design choices when the budget will not stretch?

[irving2008]

so what decides how you choose the right size motor? i presume weight of what you are driving comes in to it.. does the size/weight of the screw come in to the decision? does the materials to be cut have any sway on it? i also dont understand how the word resolution is used in cnc.. my thoughts are it is similar to a picture where there are so many dot's making curves smoother, does the same thing happen when you lose resolution in cnc? you start to get blocky curves?

You might find this useful: What size stepper motor do I-need.

Yes the size and weight of the screw is significant. The materials to be cut do have some impact, in the sense that they dictate how much force is needed and therefore, to some extent, the force able to be transmitted by the screw, but generally the materials dictate the rigidity required for a given depth of cut (DOC). To machine steel at any useful rate needs big lumps of cast iron, not ally...

The screw pitch determines resolution, i.e. the smallest step that the machine can make. For instance a directly driven 10mm pitch screw, using a 200step motor, gives a minimum step of 10/200mm = 0.05mm. This is plenty good enough for wood working, but marginal for metal working, where 0.005mm is preferred. Also if you want to move at, say, 6m/min, then you need to spin the screw at 600rpm, 10rev/sec or 2000 steps/sec. At 600rpm theres going to be relatively little torque available so there is a danger of losing steps... its all a balancing act :)