Thread: Bad accuracy with my cnc machine
Ok guys, i really need help with this. Hope some more experienced could give me some advises what to do to get more accuracy with my machine. Here i am sending you a picture from the tests i did.CIRCLES -(1) lower left 10.47-10.70,(2) lower right 10.50-10.60 ,(3)center 9.76-10.90,(4) upper left 10.25-10.40,(5) upper right 10.12-10.70
Every next circle is deeper from the previous one 1-st one 5.4mm-last one gets up to 9.5mm(program is 5mm depth for all cirles, i check my table is parallel )
1-st square (lower left corner) x=10.35 , y=10.39
2-nd square(lower right corner) x=10.38, y=10.40
3-th square (center) x=10.50, ( this square doesn look like square like the 1-st and the 2-nd one, x=10.50 lower and upper side of the square, Y-left=9.97, Y-right =10.43)
4-th square(upper left corner) x=10.49, Y-left =9.97, Y-right=10.50
5-th square(upper right) i didnt make it coz the tool went too deep in the wood and i was afraid not to broke the tool, but i ques the results would be similar to the 4-th and 5-th.
The easiest way to get that level of distortion is to have a weak machine, especially in one axis.
This can be difficult to resolve because even if you use a very low feedrate (which is not great long term anyway) you can get pulling of the bit in the material if the machine frame stiffness is low.
If you lean your weight on the spindle can you deflect it?
I don't think it is lost steps etc.
That would be my first guess anyway. What machine is it? Can you post pictures of the machine?
Yes, true, my x-axis is not rigid enough,, its too flexing. Thats what i think, thats where the problem comes from. I already ordered a new sbr 16mm rails. Here are some pictures from my machine
I do a test 50mm square and 25mm circle inside, profiled from the outside. This dimensiones are easier to measure. I use phenolic paper board as it does not change dimensions after the cut like MDF, wood and some plastics do.
The way you did the test tells me really nothing.
-may be you did not measure it properly as its very small piece
-the MDf is getting bigger just 1 min after the cut.
-such small pieces can show small deviation which rapidly could translate to much bigger when a 10cm piece is cut.
Obviously your machine far from strong. But even that could be compensated to some extend in Mach 3 via the step calibration
/example - http://www.instructables.com/id/Auto...-Using-Mach-3/ or search google and read the Mach3 manual.
Now the problem with compensation such a machine lies that every time you change depth of cut and material cut you will have again to do that. I have small belt driven machine where i have 3 different screens with different calibration, one for plastic, one for wood and one for aluminum. That all to compensate the belt stretch for different materials.
Why i do a test circle and a square. The circle is obvious, the square is to help identify the specific problem. Some times the square will be Ok and the circle not. Funny, yes. That means the plunge rate is too much for the weak machine and the depth of cut is way to big.
SBR16 rails will be an improvement and for wood cutting will be OK. BUT, the gantry and frame of the machine also needs significant upgrading. You will not gain anything just by changing the rails.
Have a look at the build logs. Here is an example of my wood cutting machine:
Here is one for cutting aluminium:
Last edited by routercnc; 19-04-2015 at 08:53 PM. Reason: typos+link
And what is ur explanation for plunging further and further into the material with every next circle? Could it be bad cracked CAM program( i use Aspire 3.0) ??! My shielded cables are not grounded yet, could this cause a missing steps?!
I ll reinstal both, and see what happens.
And what are ur advices for upgrading my machine.Beside changing the rails what else would help improving the machine?
#1 Mechanical issue so slipping coupler or something.
#2 Electrical noise from spindle affecting signals.
#3 Incorrect setup of Steps per milimeter
#4 Pulse signal is set wrong in drive so changing on wrong side so losing 1 step on every directiuon change.
#5 Low power parallel port voltage so not enough voltage differential between high and low signal causing lost steps.
My suggestion is first check all mechanical connections and wire connections. Then identify if noise by running G-code in air without spindle running. Measure starting height and check when finished.
If ok then check the Active edge set in the drive.
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