First article inspection in CNC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a basic CNC probing cycle

What is the output of the following CNC probing cycle command sequence when the probe touches the part surface at Z=5.0?

CNC Programming

G38.2 Z-10 F100
#100=0
IF [#100 EQ 0] THEN
  #101=5.0
ENDIF
PRINT #101

A 5.0

B 0

C 10

D Error: Undefined variable #101

💡 Hint

The probe sets a variable when it touches the surface; check the IF condition.

The probing command G38.2 moves the probe until it touches the surface at Z=5.0. The variable #100 is set to 0 indicating success, so #101 is assigned 5.0 and printed.

📝 Syntax

intermediate

Identify the syntax error in macro variable assignment

Which option contains a syntax error in assigning a macro variable in CNC code?

A #200=100

B #201:=200

C #202= #200 + 50

D #203= [#201 * 2]

💡 Hint

Check the assignment operator used in CNC macro variables.

In CNC macro programming, the assignment operator is '=', not ':='. Option B uses ':=' which is invalid syntax.

🔧 Debug

advanced

Why does this probing macro fail to update the variable?

Given the following macro code, why does the variable #300 remain zero after execution?

CNC Programming

G38.2 Z-20 F50
IF [#100 NE 0] THEN
  #300 = #101
ENDIF

A The feed rate F50 is too slow to trigger the probe

B The variable #101 is not assigned by the probe command

C The condition should check if #100 equals 0, not not equal to 0

D The macro syntax requires parentheses around the condition

💡 Hint

Check the meaning of the probe success variable #100.

The probe command sets #100 to 0 on success. The condition checks for not equal to 0, so the assignment never happens. It should check if #100 equals 0.

🚀 Application

advanced

Calculate the offset for first article inspection

You run a probing cycle to find the surface at Z=12.5. The nominal programmed Z is 10.0. Which option correctly calculates the offset to adjust the program for the next run?

A offset = 12.5 - 10.0

B offset = 10.0 + 12.5

C offset = 12.5 + 10.0

D offset = 10.0 - 12.5

💡 Hint

Offset is actual surface minus programmed surface.

The offset is the difference between the actual measured surface (12.5) and the programmed surface (10.0), so offset = 12.5 - 10.0 = 2.5.

🧠 Conceptual

expert

What is the main purpose of first article inspection in CNC automation?

Choose the best description of the main purpose of first article inspection in CNC automated machining.

A To clean the machine tools before starting production

B To speed up the machining process by skipping quality checks

C To calibrate the CNC machine's spindle speed automatically

D To verify the initial part dimensions match the design before full production

💡 Hint

Think about quality control at the start of production.

First article inspection ensures the first part made matches design specs, preventing errors in full production runs.

Practice

1. What is the main purpose of First article inspection in CNC programming?

easy

A. To program the CNC machine without running it

B. To check the first part made by the CNC program for errors and quality

C. To clean the CNC machine before starting production

D. To speed up the CNC machine for faster production

Solution

1. Step 1: Understand the term 'First article inspection'

   It means checking the very first part produced by the CNC machine to ensure it meets quality standards.

2. Step 2: Identify the main goal of this process

   The goal is to catch errors early and confirm the part is made correctly before making many parts.

3. Final Answer:

   To check the first part made by the CNC program for errors and quality -> Option B

4. Quick Check:

   First article inspection = check first part quality [OK]

Hint: Remember: First article means first part check [OK]

Common Mistakes:

• Thinking it speeds up production
• Confusing it with machine cleaning
• Assuming it is programming without running

2. Which of the following is the correct sequence for performing a first article inspection?

easy

A. Run program, measure part, adjust program if needed

B. Measure part, run program, adjust program

C. Adjust program, run program, measure part

D. Run program, adjust program, measure part

Solution

1. Step 1: Identify the logical order of steps

   You first run the CNC program to make the part, then measure it to check accuracy.

2. Step 2: Adjust the program if measurements show errors

   If the part is not correct, you adjust the program and repeat as needed.

3. Final Answer:

   Run program, measure part, adjust program if needed -> Option A

4. Quick Check:

   Run -> Measure -> Adjust = A [OK]

Hint: Think: Make part first, then check and fix [OK]

Common Mistakes:

• Measuring before making the part
• Adjusting before measuring
• Skipping measurement step

3. Given this CNC program snippet for first article inspection:

G01 X10 Y10 F100
M30

What will be the expected output after running this program once?

medium

A. The tool moves in a circular path to X=10, Y=10

B. The tool will not move because feed rate is zero

C. The program will cause a syntax error and stop

D. The tool moves in a straight line to X=10, Y=10 at feed rate 100

Solution

1. Step 1: Understand G01 command

   G01 means linear move to specified coordinates at given feed rate.

2. Step 2: Analyze the given coordinates and feed rate

   The tool moves straight to X=10, Y=10 at feed rate 100 units/min.

3. Final Answer:

   The tool moves in a straight line to X=10, Y=10 at feed rate 100 -> Option D

4. Quick Check:

   G01 = linear move, feed 100 = speed [OK]

Hint: G01 means straight line move [OK]

Common Mistakes:

• Confusing G01 with circular move (G02/G03)
• Assuming feed rate is zero
• Thinking program has syntax error

4. You run a first article inspection program but the part dimensions are off. The program code is:

G01 X20 Y20 F150
M30

What is the likely error if the part is smaller than expected?

medium

A. Coordinates are incorrect, should be larger values

B. Program missing M03 spindle start command

C. Feed rate is too high causing tool deflection

D. M30 command is placed too early

Solution

1. Step 1: Check coordinates vs part size

   If the part is smaller, the tool likely did not move far enough, so coordinates are too small.

2. Step 2: Evaluate other options

   Feed rate affects speed, not size; missing spindle start or M30 early stop won't cause smaller size directly.

3. Final Answer:

   Coordinates are incorrect, should be larger values -> Option A

4. Quick Check:

   Wrong coordinates = wrong part size [OK]

Hint: Check coordinates first if part size is wrong [OK]

Common Mistakes:

• Blaming feed rate for size error
• Ignoring coordinate values
• Assuming spindle commands affect size

5. During first article inspection, you notice the part passes all measurements except one hole diameter is slightly too small. What is the best next step?

hard

A. Increase feed rate to cut the hole faster

B. Ignore the small hole difference and start full production

C. Adjust the CNC program tool path or tool size for that hole and rerun inspection

D. Replace the entire CNC program with a new one

Solution

1. Step 1: Identify the issue with the hole diameter

   The hole is too small, so the tool path or tool size needs adjustment to correct it.

2. Step 2: Decide the best corrective action

   Adjusting the program and rerunning inspection ensures quality before full production.

3. Final Answer:

   Adjust the CNC program tool path or tool size for that hole and rerun inspection -> Option C

4. Quick Check:

   Fix program, then re-inspect = correct approach [OK]

Hint: Fix program for errors before full production [OK]

Common Mistakes:

• Ignoring defects and producing many parts
• Changing feed rate without reason
• Replacing program unnecessarily