CNC Speeds, Feeds & Machining Parameters 1 — Questions and Answers
Question 1: What formula is used to calculate spindle RPM when surface footage (SFM) and tool diameter are known?
- RPM = (SFM × 3.82) / Diameter (Correct answer)
- RPM = (SFM × Diameter) / 3.82
- RPM = SFM / (Diameter × π)
- RPM = SFM × Diameter × 3.14
Correct answer: RPM = (SFM × 3.82) / Diameter
The standard formula is RPM = (SFM × 3.82) / Diameter, where 3.82 is derived from 12/π.
Question 2: When machining 6061 aluminum with a 1/2-inch HSS end mill, which SFM range is most appropriate?
- 50–100 SFM
- 200–300 SFM
- 400–600 SFM (Correct answer)
- 800–1000 SFM
Correct answer: 400–600 SFM
Aluminum is a soft, free-machining material and HSS tools can run 400–600 SFM without excessive heat buildup.
Question 3: What effect does increasing feed rate have on chip thickness in CNC milling?
- Chip thickness decreases
- Chip thickness increases (Correct answer)
- Chip thickness remains the same
- Chip thickness becomes variable
Correct answer: Chip thickness increases
Higher feed rate means the tool advances farther per tooth engagement, producing thicker chips.
Question 4: The recommended starting chip load for a 1/2-inch carbide end mill in mild steel is closest to:
- 0.001 inch per tooth
- 0.004 inch per tooth (Correct answer)
- 0.015 inch per tooth
- 0.030 inch per tooth
Correct answer: 0.004 inch per tooth
A 0.004 inch-per-tooth chip load is a typical starting point for a 1/2-inch carbide end mill in mild steel.
Question 5: What is the primary reason for reducing spindle speed when drilling deep holes?
- To increase tool life only
- To allow adequate chip evacuation and prevent drill breakage (Correct answer)
- To reduce power consumption
- To improve surface finish at the hole bottom
Correct answer: To allow adequate chip evacuation and prevent drill breakage
Deep holes trap chips, and reduced speed combined with peck drilling cycles allows chips to clear before the drill binds or breaks.
Question 6: Which of the following best describes 'axial depth of cut' (ADOC) in CNC milling?
- The width of the cut measured radially from the tool center
- The depth the tool engages the workpiece along the spindle axis (Correct answer)
- The total length of the tool path
- The feed rate in the Z-axis direction
Correct answer: The depth the tool engages the workpiece along the spindle axis
Axial depth of cut (ADOC) is the engagement depth measured along the tool's rotational axis (Z direction), controlling how deep each pass cuts.
CNC Speeds, Feeds & Machining Parameters 1
What formula is used to calculate spindle RPM when surface footage (SFM) and tool diameter are known?