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Author Topic: deviations of 0.3 mm, in small parts: MACH3  (Read 7343 times)

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Offline BUG

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deviations of 0.3 mm, in small parts: MACH3
« on: January 06, 2010, 01:16:27 PM »
when programming small pieces (28x28) mm, the end result is a piece of (27.7x27.7) mm. But if the piece is large (150x150) mm, the end is accurate result (150x150) mm.
anyone knows what happens?

I modified the acceleration ramps and not get any results.

Thank you!!

Offline ger21

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Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #1 on: January 06, 2010, 01:25:36 PM »
Screw error, or flex in the machine.
Gerry

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JointCAM Dovetail and Box Joint software
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Offline BUG

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Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #2 on: January 06, 2010, 01:33:56 PM »
but.... why the large piece it's mechanixe ok?? and why the small peace it's wrong??

Offline Hood

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Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #3 on: January 06, 2010, 02:04:54 PM »
Try the same small code but use the scale DROs to make it 5 times bigger or use the big parts code and scale it down and see what happens.
Hood
Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #4 on: January 06, 2010, 02:45:17 PM »
Hi all,

just a few questions:

- How is the drive setup of the table?  Conventional screws or ballscrews? Steppers or servos?
- Is the machine new, or a used one with CNC retrofit? Possibly there is some wear of the screws causing your trouble.
- Of course, flex could be a possible cause.

Maybe, try to check wether the drive motors do the exact steps needed to move the table precisely, by marking "0°" on a flywheel or similar and make a part which needs exactly x revolutions to make the necessary travel.



Have fun
Konrad
« Last Edit: January 06, 2010, 03:23:27 PM by Konrad K »

Offline BUG

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Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #5 on: January 06, 2010, 03:50:09 PM »
The machine is a Zenbot.

Looking to analyze the information and evidence I have made.
« Last Edit: January 06, 2010, 03:56:14 PM by BUG »

Offline ger21

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Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #6 on: January 07, 2010, 07:31:50 AM »
Try cutting two small pieces in different places on the machine, 150mm apart, and see if they are the same size.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline RICH

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Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #7 on: January 07, 2010, 09:41:04 AM »
Probably screw error as Gerry said along with different backlash.
I would profile the screw some by checking the differences if in actual movements.
Say, 1" movements in one direction over 12" and then reversing direction.
Usualy you will see constant gain or loss, but it could  also be more promounced in one section
of the screw. Check the backlash at different locations. Your  .3mm ( .012") seems excessive for just the
screw since many are in the range of .004" per foot accuracy. So seems like something more going on here.
Timing belt tension also can have an influence. Try to isolate the contributing problem.
RICH
Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #8 on: January 07, 2010, 12:30:07 PM »
Hi all,

I just had a look on the website www.zenbotcnc.com....

The Zenbot mill is made from plastic material and furthermore is belt-driven. So, there is no threaded rod (screw) at all at the axis' drives.

Under all circumstances, most tooth-belts have remarkable backlash, depending on material and pulley geometry used.
(of course, there are "low-backlash" belts, but even the pictures on the zenbot website show a very small belt, which cannot be low-backlash.)

In other words: For this type of machine (and price...) the reported tolerance seems to be normal  >:D


 ::) On the other hand, it seems to be a very good deal for beginners to get familiar with CNC-technology as the price is very low.


best regards
Konrad
« Last Edit: January 07, 2010, 12:32:17 PM by Konrad K »

Offline BUG

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Re: deviations of 0.3 mm, in small parts: MACH3
« Reply #9 on: January 07, 2010, 04:20:27 PM »
I've adjusted so that the piece of 28x28mm is correct, then the large scheduled to 150x90, the result is 152x91.

Also changed the speed of 45000Hz Kernel to 25000hz and the summary is identical