The Hendey quick change is two parts, the primary, which is 3 speeds, and the secondary, which is 12 speeds. The ratios in the primary are 4:1, 1:1, and 1:4. Each change is a factor of 4. The input shaft turns at the same speed as the spindle through the two 48 tooth gears. So, if I install a 25 tooth gear on top and a 100 tooth gear below, I will slow the input shaft by a factor of 4. Then position 2 becomes 1, and position 3 becomes 2.
The first Question: Do I REALLY need from 1 to 3 1/2 threads. Very unlikely.
Out with the old,
Keep the old gears in case you ever need to cut from 1 to 3 1/2 thread. They're easy to swap out.
And in with the new
I cut the replacement gears on my turret mill. They are 12 DP, 25 and 100 tooth. Not wanting the expense of buying steel or cast iron discs, I used brass for the 25 tooth, and graphite impregnated nylon for the bottom one.
A previously unknown benefit to the nylon gear is the loss of the flywheel effect when I brake the spindle. No more Wham! as the dog clutch spins around.
Note the additional clearance cut from the primary box for the new gear.
