G93 vs G94: Understanding CNC Feedrate Modes for Precise Motion Control

G93 vs G94: Understanding CNC Feedrate Modes for Precise Motion Control

Feedrate in CNC controls how fast your tool moves through space — but did you know your controller can calculate it in two very different ways?
That’s the role of G93 and G94.

In this guide, we’ll break down the difference between these feedrate modes and when to use them.

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📘 Quick Definitions

G-Code	Mode Name	Feedrate Unit
G94	Units Per Minute	mm/min or inch/min
G93	Inverse Time Mode	1/time (min⁻¹)

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📐 G94 – Standard Feedrate Mode

G94
G01 X50.0 F500

• Feedrate is in mm/min or inch/min
• Most commonly used mode in milling, drilling, turning
• Easy to understand: “Move X at 500 mm/min”

✅ Best for most general CNC work

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🕒 G93 – Inverse Time Feed Mode

G93
G01 X50.0 Y50.0 Z-10.0 F0.1

• Feedrate is interpreted as inverse time
• F0.1 means: complete this move in 10 minutes
• F1.0 means: complete move in 1 minute

Useful when:

• You need precise timing, especially on multi-axis (4/5-axis) machines
• Creating motion-synchronized toolpaths
• Running NURBS-based CAM paths or simultaneous contouring

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🎯 Why Use G93?

• CAM software often outputs G93 for 5-axis contouring
• Ensures exact tool movement duration — vital for surface finish and blending
• Required when using trajectory-based control or machine-integrated motion paths

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⚠️ Important G93 Considerations

• G93 calculates time per block. You must define F for every motion block:

G93
G01 X10 Y10 F0.5
G01 X20 Y20 F0.4

• Forgetting to include F will result in controller errors or zero feed.

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🔁 Switching Between Modes

G94 ; Set back to normal feed

Always switch back to G94 after using G93 if your controller expects conventional feedrates elsewhere in the program.

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💡 Example: Comparing G93 vs G94

Let’s say you want to move 100 mm in 2 seconds.

• With G94:

  G94
  G01 X100 F3000  ; 3000 mm/min = 50 mm/sec

• With G93:

  G93
  G01 X100 F0.0333 ; 1 ÷ 30 sec = 0.0333

With G93, you’re programming the time per move, not the speed.

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✅ Best Practices

• Only use G93 when required by CAM or motion logic
• Always provide F value on every line
• Test and simulate carefully — wrong F values can cause tool breakage
• Remember to switch back to G94 when done

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🧠 Summary

Feature	G94	G93
Feedrate Unit	mm/min or inch/min	Inverse time (1/min)
Usage	General machining	4/5-axis synchronized
CAM Output	Most 2D/3D toolpaths	High-end surface machining
F Value	Feed per minute	Time per move (inverted)

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▶️ Coming Up: G95 – Feed Per Revolution Mode Explained