A Great Question About Center Holes

A clock builder asks:  "Should I be worried about this being too far off?  When I sand this wheel on a mandrel some of the printed lines for the teeth will fully disappear while others will be just showing.  Thanks."
Center Wheel Hole Off--Problem?

Clayton answers:  Euuuuuuw!  I really hate seeing that.  Did someone just kick me in the stomach?

These mechanisms are extremely forgiving of front to back wobble, as in the ply being warped.  As shown in the picture below, as long as the wheel can stay on the pinion the mechanism can be made to work.

Click on picture to enlarge.  Front to Back Wobble

But these mechanisms are extremely UNforgiving of up and down wobble, as in having the center hole for the arbor off center.  Those PD lines really must align perfectly all the way around the wheel.  As much as that hole is off from center one way, it is off double that amount on the other side.

The center hole is probably THE most important hole in the entire clockmaking project.  If, for example, a frame hole is off a bit, the PD's are still aligned perfectly all around.  The teeth are simply just a bit further apart ~  But if the center hole of the wheel is off, you've created a cam.  On the down side the cam will have too much clearance, and on the up side the cam will jam.

Click on picture to enlarge.  Center Hole Drilled Correctly

Click on picture to enlarge.  Center Hole Drilled Incorrectly

Now, that being said, yes, you can, with a lot of work, probably save that wheel.  You'll need to depth it, and in the process, you'll need to deepen the dedendums of both the wheel and pinion, and the addendums will already have been shortened by the process of spinning the wheel and sanding it perfectly round at the sander.  You may also need to remove some of the BACK side of each tooth in the area of the out of center hole of the wheel has caused the PD's cross each other.  That is because the tooth gets wider at its bottom.  The back side of the tooth is the non-contact side.  You'll need to look at your mechanism and see which way the wheel is traveling and determine which side is the non-contact side, and remove some "meat" from that side.  (as I mention in my book, the back of each tooth can be nearly ANY shape because clock gears only turn in one direction.)  This will, of course, show as asymmetry in the overall wheel's appearance unless ALL of the backs of the teeth are treated the same way.

The center hole in the picture you sent looks terribly off, but if in reality it is not really off by that much, give the wheel a try, and spend some time practicing your "depthing" technique.  You may even eventually qualify for your honorary Doctor Of Dentistry degree from all the additional tooth work.


Aloha, Clayton