Carbon Fiber Chain Ring Clock

The chain ring (sprocket) is cut from the carbon fiber. I used a 1/16 diameter", (2) flute, spiral upcut, solid carbide end mill. You could use a larger diameter end mill because the smallest inside radius in the design is 0.156". The material cuts easily in a single pass at about 40-50 inches per minute. I have heard carbon fiber can be cut on high power lasers (100’s of watts), but you should not try this on lower power hobby lasers.

The dust is messy and probably dangerous to breathe, so keep a vacuum on at all times and use a mask and eye protection.

The dxf file has the outline. It also has some construction lines on layers you can turn off. The chain ring is actually a geometrically accurate sprocket. If you want to make your own size or just like to geek out on this stuff like this (like I do), see the final step.

The cut edge quality was quit high as you can see from the second image.

The hands were laser cut from brushed silver on black acrylic. They could also have been cut on on the router.

Attach the clock mechanism to the chain ring using the nut on the shaft of the clock. The hands press fit onto the shaft.

I had as much fun learning about chain rings as cutting the parts. If you want to make your own geometrically accurate chain ring, follow these instructions.

You will need (3) parameters.

1. The pitch (distance between rollers) of your chain. I chose 0.50".
2. The diameter of the rollers on your chain. I chose (0.31").
3. The number of teeth on your chain ring.

I wanted a clock that is about 6.5 inches in diameter. You calculate the diameter with this formula.

Pitch Diameter = Pitch ÷ sin (180° ÷ ToothCount)

It turns out, 40 teeth gets me pretty close.

I used Draftsight, a free AutoCAD clone, to draw my part because it is easy to draw accurate angles and offsets. Here is the process:

1. Draw a long line up from center.
2. Draw (2) angled lines from the center up, representing the angle created by one link. That is determined with this formula: 360/ToothCount. In my case that is 9 degrees, so I draw the lines up at 4.5 degrees on each side.
3. Offset a line on each side of the center line at half the pitch. In my case that would be 0.25".
4. The point at which the offset lines intersect the angled lines is where the link roller goes.
5. Draw a circle the diameter of the link roller as these two points.
6. Now draw a circle from the center of one roller to the tangency point of the the other roller. This is the path a link travels as it comes off the sprocket.
7. Now you should have enough geometry to create a tooth. The tip of each tooth will be sharp. Typically this is radiused. I used a radius of about 0.30".
8. Array the rest of the teeth around the center.