Feed rate (F word) specification in CNC Programming - Step-by-Step Execution

Concept Flow - Feed rate (F word) specification

Start CNC Program

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Set Feed Rate with F word

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Machine moves tool at specified feed rate

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Continue machining operations

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End Program or Change Feed Rate

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Stop or Repeat

The CNC program starts, sets the feed rate using the F word, then the machine moves the tool at that speed during machining until the feed rate changes or the program ends.

Execution Sample

CNC Programming

N10 G01 X50 Y50 F100
N20 X100 Y100
N30 F200
N40 X150 Y150

This CNC code sets feed rate to 100 mm/min at line N10, moves tool, then changes feed rate to 200 mm/min at N30 and moves again.

Execution Table

Line	Code	Feed Rate (F)	Action	Tool Movement Speed
N10	G01 X50 Y50 F100	100	Set feed rate to 100 and move tool to X50 Y50	100 mm/min
N20	X100 Y100	100	Move tool to X100 Y100 using current feed rate	100 mm/min
N30	F200	200	Change feed rate to 200	No movement
N40	X150 Y150	200	Move tool to X150 Y150 using new feed rate	200 mm/min
End		200	Program ends, feed rate stays at last set value	Stop

💡 Program ends after N40, feed rate remains at last set value 200 mm/min

Variable Tracker

Variable	Start	After N10	After N20	After N30	After N40	Final
Feed Rate (F)	undefined	100	100	200	200	200
Tool Position	(0,0)	(50,50)	(100,100)	(100,100)	(150,150)	(150,150)

Key Moments - 3 Insights

Why does the feed rate stay the same at line N20 even though there is no F word?

Because the feed rate set at N10 (F100) remains active until changed. The execution_table row for N20 shows feed rate is still 100.

What happens when the F word appears alone on line N30?

It changes the feed rate without moving the tool. The execution_table row for N30 shows feed rate changes to 200 but no movement occurs.

Does the feed rate reset after the program ends?

No, the feed rate stays at the last set value (200). The exit_note and final variable_tracker values confirm this.

Visual Quiz - 3 Questions

Look at the execution_table at line N20. What is the feed rate used for the tool movement?

A 200 mm/min

B 100 mm/min

C Undefined

D 50 mm/min

💡 Hint

Check the Feed Rate column for line N20 in the execution_table.

At which line does the feed rate change from 100 to 200?

A N30

B N10

C N20

D N40

💡 Hint

Look for the line where the F word changes from 100 to 200 in the execution_table.

If line N30 did not have the F200 command, what would be the feed rate at line N40?

A Undefined

B 200 mm/min

C 100 mm/min

D 0 mm/min

💡 Hint

Refer to variable_tracker to see how feed rate changes only when F word is specified.

Concept Snapshot

Feed rate in CNC is set by the F word.
It controls tool movement speed in units/min.
Once set, feed rate stays until changed.
F word can appear with movement or alone.
Changing feed rate alone does not move tool.
Always specify feed rate before moving tool for control.

Full Transcript

This visual execution shows how the feed rate is set and used in CNC programming using the F word. The program starts by setting feed rate to 100 mm/min at line N10, then moves the tool at that speed. At line N30, the feed rate changes to 200 mm/min without moving the tool. The tool then moves at the new speed at line N40. The feed rate remains at the last set value until the program ends. This helps beginners understand that the F word sets the speed and stays active until changed, and that feed rate changes can happen with or without tool movement.

Practice

1. What does the F word specify in a CNC program?

easy

A. The feed rate, or how fast the tool moves while cutting

B. The spindle speed, or how fast the tool spins

C. The coolant flow rate

D. The tool change command

Solution

1. Step 1: Understand the role of the F word

   The F word in CNC programming sets the feed rate, which controls the speed of the tool's movement during cutting.

2. Step 2: Differentiate from other commands

   Spindle speed is controlled by the S word, coolant by M codes, and tool change by T codes, not F.

