Turning and Milling Datong Okuma: How Combined Machining Works

• 1
  What a Datong Okuma Mill-Turn Platform Actually Does
• 2
  Which Parts Belong on a Mill-Turn Center
• 3
  Tool Selection, Coolant, and Thermal Drift
• 4
  Holding ±0.005 mm Across a Combined Cycle
• 5
  Combined Machining vs Two Separate Operations

A Datong Okuma mill-turn platform is not a lathe with a drill head bolted on. The B-axis tool spindle swings to any angle between turning and milling, so the same tool can face a flange, interpolate a pocket, and drill an off-axis hole without a second setup. On our 16 mill-turn centers the Ø400 mm rotary table carries the part while the tool spindle works in five axes.

The practical consequence is that every feature defined in one work coordinate system stays in that system. There is no re-chucking error between operations. When a part has a turned bore and a milled bolt pattern on the same face, positional tolerance between them is limited by machine geometry, not by fixture repeatability. That is where combined machining earns its place.

Accuracy reaches ±0.005 mm on critical diameters and bore-to-bore relationships when the thermal state is stable. Surface finish lands at Ra 0.8–1.6 µm on turned surfaces and Ra 1.6–3.2 µm on milled surfaces without extra passes. Both numbers assume the tool is fresh and the coolant is directed at the cut, not around it.

Chip evacuation is the constraint most people underestimate. In a combined cycle the tool spindle is often horizontal or angled, and long stringy chips from low-carbon steel wrap around the B-axis housing. We break chips with peck cycles and higher feed per revolution rather than letting them run.

• 1
  One setup, one coordinate systemTurned and milled features share the same datum, so stack-up disappears.
• 2
  B-axis interpolatesThe tool spindle tilts continuously instead of indexing to fixed positions.
• 3
  16 mill-turn centersCapacity for parts that mix rotational and prismatic geometry.
• 4
  Ø400 mm rotary tableSets the practical envelope for a part that turns while it is milled.

The clearest candidate is a part with rotational symmetry plus features that are not on the axis of rotation. A hydraulic manifold body, a motor housing with a bolt circle and a cross-drilled port, or a sensor boss with a milled flat and a threaded hole all fit. The more features you can reach from one orientation, the more you save.

A second signal is positional tolerance between turned and milled features. If the drawing calls for a bolt circle concentric to a bore within 0.02 mm, a two-machine route needs a precision fixture and a careful datum scheme. Combined machining removes that fixture from the equation entirely.

Parts that do not belong here are simple shafts with nothing but diameters and threads. A mill-turn center will make them, but a turning center makes them faster and cheaper. The same applies to thin-walled tubes where the milling force deflects the wall; a dedicated milling operation with better support often holds the wall thickness more reliably.

Part size decides the rest. Our mill-turn envelope reaches 4,000 mm on the long axis for elongated work, while compact cells run 500 × 500 × 450 mm and 500 × 310 × 200 mm. If a part sits far inside the compact envelope and needs only turning, sending it to a mill-turn center ties up capacity you may need later.

Tool selection follows the feature, not the machine. For interrupted turning on 17-4PH or 4140 we use negative rake inserts with a strong edge, because a sharp positive insert chips on the first interrupted pass. For finishing a bore that must not bell-mouth, a boring bar with the shortest overhang that reaches depth beats a longer bar every time.

Milling cutters in a mill-turn spindle face a stiffness penalty compared with a dedicated mill. The tool is held further from the spindle nose, so we reduce radial engagement and raise spindle speed to keep chip load per tooth in a workable range. On aluminium 6061 and 7075 we run higher surface speed; on Inconel and titanium TC4 we drop speed and accept a shorter tool life rather than risk chatter.

Coolant has two jobs in a combined cycle: remove heat and break the chip. High-pressure through-tool coolant does both on deep holes and pockets. On external turning, a directed nozzle aimed at the insert tip keeps the heat out of the part instead of spreading it into the casting.

Thermal drift is real on any machine that runs a turning cycle, then a milling cycle, then a finishing pass. Spindle growth over a long cycle moves the tool relative to the part. We rough, let the machine settle, then finish. On tight bores we probe the datum again before the final pass rather than trusting the pre-cycle number.

• 1
  Negative rake for interrupted cutsStrong edge geometry survives scale and hard spots.
• 2
  Shortest overhang winsA rigid boring bar holds bore straightness better than a long one.
• 3
  Through-tool coolantClears chips and pulls heat out of deep pockets.
• 4
  Rough, settle, finishSeparating the finish pass from the roughing pass limits thermal error.

A combined cycle produces a finished part from one setup, so there is no second-operation inspection to catch a datum error. That shifts the burden onto in-process checks. We probe critical diameters and face positions inside the cycle, and the machine compensates before the finishing pass runs.

For bore-to-bore relationships we measure with an air gauge or an inside micrometer, not calipers. A caliper reads the high point of a bore and hides taper. On a part where two bores must be parallel within 0.01 mm, that difference matters.

Roundness and cylindricity are checked on a form tester when the drawing calls for them. A three-jaw chuck can push a thin ring into a triangle, so we use soft jaws bored to the part diameter or a collet when the wall is under 2 mm.

Every part gets a final inspection before shipment, and we keep raw material certificates and in-process records on file. Reports are available on request. If a feature cannot be verified on the machine, we send it to the CMM rather than guess.