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Brake Drums

A website dedicated to the construction of an accurate 1/2 scale replica of a 1937 Aero Douglas Motorcycle

The brake drums are made from cast iron and are held onto the wheel hubs with a splined taper and a large nut.  The rear brake drum has the sprocket attached to it meaning that all the drive goes through these splines.  The drums are machined from a solid billet with the central cast internal webs machined on the cnc mill.  It's alot of faffing around but I do not have casting facilities so this is the only way. 

The cast iron billet was purchased at the Midlands Model Engineering Exhibition using some of my student loan money, so at least you know some of it is going on something useful.  The 3 jaw chucks did not have the holding capacity for this 4-3/4" diameter billet, so the 4 jaw chuck was used and roughly trued by bringing a tool up to the edge when slowly rotating it.  The billet is no where near round to start with and due to its size, the top speed used will not be sufficient to cause any issues with vibrations.  Tungsten Carbide tools which are suitable for cast iron are painted red.  This is not saying that tools not painted red aren't suitable as im sure some will be, but for instance, tungsten carbide tooling brazed onto a mild steel shank painted gold are designed for steel. 

The ML7 does not have any power feeds and so it took quite a long time on top back-gear to remove the amount of material shown in photo 2.  This is roughed out to a stage where it is worth sharpening your tools for finishing cuts.  A diamond or green grit wheel should be used.  If you don't have any of these (which I don't) then someone at your local club will.  To ensure that everything stays running true, the billet should not be removed from the lathe.

The inner webs were produced on the CNC mill. 

The rear of the hub was machined hollow using the topslide set to 6.5 degrees. 

Brake Drum Cover

The brake drum covers are cast in aluminium and carry the shoes, springs and adjusting nuts.  

My trusty bandsaw - very small but because of a countershaft intermediate drive can go through 1/2" aluminium plate without too much difficulty. The rough cut plate was then chucked in the 4 jaw and the initial faces skimmed flat and smooth.  The vertex dividing head is probably the most praised tool in my workshop!  The ability to remove the chuck from the lathe, use it on the dividing head, then replace it on the lathe is invaluable.  Throughout this process it was never taken out of the chucks afte initial set up.  After the head was set up vertically and centered about the spindle the brake lever hole was drilled 3/8" using progressive drills and then a new 3/8" slot drill.  A top hat plug was then made to fit the lever hole as a guide for the front recess.  A specially ground tool (a 10mm slot drill with an 1/8" radius hand ground on the corners) was used to recess the front face about 50 thou using the guide as a stop.  The top hat bush whas then reversed, held in the chuck and replaced the 4 jaw in the vertical dividing head.  Note the previouslly machined thread.  A nut was made and the cover plate fixed onto the spigot.  A 1/4" ball nose was then used to clean up the edges around the lever hole

The next stage is to do the rear of the plate.  This has a few protusions and some steps.
The soft jaws were machined to accept the reversed plate.  An old outer race was used to lock the jaws during machining.  The chuck was then removed from the lathe and then screwed onto the already set up dividing head.  After setting the lever hole in the y-plane, the chuck was accuratly rotated and offset half the width of the protrustion.  An end mill was then used to remove the unwanted material between the protrusions by rotating the dividing head like a rotary table in the same way as the front was done.  Great care was taken to try eand get it to blend as much as possible with the previously turned face.  

The bonus of the vertex dividing head - it was brought down to horizontal without altering any settings to drill and tap the shoe-adjustment screw holes.  It is still not finished as I ran out of time - just the webbing in the centre to finish.