What lathe do you have as a small one won’t have the grunt or rigidity to handle the amount of tool engagement that a 16mm dia insert will require.

I’d probably work out a few co-ordinates to rough out the waste and then make a form tool either half round end or maybe a 1/4 of the circle and do one side at a time.

A form tool after roughing out to somewhere near, but why the need for such accuracy ? Made some pulleys for a winch which were in steel this way. Good luck. Noel

Why are you using a ball nose cutter?  Use a plain 16 dia end mill, plunge into the side and then rotate the work.  A 16 cutter working on the side face will rip metal off pretty quick.

Martin

AH YES ! Bending thin wall is never easy and even with tight formers there is a limit. Again, good luck. Noel.

An option might be to use a 5/8″ cutting tool and do the final finishing with abrasives. That would put a good surface finish on the roller, which could be difficult to achieve with other methods.

Having both imperial and metric tools in mind could give a wider choice for buying something.

Delrin might also be an alternative material and allow use of tools designed for woodworking.

I’ve made aluminium pulley sheaves using a silver steel form tool. Turn the disc with, say, a 12 – 14 deg. inc. taper for front clearance. Hardening & temper might not be needed for a one-off. Cheap as chips.

You may find that a full radius form tool chatters horribly.

There’s the option of ‘machining by numbers’ by making plunge cuts with a smaller radius insert – this is a 6mm diameter insert cutting a 20mm radius.

Steps of 0.2mm between passes meant that minimal hand finishing was needed to blend the surface:

(More info here )

I have a spreadsheet that can calculate the coordinates if you are interested in this approach.