Key Factors that Influence Surface Roughness in CNC Precision Part Machining
Cutting parameters
• Feed rate – higher feed leaves thicker "cusp" peaks; doubling feed can raise Ra from 0.8 µm → 1.6 µm.
• Cutting speed – too low causes built-up edge (BUE) and tearing; too high accelerates tool wear; both increase roughness .
• Depth of cut – heavy roughing marks must be removed by a 0.05–0.1 mm finishing pass or Ra jumps > 50 %.
Tool geometry & condition
• Nose radius – larger radius spreads chip load; changing from 0.4 mm to 0.8 mm can cut Ra by 30 %.
• Rake & relief angles – sharp positive rake lowers cutting force and vibration marks.
• Tool wear – flank wear VB > 0.05 µm turns a mirror finish into 1.6 µm Ra almost overnight.
Work-piece material
• Hardness & ductility – soft 6061-T6 Al can smear, whereas 316L work-hardens and gives torn peaks; both raise Ra unless speed/feed are matched.
• Micro-structure – fine-grain heat-treated steel machines to 0.4 µm Ra, as-cast coarse grain struggles to reach 1.6 µm.
Coolant / lubrication
• Flood coolant at 8 % concentration keeps cutting zone below 200 °C, suppressing BUE and chip welding; dry cutting can add 0.2–0.4 µm to Ra.
• High-pressure through-spindle coolant (70 bar) breaks chips and prevents secondary scratching.
Machine & fixture dynamics
• Spindle run-out > 2 µm or bearing wear creates chatter lobes; Ra can spike from 0.8 µm → 2.0 µm even with perfect tooling.
• Overhang > 3 × diameter on end-mill acts like a tuning fork; vibration marks visible at 10× magnification.
Residual area & chip flow
• The theoretical peak height h ≈ f ²/(8×R) where f = feed, R = nose radius; reducing feed from 0.2 mm/rev to 0.05 mm/rev drops h by 16×.
• Improper chip breakers let chips scratch the just-cut surface, adding 0.1–0.3 µm Ra.
Process sequence
• Skipping a 0.1 mm semi-finish pass before the 0.02 mm finish pass leaves 0.4 µm Ra instead of 0.2 µm on 45 steel.
• Final climb-mill direction gives lower Ra than conventional milling on most Al alloys.
Rule-of-thumb target map
Ra 0.4 µm → sharp insert, 0.05 mm/rev, 0.2 mm nose rad, 150 m/min, flood coolant, < 1 µm run-out.
Ra 1.6 µm → OK to relax feed to 0.15 mm/rev, nose rad 0.4 mm, no special coolant pressure.
Ra 3.2 µm → roughing parameters; no finish pass needed.
Bottom line
"Surface finish is not an accident-it is the sum of feed, tool radius, speed, coolant and stiffness; control them and you control Ra."
