CRANKSHAFT- using the mill instead of lathe
My first attempt at making a crankshaft for the triple expansion steam engine involved turning the workpiece between centres.
It worked in a fashion, but only at 200rpm. At that speed, not great finish. And frankly it was scary and hairy!
Then I discovered that I had made a 3mm mistake in the position of the middle big end bearing, so it all had to be done again.
So today, with some new steel, I decided to use the vertical mill instead of lathe. Actually, I turned the cylinder to size on the lathe, after drilling the centres on the mill. I tried to turn the big ends on the lathe, (eccentric turning, using counterweights this time) but I was still not happy with the result from the intermittent turning.
So I tried a different method, using the vertical mill, and rotary table, set up as in the photos.
After my initial problems with making the crankshaft, I asked and obtained advice from my Model Engineering Club colleagues. That resulted in the decision to machine the big ends first (thanks Stuart) and counterweight the turning when doing offset turning (thanks Malcolm). Also thanks to Peter V, for double checking my measurements this time, and jollying me along.
Still a lot to go to finish the crankshaft, but I can see that this method will work. I might motorise the rotary table before I start any more of the 8 remaining bearings.
p.s. 7 December 2019 (4+ years later) I did eventually motorise the rotary table, with a stepper motor, and it is CNC controlled, along with the XYZ axes on the mill, by Mach 3. The triple expansion engine has been running on steam for over a year, and is virtually finished except for some optional small fittings (like cylinder waste drains, builder’s label).