In-process measurement in CNC Programming - Step-by-Step Execution

Concept Flow - In-process measurement

Start CNC Program

↓

Machine moves to measurement position

↓

Activate probe sensor

↓

Probe touches workpiece

↓

Measure dimension

↓

Store measurement data

↓

Compare measurement to tolerance

↓

Within

↓

Tolerance

↓

Continue machining

↓

End or loop

The CNC program moves the probe to measure the workpiece during machining, stores the measurement, checks if it is within tolerance, and decides to continue or adjust.

Execution Sample

CNC Programming

G00 X50 Y50 Z10 ; Move to measurement point
M85 ; Activate probe
G38.2 Z-5 F100 ; Probe down to workpiece
G10 L2 P1 Z0 ; Set measured position
M86 ; Deactivate probe
; Compare measurement and continue

This CNC code moves the probe to a point, activates it, probes down to the workpiece, records the measurement, then deactivates the probe.

Execution Table

Step	Action	Command	Measurement Value	Tolerance Check	Next Action
1	Move to measurement point	G00 X50 Y50 Z10	N/A	N/A	Proceed to probe activation
2	Activate probe	M85	N/A	N/A	Ready to probe
3	Probe down to workpiece	G38.2 Z-5 F100	Z = -3.2 mm	N/A	Record measurement
4	Set measured position	G10 L2 P1 Z0	Z = -3.2 mm	N/A	Deactivate probe
5	Deactivate probe	M86	N/A	N/A	Compare measurement
6	Compare measurement	Custom logic	-3.2 mm	Within ±0.05 mm	Continue machining
7	Continue machining	Next machining commands	N/A	N/A	End or loop

💡 Measurement within tolerance ±0.05 mm, machining continues

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 6	Final
Probe Position Z	N/A	-3.2 mm	-3.2 mm	-3.2 mm	-3.2 mm
Measurement Stored	No	No	Yes	Yes	Yes
Tolerance Check	N/A	N/A	N/A	Within ±0.05 mm	Within ±0.05 mm
Machining Status	Idle	Probing	Probing	Checking	Continue

Key Moments - 3 Insights

Why does the probe move down with G38.2 command instead of a normal move?

The G38.2 command is a probing move that stops automatically when the probe touches the workpiece, capturing the exact measurement (see execution_table step 3).

What happens if the measurement is out of tolerance?

If out of tolerance, the program would trigger an alarm or adjust machining parameters instead of continuing (not shown in this trace but implied after step 6).

Why do we store the measurement with G10 L2 P1 Z0?

This command sets the measured position as a reference offset for future machining, ensuring accuracy (execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the probe position Z after step 3?

A -3.2 mm

B -5.0 mm

C 0 mm

D Not measured yet

💡 Hint

Check the 'Measurement Value' column at step 3 in the execution_table.

At which step does the program store the measured position?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look for the command G10 L2 P1 Z0 in the execution_table.

If the measurement was out of tolerance, what would likely change in the execution table?

A Next Action would be 'Continue machining'

B Measurement Value would be 'N/A'

C Next Action would be 'Adjust or alarm'

D Probe would deactivate earlier

💡 Hint

Refer to the 'Next Action' column in the last rows of the execution_table.

Concept Snapshot

In-process measurement in CNC:
- Move probe to position (G00)
- Activate probe (M85)
- Probe down to workpiece (G38.2)
- Store measurement (G10 L2 P1)
- Deactivate probe (M86)
- Compare measurement to tolerance
- Continue or adjust machining accordingly

Full Transcript

In-process measurement in CNC programming involves moving the probe to a specific point, activating it, probing down until it touches the workpiece, recording the measurement, and then checking if the measurement is within tolerance. The program uses commands like G00 to move, M85 to activate the probe, G38.2 to probe down, G10 L2 P1 to store the measurement, and M86 to deactivate the probe. After measuring, the program compares the value to the allowed tolerance and decides whether to continue machining or make adjustments. This process ensures precision during machining.

Practice

1. What is the main purpose of in-process measurement in CNC machining?

easy

A. To speed up the machine spindle rotation

B. To check the part dimensions during machining to ensure accuracy

C. To change the tool automatically

D. To cool down the cutting tool

Solution

1. Step 1: Understand in-process measurement

   In-process measurement is used to check the size or position of a part while it is being machined.

2. Step 2: Identify the main goal

   The goal is to ensure the part is accurate and meets specifications by measuring it during machining.

