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Sunday, July 17, 2011

Homemade Wood Clock Design

Awhile ago I was looking at Roman numeral clocks and wondering why there is no standard way of putting the numbers on the face, specifically the number IV being 'upside down' on some clocks and not on others. I wrote about that HERE and since then have been thinking about making my own clock. Well now that I have had some time to think about it I have decided to go ahead and make a wood clock. This post is the first in what I expect to be several detailing how my wood clock design project goes. I have never built a wood clock and have never made gears from wood so this whole project is an experiment and learning experience for me (and my young son). Designing the clock movement and the gears from wood seems like a fun project!
Because of the Roman numeral clocks that I have seen with the IV sometimes upside down I thought that it would be neat if all the numbers were right side up. Having all the numbers right side up isn't too interesting but I also was thinking that the hour hand should always be pointing up too. After all the hour is probably the most important thing that you want to know first. So to have the hour hand always pointing up and all the numbers always right side up, would mean that the face of the clock would have to turn and all the numbers would as well. The minute hand of course would be rotating around but because the clock face is rotating too the ratio of the hand to the face would be different that a conventional clock. To get all this to work I decided to do a CAD model in SolidWorks of the clock movement (see 'click here to read more' link below) and do some research on making gears out of wood. While doing some web surfing research I ran across a really neat Woodworking for engineers site that I have linked to in the "sites I like" where I found a neat on line tool for generating gear templates. Below is a picture of a couple of the first gears that I made for my clock using the on line generator. they came out nice and seem to spin pretty well. It didn't take too long to cut them either roughing them out on a band saw and finishing the teeth with the jig saw. 
My First Attempt at Wood Gears for My Clock
I have designed several gear trains professionally but never for a clock and never from wood. As it turns out (no pun intended) getting the hands to move at the right speed and in the right direction is an interesting design problem. To figure all this out I did a CAD model in SolidWorks to make sure that the gear ratio was going to be correct, the minute hand and the face were turning at the right gear ratio and in the right direction. I also wanted to see how the whole thing was going to look at different hours of the day. Have a look below at the design and a short video that I made of the clock running.
A conventional clock has a gear ratio of 12:1 between the hour and the minute hands. Because the numbers of my clock are moving at one revolution every twelve hours counter-clockwise the minute hand has to move 330 degrees clockwise so that every hour the minute hand is pointing at the 12 - remember that the 12 is moving! For this to work right the gear ratio between the face and the minute hand must be 11:1 and they have to be moving in the opposite direction. To complicate things a little more the first and the final gears have to be coaxial at the center of the clock. Of course the gears can't be so big that they 'cross over' the center of the clock either. To make things even a little more complicated the teeth have to be big enough that I can cut them out of wood and have them mesh correctly... lots to think about.
Homemade Clock Gear Movement
Above is a cut away of the CAD model that I did to work out the gears. I ended up using a total of five gears in the gear train to make everything turn correctly. You can see four of the gears in the picture above. The 21 tooth gear is connected to the clock face and is meshed with the 7 tooth gear which is connected or 'ganged' to the 33 tooth gear which is meshed with the 18 tooth. There is a 9 tooth gear that you can't see in the picture that is driving the minute hand and meshed with the 18 tooth gear. This all works out to (about) 11:1 like this:
(21/7)*(33/18) = 5.4999 * 2 = 11
I think that this is going to be work out well. (UPDATE: See comments below) Anyway those are the gears that I'm going to try for my first clock - if it doesn't work out exactly right then I'll fix it, or build another one! If anyone reading this has any clock making experience I'd appreciate some feedback or advice at or leave a comment.
The gears for the motor that will be driving the clock aren't shown in the CAD model. I am planning on driving the clock with a brushless motor that has a speed control adjustment built into it. I've decided to use a brushless motor for this clock project not because of the extended life that a brushless motor affords but because I have a box of them in the garage.
Usually you know more or less what hour it is and just glance at a clock noticing where the minute hand is to see what time it is. Not with this clock, that's not going to work. Below are a few pictures of what different times of the day will look like:
What time is it? 5:30 According to the above picture
This picture is showing it's 10:30
3:10 with my clock design
Notice that the numbers are always facing up. The idea is that the numbers will be free spinning on the face of the clock in little bearings and they will be heavier at the bottom so they will always be right side up like baskets on a Ferris Wheel. Another thing that might look odd about this clock is the the hour hand is on the outside and the minute hand is behind it. I didn't think that would look odd but when I created the CAD simulation and ran the clock it caught my eye. For some reason it looks strange to have the hour hand on the outside but putting it behind the minute hand isn't so easy because of the way everything has to turn. I thought about putting the hour hand on a bearing, allowing it to spin relative to the face and minute hand and making it heavier on the bottom like the numbers so it will always point up.
I decided against using a bearing on the hour hand and instead will attach it to a rod that runs through the center of the clock and attaches to the back. The minute hand will be attached to a tube with the hour hand rod inside and the face will be attached to a tube outside the minute hand tube. All these concentric tubes spinning around each other it nothing new, all clocks with hands work like this. The only difference is in this case the center rod is fixed to the hour hand and doesn't spin.
Below is a short video that I made of the CAD model 'running' with the gears spinning to see how it's going to work. As I mentioned above the gears for the motor aren't in the CAD model so I have a little bit of design work to do. I'll write more as I get more done. If you have any questions (or advice for a first time clock builder) let me know at or leave a comment.


1 comment:

  1. UPDATE: A reader Matt Mylar sent me an email pointing out a truncation error in my math! A big thank's to Matt Mylar for not only reading my blog but emailing me about my boo-boo ;-)
    The gear ration is exactly 11:1 but due to lack of sleep I made the mistake of multiplying the fractions wrong....