Machine–inherent errors
• Positional – linear scale accuracy, ball-screw lead error, backlash (±2–5 µm typical).
• Geometric – squareness between axes, spindle tilt, straightness of ways (creates taper or banana shape).
• Thermal – 1 °C change ≈ 11 µm/m on steel ways; growth of spindle & part cause drift during long runs.
Tool & cutting errors
• Tool wear – flank wear VB > 0.05 mm enlarges diameters, rounds edges.
• Deflection – L/D > 4:1 causes 5–20 µm bell-mouth or hour-glass.
• Chip re-cutting – micro-chips scratch surface, raise Ra 0.2 → 0.6 µm.
Workholding & datum errors
• Fixture clamp force variation – elastic part compression 2–8 µm; released after un-clamp.
• Datum shift – uneven seat or chip under locators moves zero 5–15 µm.
• Chuck jaw run-out – 3-jaw scroll gives 2–6 µm concentricity error on re-chuck.
Measurement & sampling errors
• CMM stylus bending – 3 mm stylus, 0.2 N force ≈ 1 µm deflection.
• Temperature mismatch – part 5 °C warmer than CMM adds ~6 µm on 100 mm Al.
• Sampling bias – measuring only "good-looking" parts inflates Cpk by 0.2–0.3.
Material & stress errors
• Residual stress release – after 0.5 mm skin removal, Al 7075 parts warp 10–40 µm on 100 mm span.
• Phase change – 420 SS heat-affected zone expands 0.01 %, changes hole size 2 µm per 20 mm.
Environment errors
• Shop-floor temp swing 2 °C/hour – 200 mm steel part grows 4 µm.
• Vibration – nearby forklift or punch press can leave chatter marks 1–2 µm deep.
Human / programming errors
• Tool offset typo – 0.1 mm key-in mistake still passes patrol check if gauge is 0.15 mm loose.
• Wrong cutter comp direction – mirror-image feature off by twice cutter radius.
Rule-of-thumb stack-up
For a 50 mm aluminum part on a 3 µm machine, expect:
Machine 3 µm + Thermal 2 µm + Tool wear 2 µm + Clamp 2 µm + CMM 1 µm → ±10 µm total.
Design tolerance must be ≥ 20 µm to hit Cpk 1.33.
Bottom line
"Precision" is not the machine spec; it is the sum of all error sources you identify, measure and control.
