Mill-Turn Tolerance Guide
The value of mill-turn isn't just convenience — it's the geometric accuracy that comes from never removing the part from the spindle.
Turned Feature Tolerances
Mill-turn machines are fundamentally lathes. Turning accuracy is equivalent to a dedicated CNC lathe of the same class. ±0.0002" on diameters and ±0.0005" on lengths are achievable in production on quality machines (Okuma MULTUS, DMG Mori NTX, Mazak Integrex tier). Surface finish on turned features is Ra 16–32 μin typical, Ra 8 μin with fine finishing passes.
Milled Feature Tolerances
Milling accuracy on a mill-turn is slightly less than a dedicated VMC because the milling spindle on a mill-turn is typically less rigid than a dedicated mill's spindle. Expect ±0.0005" to ±0.001" on milled features — adequate for most applications but not as tight as what a high-end VMC achieves.
The Real Advantage: Cross-Feature Tolerances
The tolerance that matters most on mill-turn is the relationship between turned and milled features. A true position of ±0.001" between a turned bore and a milled slot is routine on mill-turn because both features are cut from the same datum without re-fixturing. Achieving that same tolerance on separate machines requires careful fixturing and often process capability studies to validate.
Concentricity and Runout
Milled features referenced to the turning axis — flats, keyways, cross-holes — achieve runout to the turning centerline of 0.0005" TIR or better because the part is still centered in the turning spindle when the milling operation occurs. On separate machines, runout depends entirely on how accurately the part is chucked or fixtured for the second operation.
The tolerance table matters less than the datum structure. A mill-turn machine holding ±0.0005" on a milled feature isn't impressive on its own — any decent VMC does that. What's impressive is holding ±0.0005" on a milled feature relative to a turned diameter, because that eliminates the re-fixture error that every separate-operation setup introduces.
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