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Calibration of Linear Motion System Driven by Stepper Motors
Axis calibration
So, you don't have a dial indicator to test the accuracy of your machine. You are finding that your cuts are a little off. You're racking your brain, trying to figure out the issues that are causing this phenomenon.

Calibrating a machine is the first process after the machine is built. You want to get your end mill to exactly the place you want it. There are two main methods of calibrating a machine. One method is to use Mach3 (axis calibration under the settings tab) to calibrate, which is the preferred method. The other method, shown in the video, calibrates the axis over a long distance and shows all of the wonderful calculations to get this done. This method is not necessary if you have Mach3, but will help you understand the underlying details to get an axis calibrated.

Calibrating an axis of a CNC machine, or any other linear motion system can be a bit daunting and confusing is done mathematically. It is important to understand that mechanics are not perfect and should be calibrated, even if the correct method to derive the steps per inch was determined from the mechanical characteristics is used (pitch, number of teeth, lead screw TPI, etc.).

If the axis is calibrated over only 1/2" or so, then any slight imperfection will be magnified over the extent of the travel. That is to say, if the axis was calibrated at 1/2" and the calibration is off by .001", then over a span of 8 feet (96", or 192 half inches), the error over then entire span would be 192*.001", or .192 inches. .192 inches is huge!! That's almost the width of a pencil.