CNC Basics 3 — Questions and Answers
Question 1: Which material property most directly influences the recommended cutting speed for CNC operations?
- Color and texture
- Hardness and machinability rating (Correct answer)
- Weight and density
- Electrical conductivity
Correct answer: Hardness and machinability rating
A material's hardness and machinability rating determine how aggressively it can be cut, directly setting the recommended surface feet per minute.
Question 2: What is the function of cutter radius compensation (G41/G42) in CNC programming?
- It adjusts spindle speed for tool diameter
- It offsets the tool path to account for the physical radius of the cutter (Correct answer)
- It compensates for thermal expansion of the workpiece
- It automatically selects the correct tool from the magazine
Correct answer: It offsets the tool path to account for the physical radius of the cutter
G41/G42 shifts the programmed tool path left or right by the cutter's radius so the part profile is cut to exact dimensions.
Question 3: What does 'chip load' mean in CNC cutting?
- The total weight of chips produced per hour
- The thickness of material removed by each cutting edge per revolution (Correct answer)
- The number of chips that must be cleared before the next pass
- The ratio of chip length to tool diameter
Correct answer: The thickness of material removed by each cutting edge per revolution
Chip load (feed per tooth) is the thickness of material removed by one flute in one revolution, calculated as feed rate divided by (RPM × number of flutes).
Question 4: Which of the following best describes a 'work offset' (G54–G59) in CNC machining?
- A correction applied to tool length
- A stored coordinate that defines the workpiece zero relative to the machine zero (Correct answer)
- An emergency stop offset for collision avoidance
- The distance between two fixtures on the table
Correct answer: A stored coordinate that defines the workpiece zero relative to the machine zero
Work offsets store the distance from the machine's home position to the workpiece zero, allowing programs to reference part coordinates independently of machine position.
Question 5: What is the main advantage of using carbide inserts over high-speed steel (HSS) tools for CNC cutting?
- Carbide is cheaper and easier to sharpen
- Carbide withstands higher cutting temperatures and speeds (Correct answer)
- Carbide is more flexible and resists chipping better
- Carbide produces smoother finishes at lower speeds
Correct answer: Carbide withstands higher cutting temperatures and speeds
Carbide's higher heat resistance allows it to run at significantly faster cutting speeds than HSS, improving productivity and extending tool life.
Question 6: What happens when the radial depth of cut (stepover) is increased during CNC milling?
- Surface finish improves and tool wear decreases
- Cutting forces increase and more heat is generated (Correct answer)
- Spindle speed must be reduced to maintain chip load
- Tool life is unaffected but cycle time increases
Correct answer: Cutting forces increase and more heat is generated
A larger stepover engages more of the cutter simultaneously, raising cutting forces, heat generation, and tool wear.
Question 7: In CNC programming, what does a negative Z value typically indicate when using a standard right-hand coordinate system?
- Movement toward the operator
- Movement downward, into the workpiece (Correct answer)
- Movement to the left of the spindle centerline
- Movement away from the tool changer
Correct answer: Movement downward, into the workpiece
In the standard CNC right-hand coordinate system, the Z-axis positive direction is away from the workpiece (tool retracting), so negative Z moves the tool into the part.
CNC Basics 3
Which material property most directly influences the recommended cutting speed for CNC operations?