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Main Frame

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

The Frame on this model is made up from solid steel bar of 7/16", 1/2", 9/16" and 5/8" diameters and is in mostly straight sections.  The bottom rails allow the engine and gearbox to be slid backwards and forwards for chain adjustment.  The cast knuckels are built up from steel and the whole assembly will be silver soldered.  The headstock houses a deep groove ball bearing and an angular contact ball bearing to take the weight.  Some joints within the frame are just butt-brazed, the model has counterbores in the relevent sections to privide location and extra strength for the silver solder.

The rails which the engine and gearbox are attatched to are 1/2" diameter and have a tight 180 degree bend at one end.  The full size rails are made of tube but this would've folded badly when bent so solid bar was used.  A metal former was made in the dividing head using the side of a 1/2" end mill, this was made the diameter needed because the distance between the bars must match that of the feet on the engine and gearbox.

The rod used was far longer than finally needed so that I had something to grip and was held in the vice clamped between the former and a steel plate.  A drill press vice was used to grip the rod so that it could be manouvered in, out and rotationally about the former.  This was a two person job, one using the large propane blow-torch and the other holding the drill press vice and pulling the heated section back towards the former.  A steel pipe was then slid over the end sticking out and the bar pulled round.  It required several attempts to get it all the way and after alot of faffing we got it as I wanted. 

The U shape piece was then shortened and the ends milled square in the milling machine. 

The joints in the frame are either held together with cast sections and are pinned and brazed, or are just plugged together and brazed.  The steel assemblies below are the joints between the lower rear wheel rails and the cross bar which is brazed to the engine rails and is shown in pic 1.  The photographs explain pretty well the process of manufacture although they do not show the expanding mandrel that was made for this job.  An expanding mandrel is shown further down the page and is pretty self explanatory but a description of how it is made is given.

As stated the frame is made up of mostly straight steel tube with one or two exceptions.  One exception is the main rail which runs vertically from a cast section attached to the main engine rails upto the cast boss beneath the seat.  This scale 9/16" diameter bar has a 20 degree bend in it towards the bottom end.  This is quite a large diameter to bend normally so a jig was made which is basically a block with a hole through it and a radius on the edge.  The hole was made to be larger in diameter by the possible expansion of the rod whilst being heated so that the only chance of bending is at the surface.  The rod was then heated to a cherry red colour before being placed in the jig, a steel pipe was placed over the end and bent. 

Even after heating to a cherry red it was still very hard to bend and the heavy cast iron table moved under the force.  A protractor was used to get the angle correct making this a two person job.
   Marks out of ten for my socks on the back of a postcard.

The headstock is made up of four sections and TIG welded and silver soldered togeather.  TIG is used to join the main parts together and fill in the gaps with steel filler rod to make it appear to be a casting.  Silver solder was used to attatch a flat plate on the vertical tube for the steering damper clamp.

As shown below, the main parts are designed so that they push into the bearing housing, giving them location whilst they are joined.  One of these days I will get around to actually wighing before and after to see how much metal I removed from the bearing housing as I had piles of swarf everywhere.  A tilting vice or dividing head is a must with all the angles on the frame, and using a vice the holes were put into the bearing housing into half it's thickness. 

I do not have a TIG welder as they are horrendously expensive so took the opportunity to visit a friend, see what he was upto and use his TIG.  I had not done any TIG welding for over two years so this was a bit of a gamble, but it's like most things once you know it comes back pretty quickly.  I used filler rod to fill in the gaps between the tubes and the housing to simulate the casting, somewhere in there is over one and a half lenghts of 1/16" filler rod!  I also took this opportunity to weld up the first part of the underseat knuckle for its second op'. 

Once the job had cooled and tea was drunk, I went back to my own shed and got the dremel out.  Using a mixture of dremel, red marker, needle files and sore fingers I managed to get it looking like it does at present.  It is not finished yet as I still have the fixing plate for the steering damper to attatch and some final polishing etc. 

Oh Bugger...I had previously machined both sets of seats for the bearings before I welded the unit up.  Well, during the welding process it has bent the housing slightly so the bores do not run true anymore.  I have yet to do anything to resolve this but current thoughts are to silver-solder a block of steel into the bored-out lower bearing and then re-machine.  What a plonker - wont be doing that again.