Hi,
garyhlucas poses a very good question.
My own machine is probably even smaller than your Sherline, the X,Y,Z is 200mm/200mm/200mm. I use 5 phase Vexta steppers with low backlash
(less than 3 arc min) 10:1 planetary gearboxs. The gear reduction means they have heaps of thrust and great resolution (1um) but are slow,
G0s are 1200mm/min (60in/min). Given the size of the machine I don't really consider the slow rapids to be too much of a disadvantage.
Like garyhlucas my steppers never miss a beat either. If I replaced them with something faster I might be able to use some of the High Speed Machining
tooplaths talked up by all the major CAM manufacturers but that's about it. That's a lot of money to spend to achieve so little. Might make more sense
if this were a production machine but it isn't and will never be a production machine.
I have started to buy up components to speed up my machine... 34 size 5 phase steppers from Vexta at 900 oz.in. If I were to use half steps, entirely adequate
with the smoothness of 5 phase steppers and drive the ballscrews directly without the gear reduction I would have a resolution of 5um, still adequate and
based on published performance curves G0s between 7500mm/min and 12500mm/min. Vexta steppers and drivers are expensive and don't come up that often,
given that the benefits are fairly limited I've become quite tepid about shelling out all those bucks when they do come up!
I did however need a decent high torque spindle for machining steels and did get around to spending $1500 or so on the parts to make one, it does get
used and represents a better use of my budget than faster steppers/servos. Yes garyhlucuas raises a very interesting question, do you really need to improve
on what you already have?.
Craig