Making Wheel Brackets for a Model Armstrong Cannon using Subtraction.

by John

For my previous model Armstrong 80pr cannons I made the iron carriage and slides using metal casting of 3D printed PLA filament for the complex castings. The results were OK, but I was not satisfied with the surface finish.

So, I bought a resin printer, and I have been very impressed with the results of the resin prints.

But, to date, I have been unable to get any castable wax resin suitable for the resin printer, with which to make the bronze castings.

So, I decided to revert to traditional machining methods, using reductive technology. Milling, lathe, etc, removing brass chips from bar stock to end up with useable parts.

This is what I am trying to make at 1:10 scale.

The wheel bracket appears to be made of cast iron. Possibly the wheel also, but it was probably turned in a lathe.
These are the brackets which I have milled and turned from 38mm brass bar stock. Not quite identical with the originals, but close enough I have decided.
Billets cut to length, with an allowance for holding in chuck. OAL 50mm.
The external shape was CNC’d.
The wheel slot was cut with a 3.5 mm thick slotting saw. 3 cuts to get the full 9.5mm width. The axle holes were spotted and drilled.

Then, I pondered long about how to remove the 20mm of stock which was allowed for the chuck jaws. I realised, too late, that I should have allowed another 10mm or so, because the parting line leaves too little to be held in the lathe chuck while parting.

So, I came up with this work holding solution…..

I drilled the hole in the bracket which will eventually house the mounting bolt on the model. 5mm diameter. Then drilled a 5mm hole in a piece of scrap, and bolted the 2 pieces together.

Actually, 5mm allthread is not much to hold a 36mm diameter piece for parting. So the thread was nutted and lock-nutted at each end. And torqued as tightly as I dared.

Holding the bolted extension in the 3 jaw, then slowly parted off the bracket. I stopped at 7mm, so the bolt holding the parts together did not crush the parts together and jam the parting tool.
Removed the bolt, and hacksawed the bracket from the bar. Then some belt sanding and finishing on a flat plate.

After parting the first part by hand winding the cross slide, I became more adventurous with the next three. Made sure that the gibs were tight, the carriage locked, and setting the spindle at 500rpm, used the power feed to do the parting automatically. With plenty of coolant-lubricant (my home made mixture of olive oil and kerosene.). But still finishing with a hacksaw.

With end result shown in photo 2. All good.

Next to make the wheels and axles from steel. Those brass bar offcuts will go into the “might be useful oneday” container.