Wednesday, 21 September 2011


I have sore fingers. Since I spend many hours at a keyboard this has made the working day even more fun than usual. Most of my shed time this week has been spent making powdered steel. I don't actually want the stuff, it's just an inevitable by product of shaping metal. I also made a lot of swarf, I don't want that either.

The first job was to make the upper and lower legs fit nicely onto the knees. With the tangs milled to width and depth it simply meant carefully filing the stubs to the correct elliptical profile. As the correct size and shape is approached it is important to go carefully as it is quite difficult to file metal back on again. The fit needs to be reasonably snug so that the braze will get sucked up into the gap by capillary action. The upper legs were easier than the lower legs since the lower legs are tapered. One of the upper legs has a very slight twist from the rolling process described earlier, this is only a few degrees and by filing the tang to compensate, I've been able to remove the error at the top end of the tube. The result is not noticeable and will mean the head will be easier to make when I get to that part.

I treated myself to two new files this week.
Fitting the tubes
The next job is to smooth the transition between the tubes and the bearing. This is the fun part as time spent here will pay dividends in the overall appearance of the finished machine. I'm aiming for an authentic reproduction and I don't want anything to look out of place.

Rough cuts, note the tube off cuts protecting the tangs
Smoothing cuts
A lovely pair of knees. The channel in the side of the
lower tang is to allow the inner weld bead to remain intact.
Still to do is to hollow out the ends of the tangs for about half of their depth. This has several advantages other than the obvious weight loss. Firstly it will make the braze up easier as the braze will be able to flow to the extreme end of the joint more easily without having to over heat the tube. Secondly and more importantly, it removes a stress riser at the end of the tang. This was well understood back in the day, early bikes tend to have a solid stub brazed into tubes, whereas later bikes have a hole bored into it and the latest examples have feathered edges. The most obvious joint where this is a problem is where the neck casting is brazed into the backbone. Paul N. Hasluck describes the problem and the cure in 1897. This evolution can also be clearly seen in contemporary drawings.

Cycle Building and Repairing - Paul N Hasluck 1st Ed. 1897
Click for a readable view
I have experienced this issue myself when I cracked one of the fork blades on my racing penny half way around the 'Cambridge 50' some years ago. The crack was right at the end of the tang on the sharp V edge of the fork blade. Interestingly, It also demonstrated to me how squirrely that particular bike is (22bwg tubing) since I only noticed when the crack had propagated fully half way through the width of the blade. Fun to ride though.

I had to walk 15 miles because I failed to spot this early enough
With all the filing complete, I can do a trial fit up. I've made another of the male threaded blanks with the same recess and also machined off square a tube that is a snug fit into these recesses. By installing a blank in each knee and bolting up with the tube in the recesses I can ensure that both knees are spaced at the correct distance, parallel and concentric to each other.

Holes for the oilers and inner bearing locking grub screws will be drilled after brazing
The top ends of the upper legs are spaced at the correct distance for the head. The lower legs are correctly spaced but are slightly out of line with each other, probably only by 1 degree, but I'm a pedant and I'll need to correct that with some more careful filing. I won't braze anything up until I have the head machined since I'll probably need the jiggle factor of all the joints to get everything in the right places.

In other news, I managed a short ride at the weekend. Sunday was a fantastic day, so I borrowed my wife's recently completed Royal Enfield (with permission) and took the kids on a gentle 22km potter around the Canterbury Plains, we had plenty of breaks and snack stops to keep the kids and my backside happy. The Enfield was the perfect machine for the experiment and I'm pleased to report it went very well. I'll try and ease back into regular cycling over the next couple of weeks or so. You have no idea how good it felt to be back in the saddle.

I've had a request for a photo of my wife's shoes.

Red ones, naturally
She bought some more at the weekend too, she doesn't know that I've found them yet but that's OK because I've hidden a new bench grinder behind the door in the workshop.


  1. I have just discovered something about Blogspot photos. If you right-click over the small image, among the options is Copy Link Location, and if you click on that, and open a new tab, and paste in what you've just copied, you get the photo at full size and in detail. So I can now inspect the threads of your knees with a microscope.

  2. I have just discovered something else. You can just left-click on the image and it springs up full-size anyway. I never knew that before. - Nice shoes, Mrs K, BTW.