OK, having made the crankcase and the flywheel end bearing I wanted a moving bit so I made the crankshaft. This is the first I.C. engine crank I've made and also the first with more than one throw so I was nervous. Another reason for making this was that it got what I perceived as a difficult bit out of the way.
As Len Mason points out in the book carving big, asymetrical, chunks out of BMS flat can lead to stresses relieving themselves all over the place and the finished part distorting; definitely something to be avoided. Many years ago I managed to "catch" a muffle furnace as it was being thrown out at work, apparently it no longer got hot enough to be useful. The crank blank was a chance to try this out for stress-relieving steel and the good book told me I'd need to hold the blank at ~900C for an hour or two. At first sight the thrower of the furnace was right, the best I could get on the meter was 400C in a furnace that should reach 1200. I wasn't totally convinced and reached for the multimeter and a thermocouple - actual temperature close to 900 and a (very quick and indirect) look inside showed an appropriate colour so I slid the bar in without any more thought. Of course I'd forgotten about scale and when the whole lot had cooled down the blank looked very rough and flakey (sorry, no picture of this) and had to be machined all over to recover:
A lot of this machining was needed anyway but I need to find an Australian source for anti-scaling compound. I've bought "Cherry Red" case hardening compound from a bloke in Qld but he doesn't look to sell the other stuff.
Apart from using Metric fine threads I haven't changed the design of the crank so I won't include a drawing here. The machining mostly followed the book but modified for the available kit which makes thing a lot easier then it would be with an ML7 and a vertical slide. The next step was to put in the three centres at each end:
using an angle plate nest on the X3 drill-mill, drilling one centre then turning the blank end-for-end to drill to corresponding one should leave these very well aligned.
After some drill and hacksaw work the middle centres were used to rough turn the journals with R and L-hand tangential tools and turn the outside of the webs:
No pictures of the Eccentric Engineering tangential tools in action but these are well worth the money. I was sceptical of these in the past but I bought the Aciera F3 from Des Burke who was the original designer of these and, being a real gentleman, Des gave me one of the tools with the mill. They work!
More drilling and sawing to rough out the crankpins and then a long series endmill cleaned up the flanks of the webs (less nerve-wracking for me than turning this skinny, wobbly, piece of metal between centres) :
Eventually I had to get on and, very carefully, turn the middle crankpins with conventional L and R-hand HSS tools that were stiff enough to reach in:
For the other two crankpins I ground a 1/8" parting blade with a "forked" end as recommended by LM:
and in close-up:
I'm pleased with the finish this gave and was very relieved to get this sequence over with. The keyway for the crankshaft timing gear needs a single point tool making. There's no point in trying to file this across a diameter when the real thing is available:
I think I left it too soft after tempering and the cutting edge didn't last too well. However, the keyway cleaned up close to the correct width:
The two threads were screw-cut with 0.75 pitch and finished with dies but I didn't take pictures. The VFD on the Hercus gives almost instant stop and easy reversing and this has taken much of the stress out of screw cutting as well as speeding things up a lot. The last operations were to drill the oil passages in the crank and the tension rose again, breaking off a drill in the work at this stage would not be good. New long series drills cut well and gave plenty of clearance and visibility and all went well:
The finished product:
The proof of this particular pudding was in the mounting in the crankcase and the crank turns smoothly with very small clearances. No post-machining distortion has occured so either the metal was good from the start or the stress-relieving worked.
Now I will start making more ali swarf and carve out the base and sump.
DT