CNC Basics 4 — Questions and Answers
Question 1: What is the role of a 'tool length offset' (TLO) in CNC machining?
- It sets the maximum feed rate for that tool
- It compensates for the difference in length between tools so Z-axis moves are accurate (Correct answer)
- It tracks how many minutes the tool has been in use
- It adjusts spindle speed based on tool diameter
Correct answer: It compensates for the difference in length between tools so Z-axis moves are accurate
Tool length offsets tell the controller the actual length of each tool so Z coordinates programmed relative to the workpiece surface remain accurate regardless of which tool is active.
Question 2: Which cutting parameter should be reduced FIRST when machining a thin-walled workpiece to prevent deflection?
- Spindle speed (RPM)
- Radial depth of cut (stepover) (Correct answer)
- Coolant flow rate
- Number of tool flutes
Correct answer: Radial depth of cut (stepover)
Reducing radial depth of cut lowers the lateral cutting force that causes thin walls to flex or chatter.
Question 3: What does the CNC term 'dry run' mean?
- Running the program without coolant
- Executing the program at reduced feed with no workpiece installed to verify motion (Correct answer)
- Running the machine without lubrication on the ways
- A test cut made only in air with the spindle off
Correct answer: Executing the program at reduced feed with no workpiece installed to verify motion
A dry run cycles the program at reduced or rapid speed with the workpiece absent to verify the tool path and prevent crashes before actual cutting.
Question 4: When is conventional (up) milling preferred over climb milling?
- When machining hard, abrasive materials with no backlash in the machine (Correct answer)
- When machining soft materials for best finish
- When the spindle speed is below 1,000 RPM
- When using carbide inserts on stainless steel
Correct answer: When machining hard, abrasive materials with no backlash in the machine
Conventional milling is preferred on machines with leadscrew backlash or when cutting hard, abrasive materials where climb milling's tendency to 'grab' could damage the setup.
Question 5: What is the purpose of a 'canned cycle' (e.g., G81, G83) in CNC programming?
- To store tool geometry in the controller memory
- To simplify repetitive operations like drilling with a single-block command (Correct answer)
- To automatically select coolant type based on material
- To lock the rotary axis during milling
Correct answer: To simplify repetitive operations like drilling with a single-block command
Canned cycles condense multi-step operations (rapid approach, feed, retract) into one G-code block, greatly reducing program length for drilling, boring, and tapping.
Question 6: What effect does increasing the number of flutes on an end mill have when machining aluminum?
- Chip evacuation improves because there is more space between flutes
- Chip evacuation can worsen because flute gullets become smaller (Correct answer)
- Surface finish always decreases with more flutes
- RPM must be lowered in direct proportion to flute count
Correct answer: Chip evacuation can worsen because flute gullets become smaller
Aluminum produces large, stringy chips; more flutes mean shallower gullets that can pack and clog, which is why 2- or 3-flute end mills are standard for aluminum.
Question 7: What does the G28 command do on most CNC machining centers?
- Sets a new work coordinate origin
- Returns all axes to the machine reference (home) position (Correct answer)
- Activates tool length measurement mode
- Enables single-block execution
Correct answer: Returns all axes to the machine reference (home) position
G28 commands the machine to rapid all specified axes through an intermediate point and then to the machine home (reference) position.
CNC Basics 4
What is the role of a 'tool length offset' (TLO) in CNC machining?