It’s been a busy time at school during the first few months of the new school year, so unfortunately I haven’t had much time for model engineering. But now things are getting back to normal, and I’ve been able to continue working on the frames—specifically, the motion plate.
On the real locomotive, this part is a casting that holds the slide bars for the crosshead and also guides the valve spindles. I only have a few partial photos of the motion plate from the preserved loco, so figuring out the exact dimensions is a bit of guesswork. Since it’s clearly visible from the outside, I didn’t want it to be just a simple flat plate—I wanted it to look more like a proper casting.
So I designed this for the motion plate in Solidworks.
For the first step, I dismantled the frames and drilled the holes for the cylinder block and motion plate. To make the setup easier, I picked up a second-hand machine vice, which really helped with positioning the plates before drilling.
The plate was milled from 3 steel plate, partly on the conventional milling machine and partly on the cnc miller.
The final shape.
Two side strips were prepared for mounting the motion plate in the frames.
A groove 3 mm wide and 1.25 mm deep was milled to position the strips accurately in the middle.
A pilot hole was drilled at the center of the groove, used to rivet the strips in place.
The exact postion of these strips can be measured in the Solidworks drawing I've made.
With a set of gauge blocks and a knife-edge rule.
After clamping and drilling the strips are rivetted to the main plate.
Finally, the strips were silver-soldered to the main plate after riveting.
There’s always the risk that, after silver soldering, these strips aren’t perfectly square to the main plate. To allow for correction, I left a 0.5 mm surplus—so the total width was 108.5 mm instead of the 108 mm between the main frames. This gave me the option to skim off the excess and bring it down to the exact 108 mm, perfectly square. I set the zero point for the middle using the round valve guide holes.
After drilling the other small holes, the next step was adding the strips along the contour of the motion plate so it would resemble a casting. I chose copper plate for this because of its ductility and ease of working.
Once annealed, the bending can be done easily by hand and with a small hammer.
Annealing helps because it softens the metal by relieving internal stresses and reducing hardness, making it much easier to bend or shape without cracking.
It’s of course the same method we used when working the copper plates for the steam boiler.
The two lugs that will hold the slide bars are milled from solid brass.
This block actually started life as a practice piece from school. Back then, we used it to learn how to set up correct dimensions and tolerances on a conventional milling machine—a great exercise in precision. Now it’s found a second life in this locomotive, which feels like a nice connection between my training and the "real-world application".
