johnsmachines

The Armstrong 110pr model cannon trunnions sit in semicircular cutouts in the carriage cheeks. In the model the cutouts are 20mm diameter, and they are slightly deeper than semicircular. Making the “slightly bigger than semicircular” cutouts is complicated by the fact that the cheeks toe in towards the front, by 2.65º.

When I originally cut out the cheeks I made the cutouts 18mm diameter, allowing 2mm to be removed at the assembly time, and to then remove some extra material to cope with the toe-in angle. I did not know in advance how that would be done, but I figured that I would use a drill or reamer at the correct angle to remove the extra material.

Today was the day.

But when I was actually confronted with the task, I realised how difficult the job was actually going to be. I also realised that a drill or reamer was NOT going to do the job accurately or neatly enough.

1. These are the assembled carriages with the undersize trunnion cutouts, which do not take into account the toe-in angles. (
Since this photo was taken, the bolts have all been finished to length. See later photo.)

Then I had a brainwave. And I am really proud of this one. I made a round file, exactly 20mm diameter, and long enough do file the cut-outs together, exactly in line.

How to make a file?

And how to make the teeth small enough so they leave a smooth finish with no edge tear-outs?

2. A 20mm diameter piece of silver steel, long enough to allow filing movements plus handles. Here I am applying a fine knurl with the shop made tool which I made a decade or more ago. It is a clamp type, and can apply a lot of pressure. Run at 200rpm, well oiled.

Ah! But I forgot something. When I measured the diameter of the “file” the 20mm shaft now measured 20.25mm. I had forgotten that knurling INCREASES the effective diameter. So I turned off the knurls and machined the shaft down to 19.75mm, and repeated the knurling. The diameter was now 20.05mm which I considered acceptable.

Since I was only intending to file wood, I did not bother hardening the silver steel.

4. The “file” after a few minutes enlarging the cut-outs.

5. After one minute of gently rotating the file, I could see that it was working!

6. One finished – in 3 minutes, and one to go (the top one)

7. And the trunnions fit the cut-outs perfectly!

Garrison based cannons did not use trunnion caps, unlike the naval versions, relying on the weight of the barrel to keep it in place. The centre of the trunnion is just below the top surface of the carriage cheek.

The file worked well in hardwood. I would have hardened the steel if it was to be used on brass or other metal.