CNC Basics 5 — Questions and Answers
Question 1: What is 'chatter' in CNC machining and what is its primary cause?
- Surface discoloration caused by excessive heat
- Vibration between the tool and workpiece caused by resonance in the cutting system (Correct answer)
- Uneven chip formation from a dull tool only
- Coolant splashing that disrupts the cut
Correct answer: Vibration between the tool and workpiece caused by resonance in the cutting system
Chatter is self-excited vibration where the cutting forces feed energy into the machine-tool-workpiece resonant frequency, producing poor surface finish and accelerated tool wear.
Question 2: Which axis on a standard vertical machining center (VMC) controls the up-and-down movement of the spindle?
- X-axis
- Y-axis
- Z-axis (Correct answer)
- A-axis
Correct answer: Z-axis
On a VMC, the Z-axis is the spindle axis and controls vertical (up and down) movement, determining depth of cut.
Question 3: What is the recommended action when a CNC program encounters a tool that has exceeded its programmed tool life?
- Continue cutting at reduced feed until the program ends
- Replace or offset to a sister tool before resuming machining (Correct answer)
- Increase coolant flow to compensate for added wear
- Switch to manual mode and finish the part by hand
Correct answer: Replace or offset to a sister tool before resuming machining
Most CNC controllers support sister tool management, which swaps to an identical spare tool when the original exceeds its life limit, preventing scrap parts.
Question 4: What does 'surface feet per minute' (SFM) describe in CNC cutting?
- How many feet of chips are produced per minute
- The velocity at which the cutting edge moves past the workpiece surface (Correct answer)
- The linear distance the table travels per minute
- The spindle power consumption per foot of cut
Correct answer: The velocity at which the cutting edge moves past the workpiece surface
SFM (or surface meters per minute in metric) measures the tangential speed of the cutting edge relative to the workpiece, and is the primary parameter for selecting RPM.
Question 5: Why is it important to verify a CNC program using simulation software before running it on the machine?
- Simulation automatically corrects G-code syntax errors and rewrites the program
- It allows detection of tool collisions, gouges, and incorrect tool paths without risking the machine or workpiece (Correct answer)
- It calculates the exact cycle time and material cost automatically
- Simulation software uploads the corrected program to the controller directly
Correct answer: It allows detection of tool collisions, gouges, and incorrect tool paths without risking the machine or workpiece
Simulation visually replays the tool path in a virtual environment, revealing crashes, gouges, missed features, and other errors before any metal is cut.
Question 6: When referencing a CNC insert grade specification, what does the letter 'P' in the ISO classification indicate?
- It is suitable for non-ferrous metals like aluminum and copper
- It is designed for machining steel and cast steel (Correct answer)
- It is for hardened materials above 45 HRC
- It is a positive rake geometry insert
Correct answer: It is designed for machining steel and cast steel
In the ISO insert grade classification, 'P' designates grades formulated for cutting steel and cast steel, the most common application group.
Question 7: What is the primary function of a tool presetter in a CNC environment?
- To sharpen cutting tools to factory specifications offline
- To measure tool dimensions offline so length and diameter offsets can be entered before the tool is used on the machine (Correct answer)
- To automatically load tools into the tool magazine in the correct order
- To balance high-speed tool assemblies before installation in the spindle
Correct answer: To measure tool dimensions offline so length and diameter offsets can be entered before the tool is used on the machine
A tool presetter measures the exact length and diameter of each assembled tool offline, allowing accurate offsets to be loaded into the controller before machining begins.
CNC Basics 5
What is 'chatter' in CNC machining and what is its primary cause?