Turing-Welchman Bombe physical build – part 2.

July 30th, 2015

Even though I keep procrastinating about working on the Austin 7 it actually works out well since I am motivated to work on the Bombe. Following last nights few hours in the shed making the little clips, tonight I did a few more hours and started on the hubs. I didn’t complete them, it started getting late and I figured my chances of muffing something up were increasing so I came in, but I made good progress.

I started with a small bit of aluminium bar that happened to be the right diameter. This was lucky since it was the only bit of aluminium bar I had! I faced the ends in the lathe then drilled an 8mm hole through the middle for the main shaft.

IMG_4037_1 IMG_4039_1

I then glued a paper cut out of the hub to the end. While the glue dried I changed the 3 jaw for the 4 jaw chuck in the lathe. To get everything the same I wanted to drill all the holes at once in one piece of aluminium them slice that up to make the three hubs. With a 3 jaw chuck anything round you put in it will automatically be centralised as the three jaws all move in and out together. In 4 jaw chuck however the jaws all move independently which means you don’t have to have things centralised as all. So I was able to have the bar off centre and drill each hole that way.

IMG_4040_1 IMG_4041_1

Being off centre does mean you can’t run the lathe too fast. Off centre means it is also out of balance and if you go too fast everything vibrates. It took quite a while to drill the holes as they needed to be deep enough that I could do all three hubs at once. It meant much drilling then backing out to clear the bit of swarf. Including the centre hole there are ten to drill in total. The four holes for the mounting bolts are plain but the others needs to be tapped so they are drilled undersized.

Here I wasn’t quite sure how best to proceed. And I forgot to take pictures! The question is do you cut the three hubs then knurl each one or do you knurl the thing in one piece then slice it (knurling being the rough, diamond pattern on the edges of the hubs). I did the latter. I first used a parting off bit to cut grooves in the bar. I couldn’t part all the way through anyway because of the holes through it. I find when I try that the tool catches in the holes and makes a mess of things. So I just made a groove as a guide to cut the slices with a hacksaw by hand.

I did that then had to think how could I face the front and back of the hubs without crushing my knurls in the jaws of the chuck (back to the 3 jaw now of course). In the end I simply wrapped the hubs in layers of electrical tape. I was then able to grip them in the chuck without damaging them. I was then able to clean up the faces. The advantage of doing it this way is it makes the faces nice and flat. If I faced them then knurled the knurling would leave a slightly rough edge on the faces. Knurling doesn’t cut the metal, it deforms it.  I also used the parting off tool to remove the knurling from the bottom of hubs as on the originals.


Next I needed to countersink the holes for the four mounting machine screws. These go through the hub, through the Tufnol face plate, through the tin and through a plastic disc that will be inside the tin to allow me to mount it all to the motor shaft. To countersink the holes I used my pillar drill. The depth stop on it is broken but with a moveable table that doesn’t matter. I simply put the bit in the chuck then set the table height so that when the drill is all the way down it is where I need it. I set it a little short then to make adjustments I use spacers (blocks of wood, sheets of metal and so on) under the workpiece as I drill. That way you can get the depth consistent for all the holes.


To get the heads of the mounting screws just under the surface I set up the drill so they were flush then used a 0.5m sheet of brass under the hub to lift it up and increase the depth of the hole 0.5mm. The heads much be under the surface so the mounting clips don’t hit them.

I still need to tap the other holes. There are the holes for the screws that hold the little clips in place. One for a larger screw that holds the end of the clips down.  And two for little stops that stop the clips opening too far. The recreated Bombe at Bletchley Park seems to use little screws here on some drums, the heads acting as stops. But John Harper’s rebuild page shows little pegs being used which is what the US Army diagram seems to show.

I’ll probably use pegs. These little details are so tricky to get right and probably no one but me notices or even cares but it’s nice to do thing properly as far as you can. I wish I hadn’t lost the film of photos I took on my last trip to Bletchley as I know there were some close up shots of drums on that.

The hubs will look something like this once on the drums.

IMG_4050_1 IMG_4051_1

The other thing I did today was try to find some Letraset dry transfers that I can use to do the lettering on the edge of the discs. Seems this stuff is now hard to find (in NZ at least). I found some places online selling something but their web sites are crap and they don’t specify what exactly they are. I need 1/8th (3mm) white letters. I will try Gordon Harris tomorrow and I think maybe Spotlight has something as well?

I am so glad I only have to make three of these damn things. Makes you appreciate the work that went into creating hundreds for the rebuilt Bombe at BP. One interesting thing to note is the three indicator drums are slightly different to the main drums on the Bombe. For a start the letters go around in the opposite direction. Also the main drums and faceplates have holes in them. The three indicator drums have no holes. I am guessing it’s something to do with the indicators not having any brushes?

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