Wednesday 10 June 2020

Rear Sprocket and Brake Drum

On these early AJSs, the brake drum and sprocket are combined into one piece and this is bolted to the cotton reel hub that supports the bearings and spokes.

Rather than buy a huge piece of steel, cut the teeth on the sprocket and then machine the brake drum, I have found it much easier to buy a triplex sprocket and machine off 2 rows of teeth. Why a triplex sprocket? ….because this has sufficient material for the brake drum and I don’t have the tedious job of cutting the sprocket teeth on the milling machine, which is what I would have to do if I started with a large piece of round steel.

These are made of EN8 (080M40) steel and can be ordered online but, because this is a really heavy item, I prefer to buy it “over the counter” and collect it from a local supplier, Redhill Bearings

Although these sprockets are one piece, I have found that the sprocket teeth and the hub are often not machined concentrically. Putting the hub in the lathe chuck (my chuck is pretty accurate) and with a dial gauge on the tips of the teeth (carefully!) the eccentricity can easily be greater than 0.030”. If machining is carried out without being aware of this, the eccentricity would translate into the finished item and proper chain adjustment on the bike would be impossible – slack – tight - slack etc…

The first step, therefore, is to support the sprocket in the chuck using the teeth, check that the teeth are on-centre, and lightly skim the hub to have a concentric OD that can held in the chuck for the main machining.

The next step is to mount the sprocket in the lathe using the now on-centre hub and to machine the brake drum.

In this case, the centre row of teeth is to be retained and the rows either side are removed. The final lathe operation is to machine off the hub.

The sprocket is then set up on a rotary table on the milling machine and the circumferential slots and the locating holes and threads (6 in total, 3x locating spigots and 3x studs) are machined.

....and then to dispose of 3 dustbins of swarf!

And finally..... together with some new long nuts (copied from originals) and locating studs, the wheel is complete.

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