The second wheel (that driven by the mainwheel) has 80 teeth, and is made from 1/16" leaded brass. Since I didn't have an 80-tooth division plate handy, I decided to make one. I used the 120-hole division plate already made for the mainwheel, and simply added an inner ring with 80 holes, using the Sherline rotary table to make them.
I've made a stud which permits screwing the division plate directly to the rotary table for drilling. The rotary table has a center hole tapped with 3/8" coarse threads, with a shoulder. The stud screws into these threads, and it has an upper part made 1/4" in diameter, with fine threads on the end. Between the division plate and the upper surface of the rotary table, I set a piece of 1/4" masonite, to protect the table from being drilled.
Then, I made a special modified 1/16" drill to drill the holes in the division plate. An ordinary 1/16" drill is far too flexible to start and drill holes accurately -- it will always skate to one side. If the drill is short enough, however, it will be sufficiently stiff.
To make a short but effective drill, I first used the 1/16" bit to drill a lengthwise hole in a piece of 3/16" brass in the lathe. I left the hole blind, that is, I stopped short of drilling all the way through the 3/16" brass rod. Then, using a Dremel cutoff wheel, I cut off the 1/16" bit, making it just long enough to extend 1/4" out of the hole when the stub is inserted in the 3/16" brass rod. The drill stub bottoms out in the blind hole in the 3/16" brass rod. The drill is secured in the hole with Loctite. A tap with the hammer against the side of the rod also makes the drill secure in its new holder. The drill is inserted into a 3/16" collet in the mill spindle.
The rotary table is indexed 4.5 degrees to bore each hole. (360 degrees divided by 80 teeth = 4.5 degrees per tooth.) To keep track of the indexing, I use a scientific calculator. Most of these have the ability to store a constant, which is 4.5 in this case. You simple enter 4.5 and press the + sign twice. Then, each time you press the = sign, the constant is added to a running total. First the calculator shows 4.5, then 9, then 13.5, and so on. This is the ONLY way I know to avoid indexing mistakes. Go slowly, and check twice before you drill. For cutting gears, I use dividing plates for indexing rather than direct indexing with the rotary table, because it's easier to correct mistakes made on a dividing plate than on a miscut gear.
The gear blanks (I made four 80-toothed gears) are slipped onto an arbor, turned to size between centers in the lathe, and inserted into the indexing fixture, as already described. The runout here is less than a thousandth of an inch.
Before cutting the gears, I secure the mill's vertical axis with a piece of masking tape wrapped around the handle. Before securing it, however, be sure that the last adjustment of the axis is upward. This removes the backlash and prevents the axis from sagging while cutting. The Taig mill has a vertical axis lock, but it tends to displace the axis when tightened.
I also always mark the limits of the outer and innermost travel of the axis which will be cranked back and forth while cutting. If you don't mark the limits, it's easy to ruin a gear by indexing when the gear is still with reach of the cutter.
Run fly cutters at the mill's top speed, about 5500 rpm on the Taig. The sound of a flycutter is indicative of how well the cut is being made. It should make a buzz, not too loud. A loud hammering sound means a dull cutter.
There will be a small burr on the back side of the wheel where the cutter passes through. This burr is normal, and should easily break off. If there is a nasty burr, then the cutter is dull. Unleaded brass will also have a nasty burr, even if the cutter is sharp.
Todays' links:
Rotary table stud
Rotary table with stud
Drill bit modification
Drilling the division plate
Arbor with gears
Cutting teeth
Vertical axis taped
Moving axis limit marks
Four gears