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Sunday, August 7, 2011

Gear Runout and my Wood Clock

I am still working on the clock movement (or gear train) for my wood clock that I wrote about a couple of weeks ago and the gear runout is a slight problem. This isn't turning out to be one of those projects that I can just "cut and glue" and be finished, it's requiring a lot of adjustments and finesse. Getting the hole in the center of the gear (or runout) has turned out to be a little more trouble than I thought... So I used a dial indicator and a couple of C-Clamps to check and adjust the runout by drilling an undersized hole for the shaft then filing it appropriately until the gears turned without much indicator movement. If you have not read the previous posts about the clock HERE and HERE are the first two. Check them out so you can see what I am trying to build.

Checking Gear Runout on a Clock Gear
In the above picture you can see what I am talking about. On the right hand side of the picture there is a dial indicator sitting on a base that is C-Clamped to the bench. To the left is a piece of scrap wood with a hole in it that the brass tube fits snugly in. So with this setup I can spin the gear and place the dial indicator tip on the gear tooth crests and valleys to see how centered the hole is relative to the gear teeth, that's runout.
In order to get the the point where I could check the gear with the 'real' brass tube shaft the I am going to use in the clock I had to get the hole made to the right size and in the right place. To do that I started out by drilling a small hole in what should be the center of the gear and I put a straight finishing nail in it. The scrap wood under the gear in the picture above has a hole in it that the finishing nail fits snugly into and with that I could initially check the runout by spinning the gear on the finishing nail. Out of all the gears I made all except one had the finishing nail hole a bit off center, by about 0.03". To remedy that I drilled another hole again, slightly undersized to the brass tube shaft, and compensated for the runout that I saw with the finishing nail hole. By compensated I mean that I moved the drill off the finishing nail hole just a bit. This hole is also undersized a bit from what the final hole will be and I have a shaft that will fit in this undersized hole just like the finishing nails did. Again I can check the runout of the gear with this slightly smaller hole and a slightly smaller shaft and if my "compensation" drilling isn't quite right I can make up for it by enlarging this hole to the final size with a small rat-tail file.

I went through this with all the gears (except one) until I had all the holes in the gears the right size and within about 0.01" in total runout across the teeth. Lots of detailed work but I think it going to be worth it if I want this clock to run smooth. The one gear that I didn't have to do this with I got right on the first try.
Checking Runout on a Wood Clock Gear

After getting all the holes in the gears centered and trued up I mounted the hollow brass tubes into the gears. To mount the tubes I roughed up the brass tube a bit with a file and used the Loctite Super Glue Gel that I have mentioned before. That stuff works great for things like this because it's super glue and unlike regular super glue which is watery and runny this stuff is in a gel form so it doesn't go all over the place.
All of the moving parts are mounted to brass tubes that have brass shafts inside them acting like bushings. Ultimately this probably isn't the best way to go with brass on brass but there is very little force in this gear train and I'll add a small amount of lubricant once I do the final assembly.
Wood Clock Movement Assembled For Fit Check
I put everything together to check the fit. The wood posts in the picture above are glued to the lower piece of wood and there are snug clearance holes in the top piece. If this looks a little confusing click HERE to see a picture of the CAD layout that I did. There is also a video of the CAD model with all the gears running HERE if it still doesn't make sense. Everything seems to fit together pretty well and the gears turn smooth. If you have any questions send me an email at or leave a comment.
Wood Clock Movement Detail
I have a little bit of sanding that I need to do on the gears and cleanup on the flat wood pieces. In the two pictures above you can see the back of the clock face that needs to have holes drilled into it for the numbers to pivot. I'm planning on using nylon shoulder bushings in the clock face holes for the numbers to pivot and I'll write more about that when I get it done. I will also write more details about the concentric tubes and how the hand and face are going to move.
So far this project is coming alone pretty well and I think it's going to work! Time will tell though...

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