I was just reading a message where a guy said that 150oz motors were "absolute minimum" for a Sherline and that up to 250oz were still in the "sweet spot." I am genuinely curious about why people seem to over-spec steppers so heavily, maybe I am missing something?
I've been building CNC machines for about a year now and am on #3, an X1 conversion, after building two router tables, one for a Dremel, and one a Porter Cable. All three have been run using the same 125oz unipolar motors and a 24V Hobby CNC setup. The larger router used 1/2-10 Acme and could reliably jog at 90IPM, and generates enough sideforce to stall the router. The X1 will move the X/Y axes at 14ipm at least, which is quite tolerable on a machine this size.
I have seen people putting 425oz motors on these mills which has me completely flabbergasted. I suppose you could put brass gibs in and tighten them up like the heads on your car and gain something that way, but more oft I suspect people over-spec steppers because they think that if 125oz will work, then 425oz will be four times better.
Granted, the price difference if buying new steppers is pretty small and you can go from 125 to 300 for $10 or so per motor. So, why not?
Well, one thing, which some may think is silly, is that on my X1, those puny motors are just strong enough to cut about as hard as the machine should. Which means that, when (not if) the operator (that's me!) crashes it, the first thing to give will be the motors. While I don't count on this as a safety mechanism, it has prevented me from so much as stripping the infamous "cheese gears" such as when I mistakenly sent a .250" endmill straight into the side of a .750" block of Al at 12ipm. Oops!
At some point things start to add up. Heavier motors require more work to support, and perhaps force you to use more costly drivers to operate. But the real cost I think comes from the lazy thinking that leads to over-specing. There are times and places for servos, ball screws, and many more costly things, and if your budget is open-ended, then by all means do your part to keep the American industrial worker employed.
Anyway, I looked on Sherline's site, and it seems like they sell their mills with 136oz steppers. Are they under-powering their machines?
/rant (wrong)