Today I began making the short train of gears which drives the hour hand. There are four gears -- two wheels and two pinions. The pinions have 12 and 15 teeth, and the wheels have 45 and 48 teeth. All are made with involute tooth-form commercial wheel cutters in a 48 diametrical pitch size. The cutters are mounted in a homemade arbor which fits in the 1/4" collet in the Sherline mill.

The cannon pinion has 15 teeth and drives the minute wheel, which has 45 teeth, for a ratio of three. The minute pinion has 12 teeth, and drives the hour wheel, which has 48 teeth, for a ratio of four. Taken together, they make a 1:12, ratio which is what you want between a minute hand and its hour hand.

The blanks were made from 1/16" leaded brass, sawed into 1.125 squares with a jewelers saw. The centers were marked using a square, and drilled undersize 3/16" and then reamed with a 3/16" reamer. The blank is held on the wooden drill pad with a toggle-snap clamp used in jig tools. The holes are deburred with a counter sink. To avoid leaving chatter marks in the wheel holes, this deburring job I always do by hand.

The wheels were cut using the Sherline rotary table mounted vertically on a bracket which Sherline sells for this purpose. The other end of the cutting arbor is supported with a special tailstock, also sold by Sherline for this purpose.

This was the first time I've used this setup, so it required making a new arbor. The arbor is a piece of 1/2" cold-rolled steel which is trued and reduced to 3/16" on one end, for a distance of about 0.75". The 3/16" end is threaded with 10-32 threads, and a packing washer is made. One end is left in it's 1/2" size because this pemits using the tailstock to force the arbor against the back of the Unimat lathe chuck when the wheel blanks are trued, and then and against the back of the Sherline chuck in the rotary table when the teeth are cut. Otherwise, if the chuck jaws are all that's holding the arbor, it will slip backwards as it is used.

Since it's just as easy to make several wheels as it is to make just one, I made extra sets for use in later clocks. To cut the 45-toothed wheels you index the rotary table 8 degrees, and to cut the 48-tooth wheels, you index 7.5 degrees.

Next time I will make the pinions using the same rig.

Today's links:

Train drawing.
Multitooth cutters.
Multitooth arbor.
Marking blanks.
Wheel blanks and hole deburr.
Toggle snap clip.
New arbor.
Arbor loaded.
Cutting setup.
Wheels cut.