Author Topic: How to increase precision?  (Read 539 times)

nibbler

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How to increase precision?
« on: November 07, 2018, 01:09:18 AM »
I just did some vector tests and noticed that the repetition precision is pretty bad so I tightened the belts
the result was much better but I cannot tighten them more now.

As you can see I did 3 passes to the inner circle by restarting the g-code. So it moved to 0,0 between the passes.
I expect a bit better result if I do the multiple passes right after the other but still there is a big "gap" between the passes.

What did you do to increase the precision?
Note, this is one of these chinese 60x65cm tables so the frame is a bit shaky ut I think the problem is because it is belt driven.

Zax

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Re: How to increase precision?
« Reply #1 on: November 07, 2018, 04:32:06 AM »
It should be MUCH better than that, are you sure the carriage isn't loose? The wheels may need to be adjusted.

ggallant571

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Re: How to increase precision?
« Reply #2 on: November 07, 2018, 05:24:34 AM »
I'm 100% in agreement with Zax. What material are you etching? I use Neon colored card stock (index cards) for test burns. They are cheap are react at relatively low power levels so you can run at high speed. Other possible faults:

  1. Bad stepper motor driver.
  2. Wrong current limit setup on stepper motor driver
  3. Really bad wheel bearing
  4. Miss-aligned wheel/axle (my assembly fault - had different length spacer on one wheel))
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nibbler

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Re: How to increase precision?
« Reply #3 on: November 07, 2018, 07:46:53 AM »
Hmm, note the bigger circle is not part of the repetition and it travelled about 100x100 mm back to 0/0. If I do an instant repetition it IS much better.

This is 160g paper I used for the tests. I didnīt have any assembly manual so I assembled the whole thing as I expected it be correct and compared to the product pictures it looks quite correct ;-)

I am used to way more stable constructions I built on my own. The laser is the 4th CNC machine I own and the first one that came as a set.
I did buy the laser for fun because it was so cheap. But once I set it up I already thought the axes could be too shaky as there actually is no precision at all, the wheels move in a gap of an aluminium profile, well, even a drawer extension is more precise ;-)

My other machines use the ones I attached in this post, you can imagine that those nearly do not wiggle at all.
I am also suprised that the small drivers are even capable of handling the steppers.

Anyway, long story short point. What I want to say is I wouldnīt be too surprised if it is like this when doing the repetition with returns to 0/0 between the repetitions.

But if you say it should be way better then I obviously made something wrong when assembling it. Maybe I can tighten the wheels a bit, though there isnīt too much room for optimization.

ggallant571

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Re: How to increase precision?
« Reply #4 on: November 07, 2018, 05:59:53 PM »
Attached is a picture of my favorite test pattern etched on 0.2mm cardstock. I live in a country without nice units so it took a while to find out what 160 GSM is!!!. It was etched on my most imprecise machine, a 2.5W A3 with sloppy wheels. It does have limit switches but they were used only on startup.
Lower image is one pass at 1200mm/min with power 100 out of 255. Upper image is four passes at 1200mm/min power 50. The straight lines are not great but completely repeatable. Also attached the gcode file. After I did the homing cycle I used the simple GrblController utility for the burns.

BTW - I built a core-xy with the linear rails and it is super smooth and precise.
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nibbler

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Re: How to increase precision?
« Reply #5 on: November 07, 2018, 11:24:46 PM »
Thanks for the pattern, I did a test on the 160g paper and thought it might be an issue of the paper not being flat enough on the  table so I did another test on mdf.
However as you can see both have misplacement issues.

As you can see the on the pictures the errors happened on the two-motors (y) axis.

I am not sure if this is really getting any better as I donīt trust the usage of two motors on one axis. This is never going to work in 100% sync.
A delay of some ms on one side will already result in a big distortion because of the length of the axes.

My best bet probably is to draw in the middle of the machine if I donīt want to change anything to the machine.

Iīll try some tweaking on the frame. If this doesnīt change anything I can remove one motor and make one belt driving the other side.
However this means usage of a stronger motor, controller and a lot of other changes... not really what I wanted to do :-/

ggallant571

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Re: How to increase precision?
« Reply #6 on: November 08, 2018, 10:00:03 AM »
Have you done the current limit setup? A4988 drivers need approx twice the current for the Y axis vs the X.

I have no concern regrading the synchronization of the 2 motors. They are wired in parallel and whatever the difference is, it gets mitigated by the span and should be constant.

The nick at 10 o'clock looks ominous. Was this multiple passes?

Perhaps it is the camera but your straights don't look very good.

