Problem with precision in 6040 cnc machine

Hey all,
This is my first post ????

I have a problem with my cnc machine
It's a 6040 with servo motors and a cooling option with a bath.

Trying to mill a part but all of them have a problem with the dimensions of I mill a square 50*50mm it will be 50.2*50.6
And if I do a circle of D12mm or will be 12.2 on x axis and 12.6 on y axis

Did calibrate the steps using an indicator and have 0.02mm error

Did cut a 90deg side in wood and it looks ok

What can be the problem?
And where I can find more info about the machine and how to calibrate

Thank you very much!

Hi,
most probable explanation is that the machine is flexing under cutting forces.

To establish this, or disprove it, do a 50mm square but VERY slowly with the lightest possible cut and measure the result.
Now run the same toolpath but at regular cutting speed and much more tool engagement: measure the result.

If the two runs differ significantly then machine flex is the cause.

Craig

Forgot to add this info
Using 3 mm flat tool
1 mm engagement
1 mm DOC
And 200mm/min
Is it fast for wood?

And this is a new machine so maybe I'm missing something in the settings?
Using usbcnc program

Hi,

Quote:

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And this is a new machine so maybe I'm missing something in the settings?

---

Yes, that is possible but it's also possible that the machine is flexing. Are you going to try to determine if that is the case?

Craig

Hey
I did a 10*10 square and on the x axis it's 10.05 and on y axis it's 10.3
Did it at 50mm/min speed extremely stow??

Hi,
do it at two speeds, or rather more importantly, two loads; lightly loaded and then heavily loaded and compare
the measurements.

Craig

Normally machnists use indicators. Can be precission 10 microns or 2 microns or 1 micron. You should have tho one as well. With this indicator you can check accuracy, repeatability of the movement easy. You can try to push the spindle nose by hand and see on the indicator how rigid your machine is.

Yes I did this test and it's not great its 150-200um of movement
But as far as I understand if I goo slow it should not affect the mill no?

check repeatability as well ... backlash ... and so ...

Hi,

Quote:

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Yes I did this test and it's not great its 150-200um of movement
But as far as I understand if I goo slow it should not affect the mill no?

---

Ok, that gives a rough idea of how much your machine flexes when cutting forces are encountered. Its a fact of life. Even huge cast iron machines
of 30 tons flex under load....but much less...say 1um per 750N of force....but they still flex.

What you have to decide is how accurate you need any given part needs to be and then machine in such a manner that the flex does not exceed
that tolerance. Regrettably small Chinese machines like this disappoint when that limit is established.

Craig

Thank you for the tips
After re calibrating the steps
I did try to make a part with low speed end passes with small DOC and the x and y dimensions are the same
But they are smaller then needed

Do you recommend doing something to the machine or better compensate it in cam?

Hi,
have you positively established that flexure is causing the inaccuracy....or is it the machine settings, or perhaps a combination of both?
You absolutely must dial the machine in to be accurate firstly without any cutting forces, ie spindle off and the tool above the material, then establish
what inaccuracy occurs as you start cutting, and again when you are doing heavy cutting.

I suspect a considerable fraction of the inaccuracy is due to flexure....and there is really nothing you can do in CAM to solve it, and often a complete rebuild with better materials, design
and components required to improve the machine. More often than not its just not practical to improve these cheap machines. You'd be better off working to get the best possible
accuracy by understanding the flexure and work within those limits.

If that's not good enough for your work....get a better machine.....and this time pay particular attention to rigidity. Its the one aspect of of machine that cannot be corrected, its either built
in or its not.

Craig

Quote:

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Originally Posted by Eldarpoli

Hey all,
This is my first post ????

I have a problem with my cnc machine
It's a 6040 with servo motors and a cooling option with a bath.

Trying to mill a part but all of them have a problem with the dimensions of I mill a square 50*50mm it will be 50.2*50.6
And if I do a circle of D12mm or will be 12.2 on x axis and 12.6 on y axis

Did calibrate the steps using an indicator and have 0.02mm error

Did cut a 90deg side in wood and it looks ok

What can be the problem?
And where I can find more info about the machine and how to calibrate

Thank you very much!

---

what software are you using, are you in exact mode or continuous mode

Quote:

---

Originally Posted by Eldarpoli

Thank you for the tips
After re calibrating the steps
I did try to make a part with low speed end passes with small DOC and the x and y dimensions are the same
But they are smaller then needed

Do you recommend doing something to the machine or better compensate it in cam?

---

How did you set the Steps/Per you need to set it over a long distance like150mm to 300mm to get it accurate, you need a precision gauge block or a Micrometer Calibration standard at these lengths to do the setup to get good accuracy, these can be had quite cheap, on Ebay but only choose one that has the protection on it, like the snip below.

Once you get this correct and you have checked for any backlash in each axis drive system, then you can make adjustments in your cam, the easiest is to change the tool size to get your part the right size, or setup using cutter comp if your control can handle it.

Don't rule out any flex this can be in the cutter you are using and in the machine, use the biggest cutter you can