I cannot claim to be much of a model engine builder, I have reconned more fullsize ones, but I'm constantly amazed by the skill, patience and perseverance of the builds on here. In stevehuckss396 thread on the Small V8, I suggested that it might be possible to machine the crank in a big bore 4 jaw chuck. Nobody said "thats how I do it", so it's been bothering me and I had to try it. The advantage is that you can do all the heavy cutting close to the chuck, lestening the probability of chatter.
My 6 ins 4 jaw has a bore of 1 3/4 ins so, there is enough to offset the crank. Not trying to make a crank for an engine, just try out the principle. Material to hand was CRS 25mm dia. The crank has 6mm offset, so on the mill I drilled one centre and 4 offsets. All other sizes are 6mm for webs 10mm for journals, with the end mains 12mm.
This is how it turned out, with some photos of the method. (I hope - haven't posted photos yet.)
My 6 ins 4 jaw has a bore of 1 3/4 ins so, there is enough to offset the crank. Not trying to make a crank for an engine, just try out the principle. Material to hand was CRS 25mm dia. The crank has 6mm offset, so on the mill I drilled one centre and 4 offsets. All other sizes are 6mm for webs 10mm for journals, with the end mains 12mm.
This is how it turned out, with some photos of the method. (I hope - haven't posted photos yet.)