The Making of Pocket-Safe Hammer Shrouds: Part 8

In Part 7 of my series on the making of Pocket-Safe hammer shrouds, I discussed the trouble that loss of the laser cutter at Club Workshop caused me, and the various cutting methods I tried to get hammer shrouds from thermoformed sheet to consumers without spending thousands and thousands of dollars.

During all these struggles with shroud cutout, I had successfully set up my CNC mill and used it to cut a mold for the grip of the Triple-O knife, and then used that mold to put a grip on a sample I had made for me in China. That sample is what you see in the “in development” pictures. As is clear from those pictures, the mill can cut quite complicated organic shapes without too much trouble.   So I looked at the mill as a possible way to cut out the shrouds.

The great difficulty of cutting hammer shrouds on the mill is workholding. How do you grip a workpiece with a kind of weird organic shape, especially as it tapers slightly from the cut surface to the back, meaning that a tight grip will have a slight tendency to push it outward towards the tool? You don’t need any kind of grip when cutting with a laser, but the forces from a rapidly-spinning endmill are considerable. I solved the problem, sort of, by using the plastic mold I had made of the back of the shroud for the hot-wire cutter as a fixture. That held the shroud snugly, and if I could clamp down on it, perhaps it would work.   I trimmed the aluminum sides for clearance from the end mill, mounted the whole thing on a flat aluminum rectangle, and clamped the rectangle in a vise.

Mill jig
A Sherline mini-vize with the modified hot wire jig clamped in it, to support shrouds while being cut on the CNC mill.

I then used a clamp strap and triangular clamp rest to put pressure on it, and ran the mill over a profile I created that I thought was close to what I needed. It took 11 tries to get the right profile, and even then, a higher-than-acceptable proportion of the results had to be discarded due to excessive thickness variation.

Kydex temp variation
A sheet of KYDEX that spent too long being heated. Hot spots got holes or excessive wrinkles, while the cold spots at upper right and lower left are tougher to see.

You may recall that I said that KYDEX ® is prone to hot and cold spots unless heated very evenly. Well, the thermoformer as designed heats evenly enough for ABS, but for KYDEX ® there were two very hot spots and two very cold spots, plus several somewhat hot and somewhat cold spots.

This variation meant there was no way to achieve a uniform thickness of all the formed parts on a single sheet. Since the mill had to hold the shrouds from the outside rather than the inside, the dimensions of the part that fit into the fixture varied quite a bit (because, while all the insides were identical, formed on identical dies, the outsides reflected die + material thickness). That variation meant that the relationship of the shroud to the cutout profile varied, which in turn meant that some shrouds had an excessively long skirt, and some had hardly any at all. Close to 20% of the early batches had to be discarded due to these issues; subsequent changes to the thermoformer have greatly reduced reject rates.

In addition to the issue of thickness variation, I had the problem of the tool pulling the workpiece out of the fixture and banging it up. I tried to solve this by clamping tighter and tighter, but in the end it still happened, and I accomplished no more than ruining the T-nut and stud that I was using on the clamp strap. In the end I switched to a spring clamp that grasped both sides of the fixture, which worked as well but was easier to use.

When it worked, it worked pretty well. The finished part was left with a “fuzz” clinging to it on either side of the cut, which I scraped off with a hobby knife. When it didn’t work, I threw out perfectly good parts that were simply unlucky enough to work loose and get chewed up. A smaller-diameter cutter helped a little, but it broke shortly after entering service due to a sharp corner (causing stress risers where the cutting tool met the shank).

This was the process that produced the shrouds that are available today. As promised, it was labor-intensive but relatively capital-light (the mill was expensive, but less than paying a commercial laser cutter for just setup, never mind per-unit!). When sales volume justifies it, the milling process will be replaced with a proper laser cutter, which may also find a use laser-marking the Triple-O and any other products that come out later.


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