Home‎ > ‎Part Index‎ > ‎

Magneto and Ignition

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


Magneto Housing


I really wanted the magneto to be as self-contained as possible, and as i am unable to make a working magneto in this scale, i have to stuff some electronic ignition inside.  The full size magneto can be broken down easily into it's parts with the main body being tackled first.  A block of aluminium was machined square and to the approx size required.  A 3D model was made for the HT lead end of the mag, mainly because it has some nice 3 dimensional curves and a little raised bitand it would've been a pig to do on the rotary table, then Cut3D from vectric was used to convert it to GCode for the mill.  The result was pleasing but a little rough - the file comes out again!

It was then milled to width leaving a wide section on the bottom for the feet.  It was then replaced in the CNC mill and a simple 14-line program written by hand to do the other end which has rounded corners.  Below is a quick video of the mill in action with the program shown at the end.  40 thou deep cuts were taken and manually put on after each pass to a depth of 5/16".

Another simple program was written to do the feet.  A 3/32" long-series throw-away cutter was used giving quite a small radii in the corners.  This is the main body complete with everything else being attached by 8BA countersunk screws.  The plate which is cast to the bottom of the body on the full size magneto is a separate plate on this one and will be made when the magneto is plugged into the crankcase so that the thickness it must be can be found.  This must be accurately done otherwise the magneto will not plug into the case when sat on top of the engine.  It was then placed in the 4-jaw on the myford and a 1-1/16" hole bored most of the way through leaving a flange on the HT lead end for the locator plug to be screwed to.  The electronic ignition is going to have to be small bacause it must fit within the bore and in the little space inside the fake horse-shoe magnet!




Advance/Retard Sensor Ring

The advance/retard end of the magneto works as the full size one does, look likes the full size one does but the operating mechanisms both mechanical and electrical are different.  It will be operated by a hall effect sensor which must be rotated about the magnet disk, this sensor will be embedded in the pinion ring.  This was the first part of the magneto to be made and was not meant to be the real one but it worked so well that it was used with the housing made to fit the ring!  The teeth were cut using a specially made jig and a 1/4" x 20tpi tap in the lathe.  This method, although maybe shunned by the theory engineers, is used alot in the small model railway scene and is perfect for this job. 

The jig consisted of a half inch square bar with a bit of 3/16" silver steel held in the end with a grub screw.  The blank just sat on the bar and because the cutting force is still downwards, with a little oil, it rotated freely and didn't jam when being cut.  10 thou cuts were taken with some shadow-cuts taken afterwards to relieve any spring in the tap.  I really didn't know if it was going to work as there was nothing to rotate the blank like in a hobbing (gear cutting) machine but it seemed to work and it produced a stunning result (just make sure the tap is sharp - not just sharp to the touch, sharp under a loupe) I took a video of a cut just to show the proceedings.  As it only has to be advanced/retarded through 40 degrees at the most extreme maximum the teeth only span 1/3 of the circumference to allow for run in and out.  The 'rack' will be specially made 1/4" x 20tpi and be prevented from rotating when in the correct position.




Points Housing

The points housing will contain the magnets and advancing/retarding hall effect sensor ring.  It is made from brass with all it's bulges silver-soldered on.  Finally it will be chromium plated to look like the original.
The main body was made from solid brass with the outside diamaters and external square including the mini bulge cut-outs milled at the correct angles.  The bulges were made from brass and milled then filed to the correct shape.  The rack housing is drilled through 1/4" with 5/16" x 32 thread in the upper end for the cap.  The rear of the block was drilled and tapped M4 so that a rough jig made from copper lightning strip could be attached.  A jig was required to hold both pieces in position whilst they were silver-soldered, it is a total waste of time trying to silver solder pieces on like this unless they are held in position because they WILL move and wreck the job.  During the silver soldering process the whole lot was covered in easy-flo flux paste.  This keeps it clean meaning that after the soldering process it can be cleaned with a brillo-pad and it will come up lovely.

Once it was soldered and cleaned it was placed back in the lathe and the bore for the ring and bearing was machined.  When it was roughed out and the flats machined on the billet, it was not removed from the chuck at all - the chuck was removed from the lathe then screwed onto the deviding head for that operation.  Scribed lines were then made around the jaws and then numbers scratched on so that it could be replaced as accuratly as possible in the chuck (doing it up with the same keyhole) after soldering - it worked as it was only less than 1/4 a thou out in total!  As can be seen the bore broke through into the 1/4" hole for the rack.  Careful design, measurement and machining meant that the engagement of the rack with the pinion ring was backlash free although if it was too tight/loose a fit, a new pinion could be made or the original modified to suit. 

After a quick rummage I found a circlip which only required a 15 thou deep groove to sit in which is more than plenty.  It is a break from original but it will be behind a ornate brown cover and it does the job perfectly.  According to the literature there should be a 30-60thou gap between the hell effect sensor and the 1/8" x 1/16" rare earth magnet trigger, the only issue i have at the moment is whether the steel ring with magnets spinning within it will induce funny magnetic currents and fool the sensor.  I may have to make a bronze ring instead to get around this. 




Electronic Ignition

Yes it really could be this simple!
Im hoping to get the entire ignition system including a simple distributor inside the body of the magneto - which is tiny.  So far no commercial system will do so I will have to design my own. 




Other Parts

The magneto plate.  I decided to make the numbers the same as the
duff  full size one that i have as a rough 'spare'.  In the photograph the plate looks very liney but please remember that the plate is less than 5/16" high and in real life it looks fantastic, its clear and you cannot see the machining marks they are so close together. 

Now that the main body has been shaped and I've had my fun engraving the plate - its back to business.  The magnet which sits over the magneto is approx. 1/4" thick on the full size model giving a thickness of 1/8" steel plate which needs bending in a smooth curve.  There is a pressed aluminium sheet which is screwed to the top of the magneto body and covers the archways at the front and rear of the magnet and these are ornately engraved with the logo of BTH.  This would be very difficult to replicate so I decided to make the whole lot from the solid.  Once the magnet has been painted black you will never know.   

Now that the magnet and side covers are one thing, I do not have screws going into the top of the body anymore allowing me to cut a hole in the top and give me more room for the electronics.  The top of the magnet block will also be hollowed out giving me extra space.  As room is so tight all the space I can get is a bonus.   It looks quite spacious now and i should be able to redesign that circuit with the coil poking out of the top under the magnet and the other components on two boards either side of the shaft running through the centre.  The cutter used was a long series ball nose 3/16".