3. Final Answer:

   To check the part dimensions during machining to ensure accuracy -> Option B

4. Quick Check:

   In-process measurement = Checking part size during machining [OK]

Hint: In-process means measuring while machining, not before or after [OK]

Common Mistakes:

• Confusing measurement with tool changes
• Thinking it controls spindle speed
• Assuming it cools the tool

2. Which of the following is the correct syntax to call a probe macro with parameters in CNC code?

easy

A. G65 P9000 X10 Y20 Z5

B. G65 P9000, X10, Y20, Z5

C. G65(P9000 X10 Y20 Z5)

D. G65 P9000; X10 Y20 Z5

Solution

1. Step 1: Recall G65 macro call syntax

   The G65 command calls a macro with parameters listed after it separated by spaces, no commas or parentheses.

2. Step 2: Check each option

   G65 P9000 X10 Y20 Z5 uses correct syntax: G65 P9000 X10 Y20 Z5. Others use commas, parentheses, or semicolons which are incorrect.

3. Final Answer:

   G65 P9000 X10 Y20 Z5 -> Option A

4. Quick Check:

   G65 macro call uses spaces, no commas [OK]

Hint: G65 macro calls list parameters with spaces only [OK]

Common Mistakes:

• Adding commas between parameters
• Using parentheses around parameters
• Separating parameters with semicolons

3. Given this CNC snippet for in-process measurement:

G65 P9000 X50 Y25 Z-5
IF[#506 EQ 1] THEN
  GOTO 100
ENDIF
GOTO 200
100 M30

What happens if the probe detects the part correctly (sets #506 to 1)?

medium

A. The program continues to line 200

B. The program repeats the probe command

C. The program stops immediately with an error

D. The program jumps to line 100 and ends

Solution

1. Step 1: Understand the IF condition

   If variable #506 equals 1, the program executes GOTO 100.

2. Step 2: Follow the program flow

   When #506 is 1, the program jumps to line 100, which contains M30 (program end).

3. Final Answer:

   The program jumps to line 100 and ends -> Option D

4. Quick Check:

   Probe success (#506=1) triggers jump to end [OK]

Hint: IF #506=1 means probe success, jump to end [OK]

Common Mistakes:

• Assuming program continues to line 200
• Thinking it causes an error stop
• Believing it repeats the probe command

4. Identify the error in this CNC in-process measurement code snippet:

G65 P9000 X30 Y15 Z-3
IF[#506 = 1] THEN
  GOTO 150
ENDIF

medium

A. The GOTO command should be lowercase

B. The G65 command is missing the P code

C. The IF condition uses a single '=' instead of 'EQ' for comparison

D. The Z value cannot be negative

Solution

1. Step 1: Check IF condition syntax

   CNC macro IF conditions require 'EQ' for equality, not a single '=' which is assignment.

2. Step 2: Verify other parts

   G65 has P9000, GOTO is case-insensitive, and Z can be negative for probe approach.

3. Final Answer:

   The IF condition uses a single '=' instead of 'EQ' for comparison -> Option C

4. Quick Check:

   Use 'EQ' for equality in IF, not '=' [OK]

Hint: Use 'EQ' for equality in CNC IF, '=' is assignment [OK]

Common Mistakes:

• Using '=' instead of 'EQ' in IF
• Thinking GOTO case matters
• Believing negative Z is invalid

5. You want to measure a part diameter during machining and adjust the tool offset automatically if the diameter is too large. Which approach best uses in-process measurement macros?

hard

A. Use G65 to probe diameter, compare measurement, then update tool offset with G10 if needed

B. Use G65 to probe diameter and immediately stop the machine if size is off

C. Manually measure after machining and adjust tool offset in next run

D. Use G65 to probe diameter but ignore the measurement results

Solution

1. Step 1: Use G65 macro to measure diameter

   G65 calls a probe macro to measure the part size during machining.

2. Step 2: Compare measurement and adjust tool offset

   If the diameter is too large, use G10 command to update the tool offset automatically to correct the size.

3. Final Answer:

   Use G65 to probe diameter, compare measurement, then update tool offset with G10 if needed -> Option A

4. Quick Check:

   Probe with G65, adjust offset with G10 for accuracy [OK]

Hint: Probe then adjust offset automatically for best accuracy [OK]

Common Mistakes:

• Stopping machine immediately without adjustment
• Ignoring measurement results
• Adjusting tool offset manually after machining