Generally regarded as a Good Thing in an Englishman, eccentricity is rarely desirable in pulleys.  The drive mechanism of my printer uses six pulleys, all of which have themselves been printed.  In my ongoing pursuit of accuracy, I have discovered that some of these pulleys are not entirely circular, nor are they mounted entirely concentrically on the motor spindles (though exactly what ‘concentric’ means if the pulley is not circular is perhaps open to debate).  I’d really like to machine some small tolerance parts out of aluminium, but I don’t have access to a lathe (if you’ve got one, I’d love to use it for an afternoon…).  So how do I work with what I’ve got?  I need some way of ensuring circularity and accuracy of diameter.  The material is PLA, so it’s not too difficult to shape; I should be able to use the stepper motor itself as a lathe spindle, and turn the pulley in place.  My Idea is to print a bracket which will allow me to mount a dial gauge for measuring the pulley, and a makeshift cutting tool to pare it down.  I designed and printed one.


Wonder of wonders, it printed pretty well!  I must be doing something right.  Now I can measure the variation in the pulley radius as it rotates:


It turns out that the deviation is about half a millimeter over a full revolution.  Not, perhaps, a huge amount, but on a 20mm diameter pulley this is 2.5%. This does not mean that over a 100mm movement the carriage will move 2.5mm to much or too little – indeed, it’s possible that it could be completely accurate at specific points – but the linear motion will not be consistent with relation to the stepper motor steps.  Straight lines will wobble.  So now all (all?) I need to do is to drive the stepper motors at constant speed and pare down the pulleys to a constant and known diameter, than that’s one more inaccuracy removed.  We’ll see how that goes.

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