Turing-Welchman Bombe physical build – part 3.

August 3rd, 2015

More work on the Turing-Welchman-Keen Bombe. I think Harold Keen needs more recognition after looking at what’s involved in actually producing these things! I’ve been working on this a lot recently since it’s going well and I would like to see it done so I can get back into my vintage car stuff. I am having great fun building this as it requires a lot of fiddly metalworking and trying to work out the best way to achieve things. One thing I am finding tricky is the lack of reference material. The main dimensions I am getting from the 1944 US Army Bombe report. I am also using John Harper’s rebuild pages a lot as well as pictures of the Bombe, mainly from Dermot Turing’s book (if you have any interest in this buy that book, it’s a good intro and it helps Bletchley Park!). This film also has some nice shots of the rebuilt Bombe.

I am finding differences between all the sources though. For example the US army report and John Harper’s page show little pegs on the drums (the spring attachment posts and the limit stops for the bunny clips (they look like a bunny rabbits). But some pictures of the Bombe rebuild show these as small screws. Possibly a change made to make them easier to build? The rebuilt Bombe also has black faces on the indicator drums. It looks like that black plastic, that has a white middle and a black face, that gets engraved to make labels. I am using brown Tufnol material and will hopefully soon have dry transfers to do the lettering (that stuff is getting VERY hard to find these days). Some of the rebuilt Bombe drums seem to have cross head screws in place of slotted ones in some of the hubs. None of these differences really matter. It would just be interesting to know why!

You really know you’re deep into a project when you’re paying attention to details such as individual screw heads. Rivet counting literally.

Anyway, where am I up to? I made the little stops for the bunny clips from brass. These were just a push fit into the hubs although to be sure they wouldn’t come out I used a touch of Araldite too. In the rebuild some of these seems to be screws.


Then I went back to finishing the bunny clips. I machined up little spring posts for them from brass. I could have used screws as on the rebuild but I only needed to make six of these things so I made posts. I think the originals might have been drilled to hold the springs. I did try doing that but cross drilling a 1mm hole in a 2mm round post and getting it right is tricky (I did try!) so instead I cut a groove in them to locate the spring ends. I need to find springs still. I did find the perfect ones in my spring box (whenever I pull something apart I keep all the springs) but of course I only had two of them. I then soft soldered the pins to the clips. I drilled a small hole in a steel plate to hold the clip then just propped up the clip to be square with scraps of metal and washers.

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I then zinc plated the clips. Now this process is described in various places on the net if you search about. But the basic idea is to electroplate them.

First you make a plating solution consisting of white vinegar, sugar and Epsom salts. For 1 litre of vinegar use 120 grams of sugar and 100 grams of Epsom salts. Next you need some zinc anodes (positive terminal). For these I use cheap D cell batteries (preferably dead ones!) that I cut open to remove the zinc casings. You can open them up then flatten the case to make zinc sheets. You fill a container with the solution and place in it your anodes. This is where I find the zinc sheet handy since you can bend it to suit the container. The plating works better if the anode surrounds the parts to be plated.

The parts you are plating make up the cathode (negative terminal). The easiest way to arrange this is a piece of copper water pipe sitting on top of the container from which you hang the parts to plate on little copper wires. I get those by stripping some mains cable or other heavy cable to remove the individual conductors.

The parts need to be scrupulously clean. I sand them all then scrub them with steel wool then finally wash in acetone. Wear rubber gloves when doing all this to avoid putting greasy fingerprints all over the metal! If the parts are rusty some people dip them in hydrochloric acid first (nasty stuff!).

With the parts perfectly clean you attach the copper wires then hang them off the copper pipe.  Then you attach a current limited supply to the setup, positive to the zinc plates and negative to the parts you are plating. Now the current and voltage required takes some playing about with. The number I have seen is 20 amps per square foot. What that means when plating a few small bits I do not know! I try to get a current somewhere less than an amp and a voltage of about 3 volts.  This all depends on the size of the container, how many parts you have, your power supply, phase of the moon,, how you hold your tongue, etc, etc. I think it’s better to err on the lower current side. Too much and the coating apparently deposits too quickly. Too little and it just takes longer to plate.

This is what this setup looks like.

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You will see bubbles of gas coming off the parts as they plate. Leave them for 15 or 20 minutes then pull them out. They should be a dull grey colour which is the electroplated zinc.


You can clearly see the zinc on the copper wire.