My A3 controller uses a 32bit Arm processor and updates the steppers at a higher frequency than a standard AVR grbl. Be nice if someone else would try a test burn and post results.
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nibbler

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Re: How to increase precision?
« Reply #7 on: November 08, 2018, 10:58:00 AM »
It is scanned :D
It is clearly visible that the Y axis does the errors. I already have an idea for a simple fix, the motor drivers can handle up to 2A so I probably can replace a motor with a stronger one and connect it to the other side but anyway before I do that:

current limit setup .... no :-/ you know I had no manual or assembly instructions at all.
How do you do that ?

Zax

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Re: How to increase precision?
« Reply #8 on: November 08, 2018, 12:22:17 PM »
The drivers are rated up to 2A, but that requires both passive (heat sink) and active cooling (fan).

These machine work perfectly well with dual Y-motors from the same driver, stepping is synchronized and often works better than a single motor with physical connection.

There's a sticky thread on this forum with instructions, or you can search "A4988 current adjustment". The standard driver uses a 0.8V ref so 0.6V is typical for the Y-axis (0.6/0.8 = 0.75A, which is within the 1A rating without a heat sink). The X-axis should be ~0.3V since it's a single motor.

ggallant571

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Re: How to increase precision?
« Reply #9 on: November 08, 2018, 02:11:55 PM »
The latest set of A4988 drivers all came with the voltage at at 0.5V. I like to use 0.5 for the X and 0.8 for the Y. It runs the motors a little warmer than necessary.

As Zax said there are lots of tutorials online for current adjustment. I think the "best" is at polulo.com.
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nibbler

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Re: How to increase precision?
« Reply #10 on: November 08, 2018, 11:21:00 PM »
Alright, looks like I need to get a better multimeter. I always try to avoid the electronics stuff, no big deal soldering but I suck at the math part :D

But if I get this right, this is just adjusting how much current the motors can draw and with that can "perform".

I never blew up a stepper because I have always chosen some with more torque than required.

If this is a precision issue here it tells me the motors are actually too weak if it is needed to drag them to their limits :-/

Whatever, I need a better multimeter for a long time already... I guess this is the time to buy one.



ggallant571

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Re: How to increase precision?
« Reply #11 on: November 09, 2018, 05:53:28 AM »
Torque is the ability to effect change in motion. Each motor has a specified torque at a specified voltage and current. If you do not supply enough current the motor will not react properly. The motors on these little machine are more than sufficient for the job. If your fixture has a bad spot in a bearing(s), flat on the wheels, twists in the frame, there will be times when it requires more torque to go forward.

If you are lucky the current limit was set a little too low.

Bearings will loosen with use.

Another experiment would be to change both the speed and power settings.
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nibbler

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Re: How to increase precision?
« Reply #12 on: November 12, 2018, 12:26:03 AM »
0,64 and 0,46

I set them to 0,79 and 0,5 now. Still I donīt believe this is the problem but will do a testpattern later today.

EDIT:

As I expected this didnīt change much. I tightened the wheels on both axes and what came out exactly proved what I already thought, the motors donīt have enough torque for this construction, at least the one motor (X) axis.
Once I "optimize" the carriages to the possible limit, the wheels are too tight for the motor. So I had to losen it again for the X axis.
The Y axis with dual motor setup is able to handle the tight setting but it ainīt getting much more precise. I am out of ideas apart from the two choices kicking the sh-t in the corner or builting a whole new table.
I could also reduce the size of the workarea but that ainīt going to help me, plus cutting the rails without high precision will not make things better either.

Once again ... buy cheap, buy twice. :(


« Last Edit: November 12, 2018, 03:30:10 AM by nibbler »

ggallant571

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Re: How to increase precision?
« Reply #13 on: November 12, 2018, 09:30:57 AM »
It got rid of the the nick in the circle at 10 o'clock!! I suspect the wave in the straight lines is due to wiggle in the laser mount carriage. I have it on my A3 fixture that has the carriage suspended on wheels (probably your configuration) and not on the core-xy fixture where the laser carriage is on linear bearings. Use the laser for etchings with lots of curves. Try the attached image. It is 0.1 mm resolution. On my 2.5W fixture I use speed 1200, power 255 for a dark burn on wood. 1500 might show the lines better
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nibbler

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Re: How to increase precision?
« Reply #14 on: November 12, 2018, 10:56:04 PM »
I think the nick was just a random occurence. But it is true it could also happen due to wiggle in the laser mount that would indeed explain the curved lines.
Images like this example usually work ok because precision isnīt that visible.

Btw, my x/y setup is more like A2, so small errors get even bigger with that.