Non circular holes and lack of precision?

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morten_denmark
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Non circular holes and lack of precision?

Post by morten_denmark » Fri May 09, 2014 5:53 pm

hi

When i cut holes especially small holes, the holes is not circular?
When i make a large object this is not a problem or maybe it is just more difficult to see?
IMG_3384l.jpg
IMG_3384l.jpg (467.77 KiB) Viewed 1966 times
When i created the Gcode in CAMBAM i produced the operation at a profile operation.

Here is the setting in CAMBAM:
setting.PNG
setting.PNG (8.35 KiB) Viewed 1966 times
and here is my GRBL settings:
grbl setting.PNG
grbl setting.PNG (19.86 KiB) Viewed 1966 times

morten_denmark
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Re: Non circular holes and lack of precision?

Post by morten_denmark » Fri May 09, 2014 5:57 pm

The object i cut is also undersized. This should have been 40mm:
IMG_3380l.jpg
IMG_3380l.jpg (393.84 KiB) Viewed 1964 times
I have tried to adjust the current for the stepper motors and i also have a fan on the GRBL.. and that didn't help.

Thanks in advance :)

Regards Morten

WillAdams
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Re: Non circular holes and lack of precision?

Post by WillAdams » Fri May 09, 2014 6:08 pm

Things to check:

- is the spindle mounted plumb and square? http://www.shapeoko.com/wiki/index.php/ ... he_Spindle
- deflection of the gantry along the Y-axis / belt tension? http://www.shapeoko.com/wiki/index.php/ ... m_Expected )

Have you tried cutting a circle-diamond-square and calibrating the # of steps based on that first?
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morten_denmark
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Re: Non circular holes and lack of precision?

Post by morten_denmark » Fri May 09, 2014 6:53 pm

I will try to double the spindle tomorrow, it dos also make a loud squeaking noise when cutting, heres a video:

https://dl.dropboxusercontent.com/u/237 ... G_3376.MOV

And there is a bit of deflection on the X-axis, but i don't know if thats more than normal?

https://dl.dropboxusercontent.com/u/237 ... G_3375.MOV

I will try to make the circle-diamand tomorrow

twforeman
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Re: Non circular holes and lack of precision?

Post by twforeman » Fri May 09, 2014 8:59 pm

Any out of square between the X and Y axis, and any out of plumb in the Z axis will cause non-round circles. So will deflection.

Also, if they are under sized, you may need to change your steps/mm settings. The calculated values are a starting point. Belts stretch and changes in length will affect accuracy.

This is how I calibrated my machine last time I did it: http://timf.anansi-web.com/wp/ferrules- ... -shapeoko/
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cvoinescu
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Re: Non circular holes and lack of precision?

Post by cvoinescu » Fri May 09, 2014 9:08 pm

The non-round holes are usually caused by flex, loose V-wheels, loose belts, or loose pulleys -- basically, anything that allows the tip of the tool to deviate from the commanded position under load. Flex can be made worse by a Z axis that's out of vertical, because it increases load as the tool digs into the material more than intended.

On the other hand, X and Y not being square to each other, steps-per-mm being set incorrectly, and spindle runout will not cause that shape of hole (they'd be slightly elliptic at worst, not irregular like that). However, incorrect part dimensions can be caused by all of the above.
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morten_denmark
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Re: Non circular holes and lack of precision?

Post by morten_denmark » Sat May 10, 2014 5:37 pm

Thanks for your inputs :) I did a little work on my Shapeoko today.

The spindle wasn't plump and the machine wasn't 100% square. I fixed both and i didn't help on the non-circular holes.

I think the problem is flex caused by the 1000mm extended X-axis, so that i next on my list.

A first i would try to stiffen the X-axis using LouisV method, by adding and extra 40x20 aluminum profile:

http://www.shapeoko.com/forum/viewtopic ... 4&start=30

But before i try to source al the parts needed. I will try to add a 1000X40X4mm flat steel bar between the two x-axis maker slide
x.JPG
x.JPG (118.76 KiB) Viewed 1866 times

Nigel K Tolley
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Re: Non circular holes and lack of precision?

Post by Nigel K Tolley » Sun May 11, 2014 9:39 pm

Not being mean or anything, but how accurate were you expecting the machine to be?

40mm planned and you got 39.19mm - that's almost exactly 2%, which isn't a lot of error.

How close would you get that if you were cutting by hand?

How far out are the circles?

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twforeman
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Re: Non circular holes and lack of precision?

Post by twforeman » Mon May 12, 2014 2:35 am

40mm - 39.19mm = 0.81mm off. That's a lot. For the old, US people, that's 0.032" = 1/32".

I can achieve much better accuracy than that. I shoot for +/- 0.010" (0.25mm) and usually see +/- 0.005" (0.13mm)

However, I also have issues with circles not being very round...

Tonight I did an experiment. I have 1000mm rails on my long axis, so I mounted up my test indicator in the center of the rail to see how much deflection I could generate. I was astonished to see that I could easily push the rail in 0.030" (0.76mm) with some force and could wiggle it back and forth half that about with almost no effort. So, in my brain, I see that if I'm cutting a circle and the mass of the trim router and Z assembly is moving back and forth at a pretty good clip (I run my machine pretty fast) it could deflect back and forth and maybe even resonate.

I have some 1/8" x 1" aluminium angle that I'm going to bolt to the inside of the rails. That should really stiffen it up. Just what I wanted, another project. :)
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Nigel K Tolley
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Re: Non circular holes and lack of precision?

Post by Nigel K Tolley » Wed May 14, 2014 8:03 am

Tw, I was more wondering what the expectations of the OP were.
Also, if that is still 2% at the full travel, yes, it will likely suck, but then it would be easily corrected by software tweak.

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