Spoking a gear wheel with the Sherline CNC Milling Machine

One of the tedious tasks a clockmaker faces is the spoking of gear wheels. Traditionally this is done by cutting the spokes with a small jeweler's saw and then filing them smooth. It's a perfect task for a CNC machine.

Sherline has recently introduced a CNC version of their 5400 milling machine. It's small size is perfectly suited to spoking a gear wheel. (With the Sherline CNC rotary table, it will automatically cut the gear teeth too.)

I took my pattern for clock wheel spokes from the work of W.R. Smith, who has written many books on clockmaking. In his book "How to Make a Skeleton Wall Clock" he describes the spoke as having a 5 degree taper to the rim, rounded to a 3/8" radius at the root. Here's a diagram from his book.

The geometry of the spokes is not all that complicated, but it's a difficult computation for those of us who are not proficient in trigonometry and algebra. The hard part is that the spoke arms are tapered, and you have to locate the center of a circle that is tangent to these tapered arms. The shape must be precisely specified mathematically to write a program for the CNC machine. I turned to a very smart man who has helped me in similar situations in the past.

David Smith is a senior lecturer in mathematics at the University of Auckland, and a clock enthusiast. I presented him with a verbal description of the problem and he provided me with the mathematical computations needed to define the shape for the CNC program.

With this in hand I wrote a Visual Basic program that takes inputs for the specifications of a particular gear blank. This screen shot shows the program screen. After you have specified the parameters and clicked on "Process", a file of g-codes is written that will cut the gear blank specified. "G-code" is the language used by CNC machines.

To see the g-code file click here.

The g-code file is then moved to the Sherline mill and previewed on the CNC computer screen. The screen shot below shows the EMC software with the tool path for cutting a gear blank.

The final test is the gear blank itself. Call me cheap, but I tested my part with a plastic blank instead of using up real brass! The blank is perfect and was easy to make. I must point out, however, that it's not complete yet. A traditional clock wheel has sharp corners where the spokes meet the rim. These must still be added by a little hand filing.