Ev-technologyHow-ToBeginner · 4 min read

CNC Programming Interview Questions: Key Concepts and Examples

Common CNC programming interview questions focus on understanding G-code syntax, toolpath generation, machine setup, and troubleshooting. Interviewers often ask for examples of G-code commands, how to optimize programs, and how to handle errors during machining.

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Syntax

G-code is the language used to control CNC machines. It consists of commands starting with letters like G or M followed by numbers. For example, G01 means a linear move, and M03 starts the spindle.

Each line usually has a command and parameters like coordinates or speeds.

gcode

N10 G21 (Set units to millimeters)
N20 G90 (Set absolute positioning)
N30 M03 S1000 (Start spindle at 1000 RPM)
N40 G01 X50 Y25 F200 (Move linearly to X=50, Y=25 at feed rate 200)
N50 M05 (Stop spindle)
N60 M30 (End program)

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Example

This example shows a simple CNC program that moves the tool to a point, starts the spindle, cuts a line, and stops the spindle.

gcode

N10 G21
N20 G90
N30 M03 S1200
N40 G01 X100 Y0 F150
N50 G01 X100 Y100 F150
N60 M05
N70 M30

Output

The machine moves in millimeters, starts the spindle at 1200 RPM, cuts a line from the origin to X=100 Y=0, then to X=100 Y=100, stops the spindle, and ends the program.

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Common Pitfalls

Common mistakes include forgetting to set units (G20 for inches or G21 for millimeters), mixing absolute (G90) and incremental (G91) positioning without care, and not stopping the spindle (M05) before program end.

Also, incorrect feed rates or missing safety commands can cause machine errors or damage.

gcode

Wrong:
N10 G90
N20 G01 X50 Y50 F100
N30 G91
N40 G01 X10 Y10 F100

Right:
N10 G90
N20 G01 X50 Y50 F100
N30 G91
N40 G01 X10 Y10 F100
N50 G90 (Return to absolute positioning)

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Quick Reference

Command	Description
G00	Rapid positioning (fast move)
G01	Linear interpolation (cutting move)
G02	Clockwise arc
G03	Counterclockwise arc
G20	Set units to inches
G21	Set units to millimeters
G90	Absolute positioning
G91	Incremental positioning
M03	Spindle on clockwise
M05	Spindle stop
M30	End program

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Key Takeaways

Understand basic G-code commands and their syntax for CNC programming.

Always set units and positioning mode explicitly to avoid errors.

Use spindle and feed rate commands carefully to ensure safe machining.

Test simple programs to verify machine behavior before complex jobs.

Be aware of common mistakes like mixing positioning modes or missing stops.