In Part 13 of my series on the Making of the Wall-Saver safety cylinder I discussed what I thought was the final process for manufacture and assembly—and a serious problem. Less than 10% of plastic inserts were coming back out because of spring pressure—but that meant rejection of about 40% of cylinders, and the fear that the others might fail at some point in the future.
The day after I discovered this, I hit Ace hardware and picked up some little cylindrical nylon spacers meant for use with machine screws. They cost a whopping $0.30 each (the people at that Ace know me—I wonder if they make snide remarks about the guy who puts $2.00 worth of goofy random stuff on his business credit card twice a week?).
Nylon is a great material, in that it can be both hard and resilient. I picked up spacers about ½” in diameter, which is substantially larger than the .420” of the widest point in the chambers. I used the mill and rotary table to bore them out, cut them down to the right diameters (main body and retention shelf) then to put a bevel on the wide part to facilitate insertion. Then I added a circular cutout between the center hole and the outer rim to promote flex of the retention shelf upon insertion. I made two different examples with different thicknesses so I could see how they worked.
Both worked very well. They slip in without the need to use a vise and have no trouble popping back into shape and holding against spring pressure.
The major downside is that nylon can only be injection molded, not cast, and such molds are very expensive. That means that each and every nylon insert must be machined from a solid piece of plastic—a process that is even more troublesome than machining brass. Fortunately, raw material is very cheap, and a program to automate the machining was simple to create. So while it slows things down, it improves quality and greatly reduces the QC reject rate. I liked the results so much I sent one of the new cylinders to everyone who had one of the old ones.
I did think to try the machining process on the old polyurethane inserts, both because I had a big pile of them I’d like to use, and because using them going forward could substantially reduce the necessary machining time. But it was a complete failure, because the polyurethane is so hard and brittle, instead of compressing and springing back, it just broke. So nylon it is.
When the day comes that cylinders are popping out of injection molds 4 times a minute, the nylon inserts will be just another cavity in that mold and all this extra CNC mill time will be just a memory. Until then, that’s how the cylinders you order are made!