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DRO error in mm
« on: May 24, 2009, 04:40:36 PM »
I have been working in inches for some time and the DRO's always read exactly what I commend.  ie: X1.2345 reads X1.2345 after the move.  I just switched to a project in mm by starting my code with 21.  I set X=0 and then moved X=10.  The DRO reads X 10.0013.  It happens in the Y axis also - and the incremental change is not always the same amount.  ie: X54.0635 becomes X54.0639 but
Y29.93565 becomes Y29.9371.   Can anyone tell me what the problem might be.

Thanks

Murray

Offline Hood

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Re: DRO error in mm
« Reply #1 on: May 24, 2009, 04:58:18 PM »
Could be the resolution of your machine, remember that 0.0013mm is equal to 0.0000511"  ie not a lot ;)
Hood
Re: DRO error in mm
« Reply #2 on: May 24, 2009, 05:43:44 PM »
Thanks.  That's probably all it is.  It's a Taig milll and I can usually get .001 inch out of it.  Siince that is only .025 mm, that may be the problem.

Offline Hood

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Re: DRO error in mm
« Reply #3 on: May 24, 2009, 05:46:35 PM »
What are your steps per unit?
Hood
Re: DRO error in mm
« Reply #4 on: May 24, 2009, 07:39:03 PM »
I'm not sure.  Where do I check (change) them?  I'm using the Taig mill with the R3 version of Mach 3

Offline Hood

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Re: DRO error in mm
« Reply #5 on: May 25, 2009, 01:54:35 AM »
The steps per unit are in your motor tuning (config menu then motor tuning)
You dont want to change that as that is the number that makes your machine move the distance you told it to move, I was asking so that I could tell you what your resolution was. For example if it is 20,000 steps per unit and you are set in inches then that is 1/20,000 =0.00005" or 0.00127mm
Mach may manage to move your machine to the exact position you have commanded but more likely is will be within one step of that position as moving another step would be further out of position. You see the small decimals out of position in the DROs but as mentioned they are extremely small but because they are in mm and you think in inches they look bigger, I work in mm and think the opposite when I see a decimal inch, I think its tiny but when I convert to mm its actually quite a large number :)
Hood
« Last Edit: May 25, 2009, 02:34:49 AM by Hood »
Re: DRO error in mm
« Reply #6 on: May 25, 2009, 02:14:32 AM »
You just demonstrated why the US was not able to switch to metric. Instead of just switching we kept trying to convert inches to metric and getting nonsense numbers. The only industry that came close to making the switch is the beverage companies with the two liter pop containers.

Offline jimpinder

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Re: DRO error in mm
« Reply #7 on: May 25, 2009, 03:18:47 AM »
It stands to reason that not every number can be divided by a whole number and come out to an exact whole number.

It your machine is set up in inches, then usually this is becasue the leadscrew is in inches, so a finite number moves the correct distance. The same applies to a machine set up in metric - where the leadscrew is metric - again a finite number will give a finite answer.

Whe you use one to produce the other , the divisor is 25.4 hardly a simple number, and therefore some rounding up must occur. Mach calculates to 12 places of decimals (depsite what is shownon the DRO's) and the error, as Hood said, is very very slight, and I doubt your machine will cut that accurately anyway - we are way past the micron stage.
Not me driving the engine - I'm better looking.
Re: DRO error in mm
« Reply #8 on: May 25, 2009, 08:35:34 PM »
Thanks to everyone.  I think I understand now.  Especially since I know my lead screw is in inches not metric.  That explains why so many units in inches always comes out as commanded, but metic does not.  I just assumed that the DRO's would track the input commanded. 

I appreciate all the help.  I wasn't sure whether to correct the errors (by resetting the DRO's ) or leave them as they are.  I can see that I'm more accurate to leave them alone.