[Goemon]

Different pitches mean potentially different dynamics for each axis. Using a coarser pitch may necessitate using a larger motor to compensate for the decreased "gear reduction." Also you machine will only be as fast as your finest pitch screw when it comes time to 3D machining. And speed is not your primary concern, so why not have more available torque and mechanical resolution?

My choice was largely based on the availability of good deals on quality ball screws. I also figured that the larger diameter wouldn't hurt on a longer axis and the one that carries the most weight.

I guess I'll have to see if it causes problems for me. If it does then I'll have to replace it with a fiber pitch screw and use the old one on another build.

You are right that speed is of little concern to me unless it causes a cutting problem. My main goal is to be able to cut a mold cavity in 6061 and getting it right first time. 6061 is not super expensive but I can't afford to be screwing up 1.5" x 8" x 36" slabs on a regular basis.

It would probably be cheaper to upgrade a stepper motor than to buy a new ball screw though. When I bought my stepper motors I was building a different machine with slightly different cutting goals and a lighter spindle. They were on my list of components to revisit to see if there were opportunities to improve anyway.

I keep meaning to read up more on why metal cutting machines use much larger stepper (or servo) motors to see if I was missing something. I can't shake the nagging thought that more holding torque would be helpful for cutting metal.