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Author Topic: Trouble cutting accurate parts  (Read 15674 times)

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Offline Hood

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Re: Trouble cutting accurate parts
« Reply #30 on: September 13, 2010, 03:24:59 PM »
The problem you have is your steps per unit equate to 0.0185mm per step step so really that is the minimum distance Mach can move.
Hood
Re: Trouble cutting accurate parts
« Reply #31 on: September 13, 2010, 03:30:54 PM »
So what do you suggest I do.  I am microstepping my driver (1/16 step)currently.
  Should I change it to half step and then use another value for my step setup Or am I barking up the wrong tree.

Offline Hood

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Re: Trouble cutting accurate parts
« Reply #32 on: September 13, 2010, 03:55:32 PM »
Really its not going to make a huge difference and I presume its a router so its probably more of a visual annoyance seeing the numbers in the DRO than of an accuracy issue. The problem withthe standard Mach screens is they are designed for Imperial so have 4 decimal positions, a metric one would be fine with 2 or possible 3 depending on the type of the machine, a router may even be fine with 1? You can easily change that by editing the screen and choosing the decimal places, for example I have 3 on my screens but my resolution is 0.000625mm per pulse.

Hood
Re: Trouble cutting accurate parts
« Reply #33 on: December 17, 2013, 04:15:00 PM »
Maybe you need to map the leadscrews with Mach's mapping feature.

I've been trying to find out how to map my ballscrew. How do yo access Mach's mapping feature?

Offline RICH

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Re: Trouble cutting accurate parts
« Reply #34 on: December 17, 2013, 04:42:35 PM »
Capt Meatballs,
Never tried or used the Mapping feature so someone else will need to chime in on it.
Your the second one in years to ask questions about it that I can remember.

Why do you want to map them?
I ask that because similar to backlash, a software fix for bad screws sugggests fixing the problem instead.
RICH


 
« Last Edit: December 17, 2013, 04:45:29 PM by RICH »
Re: Trouble cutting accurate parts
« Reply #35 on: December 17, 2013, 05:00:18 PM »
Well, my ballscrew is a bit erratic throughout its movement. I'll ask X to move one inch, and the mill's dro will report 1.002", then I'll ask to go to the 2" position and it will be spot on 2.000". Then the 3 inch might go 3.0035", but then the 4 might be 4.000.

My backlash is around 0.003", but this problem is a little different, it's more cyclic in nature.

My Roton ball screws are not very precise, but for my first CNC built I didn't want to go too expensive. I think I will eventually change them, but for now I'd like to train Mach 3 to recognize my ballscrew inconcistencies.

Offline Sam

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Re: Trouble cutting accurate parts
« Reply #36 on: December 17, 2013, 11:19:09 PM »
Sounds like what your seeing is a factor of your machines resolution. Mach is open loop, and therefore could not report back the differences even if it did move more or less than commanded. What your seeing is mach rounding the movement to the nearest step your machine is capable of making. Take for instance the 3.0035"....another step up or down of the motor, would have put you beyond the 0.0035 length, therefore, 3.0035 was simply the closest that could be achieved.
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Offline RICH

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Re: Trouble cutting accurate parts
« Reply #37 on: December 18, 2013, 07:12:04 AM »
My question .............Is the mapping functional or actualy implemented?

To use your need a standard  that can actualy give a true position over the total length for input.

I have checked ball screws opticaly using a calibrated scale and a jig transit. Longest scale I have is 48" long and only have
a  12" long scale that is calibrated to 0.0001" over it's length. A ball screw spec that states "X inches per foot" may not mean
that screw error is linear and it also dosen't mean that the reverse direction is the same, so one could have a ball screw
where the total error / more pronounced  is in some foot section of the length.

The Mach counting of pulses is very accurate and before doing threading tests we actualy compared the pulse to mechanics of the
machine and the other pulses along the sending stream. Used one section of the z axis do do all the tests as it was more accurate. Practicaly
speaking, I would trust the DRO indication.

Practicaly speaking, at the end of the day or machining task, one has what they have, and the part is what it is.
Want better than what one has, pick whatever you want off the shelf but once at the register you need to pay for it! 

RICH
Re: Trouble cutting accurate parts
« Reply #38 on: December 18, 2013, 08:20:34 AM »
What your seeing is mach rounding the movement to the nearest step your machine is capable of making. Take for instance the 3.0035"....another step up or down of the motor, would have put you beyond the 0.0035 length, therefore, 3.0035 was simply the closest that could be achieved.
Thank you for your prompt answer Rich, but I respectfully disagree. Perhaps I need to offer more details to allow you to appreciate my assessment.

First is the fact that I can manually step the X axis to the any number I choose, so in this example I can step it all the way back to 3.000" after its initial commanded movement.

Second, mechanically the resolution is 0.0001" per step, but obviously not the accuracy. My screw is Roton #5932, not a particularly accurate one, but ok overall. It's a 0.200 inch per revolution which combined with my 200 steps per turn stepper motor, plus 10 microsteps per step, gives me 10'000 microsteps per inch.

So, I can physically move the table at increments of 0.0001", that's just a fact. I cannot however place the table where I want it using an X#.#### command because the spiral screw is not consistent throughout its length (my assessment).

This is where the idea of mapping the screw comes in.

Now, I've seen a few people mentioning it, but never anyone actually doing it, so I wonder at this point if it is an actual feature of Mach3, or not. If it is I would like to take advantage of it, as it would allow my somewhat inaccurate screw to achieve much better positioning, at least as accurate as my mill's DROs can measure. Who wouldn't want that?!

Offline Chaoticone

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Re: Trouble cutting accurate parts
« Reply #39 on: December 18, 2013, 09:37:25 AM »
I have tested screw mapping before in a very coarse manner and it seemed to work fine. Set it up and test the results. Its straight forward. Function Cfgs, screw mapping. You jog it to a position, shows the current dro reading in the bottom dros. You measure and enter the actual distance moved in the top dros and enter a correction point. Add as many correction points as you need, save the curve for the axis and enable screw mapping for that axis.

Brett
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