issues in position accuracy

[user M_123]

hello,
I am using 16x16 ball screw and 100 steps/mm accuracy so that for 1 step=10um .
if G code G1 X140.010 so it's generate 1 step for .010
but if G1 X140.012 and G1 X140.017 for that how it's work
if any one know how it's generated position accuracy for these two please let me know

[deckingman]

@user-m_123 Well you can't have a fraction of a microstrip so if the first move ends 0.012, then the firmware will round it to the nearest microstrip which would give 0.01. However, IIRC, the firmware keeps track of the fractions of microsteps and will add an additional microstep when the cumulative fractions become greater than a single step. So if you have a number of consecutive moves which all end in 0.012, then you will get a series of moves ending in 0.01 followed by a single move of 0.02, followed by more moves at 0.01 etc. I'm not sure if the firmware rounds to the nearest microstep or if it always rounds down. I would think it would round to the nearest whole step so a move ending 0.017 will probably move to 0.02.

[JayT]

@deckingman : But does this kind of rounding cause dimensional accuracy in the part? If so, what could be the approximate dimensional variance ?
Also as you stated, if the SW keeps a track of collective moves, to add a step for cumulative fractions, then will it still round to the nearest?

[droftarts]

@jayt RRF keeps track of requested position, and moves the motor to the closest microstep. That is fundamental to repeatable motion control, and is true in other firmwares. There is no cumulative error; this would be very noticeable.

Realistically, your motors will be less accurate/repeatable than the micorstepping can be. Most motors have a full step accuracy of +/- 5%. Anything beyond x16 microstepping generally uses up extra processor power for very little gain, except for in specific situations (@deckingman 's extruder is one example). Geckodrives use x10 microstepping for this reason.

Ian

[oliof]

and just to add while full step accuracy is often +-5%, accuracy of microsteps is basically "without any guarantee" since you're shifting around within competing magnetic fields.