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Posted by Tim Auton on February 12, 2007, 8:57 pm
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[X1 CNC conversion]
> Alan, I have been wading through your writeup today whilst sitting here with a
> sore throat and lack of voice.
>
>
> You are doing yourself down...
>
> here:-
> http://www.aonx97.dsl.pipex.com/WS-page/X1CNC/X1CNCpage5.htm
>
> On page 5 you have :-
>
> So what is the error for a movement that is not a multiple of 64? Well taking
> 0.001" as the probable smallest accuracy then 1/1000 = 40640/1000 steps =
> 40.640 steps which of course cannot be achieved so there will be either 40 or
> 41 steps done.
>
> At 40 steps this will give 40/6400 =1/160 mm which equals 0.000255" and:-
>
> At 41 steps this will give 41/6400 = 0.00640625 mm which equals 0.0002522",
>
> so a 1/4 thou error can be expected. Am I bothered? (to use current jargon!)
>
>
>
> This should be :-
>
> At 40 steps this will give 40 * 4 /6400 =1/40 mm which equals 0.000984" (.016
> thou error) and:-
>
>
> At 41 steps this will give 41 * 4 /6400 = 0.025625 mm which equals 0.001009"
> (.009 thou error)
Alan's maths may be wrong, but he's probably closer to the right answer
anyway. 32 microsteps just doesn't buy you anything like 32x the
*accuracy*. It gets you 32x the *resolution*, but that's not the same
thing at all, especially when it comes to microstepping. A search on the
interweb for "microstepping", "microstepping accuracy" or similar will
find ample discussion of exactly why you don't get the accuracy (and why
microstepping is still a good idea anyway). The short version: You get
bugger all /incremental/ torque (5% for 32 microsteps) and that small
incremental torque is swamped by effects in the stepper itself
(friction, detent torque), let alone the external load.
Oh, and the maximum (resolution) error is always half the smallest
division; half a microstep in this case. So I'd just have worked out how
far that is rather than working examples. Imperial and metric are "out
of phase" so you are going to get that maximum error at some point
(actually at 1/127 points).
Tim
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> sore throat and lack of voice.
>
>
> You are doing yourself down...
>
> here:-
> http://www.aonx97.dsl.pipex.com/WS-page/X1CNC/X1CNCpage5.htm
>
> On page 5 you have :-
>
> So what is the error for a movement that is not a multiple of 64? Well taking
> 0.001" as the probable smallest accuracy then 1/1000 = 40640/1000 steps =
> 40.640 steps which of course cannot be achieved so there will be either 40 or
> 41 steps done.
>
> At 40 steps this will give 40/6400 =1/160 mm which equals 0.000255" and:-
>
> At 41 steps this will give 41/6400 = 0.00640625 mm which equals 0.0002522",
>
> so a 1/4 thou error can be expected. Am I bothered? (to use current jargon!)
>
>
>
> This should be :-
>
> At 40 steps this will give 40 * 4 /6400 =1/40 mm which equals 0.000984" (.016
> thou error) and:-
>
>
> At 41 steps this will give 41 * 4 /6400 = 0.025625 mm which equals 0.001009"
> (.009 thou error)