Turing-Welchman Bombe physical build.

July 29th, 2015

I think last update I had the Turing-Welchman Bombe working on breadboards. I have now made the electronics neat and tidy with a circuit board for the electronics and a base board to mount all the various modules to.  And I have started the rest of the mechanical build.

The whole time I am doing this I can’t help thinking of Harold ‘Doc’ Keen the engineer who actually did the physical building of the Bombe from Turings design (no, unlike that film Turing didn’t single handedly build it himself).  If you think Welchman is forgotten poor old Harold is even less well known.

The base board is made from the same stuff that I am making the drum faces and the front panel of the Bombe from. Now I don’t know what the original used but it looks to be some kind of phenolic board. I bought some of this from RS locally which seems to be similar. I got a dirt cheap Arduino clone and a prototyping shield to which I added connectors to join to the rest of the circuit.

I made a base board and mounted the Raspberry Pi 2, Arduino, three driver boards and my interface board to it. The interface board is just made from some Vero board. It contains the 4066 switches that control the signals going to the second and third drums and the transistor buffers that let me safely interface the 3.3 volt Pi to the 5 volt Arduino. It also has the voltage regulators (5 volts for the computers and 12 for the stepper motors) and heat sink for the power supply.

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The boards are all mounted with 3mm screws and small nylon spacers. They also came from RS. Jaycar (who are just down the road) sell spacers but at some ridiculous cost for what’s basically a little plastic tube. The interface board connects everything together. The 20 character, 2 line display I ordered also arrived so I wired that up too.

I made all the interconnecting wires, except those carrying power, red as the original Bombe seems to have a lot of red wiring internally. To hold the loom together I tried cable lacing. This is when you use a waxed twine to tie the cables together into a loom. Now, this being NZ – the land that time forgot, you can’t actually get lacing twine here. In the end I bought some waxed cotton cord from a bead and jewelry place. That seems to work very well.

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It’s not the neatest cable lacing in the world and I was considering redoing it all then I found some pictures online of the rebuilt Bombe at Bletchley and see it’s not perfect either so mine is in keeping with the original!

With everything mounted I was able to fire it all up and check it works. I have set up the Pi so that when it boots it automatically runs the Bombe software. That reads from a USB stick which contains the menu to run. If the menu is correct the Bombe will then start running when the start button is pressed. The start button is one of two buttons that will be on the front of the Bombe like on the original. The original Bombe has two buttons on the front but also a physical lever on the side next to the indicator unit that needs to be moved after the Bombe stops and you wish to restart it. I have simplified my interface to just a start and a stop button. Pressing the stop button will pause the Bombe. Holding it down for more than 1.5 seconds will abort the Bombe run and drop out to the command prompt. Normally however the Bombe will complete it’s run and automatically shut down the Pi once the stop button is pressed a final time. The Pi doesn’t have an on/off switch so the nice way to shut it down is with a command from the command prompt. I have a simple script that starts up the Bombe code then checks the return value when it exits to see if it should shut down or simply exit to the command line.

I did get a WiFi dongle working but since the Bombe doesn’t normally need an Internet connection to run I haven’t used that in the model. If you need to connect to it you can connect an Ethernet cable.

With the electronics done, mounted and tested I started on the rest of the model. I rough cut then machined three discs of the Tufnol material to go on the front of the drums.

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The material machines really easily. It’s easier to bolt the three together and machine them all at once so then they are exactly the same diameter.

I also drilled an 8mm hole in the base of the tins I am using for the drums. I made a rough wooden disc to sit in the tin so when it was mounted on the lathe the bottom of the tin was hard up against the wood so I could center drill then properly drill it.

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Tonight I made the metal clips that go on the front of each drum to hold it to the drive shafts. Originally these would have been stamped out I guess. I only need six of them for my three drums so to make them I cut out six pieces of 1mm steel sheet and soft soldered them together into one solid lump. I then printed out a picture of the drum from the US Army Bombe report shrunk down to my 3/4 scale. I cut out the shape of the lever and glued that to the steel block.

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I then drilled the holes right through the block and started shaping it. Rough shaping was done with a hack saw and a bench grinder. Then I progressed to hand filing and finally a Dremel with various grinding attachments.

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After much shaping I ended up with a single block in the right shape. I then simply unsoldered the pieces and wiped the solder off with a cloth. Each piece was then rubbed on some sandpaper to make them smooth.

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They are not perfect but are bloody close. I still need to make the spring mounting posts, which will be brass and soldered in place, then I will zinc plate the clips.

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I have tried to find screws (mostly from RS again) that match the appearance of the originals and luckily enough so far all the screws I have found have heads that are almost exactly the right size and shape.

Next I need to make the hubs from aluminium which will need a bit of careful machining and drilling. I ordered (again from RS) some little castors and soon I will start making the actual frame once I get my acetylene bottle back off a friend who borrowed it. I also need to machine up the little finger posts mounted to the front of the drum used to help the operators turn them to the right position.

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