Tuesday, July 28, 2015

Life at module 0.9

In the last episode, we nade some lantern pinions. Measurement revealed that they were actually module 0.9. So this is really a  blessing. Since I am making my own cutters, the larger module will be easier to work with. So I am now embarking on the process of cutting the wheels. The first problem is to make the cutter.  I use Dremel 3mm tool shanks from expended Dremel tool cutters. Cheap, and they are good steel. they can be hardened and tempered. I am making a fly cutter, a one-point cutting tool.

Now a fly cutter does not actually cut teeth. What it does is cut the space between teeth. A clock tooth is supposed to have a cycloidal profile. This is the profile generated by a circle rolling on another circle. Yuk. However, this is approximated by a straight cut with a "rounded  over" circular radius at the tip. The radius is something like 1.7 mm at module 0.9.

So as a first task I made a button gauge.

I turned down a piece of steel to the proper radius, say 1.7 mm. I am too tired to go consult my notes in the shop. I drilled two holes the proper distance apart. This was done on the mill, you could never hit it by eye. The button gauge will be used see if I am on target with the radius. There is the problem of depth of cut, but if I overdo this I can always grind it off. Off to the mill.

Here we have an expended Dremel shaft put into a homemade fixture, a piece of square stock with a setscrew to hold it in place. The fixture is clamped in the mill vise.

I have available 3mm, 2mm, and 1mm. end mills. These are diameters. Hmm. If I were to cut 1.7 radius I would need a 3.4 mm cutter. Unicorn. Uncomfortable. But the 3mm guy will go 1.5 mm aand for now that will do. It is quite difficult to center up the cutter. But above you see it taking shape. So I did this. Now we heat it up red hot and quench. This will harden the steel.

I use my handy furnace and water-quench, and then temper, a difficult job on a piece smaller than your little fingernail.

Having done this, we take a test cut on a leftover blank we happen to have. The diameter is completly off, we just want to see if the cutter works at all.

So I mount this random blank on the dividing head and cut a few teeth. The diameter is wacky. But it does work -- i.e. it cuts teeth. Spacing all wrong of course.

Next step is to turn up a proper blank on the lathe. I cut them out on the bandsaw. The scrollsaw would be better, but it melts the plastic so the bandsaw wins.

Now we can cut teeth properly. I did a whole bunch of them. There are so many errors you can make. You can forget to tighten the dividing head, for instance. This will chew the blank. You can forget to loosen the dividing head, which will mean slippage in the gear train. Maybe I should loctite the worm. But I don't want to do this yet.

Anyway, at the end of several days work,  I came up with some wheels.

The leftmost wheel is complete chowder, as Tom Lipton would say. As we go left to right, we see gradual improvement, as I correct my mistakes, so the rightmost wheel is almost usable. But there are two problems. The tooth profile is off. Also the spacing is irregular. The tooth widths vary. This is a problem with my homemade dividing plate. In the next episode we del with these problems.

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