Turing-Welchman Bombe physical build – part 7.

August 16th, 2015

This is turning into one of those projects. The never ending kind!

Lets see, things were going well. During the week I painted the switch housing with crackle black. Being winter, even with a sunny day, it wasn’t warm enough to get the paint to ‘go off’ so I cheated and used a hot air gun. That worked well.

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I had to do it in two goes, the inside then the outside. You can see when the paint is just sprayed it is very glossy and smooth. The crackle finish comes as the paint dries. The trick is getting a nice thick, wet coat without the paint running. I prefer to let it dry naturally in the hot sun but the hot air gun works too.


After the paint is dry I like to cure it in the oven. I leave it for a few hours on a low heat, 50-70C, which helps really harden the finish.

I also added the transfers to the drum faces. This took hours. It takes careful positioning and much care. It’s a good thing to do while sitting in front of the tellie. Unfortunately I did this while watching Airport. It was going great until I got to an exciting bit (Petroni giving the 707 full throttle to get it off the runway) and I made a slight mistake. Unfortunately I didn’t notice until I got to ‘W’ and realised I had 4 letters to go but 5 spaces. I’d missed a bloody ‘L’!

So I had to scratch off most of the transfers and start again from ‘K’. That all went fine but then I found I didn’t have enough ‘M’s so one of them has an M made from part of an upside down A and two Is!


I clear coated the discs and again baked them in a low oven.


I also bought a new 12 volt SLA battery and made up a small poser supply board with the boost converter. This bolts onto the main board.

With the drums dry I was able to test things out. Only I hit a snag. I was hoping to just use a friction fit between the shafts of the motors and drums. This was a total failure. There is too much mass in the drums and there was a huge amount of slippage. So I had to rethink the whole way I attached and drive the drums. I came up with this.

I remade the drum shafts with a thicker end piece so I could drill and tap it for a locking screw. If I can find some 3mm grub screws I will use them but normal ones work for now. On the side of the bigger section I soldered on a section of square U tube. That provides a key to give positive location and drive. The key fits into a slot on a washer. The washer fits on the back of the drum.

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I tapped the holes in the washer so the four screws that hold the hub in place thread directly into it.

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The shaft is permanently attached to the motor shaft. The drums are a slide fit over the shafts. And now the little rabbit clips actually do something. The drums now actually work much the same as the real ones. The slide over the shafts, there is positive location and drive and the clips stop the drums from coming off. It all works really well!

So the front of the Bombe looks great.


I may replace the face plates with some lasercut and engraved ones. I went and found out how much that would cost (less than $100NZ) if I can provide them with a suitable file. But I wanted to make sure the Bombe actually works before I do that.

Good thing too. It doesn’t bloody work! I made another mistake. The issue is how I drive the steppers. Because of the mass of the drums jiggling about I am using a holding current to keep them in position accurately. When driving steppers you make them move by switching voltages through several coils. Each step is caused by the voltage changing in a particular sequence through the coils to make the motor move. Between steps you can either keep the last voltage there or turn all the voltages off.

If you turn them off the motor is free to move. If you hold the voltage the motor locks into position. For what I am trying to do I want it to hold. My code does do this. But because of the way I share the drive signals to the second and third drum only the first drum was being held. It steps very accurately but the other two would not. I used an analogue switch to switch in the drive signals when the second and third drum need to move. But when they are not moving there is no holding current since the output doesn’t latch.

So I have redesigned the electronics. Instead of 4066 switches I am going to use 74HC373 octal latches. I think with these I can still switch the output when I want (using the latch enable) but when they aren’t changing they will hold the same voltage on the motor driver inputs so they will hold their position.

I hope.

So the back of the machine is looking rather sad.


I pulled out the main board and will modify it to take the new latches. There should be just enough space. I just need to get some desoldering braid so I can remove the old components then clean things up to rework it.

The other thing I have done is ditch the USB hub and am connecting the menu thumbdrive direct to the Pi. I am getting a small USB panel socket and will mount that to the main board so the USB thumb drive a can be plugged in easily.

One Response to “Turing-Welchman Bombe physical build – part 7.”

  1. Austin 7 Special » Blog Archives » Chummy wheels. Says:

    […] finally started on the cars again. The Turing Bombe is still ongoing but nearing the end! I keep getting distracted. I had some giant tress cut down […]