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Author Topic: Z DRO depth reading incorrect??  (Read 829 times)

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Z DRO depth reading incorrect??
« on: October 18, 2017, 08:07:25 AM »
Hi there,

I am fairly new to CNC so please forgive my noobness!

I recently purchased a 6040 that came with a dodgy copy of Mach3...but dont worry, i have not used that copy and instead purchased the real thing and am happy to provide proof if needed :)

Anyway, something is not quite right with how i have the Mach3 software setup as when its making a pass at 2mm in depth the Z DRO will always show a number such as 1.9975 instead. I have just calibrated the motors and this has improved things and the same pass will now show as 1.9988 on the Z DRO but surely it should be showing 2mm? This follows suit for each 2mm depth pass as well..so the Z DRO is showing 1.9988, then 3.9988, then 5.9987 etc etc rather than 2, 4, 6, 8 etc

I have even turned off the actual CNC machine and run the cycle unconnected and the Z DRO still shows the exact same number

I was thinking that Fusion may be the issue and that Mach3 didn't like the G code it was producing but even when i run the Mach3 wizard and use it to make a circular pocket at 1mm in depth the Z DRO shows the depth as 0.9988 instead...why does it not show as 1mm on the DRO??

And quite often, when i press the Go To Zero button it won't go fully to zero...instead it may read something like X 0 - Y 0.25 - Z 0.25

The actual cut depth of the workpiece plus the other dimensions are fairly accurate, but why does the DRO show values such as 1.9975 instead of 2 when 2 is what the pass should be?

Any help or advice would be greatly appreciated

Offline Tweakie.CNC

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Re: Z DRO depth reading incorrect??
« Reply #1 on: October 18, 2017, 08:24:02 AM »
It's really not something you need to worry about.  :)

Your stepper motors move in steps and the actual number of steps per unit of measurement may not be an exact number so the Mach3 DRO does not show an exact reading. This is not really an error because it is not cumulative and always corrected at the next move so you will always be able to make accurate parts.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Z DRO depth reading incorrect??
« Reply #2 on: October 18, 2017, 08:31:46 AM »
Hi there,

Many thanks for the reply :)

"always corrected at the next move"

Would this not mean though that the final step would / should therefore be the correct size? For example, i was hollowing out a rectangle area which had a depth of 22mm so on the final pass shouldn't the Z DRO have shown 22mm? Instead it was showing something like 21.9988

Don't get me wrong, i realise that 0.0012 is hardly much to worry about but i just don't get why it shows that value rather than the value its meant to be using. My friend has the smaller 3040 and Mach3 and when his machine makes passes the Z DRO always shows the correct intended depth...so why does my machine not do that?

Offline ger21

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Re: Z DRO depth reading incorrect??
« Reply #3 on: October 18, 2017, 09:02:28 AM »
No, not unless your steps/unit work out so that 22mm falls on a full step. Mach3 can only move in full step increments. So if the commanded position doesn't fall on a full step, mach3 will get as close as it can.
Gerry

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Re: Z DRO depth reading incorrect??
« Reply #4 on: October 18, 2017, 12:00:03 PM »
Okay guys, many thanks for the replies...much appreciated :)