In Part 5 of my blog series The Making of the Wall-Saver safety cylinder, I discussed the seemingly minor mistakes that delayed the creation of a “final” mold master, from which the “final” mold could be made. Having overcome those difficulties, I made two more masters from the “final” mold, in case something should happen to the mold, and to make it easier to make copies when the time came to ramp up production. Then I turned my attention to two more difficulties: the formation of the center hole, which slides onto the crane, and the creation of ridged plastic inserts that would fit the chambers.
The center hole turned out to be a problem that I made much harder than it really was. The crane’s shaft measures .295″. As I did not have a lathe, my basic idea was to cut a 5/16” (.313″) round plastic rod down on the mill, and use that inside the mold to form the center hole. I had a rotary table and a tailstock…what could go wrong? Well, a mill doesn’t cut very cleanly like that, so the resulting slightly smaller rod was bumpy and difficult to extract. I changed tactics to making a hexagon—which was smoother but still hard to extract. I got a couple of center rods that simply would not come out of their cylinders, which made them useless.
The solution, in retrospect, was obvious: just use the 5/16” rod unmodified. It was pretty smooth and polished as purchased, and with mold release applied, it comes right out (without mold release, it can still get stuck). Of course…this turned out not to work for production either, as will be discussed in later installments. Although I was able to reliably extract the center rod, I struggled with a number of issues: the difficulty of putting a hole in the center of the rod without a lathe, rods that were slightly curved, and the difficulty of making rods sit right in the center of the mold along their length so that the resulting hole was right along the axis of the cylinder. The length of the rod was also a problem at times, and a few were trapped by excess plastic just as the chamber inserts had been.
The ridged inserts were made in a multi-step process. First a tiny amount of silicone was poured into the chambers of the “master” castings made from the final mold. Then those castings were trimmed to the approximate size desired for the final inserts, and more silicone was poured around them to make an insert mold. Finally, polyurethane was poured into the insert mold to make insert blanks, which could then be finished with drilling and facing.
The result was a cylinder that was fully functional and durable, but which didn’t work as a commercial product. For one, the thin plastic created from the walls of the brass was brittle, and often broke under the strain of inserting the ridged inserts. Functional, but ugly. Additionally, the chambers were different lengths. Partly, that’s because silicone is flexible and can be stretched or compressed (which also alters the diameter of the chambers–huge issue for inserts!) but mostly, it’s because the master was made with the hard plastic chambers, which did not sit uniformly in the old mold. So the master was uneven, which meant the mold made from it was uneven, which guaranteed uneven parts.
Most critically, I discovered that the cylinder slipped backwards off the crane when opened. On a J-frame, the cylinder is retained on the crane by a small shelf on the frame. The latest version of the plastic cylinder was too small in diameter because there is about 1% shrinkage of the plastic as it cures. That doesn’t sound like much, but for a cylinder with a diameter of 1.3 inches, that’s 0.013” shrinkage PER GENERATION. Since the cylinder I was using was a casting of a casting, it was at least .025” smaller in diameter than the original. When was the last time you heard of gun parts with tolerances that sloppy? It’s a deal breaker.
Solving these problems required a pile of work. But surprisingly, a much, much smaller pile than I had anticipated.
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