Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: TerryS on June 04, 2022, 10:52:21 AM
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This only seems to happen occasionally, mostly after a run the indicators all show zero as they should. Sometimes however the indicators show some very low amount like 0.0025mm or 0.0031mm. Any idea why this might be as I am using stepper motors and there is no feedback so can not be a mechanical error.
Thanks in advance
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Do you work in mm and are one or more of your feedscrews Imperial?
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I work totally in metric
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Me too, but my lathe has an 8 tpi leadscrew so steps/unit is an approximation, it works out to 2015.748 steps/mm taking account of pulley ratio and microstepping. Since Mach can only generate integer numbers of steps the Z axis at least often shows very small errors like you describe. Cross-slide uses a metric ballscrew directly driven from the stepper so that one is 1280 steps/unit - but still one step is 0.00078125 mm and since again it can only do integer step numbers you can get small "zero" errors.
Another factor comes in when homing - if I home the X axis and zero the m/c coords, it usually will "rehome" very accurately (I have a hard contact home switch). But after it's switched off but then switched on again when I home the new home position is often very slightly different and I think this is because steppers lose their microstep sequence when powered off which introduces a small error. It's not a problem but it does mean that for accurate working one has to re-home every time the machine is powered up.
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Thanks for that it makes sense that is what's happening. It's effecting my Chinese router which I assume is metric but maybe its the way I have set it up. Will this error build up over time to a real error on the cutting tool position if not reset?
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Not that I have noticed.