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scissors pivots - comparing rivets and screws


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I've tried a variety of pivots, both rivets and screws. My first pivoted scissors were bonsai-type, so I used a rivet. Typically these scissors have conical copper or brass washers on the rivet. I initially thought the cone shape was acting like a belleville washer to apply some spring force. What I now understand is that the cone simply lifts the head of the rivet off the scissors blade so peening the head doesn't expand the rivet in the hole in the blade. The rivet is also an axle, so it needs to fit nicely in its hole. Most scissors don't use any spring force to hold the blades together. When wide open, most scissors are a little loose on the pivot. 

 

Here is a pair I made and a cross section of the rivet assembly. I pressed flat copper washers into a cone-shape. The rivet head fills the crater at the top of the cone. 

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I prefer using screws for a couple reasons. First, with a rivet I don't know what's moving when I work the scissors. I don't know if both blades spin around the axle or just one does. I don't know if the axle moves in the washers. With a screw I can lock the screw to one of the blades leaving the other blade to spin on the axle. Second, setting the rivet is not a precise process. There's no backing off if it's too tight. 
 
Here is a scissors with a screw. I used an 8-32 stainless shoulder screw to test this design, which I copied from a pair of tin snips. I filed a square hole in one blade and a matching square on the screw. The disadvantage of this design is that it requires a lock nut. Then the threads in the nut need to be peened to lock the nut to the screw. 
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While I like how the square hole works, I don't like making it. Lately I've started tapping a thread in one of the blades like a lot of factory scissors do. The screw needs to be locked to that blade by peening after adjustment. This is the screw design I'm currently using. I make it on my lathe from mild steel (I use a die for the thread). 
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The head diameter is 5/16", the shoulder is sized to slip in a 3/16" reamed hole, 8-32 thread. I'm still looking for a good way to peen the screw. I've tried different punch designs on the screw, but I'm thinking I should try to peen the threads in the blade, too. Either way I need to do it without damaging the head of the screw.
 

Which brings me to the design on the head of the screw. I won't use a screw that looks like a screw. I don't want to encourage someone to try to adjust it. I'm going to lock it so it can't move anyway, so I don't want it to look like it can. 

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