On Monday, June 26, 2017 at 6:40:30 PM UTC-7, bob prohaska wrote:
How were crankshafts made before the development of specialized
machine tools?

I did find http://modelengines.info/goldeng/const/crankshaft.html
but I can't figure out how the author located the offset center
holes in the blank.

That model was turned on centers, with two center-drilled pivot
positions at each end. Then, the ends were cut off so the finished
item doesn't show where the centers were.

Probably forged or cast rough items were similarly turned on centers.
After you finish the ends, the evidence is gone.