Making of large gears from aluminum sheet material
For the drive of my split ring mount and the Dobson telescope I used large toothed wheels driven by a small worm. These components are very expensive. So I had to find a way to make these wheels by myself. The procedure presented allows to make those wheels from 5 mm aluminum sheet material.
Cutting the teeth with a threaded rod
This is my gear cutting assembly. The aluminum wheel is supported by
three ball bearings and has fourth bearing as a central pivot point. The
threaded rod cuts the teeth into it. The rod has a slit along its axis
so that its threads become teeth which cut into the aluminum wheel.
Detail view of the cutting tool
The M8 threaded rod is held by two ball bearings (8mm inner, 22mm outer diameter). The bearings are in 22 mm borings in aluminum sheets, which are fixed on a piece of wood.
Notes on the procedure
First I cut a circular disk from the aluminum sheet (jigsaw, blade for metal cutting).
Then a pivot point is mounted at the disk's center.
The disk is rotated along a stationary grinder until it is an exact, smooth circle.
Therefor I removed the stone from the grinder. Instead I attached a semi-flexibe grinding wheel with sand paper (grain size 80, finish with 320).
From the first cut (without a slit in the threaded rod !) fine marks will be generated at the circumference of the wheel. If the marks after one turn don't meet the first
mark, the marks must be ground away and the procedure must be repeated.
When the marks fit around the wheel one more turn with more pressure can be made to deepen the marks. Then the slit is cut int the rod. A small angle grinder is sufficient for this task. I used a dremel tool to make clean teeth on the rod.
In the picture the boring machine has to rotate left so the wheel will not be lifted from its support. If left-hand motion is not possible the assembly can be built from the other side. One of the three supporting wheels under the aluminum sheet should be near the cutting rod to prevent the disk from springing.
The boring machine should run slowly (5-10 turns per second) because it might be overloaded if running fast (I tried and could smell it !). The rod was firmly pressed onto the disk but the whole procedure took 1-2 hours. A tapper which is harder and precisely made should do the work faster. But a tapper is to short to fix them near a big wheel (80cm diameter for my split ring mount).