Turing-Welchman Bombe physical build – part 4.

August 6th, 2015

Tonight I machined up the drum hub shafts from some brass rod. These are very simple. A beveled end and a groove for the rabbit clips to slip into on one end and a hole bored in the other that slides over the stepper motor shafts. The rod is a tight press fit into the white plastic part that everything on the hub bolts to.  All the torque of the motor drive goes through this. As there is no load on the motors it should spin the drums easily.

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The real Bombe has a fairly substantial steel frame. For my Bombe I decided to make a slightly lighter weight, but still steel, frame. Since it was a fairly quiet day and there was a break in the weather I went for a drive at lunchtime to get the steel. I thought I had some left over from building the Austin 7 body frame but apparently I had used it all! I went and bought some lengths of 16mm square steel tubing as well as some other pieces of angle iron.

Last night I had drawn up a plan of how I will make the frame.  It’s quite simple, just a rectangular box with some places to mounts various plates and components. The outer covers will be steel. Since I am just doing the indicator drums I can’t exactly copy the entire housing so I am just going for a Bombe type look.

I am starting with a simple rectangular box frame. The top and bottom are simple steel squares with a vertical joining them in each corner. I first cut the top and bottom pieces. The corners are cut to 45 degrees so I can weld it all together square. I made the first carefully to the right size then used that to mark up all the others. I have to hand cut and file everything carefully.

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Then I end up with 8 pieces all the same length. The little holes in the middle are because I am going to gas braze the frame together. The hole lets heated air out when you’re getting everything hot with the welding torch. Not sure that’re really necessary but I have always done it! The little holes are handy later for painting since you can bung wires into them to hang the thing up.

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The verticals are simply cut with square ends. Everything will be MIG tacked into place so it’s all square, then I will braze it.  I do need to go and  get a new bottle of acetylene and some brazing rod first. Everything is done on a flat steel plate so it is all level. I need to tack everything just to check it’s all going to fit. It will be a tight fit but this is meant to be a desktop type model so it can’t be too big.

When I was out I popped into Supercheap Auto and bought some gold paint and some clear coat to cover it. I will paint the drums with this. This is just a test on a spare sweet tin I had. I want to see how resilient the paint is.

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I also received the other day the little castors which will go on the bottom of the Bombe just like on the original. There are two fixed and two swivel wheels on it.

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I also got some dry transfers for the lettering finally. This stuff is now very hard to find. None of the stationary or art shops have it anymore. I don’t think Letraset has been around for a long time. But you can still get similar transfers from modelling places. Woodland Scenics will be familiar to anyone who does model railways. They do dry transfer sheets for doing lettering on things suck as trains and buildings. I ordered some from a model shop online here and had it the next day.

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Since they cater to all scales there should be a size of lettering that will be appropriate for my drums (around 1/8 inch is about right). I am going to do a test first as I want to see if I can clear coat over the transfers to make them more robust.

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