Now for the long answer...
1) Take the cast crank and determine the center of each end. Mark it, center punch, and center drill both ends. This so you can put it between centers on the lathe.
2) Take a short bolt and nut (or a piece of stock cut to fit) and put it in the gap where the throw is. Adjust so its "finger tight". That is, its putting a little pressure on both sides (you should file the sides to give yourself a flat surface) but you can just pull it out. Idea is to prevent springing when you put it between centers, but you don't want to introduce and springing. (a future photo will make what I'm trying to say this a bit more obvious.)
3) Between centers in the lathe, turn the two main shafts. Don't take them to final dimension yet, leave them oversize. You're really just cleaning them up at this point.
4) Make two blocks that kinda look like this:
Let me explain what you're looking at. You have one hole which is drilled to take the axle, whatever dimension you've turned it to. You've also drilled, using a centerdrill, at the given distance for the crank throw. This is how you'll turn that part of the crank. You've used a hacksaw to cut a slot to the hole for the axle (doesn't have to be pretty, just needs to be able to clamp down on the crankshaft). Then you've drilled a hole thru, perpendicular to the slot, threaded on one side of the slot and clearance drill on the other. Tighten onto the crank, making sure both blocks are parallel. The extra bolt is to lock the tightening screw so it can't loosen up during machining.
Looks like this from the side:
What you're looking at here shows what I mean by putting a bolt and nut "finger tight". In this photo, its preventing any springing when between centers and you're turning the throw. DO NOT tighten with a wrench, or you'll introduce springing. You're just making a spacer exactly the distance from the end blocks to the web. I can slide those out with a little pull. But, you can't spring the crank with the pressure which comes from mounting between centers. When turning the mains, (step 2) a similar, smaller bolt/nut goes in the small gap in the center of the crank.
You might be wondering if they'll fly out when in the lathe. Nope. When you're turning the throw, these aren't "off center", the centerline of the bolts is the center of rotation, so there's no centrifugal force on them.
5) Make two cutting tools which look like this:
This is how you're going to turn the throw. The larger one is for rough cutting, the smaller for the finish cut. Both sides are sharp. You need to set it up so both ends make contact at the same time. Easiest way to do this is to push it against the blocks at the end, with a piece of paper between the blocks and the tool. Move the paper until you're feeling the same resistance on both ends. If it pivots on one side and the other is loose, its not right.
The width of the finishing cut tool must be less than 1/2 of the width of the throw, so you can make a pass and both ends of the tool will go beyond the center. That'll make sure you don't have a ridge in the center.
6) Put the crank between centers on the lathe, using the two center drilled holes for the offset. You'll have a fair amount of offset mass, and an intermittent cut, so you want to use a slower cutting speed.
With the rough cutting tool, cut both sides of the crank, and turn the throw oversize, but to close to final dimension. Then put in the finishing tool and turn to the final dimension.
IIRC, I then used a "normal" tool to machine the radius on the end of the web. Note you're only getting one end in this setup.
7) With everything still together, use the end blocks to put the crank in your milling machine and finish the sides of the webs. (that is, the surfaces which you can't turn in the lathe!)
8) remove the blocks. Put the spacer back between the now-machined webs. Put between centers and turn the main shaft to final dimension. Also machine the radius on the "other end" of the web.
When you're done, it should look something like this
I hope this helps. If there's any part that's confusing, ask and I'll try to explain it better.
They supply very good documentation in the form of a large exploded diagram and detail machining plans:
and I had not seen one with multiple small castings tacked onto a larger one. When I was looking at the machining drawing for some of these, I wondered how I would hang onto them while milling. Now I know.
and, of course, a bag of little bits - screws, setscrews, piston rings, etc.
