Today I made the little train of gears which drive the hour hand. I also made the hand clutch, which is part of the same mechanism. The minute hand fits on a shaft which is not directly driven by the center arbor, but is indirectly driven through a tension spring. This clutch allows one to turn the minute hand to set the clock without moving the clock's gear train.

The motion work consists of two brass pinions and two brass wheels. One pinion has 15 teeth and drives a wheel of 45, while the other has 12 teeth and drives a wheel of 48. The result is a gear ratio of 1:12, which is the ratio that the minute hand turns vis-a-vis the hour hand. The gears are made with a DP 48 involute cutter -- the only involute gears on my clocks.

The pinion stock and wheels were left from a previous project -- I make several sets of wheels at one time to save work. Gear-cutting details can be found in my crown-wheel clock project documentation.

For installation on arbors for this clock, the pinions and wheels had to be machined. For machining, I used the time-honored method of "turning in a box" -- a block of wood is mounted to the lathe face plate, and a pocket or "box" is bored in the wood to the outside diameter of the part to be machined. The part is held in the box by a friction fit to its teeth.

First, I made the minute pipe assembly. It's made from 3/16" brass which has a hole bored through for the center arbor. The minute pipe has a shoulder cut on one end which holds the cannon pinion and a spring disk.

Next, I made the cannon pinion from a piece cut from the 15-tooth pinion stock. I used a boring tool to cut a "box" in the face plate wood and slipped the pinion into it. The fit must be close enough to hold the pinion securely but not too tight either, which makes removing it a problem, and can also cause it to turn out-of-center.

I trued the ends of the pinion and bored a hole through it which fits over the shoulder on the minute pipe. I also fit a little disk of brass which is also left from a previous project. This disk will serve as an area for the hand clutch spring to press against. The assembly is rivetted together by expanding the end of the minute pipe next to the little disk.

The minute wheel consists of the 12-tooth pinion and the 45-tooth wheel. I sawed the pinion from the mother stock and inserted it into a 1/4" collet in the lathe for truing and drilling. I have a true 1/4" collet, so I didn't use the "box" method here.

I cut a shoulder on the pinion stock for the 45-tooth wheel. I enlarged the hole in the wheel center in a a box. I rivetted the pinion onto the wheel, expanding the end of the pinion to hold the wheel in place. A cutting broach will clear any material expanded into the through hole.

I also made a post for the minute wheel from 1/2" brass. The post has a flange with 3 mounting holes made in the mill as shown many times previously. The outer end has a hole drilled and tapped for a 0-80 screw, which secures a washer which keeps the hour pipe from sliding off.

The hour wheel assembly consists of a thin hour pipe with the 36-tooth wheel affixed to one end. First, I drilled a hole through the pipe material which is the size of the outside of the cannon pinion arbor, since that must slip through. Then I thinned the tube by chucking one end and supporting the other end with a live center. At one end, I left a thicker section -- the hour wheel is rivetted on this flange.

The clutch spring is made from .005" blue steel suspension spring stock. It's just an oblong of steel bent, and with a hole punched through it. I punched the hole by using a hard steel punch and a block of lead which acts as a die. A sharp rap, and you've got a clean hole through the hard steel.

I removed the center arbor from its bridge on the front plate to cut a goove in it, which will hold an E-clip. The clip will support the inside of the clutch spring.

I returned the center arbor to the front plate of the clock, and layed the plate horizontally on the bench. I slipped the minute pipe onto the center arbor and set the minute wheel post assembly on the clock plate in its intended position. I moved the post assembly little-by-little until depthing between the minute and hour wheels was just right. I used feel and sight -- just a little wiggle to indicate a little clearance. Then I clamped the post assembly to the front plate with a drop of super glue.

When the glue had set, I made sure the wheels meshed properly and turned freely. Had the fit been too loose or too tight, I could have broken the super glue clamp and re-adjusted everything. When sure, I drilled and threaded 0-80 mounting holes in the clocks front plate.

Now I can install the dial and make hands -- just a bit until the clock keeps time!

Today's links:

Raw wheels and pinion stock
Back of face plate, showing wood srewed on
Drilling the cannon pinion
Cutting a shoulder on minute arbor
Cannon assembly complete
Making a box
Boring minute wheel
Cutting shoulder on minute pinon
Minute wheel complete
Wheel and post
Thinning the hour pipe
Hour pipe rivetted
Punching clutch spring
Clutch spring
Motion works
All the parts
Setting the minute post
Finished

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