CNC Cutting Parameters and Speeds/Feeds 2 — Questions and Answers
Question 1: According to Taylor's tool life equation (VTⁿ = C), what happens to tool life as cutting speed increases?
- Tool life increases proportionally with cutting speed
- Tool life is unaffected by cutting speed changes
- Tool life decreases as cutting speed increases (Correct answer)
- Tool life first increases then sharply decreases
Correct answer: Tool life decreases as cutting speed increases
Taylor's equation shows an inverse relationship: as cutting speed V rises, the tool life T falls because higher speeds generate more heat at the cutting edge.
Question 2: In CNC turning, feed rate is most commonly expressed as:
- Inches per minute (IPM)
- Surface feet per minute (SFM)
- Inches per revolution (IPR) (Correct answer)
- Millimeters per tooth (mm/tooth)
Correct answer: Inches per revolution (IPR)
In turning, feed rate is expressed as inches per revolution (IPR) because it directly sets chip thickness relative to each full rotation of the workpiece, independent of RPM.
Question 3: If you switch from a 2-flute to a 4-flute end mill at the same RPM and want to maintain the same chip load per tooth, you must:
- Keep the feed rate identical
- Double the feed rate (Correct answer)
- Halve the feed rate
- Quadruple the feed rate
Correct answer: Double the feed rate
With twice as many flutes cutting per revolution, feed rate must be doubled to maintain the same chip load: Chip load = IPM / (RPM × flutes).
Question 4: What does the G96 code activate on a CNC lathe?
- Fixed RPM (constant spindle speed) mode
- Thread cutting cycle
- Rapid traverse positioning mode
- Constant surface speed (CSS) mode (Correct answer)
Correct answer: Constant surface speed (CSS) mode
G96 enables constant surface speed mode, automatically adjusting the spindle RPM as the tool moves radially to maintain a consistent SFM at the cutting point.
Question 5: Which cutting parameter has the greatest direct influence on surface finish quality in CNC milling?
- Axial depth of cut (ADOC)
- Cutting speed (SFM)
- Feed rate (Correct answer)
- Radial depth of cut (RDOC)
Correct answer: Feed rate
Feed rate directly determines the cusp (scallop) height left between tool passes; lower feed rates produce smaller cusps and finer surface finishes.
Question 6: Blue or heavily discolored chips produced while machining steel most likely indicate:
- Feed rate is set too low, causing rubbing
- Excessive cutting speed generating too much heat at the cutting zone (Correct answer)
- Insufficient depth of cut per pass
- The cutter has too many flutes for the material
Correct answer: Excessive cutting speed generating too much heat at the cutting zone
Blue discoloration indicates chips exceeded approximately 450°F, a clear sign that cutting speed is too high or coolant delivery is inadequate for the conditions.
Question 7: Radial depth of cut (RDOC) in CNC milling describes:
- How deep the tool plunges along the Z-axis per pass
- The percentage of the tool diameter engaged radially in the cut (Correct answer)
- The distance the tool feeds per full revolution
- The total length of the programmed cutting path
Correct answer: The percentage of the tool diameter engaged radially in the cut
RDOC (radial engagement) is the width of cut measured perpendicular to the tool axis, typically expressed as a fraction or percentage of the tool diameter.
CNC Cutting Parameters and Speeds/Feeds 2
According to Taylor's tool life equation (VTⁿ = C), what happens to tool life as cutting speed increases?