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Author Topic: Dial indicators show random amounts after retuning to start  (Read 821 times)

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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
Re: Dial indicators show random amounts after retuning to start
« Reply #1 on: June 04, 2022, 02:59:20 PM »
Do you work in mm and are one or more of your feedscrews Imperial?
Re: Dial indicators show random amounts after retuning to start
« Reply #2 on: June 05, 2022, 03:57:03 AM »
I work totally in metric
Re: Dial indicators show random amounts after retuning to start
« Reply #3 on: June 05, 2022, 06:59:13 AM »
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.
Re: Dial indicators show random amounts after retuning to start
« Reply #4 on: June 11, 2022, 06:44:18 AM »
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?
Re: Dial indicators show random amounts after retuning to start
« Reply #5 on: June 11, 2022, 06:54:21 AM »
Not that I have noticed.