Turning down a crank question (machinist)

What rpm do I turn the crank on the lathe to cut down the counterweights when going to b block?

It's a forged unit.
How do I keep from putting dent marks in the snout when chucking it up?
Thanks.

You don't want a lot of rpm on the lathe since the crank is big and heavy but speed will depend on your cutter. Shoot for the surface speed that the cutter is happy with.

I use an aluminum flange on the flywheel end and clamp on that. Then I use a centerdrilled bolt in a live center on the nose of the crank.

put some shim stock between the jaws of the chuck and the snout and tighten on that. turn the crank at whatever rpm the lathe is happy with. big lathe more rpm, small lathe, a lot less. if cutting with carbide, 300 rpm, granted you can go a lot faster but don't know what you're working with since your asking basic questions.

even though its forged the crank is still a lot softer than carbide and will cut easily. depth of cut should be at least the tool nose radius of the insert your cutting with,.

All that and your tooling and lathe must be rigid. An interrupted cut is always hard on tooling. And the machine.

I don't have it handy, but the HTRAL handbook calculates headstock RPM as a function of material X cutter distance from the work.
If in doubt, slow it down.

Under 100 rpm. Prolly 1/2 that

I'd have to double check with the machinist who does cranks for me but I think he runs them at around 300 rpm with a special carbide insert that is designed for heat treated steel. He gets a mirror finish on the counterweight and it doesn't take very much time. Only a minute or so per pass and you don't need a lot of passes to knock 100 grams off.

The last 470 engine I did we had to take a little more than 100 grams off of each end. The crank was rough balanced for about 2300 gram bobweight but the final assembly only needed a little over 2100 grams.

There should be an area between the front main journal and where the crank gear mounts where you can clamp onto it without worry. A medium or large live center on the rear flange in the tailstock.

i go with what jerry and andy recommend. above all, make sure your lathe and tool set up are as ridged as possible, as interrupted cuts create lots of vibrations and harmonics. a small[ish] lathe can do this, but it has to be a high quality unit with an equal quality chuck and headstock. the tool post must also be very ridged. not something you would want to attempt on an imported, cheapy lathe with the tool post using only 3/8" square tooling.

How much material do you remove with each pass?

Thank you for the replies.
My personal lathe is an itty bitty Logan from the 40's or 50's.
Was my grandpa's, leather belt drive. :-)

The one I'll be using is one in the maintenance shop at work and it's pretty big.

I will do search to find out what size dia. to turn it down to.

431 combo.

Unless someone can talk me into building a 440 or 451/470 instead.
I have all 3 block sizes under my bench.