Wooden gear cutting jig
There's nothing that symbolizes machinery like gears. Seeing that I like to make machines out of wood, it follows then that I should want some of these machines to have gears on them. Wooden gears, of course.I have cut numerous wooden gears on my table saw for various machines, such as my Marble machine 2. One of the biggest problems with this is precisely measuring the angle between the teeth, and rotating the gear by exactly that much between cuts on the table saw. With the gear jigged up on a shaft to hold it securely, it isn't possible to attach a protractor to measure accurately. And even when using a protractor, there is always the problem that the protractor has to be moved (seeing that it doesn't span 360 degrees), and the possibility of getting some slightly odd teeth intervals as a result.
To aid in rotating the
gear by a precise amount for the purpose of cutting teeth, I decided to build
some kind of mechanism for rotating the jigged up gear. The idea being to
have something to roughly take the role of a machinist's dividing plate, but
much smaller, lighter, and less precise.
Rummaging through my junk, I was fortunate enough to find a remote servo (for controlling valves and such), of roughly 1940's technology - (or 1960's technology in American). The box consisted of a small split phase motor, 5 stages of reduction gears to a big gear on a half inch shaft, a rheostat, and some sort of mechanical balance contraption to function a bit like a bridge circuit to compare the rheostat in the box to some master rheostat somewhere else. When unbalanced, the balance would connect either phase of the split phase motor to AC power, causing it to rotate in either direction.
I removed the motor and the the gears closest to it, leaving me with two gears of reduction. The last shaft I mounted backwards, so that it protruded out through a hole I drilled in the aluminium plate, and attached a knurled knob to it for fine adjustment. I then needed some means of measuring rotations of the knob. I originally thought of using a counter from a tape recorder, but found an old set of mechanical car odometer wheels in my parts collection. The gear attached to the last wheel was exactly the right pitch to match with those in my gearbox. Unfortunately, the first (0.1 km increments) wheel of the odometer was designed to jam up on running backwards. The rest of the wheels didn't have this property. So I moved the 0.1 km increments wheel to the other end, and had the gear drive the 1 km increment wheel directly. With the various gears, I got a 1:39.12 gear ratio.
The gear cutting jig in action on the table saw.
I then made a sturdy wooden box to attach to the aluminum frame of the gearbox, and keep my gears free from dust. The way I put the jig on a sliding piece on the table saw, the sawdust tends to get thrown directly at the gearbox, so sealing out the dust is crucial. I even made a little window for the counter, so no dust can get in that way either.
The whole mechanism runs really smoothly. A light twist to the output shaft will set the odometer counter spinning. This turned out to be a bit of a problem when cutting gears, because I always apply a slight amount of torque to the output shaft while cutting to avoid random errors from gear backlash. But with the mechanism turning so easily, I need to add a brake to the input knob so that it doesn't start spinning from this slight torque.
A gear ratio of 1:36 would of course have made the counter increments 1 degree, but I was already lucky enough to have a gearbox as suitable for the job as it was. Besides, most gears I make don't have a number of teeth that would divide nicely from 360 anyway. As it is, I have 391.2 counter increments per revolution. When I cut the gears, I calculate how many increments I need to advance per tooth, and then use the constant calculation feature of a calculator to generate the numbers one at a time, and dial them on my counter. 391.2 increments is unfortunately too few to just round to the nearest, so I have to visually interpolate fractional increments to get decently accurate gear spacing. For most gears, I have to make two cuts per tooth, so I have to dial in two angles per tooth.
Two 22 tooth gears, made while testing the jig. |
Gears with inset teeth |
Naturally, cutting the gears straight out of plywood is not ideal, because the wood grain is not aligned with the teeth. This makes for weaker teeth, with the wood grain not aligned in the direction that the teeth rub against each other. Better gears can be made by inserting individual teeth into a plywood wheel. Unfortunately, this means the teeth have to be fairly narrow, as to leave enough wood to hold them. At left is a couple of gears I made this way. The small gear only has 14 teeth, and it runs fairly noisily. This can of course be desirable for drawing attention, as in, as in "look, it has wooden gears!" |
Stepper motor and gear reduction
This works pretty good hooked up to the printer port, but so far hasn't been worth while developing further. There is always the problem that I would have to make an awful lot of gears to ever save the time I'm spending on this. I guess its more of a gears for gears sake sort of thing.
A better solution to this problem though is to just have a paper gear template of a gear glued to the piece of wood, which makes it easier to set the angles. With care, this should be more than accurate enough.
I did however end up using a computer and stepper motor in my workshop for my Computer controlled table saw jig. So I have the computer and electronics, just haven't had a need to make gears since.
Related pages:
To my Woodworking index, or go to Woodwork tools & techniques index
