MTM CNC machining overview

• 1
  What MTM CNC machining actually does
• 2
  Machine configurations you will see quoted
• 3
  Tolerances and surface finish in one setup
• 4
  When MTM CNC machining is the wrong choice
• 5
  Materials that suit mill-turn work
• 6
  Post-processing and what it costs in time
• 7
  MTM CNC machining versus separate operations

A conventional lathe spins the part; a conventional mill moves a cutter around a stationary blank. MTM CNC machining puts both in one enclosure. The main spindle turns the bar, and a tool turret or lower turret carries milling heads, drills and turning tools. Some machines add a second spindle so the back of the part can be finished without unclamping.

The key word is one setup. Every time a part moves to another machine, you add a fixture, a re-zero, and a stack of position error. On a mill-turn center the B-axis head tilts and the C-axis spindle indexes, so a cross-hole, a face slot and a turned diameter all come off the same datum. Position error stops compounding.

Multi-task machining is not a new idea. What changed is the control. Modern CNCs synchronize two spindles, a sub-spindle, a Y-axis and a lower turret in one program, and they hold interpolation accuracy while doing it. That is why a shop can now quote a shaft with milled flats and drilled ports as a single operation instead of four.

The practical result for a design engineer: fewer datums to define, fewer fixtures to design, and a shorter path from print to finished part. The trade-off is programming time and machine hour rate, which we cover further down.

Mill-turn centers come in a few shapes. The most common is a turning center with a Y-axis and a B-axis milling head. Add a sub-spindle and you get back-side work in the same cycle. A Swiss-type sliding-head machine is a different animal: the bar slides through a guide bushing, which suits long, slender parts under about Ø32 mm.

At GreatLight the mill-turn group runs 16 mill-turn centers alongside 16 simultaneous 5-axis machining centers, 27 three-axis machines and 12 four-axis mills. That mix matters when a feature is better cut on a 5-axis trunnion than on a turret head. We route the part to the machine that holds the tolerance with the least handling.

The rotary table on our 5-axis work is Ø400 mm. Maximum processing size across the shop reaches 4,000 mm, with large travels of 4,000 × 400 × 150 mm and medium travels of 750 × 1,150 × 550 mm or 600 × 600 × 600 mm. Compact cells run 500 × 500 × 450 mm and 500 × 310 × 200 mm.

Which configuration you get quoted depends on part length, the number of features on the back face, and how tight the coaxiality callout is. A short part with one cross-hole rarely needs a sub-spindle. A 300 mm shaft with ports at both ends usually does.

GreatLight holds ±0.005 mm (±0.0002 in) on mill-turn work. That number is achievable because the turning and milling features share a datum. If you split the same part across a lathe and a mill, the stack-up between the two fixtures usually eats most of the budget before the cutter touches metal.

Surface finish follows the same logic. As-machined surfaces land at Ra 1.6–3.2 μm. A high-finish pass gets you Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm. On a mill-turn center the finish pass can run right after the roughing pass without a re-chuck, so you avoid the tiny eccentricity that shows up as a visible step.

Coaxiality and perpendicularity are where the one-setup argument pays best. A bearing bore and its mating pilot diameter cut in the same clamping typically hold within a few micrometres of each other. Two setups add whatever runout the chuck and fixture contribute.

We inspect 100% of parts before shipment, with raw material checks, in-process monitoring and a final inspection. Reports are available on request. Qualification rate on this equipment runs at 99.99%.

Mill-turn is not always cheaper. The hourly rate is higher than a three-axis mill, and the programming is heavier. If a part is a simple plate with holes, a three-axis machine will beat it on cost every time. The one-setup advantage only shows up when handling and re-fixturing are the real cost drivers.

Very hard materials change the math too. Inconel and hardened tool steel cut slowly, and a mill-turn center sitting at 30% spindle load while a small cutter creeps through a slot is expensive time. Sometimes it is better to turn the body on a lathe and wire-cut or grind the features on a separate machine.

Thin-wall parts can also fight the process. On a mill-turn center the part often stays clamped in the main spindle while the milling head works on it, and the clamping force can distort a 1 mm wall. A dedicated fixture with soft jaws on a 5-axis mill may hold it better.

Finally, if the geometry is mostly prismatic with one turned bore, you are paying for turning capability you will not use. Match the machine to the feature mix, not to the brochure.

Aluminium is the easy case. 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 all turn and mill well at high spindle speeds, so a mill-turn cycle stays short. 7075 machines cleanly but is less forgiving of chatter on thin walls.

Stainless grades 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630) are common on mill-turn centers, especially for valve bodies and instrument housings. 304 and 316 work-harden, so keep the feed per tooth up and avoid dwelling. 17-4PH in the H900 condition cuts well but needs a finish pass that respects the hardness.

Steel grades 1018, 1045, 4130, 4140, 4340 and A36 cover most shaft and housing work. Copper and brass families C101, C103, C110, beryllium copper, C27400, C28000 and C36000 machine fast and leave a good finish, which makes them a natural fit for one-setup cycles.

Titanium TA1, TA2 and TC4 (Ti-6Al-4V), plus Inconel and magnesium AZ31B or AZ91D, are also within scope. These run slower and generate more heat, so the cycle-time saving of one setup matters more, not less. Plastics from ABS and POM through PEEK, PP, HDPE and carbon fibre are handled on the same platform with appropriate tooling.

A mill-turn part still needs finishing. Anodizing in clear, colour, hardcoat or conductive versions is common on aluminium housings. Electroless nickel, zinc, silver and gold plating cover the conductive and corrosion cases. Powder coating and black oxide handle the rest.

Mechanical finishes include bead blasting, tumbling, brushing and polishing. These change the surface but not the geometry, so they can run after the final inspection. Laser marking and engraving are available with a minimum character height of 1.5 mm, which is worth knowing when you place a part number on a curved turned surface.

The time cost is mostly queue, not process. A hardcoat anodize adds a day or two; plating often adds more. Build that into your schedule rather than into the machining cycle.

One caution: masking. If a turned diameter must stay conductive or must fit a bore, say so on the drawing. Anodize builds roughly half the coating thickness per side, and a 25 μm hardcoat on a ±0.005 mm diameter is enough to lose the fit.