Turing-Welchman Bombe physical build – part 5.

August 9th, 2015

Right, this weekend I had so much to do. Clean the house, cut and stack firewood, work on the car. I did none of that. I worked on this thing instead!

Earlier in the week I painted the three drums. Three coats of gold then three of clear over that. They came out great!


I also went and got more acetylene on Friday so I could braze up the frame. The plan was to spot MIG it all then braze the joints. As it happens I was out of MIG shielding gas too (so another trip to BOC this week then)! So I just gas tacked everything then brazed it. I started with the top and bottom squares then I clamped everything down to my steel plate to do the sides and was very careful to keep everything square.

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The brazing went very well and after a quick sand down I started on the various cross pieces things would mount to. I started with the front cross pieces which the brown Tufnol would attach to to make up the front. The drums are mounted on a sheet of Tufnol set back into the frame. Smaller pieces fill in the game. Some angle iron made a convenient mounting place for these small pieces. I drilled and tapped them so that the pieces can be simple screwed into place. The main plate is attached to two long, vertical bars set the correct distance back into the frame.

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AFter all the brazing was done I sanded, wire brushed the frame. To get into some of the inside corners I used a spot sandblaster. It’s very important to remove all the flux left from the brazing. It turns into a solid, glass like substance that can be hard to remove. Sand went everywhere! The front pieces attach like below. All the mounting screws are temporary. I have some slot head ones coming which will be much nicer than the cross head ones.


The pieces on the face have beveled edges so they fit nicely into the frame. It’s very hard to see how all this is done on the real Bombe so I am sort of making things up as I go now. Since I am not making the entire thing though I don’t have to be (indeed I can’t be) exact. I can just be Bombe like!

The main front plate is a sheet of Tufnol drilled to take the steppers, the pointers and the main circuit board.

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With everything temporarily mounted I was able to see how the drums (only loosely assembled) would look. I think that’s getting there! You can just see the face plate is mounted behind the vertical bars it bolts to. This is so when I make the side covers I can fold the front edge right around and bring it right back to the face plate. The bar there gives it something to rest against.

The main board attaches to the back of the face plate. I machined up some 50mm long brass standoffs to hold it up past the steppers. The space between the motors and the circuit board also provides a handy place to hide all the motor wires.


Next I made the little switch unit. On the real Bombe this seems to have two little paddle type switches. I am just using push buttons. To mount them I cut a piece of oak (same as I used on my Orwell computer case). After cutting to shape I drilled two cutouts underneath it so there was room to poke the switched through and do up the locking nut. I kept the block thicker than needed while I drilled then later planed and sanded it down to the required thickness.

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The curved cover that goes over the switches I made from steel. That was a simple job but I needed something round to bend the metal over. I looked for cans or piped or anything the right size to bend the steel around. In the end I found that the rear axle of a 1929 Riley was exactly right, so I used that! Vintage cars are good for more than just banging my shins on after all!


I then soldered on a simple steel back. The back is right up against the face plate so not really needed but on the real Bombe it looks like there is a flat metal plate just behind the keys under the cover. You can see this is painted crackle black. So I put a back on mine so when the whole cover is painted crackle black you will see the back of the switch cover is the same.

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The cover attaches to the block with four small wood screws on the sides and the whole thing bolts to the frame.


Now we’re getting an idea what this thing will look like finished.


I next need to make the covers.  They will screw to the frame. I am not looking forward to drilling and tapping all those holes! The bottom will have the castors on it. The top will have a handle so I can move the thing. I was originally going to power it from a wall type plug in supply. The main board has a rectifier and a regulator so any voltage above around 15 volts, AC or DC will work. But then I thought well, the whole thing can be portable. I am adding a carry handle after all (which the real Bombe definitely didn’t have!). So I have ordered a small boost converter and can run the whole thing off a small SLA battery that fits inside the case. The whole thing will be completely standalone then. It’s not efficient stepping the voltage up then down again but for this that won’t matter. A run only takes 20 minutes and it draws well less than 1 amp of current so even a small SLA will be able to power it for many hours. I also still need to do the lettering on the drums.

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