Automatically calibrating a CNC

What's this?

The repository contains a Python script to automatically determine how perpendicular the Z axis of a CNC machine is, using just a feeler gauge and a touch probe.

This repository also contains a simulator (written in Rust) to make sure that the movements and calculations in the Python script are actually correct.

How to use it

Hardware

1. Build the arm:
   1. 3D print the arm
   2. Insert a needle in the clamp at the end, make sure it sticks out around 20-25mm below the arm
   3. Insert an M3 bolt in the hole close to the collet nut hole and connect the needle to this bolt with a piece of wire
   4. Mount the arm to the collet nut of your router and use another M3 bolt to secure it in place
2. Build the feeler gauge holder:
   1. 3D print the holder
   2. Wrap a piece of wire around the middle of feeler gauge
   3. Put a feeler gauge on top of the holder, put the lid on top, and screw it together with 4 M3 bolts
   4. Bolt the assembly to your spoil board, with the long end parallel to the X axis and the feeler gauge on the right-hand side
3. Attach the Z probe:
   1. One wire should be attached to the wire that is connected to the feeler gauge
   2. The other wire should be attached to the M3 bolt that is connected to the needle

This is how it looks when everything is assembled:

CNC Firmware

The script only uses gcode to communicate with your CNC router. Any firmware that supports the required gcodes should work. It has only been tested with Marlin so far.

When using Marlin:

1. The Z probe should be wired and configured to trigger the z_min endstop. (It was the only way I could get G28 Z and G38 to work at the same time. If yours is triggered by z_probe, then change z_min to z_probe in the isZProbeTriggered function.)
2. Enable G38 in Marlin in Conguration_adv.h:
   1. Uncomment #define G38_PROBE_TARGET
   2. Uncomment #define G38_PROBE_AWAY
   3. Recompile and upload Marlin to your CNC router

Software

1. For the simulator:
   1. Install Rust
   2. In the simulator subdirectory, run cargo build or cargo run
   3. You can control the simulator with the keyboard
2. For the calibration script:
   1. Install Python 2.7, pandas, matplotlib, and statsmodels (you can use Conda to make this easier)
   2. Edit the settings at the top of the script (below the imports) to your needs

Running the script

1. Position the router as shown in the pictue above, with the arm parallel to the X axis and pointing towards positive X
2. Position the needle on top of the feeler gauge, a couple of millimeters to the right of the tip
3. Open feeler_gauge.py in the Spyder IDE and run it. Alternatively, run python feeler_gauge.py from the command line
4. While the script is running, keep an eye on your machine to make sure it's not doing anything weird. The full script will take up to half an hour.
5. When it's done, it will print something like this:

Spindle angle around X: -0.1563 degrees off perpendicular

Spindle angle around Y: 0.2554 degrees off perpendicular

Z axis angle around X: 0.3192 degrees off perpendicular

Z axis to spindle in X: -0.4756

Z axis angle around Y: 0.1866 degrees off perpendicular

Z axis to spindle in Y: 0.0688

"Spindle angle around X/Y" means the angle between the XY plane and the tip of the end mill. If this is zero, surfacing your spoilboard would make it perfectly flat. If it's nonzero, you would get tiny ridges.

"Z axis angle around X/Y" means the angle between the XY plane and the movement of the Z axis. If this is zero, your Z axis moves up and down exactly vertically. If it's nonzero, it also moves laterally when moving up and down.

"Z axis to spindle in X/Y" is the difference between the former two angles. This is the amount you need to shim the router mount to get it perpendicular.

The simulator in action

See this video on Youtube.