On the RACK

by John

The final gear in the cannon carriage positioning train is a rack gear. It is 198mm long, 7.2mm wide plus tabs for bolting it to the carriage of the model Armstrong cannon.

A rack gear is a flat gear, and it is cut with the same cutter (number 8) which makes a circular gear of 135 teeth or more.

The teeth of a circular gear are cut by dividing 360 degrees by the number of teeth. But the pitch of the teeth of a rack gear is determined by a formula found in Machinery’s Handbook. rack gear pitch = module x 3.1416. Which for my module 1.25 = 3.927mm. Hmmmm. 3.1416. That is a familiar number. Light bulb in brain switches on! A rack is just part of a circular gear of very large diameter.

At first I thought that I would use the same mill arbor which I had been using for the circular gears, but as soon as I started to set it up I realised that the stick out of the arbor would be ridiculously excessive. So, reluctantly, I set up the horizontal milling attachment of the mill. Reluctant, because the attachment is heavy, fiddly, and time consuming. Luckily, I had a 22mm shaft for the attachment, the correct diameter for the cutter. I had never used this shaft before, and it was missing the nut, and bronze bush. Bought it on Ebay years ago. Found a suitable nut and made a bush.

This is the setup. The 350mm shaft was not long enough to make the rack in one setup, and it took a bit of trial and error to work out the best compromise.

The horizontal attachment on the vertical mill. Setting it up takes me a couple of hours.
No CNC here. Just lots of calculations using 3.927.

I need 2 racks, so I will split this one down the middle.

Did not have a piece of bar stock big enough for this job.

So I joined 2 pieces end to end with silver solder. Will that be strong enough? My friend Stuart T insists that a well made silver solder join is stronger than the parent metal, so we will see.
It meshes nicely. The mounting holes (which I did not use) will disappear when the outsides of the 2 racks are shaped.

A bit of a story about that heavy horizontal mill attachment. When I bought it some years ago, I put it in the rear compartment of my SUV. But on the way home my SUV was T-boned by an idiot at an intersection. No injuries, but a big expensive dent to the passenger side of my SUV. Air bags activated. And the rear window was smashed. I could not figure out why the rear window was broken. After the police and fire engines had finished, and the tow trucks arrived (my car was out of action for 2 months, the other vehicle was a write off), a by-stander approached me with a familiar object which he had found in the gutter on the OTHER side of the 8 lane highway. Yep! It was the horizontal mill attachment. Been flung through the rear window by the violent impact of the collision, and across 8 lanes of the road. It was scratched, but otherwise intact. And thank goodness, it had not hit me or anyone else in its trajectory!