How to Check CNC Machine Accuracy: Simple Steps and Code

To check CNC machine accuracy, perform test cuts on a calibration block and measure dimensions with a dial indicator or micrometer. You can also run a G-code program that moves the tool to known positions and compare actual positions to expected values.

Syntax

Checking CNC accuracy involves using G-code commands to move the machine tool precisely and measuring the results. Key commands include:

• G0: Rapid positioning
• G1: Linear interpolation (controlled feed)
• G28: Return to machine home
• M30: Program end and rewind

Use these commands to move the tool to specific coordinates and then measure the actual position with a dial indicator or measuring tool.

gcode

N10 G21 ; Set units to millimeters
N20 G90 ; Absolute positioning
N30 G0 X0 Y0 Z5 ; Move to start position
N40 G1 Z0 F100 ; Move down to surface
N50 G1 X50 Y0 F200 ; Move 50mm in X
N60 G1 X50 Y50 ; Move 50mm in Y
N70 G1 X0 Y50 ; Move back in X
N80 G1 X0 Y0 ; Return to start
N90 G28 ; Return to home
N100 M30 ; End program

Example

This example G-code moves the CNC tool in a square path of 50mm sides. After running, measure the cut or tool position with a dial indicator or caliper to check if the machine moved exactly 50mm on each side.

gcode

N10 G21 ; Use millimeters
N20 G90 ; Absolute mode
N30 G0 X0 Y0 Z5 ; Move above start
N40 G1 Z-1 F100 ; Cut down 1mm
N50 G1 X50 Y0 F300 ; Move 50mm right
N60 G1 X50 Y50 ; Move 50mm forward
N70 G1 X0 Y50 ; Move 50mm left
N80 G1 X0 Y0 ; Move back to start
N90 G0 Z5 ; Lift tool
N100 M30 ; End

Common Pitfalls

Common mistakes when checking CNC accuracy include:

• Not zeroing the tool properly before test cuts, causing measurement errors.
• Using worn or uncalibrated measuring tools like dial indicators or calipers.
• Ignoring machine warm-up time, which can cause thermal expansion and affect accuracy.
• Running test cuts on uneven or unclean surfaces.

Always ensure proper setup and calibration before testing.

gcode

;; Wrong way: No zeroing
N10 G21
N20 G90
N30 G1 X50 Y0 F300 ; Move without zeroing

;; Right way: Zero tool first
N10 G21
N20 G90
N25 G92 X0 Y0 Z0 ; Set current position as zero
N30 G1 X50 Y0 F300 ; Move 50mm

Quick Reference

Tips for checking CNC machine accuracy:

• Use a calibration block with known dimensions.
• Run simple G-code moves to test linear accuracy.
• Measure with precise tools like dial indicators or micrometers.
• Repeat tests multiple times to confirm consistency.
• Document results to track machine performance over time.

Key Takeaways

Perform test cuts with precise G-code moves to check CNC accuracy.

Always zero the tool and use calibrated measuring tools for reliable results.

Measure multiple times and document to monitor machine performance.

Allow the machine to warm up before testing to avoid thermal errors.

Use simple shapes like squares or circles for easy measurement comparison.