3. Final Answer:

   The feed rate, or how fast the tool moves while cutting -> Option A

4. Quick Check:

   F word = feed rate [OK]

Hint: F word always sets tool movement speed [OK]

Common Mistakes:

• Confusing F with spindle speed (S word)
• Thinking F controls coolant or tool change
• Ignoring feed rate importance

2. Which of the following is the correct syntax to set a feed rate of 150 mm/min in a CNC program?

easy

A. F150

B. S150

C. M150

D. T150

Solution

1. Step 1: Identify the correct letter for feed rate

   The feed rate is set using the letter F followed by the numeric value.

2. Step 2: Check other letters meanings

   S sets spindle speed, M codes control machine functions, and T selects tools, so they are incorrect here.

3. Final Answer:

   F150 -> Option A

4. Quick Check:

   Feed rate syntax = F + value [OK]

Hint: Feed rate always starts with F followed by number [OK]

Common Mistakes:

• Using S instead of F for feed rate
• Adding spaces like F 150 (may cause errors)
• Confusing M or T codes with feed rate

3. What will be the feed rate set by this CNC line?

G01 X50 Y50 F200

medium

A. 50 mm/min

B. 200 mm/min

C. No feed rate set

D. Default feed rate

Solution

1. Step 1: Locate the F word in the line

   The line contains F200, which sets the feed rate to 200 mm/min.

2. Step 2: Understand the effect of F200

   This means the tool will move at 200 mm per minute while moving to X50 Y50.

3. Final Answer:

   200 mm/min -> Option B

4. Quick Check:

   F200 means feed rate = 200 [OK]

Hint: F number in line sets feed rate directly [OK]

Common Mistakes:

• Ignoring F word and assuming default feed
• Confusing X or Y values with feed rate
• Thinking feed rate is not set without separate command

4. Identify the error in this CNC command line:

G01 X100 Y100 F

medium

A. Feed rate should be set with S, not F

B. Incorrect G code used

C. Coordinates X and Y are invalid

D. Missing feed rate value after F

Solution

1. Step 1: Check the F word usage

   The F word must be followed by a numeric value to set feed rate, but here it is missing.

2. Step 2: Verify other parts of the command

   G01 is correct for linear move, X and Y coordinates are valid numbers, and feed rate uses F, not S.

3. Final Answer:

   Missing feed rate value after F -> Option D

4. Quick Check:

   F must have a number after it [OK]

Hint: F must always have a number after it [OK]

Common Mistakes:

• Leaving F without a number
• Using S instead of F for feed rate
• Assuming coordinates are wrong instead

5. You want to program a CNC tool to make a rough cut at 300 mm/min and then a fine cut at 100 mm/min. Which sequence correctly sets the feed rates?

hard

A. G01 X100 Y0 S300
G01 X200 Y0 S100

B. G01 X100 Y0 F100
G01 X200 Y0 F300

C. G01 X100 Y0 F300
G01 X200 Y0 F100

D. G01 X100 Y0 M300
G01 X200 Y0 M100

Solution

1. Step 1: Identify correct feed rate syntax

   Feed rate is set with F followed by the speed value, so F300 and F100 are correct.

2. Step 2: Check sequence for rough then fine cut

   The first line sets feed rate to 300 mm/min (rough cut), the second line sets 100 mm/min (fine cut), matching the requirement.

3. Step 3: Verify incorrect options

   The options using S and M are not for feed rate;

   G01 X100 Y0 F100
   G01 X200 Y0 F300

   reverses the order of speeds.

4. Final Answer:

   G01 X100 Y0 F300 G01 X200 Y0 F100 -> Option C

5. Quick Check:

   F sets feed rate; rough then fine = 300 then 100 [OK]

Hint: Use F for feed rate; order matters for rough then fine [OK]

Common Mistakes:

• Using S or M instead of F for feed rate
• Reversing rough and fine feed rates
• Omitting feed rate changes between cuts