CNC Speeds, Feeds & Machining Parameters 2 — Questions and Answers
Question 1: If a CNC lathe is running at 800 RPM and the workpiece diameter is 2.0 inches, what is the approximate surface footage (SFM)?
- About 210 SFM
- About 420 SFM (Correct answer)
- About 630 SFM
- About 840 SFM
Correct answer: About 420 SFM
SFM = (RPM × Diameter × π) / 12 = (800 × 2.0 × 3.14159) / 12 ≈ 419 SFM, which rounds to about 420 SFM.
Question 2: What does 'radial depth of cut' (RDOC) refer to in a milling operation?
- How deep in the Z-axis the cutter plunges
- The stepover distance of the cutter across the workpiece (Correct answer)
- The length of the cutter flute
- The distance from the spindle center to the workpiece edge
Correct answer: The stepover distance of the cutter across the workpiece
Radial depth of cut (RDOC) is the lateral engagement of the cutter, measured perpendicular to the spindle axis, often expressed as a percentage of cutter diameter.
Question 3: Which machining parameter most directly affects tool deflection and dimensional accuracy in CNC milling?
- Spindle speed (RPM)
- Coolant pressure
- Radial depth of cut
- Tool stick-out length (Correct answer)
Correct answer: Tool stick-out length
Excessive tool stick-out increases leverage on the cutter, magnifying deflection forces and causing dimensional error and chatter.
Question 4: When using a constant surface speed (CSS) mode on a CNC lathe, what happens to RPM as the tool moves toward the center of the workpiece?
- RPM decreases to maintain SFM
- RPM increases to maintain SFM (Correct answer)
- RPM stays constant
- RPM alternates between high and low
Correct answer: RPM increases to maintain SFM
As diameter decreases toward center, the control raises RPM automatically to keep surface feet per minute constant.
Question 5: Which of the following best describes 'climb milling' compared to 'conventional milling'?
- The cutter rotates against the feed direction, creating thicker chips at entry
- The cutter rotates in the same direction as the feed, starting with thick chips (Correct answer)
- The cutter moves vertically downward through the material
- The cutter engages 100% of its diameter at all times
Correct answer: The cutter rotates in the same direction as the feed, starting with thick chips
In climb milling the cutter rotates in the feed direction, starting with a thick chip that thins to zero, reducing heat and improving surface finish.
Question 6: What is the most common reason a programmer specifies a 'finishing pass' at reduced depth of cut and feed?
- To increase material removal rate
- To achieve tighter dimensional tolerances and better surface finish (Correct answer)
- To extend coolant life
- To reduce program length
Correct answer: To achieve tighter dimensional tolerances and better surface finish
A light finishing pass removes only the stock left from roughing, minimizing tool deflection and producing the specified tolerance and surface finish.
CNC Speeds, Feeds & Machining Parameters 2
If a CNC lathe is running at 800 RPM and the workpiece diameter is 2.0 inches, what is the approximate surface footage (SFM)?