Quote Originally Posted by Cret View Post
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In respect of the drivers and the current they can output & the rating of the motors, do the motors only pull the full rating when a load on them demands that? Or do they somehow draw that constantly? I'm not quite sure how that works sorry.
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As a general rule the power supply should be rated at 60% of total peak current, so 60% of 3 x 4.2A = 7.6A

Switch mode power supplies are not best for CNC work, linear supplies handle the peaky nature of steppers better and are easy to build.