Saturday 9 December 2023

Machining Castings and Progress on Wheel Building


While I was waiting for the spokes, I started on machining some of the AB2 castings that had recently arrived.

The first were the girder fork links (23F, 24F, 25F and 26F), specifically these:

The originals, from which the patterns were made, are in the upper part of the picture and the yet-to-be-machined castings in the lower part.

There are a number of machining operations including cleaning up the faces and drilling and tapping the internal holes

which is done on the milling machine - the DRO is important to accurately position the holes.

The links require an external ¾” x 26 TPI thread (for the damper adjuster) and are set up in the 4-jaw chuck on the lathe for machining to size and cutting the thread with a die.

….and the finished links…

You may ask why I used AB2 Ni-Al Bronze as a material?

There are 2 reasons: first, girder fork links are a stressed item and a material that has sufficient strength to withstand the loads, in particular the shock loads, transmitted between the road and bike is required. The quoted tensile strength of AB2 is 640 N/mm2 (MPa); for comparison, the quoted strengths of various steels are: EN1A (mild steel) – up to 400 N/mm2; EN8 (a medium strength, medium carbon steel) – 550 to 650 N/mm2; EN24T (a high strength alloy steel) – 850 to 1000 N/mm2. AB2 also has excellent shock-loading characteristics – I have previously used this material for Norton Model 18 inlet rockers that were subsequently used for thousands of miles of riding (see here for the 2 remaining unmachined castings).

The second reason is that this material is readily available from non-ferrous foundries.

It is, however, somewhat tedious to machine in the respect that drills easily bind – with or without lubricant and much heat is generated. Other bronzes, such as phosphor bronze, have similar machining characteristics.

The second components that have been machined are the rear brake arms (KS17/2).

The finished items, together with the original (again, used as the pattern for making the silicone rubber moulds) and a brake cam are shown below.

Most of the machining of these was done on the milling machine although the square hole was spark eroded and is a perfect fit on the male square on the cam.

Progress on Wheel Building

The spokes turned up

....around 250 in total - enough for 6 wheels plus a few spares. Yes, they are all mixed up together but Steve, who will build the wheels, says it is no problem to sort them out.

These need to be black but are made of stainless steel and, whilst I am able to chemically black mild steel, I do not have a process for stainless. I took them to JP Metal Treatments who provide a hot blacking process especially for stainless.

The nice shiny spokes, together with the rims and hubs, all of which have been gloss black powder coated

will shortly be delivered to Steve to ensure he doesn't get bored during the Christmas period.

More recently, I have been working on making Mk1 KTT cambox oil pumps (K102/2, K152 etc) and I’ll report progress on these in due course.


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