I'm looking at a pretty simple item: a small bolt with a loose, captive washer. Then I start thinking, exactly how did they do that?
Either 1) They form (crimp? stamp?) the washer after it was put in past the threads, so it changes the hole diameter and does not come off. or 2) The bolt is cooled with liquid nitrogen and the washer is heated... then the washer is put in... when temperature is back to normal the washer can not be removed That's the way I would do it Alex
I'm thinking they weld the bolt head on somehow. The washer hole does not look crimped in any way, a neat cut, and pretty close tolerance.
They would NOT weld a bolt head on. Way too much time and effort for something that should otherwise just be formed and be done with. I agree that the washer was "crimped" after placed over the bolt. But the crimping was done under extreme and symmetric pressures.
They could also put the washer on a piece of rod then heat it and stamp into a mould to make the hex head. I believe I've heard that called "hot forging" before somewhere in some documentary...
Good question! That there is a SEMS fastener and to grok how they make it you have to know there are two different ways to form threads on a screw. The most familiar way is to start with a shank matching or exceeding the final thread major diameter, and twist a sharp die over it that cuts the threads and makes lots of metal chips out of the removed material. The other way to form threads is called rolling, and it has some advantages if the material is suitable. For example, it doesn't remove material or create chips, leaves a better surface finish and higher-tensile-strength, cold-worked threads. It also starts with a blank that's smaller than the final thread major diameter, and the threads get squished out into the final shape. ... which allows you to stick a washer on the blank first, and have it be captive after the threads are rolled. -Chap
"SEMS" was developed by ITW (Illinois Tool Works) in the 1930's and patented by them. The patent has long expired. "SEMS" means assembled. I have made the equivalent by drilling out stainless steel fender washers with a number 8 drill on a lathe and tapped them with an M6x1.0 tap using the tail stock. With M6x1.0x20 stainless steel Philips head machine screws, it works out really well. You could also use 10mm hex head screws o the same thread pitch to replace the OEM ones. I've done that too.
So you've got a fender washer with internal threads, you can screw it onto the bolt? And once it gets to the shoulder of the bolt it's loose, able to shift around?
The concept is a little different, but the result I the same. The washer contacts the surface first. The bolt then runs down so, when the bottom of the head contacts the washer, the bolt and the washer spins together. Only a little extra torque due to the spinning of the fender washer, is needed to achieve sufficient tightening. The assembly comes off in one unit, when loosening relieves frictional contact between the washer and the surface of the part being held. For additional information; note that washers are "die cut." On that account, there is a smooth edge an a sharp edge. If you are fastening soft and flexible material, make sure that the smooth rounded edge contacts the material, as the sharp edge is more likely to cut and tear it.
Thanks Mike. Yeah, I've noticed that about washers. Fabricators would call them "punched". Sort of related: when detailers calculated minimum edge distance for holes in plates, if the plate edge was sheared, they add a quarter inch to the allowance, considering the plate edge compromised by the shear action.
