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Crankcase

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


The crankcase is a complex casting which looks annoyingly simple from the outside - it's only when you take off the front cover that you see how complicated it really is.  

I decided that the most complicated part of the crankcase should be dealt with first.  This was the inside form which would be milled from a solid block of aluminium then once this was complete, the outside form would be finished.  This block was taken to the correct outside dimensions first on the manual milling machine and 3 M8 holes were equally spaced in the 'back' face.  These three tapped holes will be used to bolt the block down to the raised table on the CNC mill.  An 1/2" aluminium plate was bolted down onto blocks so that access to the underside was given allowing bolts tp be put through from the underneat. The clearence holes in the raised table were drilled 8.3mm so that the crankcase blank can be clocked up parallel then tightened fully. 

An accurate 3D model of the crankcase was made on a program called SolidEdge then loaded into a program called Cut3D by Vectric which produces the correct code for a CNC machine.  Alot of care was taken to make sure that everything was correct as if anything is wrong at this stage there is little that can be done to correct it.  The making of the 3D model and checking the code took over 3 days.    

The corner fo the block was set to zero using a clock and the program loaded and made ready. 
The tools used are the longest I could get whilst being the smallest in diameter to preserve the sharpish corners, being 1/4" very long series, about 1.5", ball nose in tungsten carbide.  I purchased two, one for roughing out and one for the clean up cut. 
The two programs, roughting and finsihing, use the same datum in the X and Y and the top of the block in the Z so resetting the tool was fairly easy.


A neat cutting oil was used and regular chip removal was required as I was in effect milling out a box.  These photographs show the completed roughing out cut of the block bolted to the raised table. 


The finishing cut was programmed so that it would take approx 0.007" off the roughed surface in steps of approx 0.004".  This produced a much finer finish although due to the high walls and small diameter cutter there was some chatter.  The inside of the crankcase will be sandblasted to give it a cast look and feel, this will hide any chatter along with a small amount of filing. 

A larger cutter with radii ground on the corners was then used to profile the outer shape using a manual coded cnc program.  The radii were ground on an off hand grinder and using a radius gauge and oil stone a nice form was created ensuring that the cutter had the correct clearences.   

As can be seen in the photographs below, the bolt holes for the engine cover were drilled and tapped aswell as any other threaded holed within the crankcase from this side.  These tapped holed were then used to accuratly bolt the block down to do the underside by having almost no clearence in the holes in the raised table.  The block was still clocked up to make sure it was square.  The other side of the large crankcase can be seen appearing.




More to come

The tapered holes are actually at an 8 degree angle which adds more complexity to the proceedings and being tapered the holes need to be reemed the same depth or the inserts will not sit in identically.  This was solved by grinding an 8 degree point on the end of the shortened reamer allowing a reference plate to be placed over the roughed out hole in the crankcase, which is being held at 8 degrees, and the zero of the Z axis set.  The table was then wound up the same amount on each hole meaning that all four inserts went in at the same depth.