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Crankshaft

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


The crankshaft was made when the camera wasn't around so there arent any pictures showing each component during manufacture.  I will try and describe how each part was made without sending you to sleep.  I do have photographs of how the crankshaft was assembled which may be of interest.  This needs to be read in conjunction with the Connecting Rods section.  


The crankshaft was made from En16 which is a shock resistant high tensile steel and it's also what I had at the time.  The crankshaft is very well balanced and beefy, which is good as im still hoping to ride it!  It is manufactured as the full size one it, including the bearing puller slots and setting steps although these were not needed on the model.  The two outer crankwebs were machined in the myford by turning the outer diameter and all the features on the front in one go insuring that everything runs true.  There is a large radius where the crankshaft meets the web and this was done with a HSS tool ground to the radius required but because of the incredibly large cut, this tool was only used after it had been nibbled away with a smaller radius tool and even then the lathe spindle was rotated by hand using a handle in the rear of the spindle.  The myford is not rigid or powerfull enough to cut this in one go without chatter and once you have chatter marks the tool tends to follow them, making them impossible to remove. 
The flywheel end has the large diameter spigot, taper and fine thread to run in a large diameter ball race & oil seal, accept the flywheel and flywheel retaining nut.  Really i should've made the flywheel nut before I machined the thread on the crankshaft but as i hadn't and it was set up in the machine, I made the thread as close to standard size as possible having to make the nut fit the thread - which is always harder and more painful to do.  It was centre drilled at this point ready for when it is assembled.  On the timing end it was a similar story except I had a nut at hand for the screw-cutting.  The diameters in ascending order are for a deep groove ball race, clearence through the crankcase, timing gear location and timing gear fixing castellated nut.  In the very end there is a 6BA left hand thread to accept a special screw which prevents the castellated nut (which drives the oil pump) from unscrewing and bursting the timing case! 
The timing gear can be seen in the Valve Gear section.


The turned parts were then turned around in the chuck and machined to length as the material used was only just long enough in each case.  They were then bolted down to the milling machine table with a thick flat aluminium spacer and the spindle clocked up around the crankshaft.  Once the dials were set to zero the table was moved along and the crank pin hole drilled.  To size the hole finally, it was taken close with a drill then the head raised and a boring head used to take out the final few thou. 
The counterbore seen was done later on the rotary table and will allow a tab washer to locate as it stickes through a break in the edge of the web.  The crankpin was done first because the maximum number of clamps could be used to prevent it from moving whereas, when the web was machined, clamps had to be moved to make way for the cutter.  The parallel section was machined on both sides first then the job rotated the correct amount and then the radial faces machined to meet smoothly.  The parallel section will be used later for easy of assembly.  The slot and step was machined at a later stage as they are really only window dressing.    The rotary table was then used to create the raised section on the rear for the bearings to 'rub' on, this was done using a fly cutter and rotating the job after clocking up on the crankpin hole.  

The central web was a little bit more complicated as total accuracy is required - any difference in centres and the journals will not run true,  but is basically the same as any other crankshaft machining operation.  There is a very well written explanation on the Model Engine News IC page which can be found in the links section. 
The groove in the bearing cheek is designed so that when the crankweb dips in the oil puddle it picks up oil and guides it into the bearings.  The top hat bushes which act as spacers between the bearings and the crankwebs have been described in the Connecting Rods section and are designed so that they expose the open ball cages of the ball races hopefully meaning that they will get plenty of oil.  You  can see in the photographs I have already roughed out the dowel locating holes in the crankpins and outer web holes, I will not do this again but wait until it is fully assembled if I were to do another one. 


The tab washers are made from thin steel and were cut on the rotary table and then filed to have sharp corners by the tabs.  The washers were then held between two nuts on an angle plate and a 15 thou
slitting saw used to make the tabs.  The slitting saw was also used to produce the inner tab by raising it 10 thou at a time and taking a pass.  You must be careful when doing this as the cutter may bend away from taking the cut and shatter, in this situation everything went rather easily.  A few spares were made so that if the crankshaft had to come apart for any reason, I didn't have to set about making tab washers from scratch again.
The two Bolts which go through the webs, crankpins and tighten into the central web are high tensile steel.  If you are ever looking for a source of high tensile steel in bolt shape - use a commercial bolt!  I used an M10 bolt and machined a large-headed fine-threaded crank bolt from its centre - perfect as these bolts have to be done up tight.  




