Evening Stot

I'm going to throw a few pennies in here and see if it helps??
I'm assuming you have done a search of the site for "ball turner"? (in the " " marks.)
This turns up all sorts of designs and re-designs on the ball turners used by members of the site (including me)
I've made 2 of them and both using steel to steel design. I've cut a lot of stainless using the larger one I built (the mini one has a new home).
I've had no problems using mine to cut hundreds of shaped bits 'n' bobs in stainless, brass, bronze, aluminium and just about any metal I could get my hands on. I've posted a lot of pic's and videos in the gallery and other areas of the site.
Just a thought, The diameter of the turret (3") allows for a lot of lateral movement of the cutter section, but how tall are you making the cutter (using the 15mm square)?
I know this might sound obvious but, the height of the tip from the turret would have to be just over 3" to enable the cutter to reach it's maximum "safe" radius size of 3".... A 6" solid metal sphere

Now that would take some turning!!!
(IMHO "safe" being... the "L" clamped in half of the slot it slides in)
I'm also going to assume that the 5/8 base depth and the 1.5" turret depth are what you have worked out to bring the tip holder closer to centre height and keep the rigidity in the tool? (this would have to still be just over 3" to obtain optimal tool use).
These are just my thoughts..... I know I ramble a bit!
As for the use of aluminium. I would think that it would need a lot of lubricating and a much larger contact surface with the base than steel would to spread the cutting forces and prevent premature wear.... Just my thoughts.
Cast iron might be a bit odd to use too? It's a little brittle IMHO and may also need a larger contact area to prevent stresses from cracking/chipping the turret?
Steel is IMHO the way to go.
Of course when you add bearings to the mix it all alters.... I have not had to go to this extreme as my ball turner works a treat
I hope some of that makes sense,
Ralph.