CNC3020 routing larger than expected

I did not know if this belonged in the stepconf subforum or here. So if a mod wants to move it then I understand.

I just set up my cheap CNC3020 Chinese CNC machine. I am going to use it mainly for routing PCBs so I think it will do fine.

To check my calibration, I drew a 4 x 4 inch box in Draftsight. I then converted it to Gcode and opened it in LinuxCNC. The display showed a 4x4 inch box. But when I traced the x and y axis with the axis controls in Linux CNC the machine actually moved almost 5 inches for each axis as opposed to 4. I assume this is some kind of setting (maybe in stepconf) but I am not sure what. What setting would this be and is there anyway to figure out what value I need as opposed to a lot of trail and error?

Do you know how far an axis moves for one revolution of it's motor? Also, do you know how many pulses it takes for one motor revolution?

It seems to work fine in Mach3. There aren't as many setting in Mach3 so I don't know how it figures that all out.

The settings are:

Steps Per: 400
Velocity Inches per minute: 1999.8
Acceleration inches per second: 200
Step Pulse: 1us
Dir Pulse: 1

The mach3 settings are pretty much all you get.

I tried importing the Mach3 xml into Stepconf but it does not seem to import it.

I assume I can just measure that distance by watching the motor?

By no means a super accurate measurement but I turned the motor 20 revolutions and the machine traveled 3.150 inches. That means .1575 inches per revolution.

The 3020's generally have a 1204 trapezoidal screw for all axes. .1575" is 4.000mm so your measurement is spot on.

Might want to keep it simple to start.
Make sure all offsets are off.
wiki.linuxcnc.org/cgi-bin/wiki.pl?CoordinateSystems
Jog to a location and touch off.
Below in inches
F10
G1 X-.25 ;Make a small negative move
G1 X0 ;Move back to 0 (to get rid of backlash)
(Set your dial indicator)
G1 X1;Move a set amount and measure the distance the machine moved and compare that to what was commanded.
I like to make this move as long as possible for the greatest accuracy.

Calculate any difference and edit your calibration file or re run Stepconf.

Rick G

is their any belt pully gear to account for
also if the machine is built in metric , set the machine in metric helps , as it has 1204 ballscrews
you can still use in imperial in your gcode ,

it just helps with the little errors that can pop up at times , also check what microsteps you have set
steppers are 200 steps per rev , so with for example a 1/2 step microstep setup this would become 400 , which does tend to be the default with some stepper drivers .

Thanks everyone for your help! I found thios online

www.muskratcnc.com/machine_setup.html

And it appears to work. Does it seem to make sense?

Rick wrote:

Might want to keep it simple to start.
Make sure all offsets are off.
wiki.linuxcnc.org/cgi-bin/wiki.pl?CoordinateSystems
Jog to a location and touch off.
Below in inches
F10
G1 X-.25 ;Make a small negative move
G1 X0 ;Move back to 0 (to get rid of backlash)
(Set your dial indicator)
G1 X1;Move a set amount and measure the distance the machine moved and compare that to what was commanded.
I like to make this move as long as possible for the greatest accuracy.

Calculate any difference and edit your calibration file or re run Stepconf.

Rick G

I was with you up to (Set your dial indicator). Is that something I need to do? If so what. Remember I am a newb. The settings I found do seem to work just want to know for educational purposes

geoffs wrote:

The 3020's generally have a 1204 trapezoidal screw for all axes. .1575" is 4.000mm so your measurement is spot on.

How did you know that? I assume there is a formula?