The complete crankshaft can be seen in its component parts being prepared for assembly.  The ball races were cleaned deeply in petrol and then blasted with the air line.  Any tight or rough spots must be dealt with beforeit was assembled.  It took several attempts to get one bearing clean.  Then once they are fully degreased and spotless, they were filled with light oil and put somewhere clean and free of general anything.  You may have noticed all the butter pot's in the background of photo's - they make quick clean storage tubs at no cost! 
The baby press was used to assemble the crankshaft in its rough state.  When pressing home the webs, they came up tight on the top hat bushes which then come up tight on the inner race of the bearing and leave 2-3 thou clearance between the side of the connecting rod and the cheek of the web, this means that there is no way the inner race can move and it is perfectly spaced.  The crankpins on the centre web are a tight push fit in the outer webs meaning that it could be assembled and then tweaked before pinning and bolting it all up tight. Plenty of oil was used in the assembly as it was very unlikely that the engine would see engine oil for a year or two and i didnt want things to go rusty after the connecting rods dunk in petrol. 


This is where the careful machining of each component, the centres of the centre web and parallel sections of the outer webs, are worth their weight in gold.  The centre web was bolted down on a parallel on the milling machine table and the DTI was brought into use. 
The parallel section of the outer webs were tapped parallel to the centre web by a lead thumper so as not to mark the job.  This was done on both sides and gave me a good starting point.  

The assmebled crankshaft was then moved over to the lathe and the centres in the end of the outer webs used for the first time.  Very little pressure was put on by the centres as it would bend the shaft in its current state.  Two clocks were used on the journals to measure the run-out.  They were both within 2-3 thou which as a starting point was very good.  One ran out slightly more than the other so it was put back on the table and tweaked, then put back in the lathe and measured.  This was repeated a few times until the journals ran true - the machining was accurate!   It was now that the bolts were out in with large commerial nuts under the heads and nipped up, and of course the crankshaft twisted a little bit as it pulled up the last few tenths from the tweaking.  This time the crankshaft was tweaked with the bolts nipped up which was more difficult but meant that I was less likely to overshoot.  Once it was true again in the nipped up state, the large nuts were held with a spanner whilst the bolts were undone.  This meant that the crankshaft did not try to twist so much and stayed put. 



The dowels were the next thing to concentrate on.  Luckily on my part, my roughed out dowel holes lined up pretty good although i would not recommend doing it like this again.  I was able to open out the holes a little with a drill bit, enough to clean them up and make them round.  Dowels from mild steel were turned up a 'loose' push fit - one of those annoying fits where you can't push it in by hand but it will fall out on its own accord!  The ends were also spotted with the centre drill before being parted off.  This centre-drilled end allows easy location for a drill bit if the dowel has to be removed at a later date, as trying to centre a drill to remove a dowel or pin easily is impossible.  The dowels were Loctited in after the holes and pins were cleaned with an industrial degreaser on a cotton bud.  The Loctite was given longer to set as I would normally give as i really did not want a twisted crankshaft.  

After 5 or so hours (yup much longer than normal) I set about fitting the tab washers and bolts.  The tab washers can only sit in one way because of the cut-out for them.  The outer two fingers get bent down over the web and the inner finger gets bent up to prevent the nut from turning.  The Bolt was done up using the standard tommy bar and socket and was done up TIGHT.  Remember the bolt and web are high tensile steel and the amount of thread engagement is over 1/2".  Sadly the bolt did not tighten up in a 'pretty' position meaning the central finger was belt over a corner on both bolts - oh well.  It was then put back in the lathe to see if everything was still running true, there was less than 1/4 thou run out on one journal which I can live with.  Now the crankshaft is done, items such as the valve gear and flywheel can be started and the engine might start to look like an engine!