Next you need to polish the parts. For this I just use steel wool and gently polish them. It removes the dull grey and leaves a nice, shiny zinc finish. I usually then plate the parts a second time. Below you can see two polished clips and one dull one. I had six clips to do and did three at a time. The plating seems to work best line of sight between the anode and cathode so fewer parts is better so you don’t get shadowing.


The clips all came out well. You can tell if it worked by the fact that the steel doesn’t immediately start rusting again afterwards! I found cleaned steel will flash rust very quickly in a humid environment (like Auckland) so it’s easy to tell if it worked or not. If the parts are still shiny the next day they are plated. The zinc is also slightly less shiny than polished steel.

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Another thing I did was machine up the hub centres. These were just plastic (from chopping boards). These go inside the tins and the hub bolts through them. They will also hold the shaft that attaches to the stepper motors.


Next I made up the finger posts. These attach to the face plate and are there so the Bombe operators can turn the drums easily. You can see this easily in this YouTube film at about 6:50. That film has some great details of the rebuilt Bombe. I machined these from some little aluminium spacers I had. To make the groove in them I used a grinding wheel on the Dremel and spun the piece in the lathe then used the Dremel to grind the groove. This worked quite well. Then I just turned the ends to size and then centre drilled one end to take a oval head screw (the same I used on my Enigma Watch).

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The tricky part with these is once you cut off the spacer how do you face the end without marking the piece in the lathe chuck. I cut a piece of rubber fuel hose that the piece fits into and clamped that in the chuck. The finished pieces remind me of spark plug tops! They aren’t all identical since I am hand making things of course but within 0.1mm is close enough!

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With these done and the holes drilled in the face plate I could assemble the hubs. The finger posts bolt to the faceplate and there are clearance holes in the tin for the nuts. I had to do this since the screws weren’t long enough to pass all the way though.

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I still need the springs and the letters of course. One thing I did notice is when bolting everything together the bottom of the tin would distort. Since the tin base isn’t flat I needed something to fill the gap between the tin and the faceplate. I machined up three plastic discs from 2mm sheet. I am not sure what this stuff is. I can’t even remember why I have it. It’s plastic but reasonably soft and it will crack if you bend it too hard. Odd stuff. Not styrene, but not acrylic. It machines and drills easily though and was almost exactly the right thickness.

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With that done the hubs are almost complete. Just painting (gold) and lettering to do now. I did find somewhere online that hopefully has the right white lettering. I ordered some and hopefully that arrives soon. I might also look at having the discs engraved but I suspect that would be hard to get right as the letters all need to be in the exact right places. The tins are almost the right colour but covered in labels unfortunately. As well as finding three suitable springs I still need to make the drive shafts of course.

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One of the other odd differences between all the sources I have is where the lettering is in relation to the bunny ears and the finger posts. The US army Bombe report shows the finger posts at A and N but with the bunnies ears pointing to Q on the main drums (K on the indicators since they go the other way around). But the rebuilt Bombe has the finger posts at A and N still but the bunny ears pointing to D (X on the indicators). So they are 180 degrees out. Again it doesn’t make any difference to the machine itself or how it works. I just wonder what they were like in WW2 on the real ones. Maybe those had differences too? It’s actually interesting the indicator drums are almost as complex as the main drums as they didn’t need to be. I guess when you are making hundreds of the other drums it’s easy to use three more as the indicators. I am glad I only have to make three of the damn things and not the 183 needed for a real Bombe.

Since I had my plating bath all set up I also made the pointers that indicate the letter the drums are pointing too. These were made from some thin steel, cut into strips then metal bashed into the right shape. These were then zinc plated but I needed to use a deeper container. This is where using zinc sheet for the anodes works well since I could just reshape them to the new container shape.

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These will be held in place with two small brass screws.

Next I need to make the hub shafts, probably just a push fit onto the motor shafts, and start working on the main housing. I ordered some suitable castors from RS which should hopefully arrive soon. The main frame will be square steel tube brazed together. The sides will all be steel sheet and of course I will paint everything crackle black (I love crackle black)!

One Response to “Turing-Welchman Bombe physical build – part 3.”

  1. Austin 7 Special » Blog Archives » Update. Says:

    […] Just a post so people don’t think I am dead! I’ve been busy with other things. The problem is I have too many projects so I have been working on other things. Mainly building my own version of the Turing-Welchman-Keen Bombe. […]