CNC Speeds, Feeds & Machining Parameters 3 — Questions and Answers
Question 1: What is the purpose of specifying a 'dwell' (G04) command in a CNC boring cycle?
- To slow the feed rate automatically
- To pause spindle rotation at the bottom of the bore for a timed period to improve finish (Correct answer)
- To retract the tool rapidly to home position
- To index the turret to the next tool
Correct answer: To pause spindle rotation at the bottom of the bore for a timed period to improve finish
A G04 dwell at the bottom of a bore allows the tool to complete a full revolution, cleaning up any feed lines and improving roundness and finish.
Question 2: When machining stainless steel (304 SS), which approach is preferred to reduce work hardening?
- Light, fast interrupted cuts with high RPM
- Continuous cuts with adequate chip load to stay below the work-hardened layer (Correct answer)
- Dry cutting at maximum SFM
- Peck drilling with no coolant
Correct answer: Continuous cuts with adequate chip load to stay below the work-hardened layer
Stainless steel work-hardens quickly; maintaining continuous, sufficiently deep cuts keeps the tool cutting below the already-hardened surface layer.
Question 3: In high-speed machining (HSM), which strategy reduces radial load while maintaining material removal rate?
- Full-width slotting passes
- Trochoidal or circular arc tool paths with low RDOC and high ADOC (Correct answer)
- Reducing spindle speed by 50%
- Increasing feed rate without changing depth of cut
Correct answer: Trochoidal or circular arc tool paths with low RDOC and high ADOC
Trochoidal tool paths use low radial engagement but high axial depth, distributing heat over the entire flute length and reducing peak load.
Question 4: What does 'tool life' in Taylor's tool life equation primarily depend on?
- Workpiece hardness and coolant type only
- Cutting speed (SFM) as the dominant variable (Correct answer)
- Feed rate and depth of cut equally
- Spindle horsepower only
Correct answer: Cutting speed (SFM) as the dominant variable
Taylor's equation (VT^n = C) shows that cutting speed is the dominant variable — doubling SFM dramatically reduces tool life.
Question 5: When calculating feed rate (IPM) for a milling operation, which variables are needed?
- SFM, tool diameter, and depth of cut
- RPM, number of flutes, and chip load (IPT) (Correct answer)
- Tool material, workpiece hardness, and coolant flow
- Axial depth, radial depth, and spindle power
Correct answer: RPM, number of flutes, and chip load (IPT)
Feed rate (IPM) = RPM × Number of Flutes × Chip Load (IPT), which directly links table feed to the number of cutting edges and desired chip thickness.
Question 6: Which symptom most strongly indicates that spindle speed is too high when turning hardened steel?
- Blue or discolored chips curling away from the cut (Correct answer)
- Long, stringy chips wrapping around the workpiece
- Rapid built-up edge (BUE) on the tool nose
- Excessive chip breakage into tiny fragments
Correct answer: Blue or discolored chips curling away from the cut
Blue or discolored chips indicate excessive heat from too-high cutting speed, which can cause premature tool wear or thermal cracking of carbide.
CNC Speeds, Feeds & Machining Parameters 3
What is the purpose of specifying a 'dwell' (G04) command in a CNC boring cycle?