metal, but not so much that it creates those little ridges where the pin’s surface get torn up as it goes back in.
Where there’s a shaft with cross-holes drilled through it, alignment can be tricky. A curved surface causes drills to wander, creating oval holes that compromise the whole joint. Center punching the entry point, along with using a V-block, forces the drill bit to begin exactly as intended. Although small, this step realy matters in terms of how centered the hole are.
Also think about your access needs during assembly. If you want to remove them easy from both sides, use a through-hole. If not, use a blind hole. This is great for things like maintenance-heavy applications, because you won’t have to damage whatever is inside the cavity.
Lastly, position the slot properly. The slotted spring pin has a weak spot where the long groove runs through it. By turning the pin 90 degrees so that the solid wall faces the load direction, you can significantly improve shear strength. It’s a small detail when installing but it makes all the difference between catastrophic failure down the road.
Set the hole correctly from the start and follow the recommended depth limits. Then stand back and let the pin do the task it was designed for: compressing, gripping, and holding. Your assemblies will remain intact and you’ll never have to look for lost pins at the bottom of your machine shop floor again.
