When I build model locomotives, I always work using engineering measurements. That’s because being a retired mechanical engineer, I’m comfortable working in thousands of an inch. You can use any system you wish; it doesn’t matter whether it’s metrics or fractions. I build scaled down versions of actual prototype locomotives, but you have to have someplace to start. I start first by deciding what scale I want to build in and what locomotive I want to model. My scale of preference is 1-1/2” to the foot (1/8 full size). The reason for this is to utilize common wood profiles and allow parts to be, not so small that they are difficult to work with. A piece of ¾” thick wood scales down to 6”, and ¼” thick plywood scales down to 2” for example. I make driving wheels from 18mm Baltic Birch, which is the perfect thickness of the prototype wheels. I also use Baltic Birch because it’s 11 ply’s hold up without delaminating. Here is an example of a typical locomotive driver:
Making the driver as shown, requires common woodworking tools. Here’s where jigs come in. I used a router in a table with a jig to cut the general outline. Another template was used to locate all the ¼” dia holes which define the open space between spokes. Using a jigsaw with a fine tooth blade, I cut out between the drilled holes to create the spokes. I had to repeat this for the remaining 7 drivers. That is why a CNC router is in the works. Doing it all by hand, took me about a day.