Do CNC Machines Cut Metal?

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  Do CNC Machines Cut Metal or Squeeze It?
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  How Milling, Turning and 5-Axis Cutting Differ
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  Which Metals Cut Cleanly and Which Fight Back
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  When Metal Cutting Reaches Its Limits
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  Coolant, Heat and Surface Finish
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  Matching the Process to the Part
• 7
  Metal Cutting Behavior by Material

A CNC machine does not cut metal with a blade the way a saw or shear does. It drives a rotating cutting tool into the workpiece, and each flute shears off a thin layer of material. That layer leaves the cutting zone as a chip. The machine only supplies motion and stiffness. The geometry of the tool decides whether metal leaves cleanly or gets smeared.

Every metal removal operation ends in one of two modes. In the ductile mode, material shears ahead of the edge and flows up the rake face. Cast iron and some brass behave differently. They fracture ahead of the edge, so chips break into small segments and the surface can look slightly dull rather than polished.

So do CNC machines cut metal? Yes, but the cutting edge never touches the part with zero clearance. A small edge radius, usually 5 to 25 μm on a carbide insert, pushes material before it shears. If the depth of cut is smaller than that radius, the tool rubs instead of cuts. That is why a 0.01 mm finishing pass on soft aluminum often produces a worse surface than a 0.1 mm pass.

The practical consequence is that chip load per tooth matters more than spindle speed alone. For 6061-T6 with a 10 mm three-flute carbide end mill, a feed of 0.05 to 0.12 mm per tooth keeps the edge engaged and the chip thick enough to carry heat away. Drop below 0.02 mm per tooth and the tool starts burnishing the surface.

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  Shear, not scrapeThe chip carries most of the heat away from the part.
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  Edge radius sets the floorCuts thinner than the edge radius rub and work-harden.
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  Chip load beats RPMFeed per tooth controls surface finish more than spindle speed.

Milling spins the tool and moves it across a stationary part. The workpiece sits in a vise or fixture while the spindle travels in X, Y and Z. A 3-axis mill handles flat faces, pockets and holes from one direction. Add a fourth axis and the part rotates so features on four sides can be reached without re-fixturing. A 5-axis machine tilts both the tool and the table, which lets a ball nose cutter stay normal to a curved surface.

Turning spins the workpiece instead. A single-point insert moves along the rotating bar and peels off a continuous chip. Turning suits shafts, bushings, hubs and anything with rotational symmetry. A mill-turn center does both in one setup, so a part with a turned OD and milled flats does not need to move between machines. That single setup is worth more than the cycle time it saves, because every refixture adds position error.

Five-axis cutting changes what is possible, not just what is fast. Undercut features, deep ribs and angled holes that would need two or three fixtures on a 3-axis machine can be cut in one pass. The trade-off is programming time and rigidity. A tilted setup moves the cutting force away from the stiffest direction of the machine, so light finishing passes are common.

At GreatLight we run 16 simultaneous 5-axis machining centers, 16 mill-turn centers and 27 three-axis machines. Travel ranges from 500 × 500 × 450 mm up to 4,000 × 400 × 150 mm, so the choice of machine follows part size and feature access, not habit.

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  3-axisPrismatic parts, one approach direction, lowest setup cost.
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  4-axisCylindrical parts plus milled features on four faces.
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  5-axisContoured surfaces, undercuts and angled holes in one setup.

Aluminum is the easy case. Grades 6061, 6061-T6, 2024, 5052, 6063, 6082 and 7075 all machine at high surface speed, often 300 to 600 m/min with carbide. Chips are long and soft, so you need sharp flutes and enough coolant or air blast to clear them. 7075 is stronger and more abrasive than 6061, so tool life drops. ADC12 die-cast aluminum machines well but can carry hard spots that chip a small cutter.

Stainless steel is where most shops slow down. Grades 303, 304, 316, 316L, 17-4PH and 440C work-harden if the tool rubs. Keep the feed up and never let the cutter dwell. Surface speed drops to roughly 80 to 150 m/min for 304, and 17-4PH in the aged condition is tougher still. 303 with its added sulfur is the free-machining grade and gives the best finish of the group.

Titanium and nickel alloys are the hard end. TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B or AZ91D all need low surface speed, heavy feed and flood coolant. Titanium conducts heat poorly, so the heat stays at the edge instead of leaving with the chip. Inconel is worse. It holds strength at red heat and work-hardens fast, so a worn insert fails within seconds.

Steel grades 1018, 1045, 4130, 4140, 4340 and A36 sit in the middle. Low-carbon 1018 cuts freely. Medium-carbon 1045 and alloy 4140 need lower speed but respond well to coated carbide. Hardened tool steel above 45 HRC usually calls for carbide with a hard coating or, in tight corners, electrical discharge machining instead of a cutter.

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  Free cutting6061, 303 stainless, 1018, brass C36000.
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  Moderate304, 316L, 4140, 17-4PH, magnesium alloys.
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  DifficultTi-6Al-4V, Inconel, 440C, hardened tool steel.

A cutting tool cannot reach every shape. Deep pockets narrower than about four times the cutter diameter force a long, thin tool that deflects under load. The result is chatter, a tapered wall and a finish that fails inspection. A common workaround is to rough the pocket on a larger machine and finish with a smaller tool at reduced depth, but the cost climbs.

Geometry with sharp internal corners is another boundary. A rotating cutter always leaves a radius equal to its own radius. A true square internal corner needs a broach, EDM or a design change. Engineers who add a 0.5 mm corner radius at the drawing stage save real money, because the shop can then use a standard end mill instead of a special process.

Hardness sets a second limit. Carbide holds up to roughly 45 HRC in interrupted cuts and higher in light finishing. Beyond that, the edge chips. Hardened 440C or tool steel above 55 HRC is normally ground or cut on a wire EDM.

Size is the third limit. GreatLight machines parts up to 4,000 mm in the longest travel, with a Ø400 mm rotary table for round work. Parts larger than that need a different process route. On the small end, a 0.5 mm cutter is practical in aluminum but fragile in stainless, so tiny features in tough alloys are often better formed by EDM.

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  Depth-to-diameterKeep pockets shallower than 4× cutter diameter when possible.
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  Corner radiusA 0.5 mm radius on the drawing avoids a special process.
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  HardnessAbove 45 HRC, carbide edges chip in interrupted cuts.

Cutting metal generates heat at three places: the shear zone, the chip-tool interface and the flank. Most of that heat should leave with the chip. If it does not, the part grows, the tool wears and the finish turns rough. Flood coolant is the default for stainless, titanium and steel. Aluminum often runs dry with a strong air blast, because thermal shock can crack a carbide edge.

Surface finish is predictable once the tool, feed and speed are set. A sharp carbide end mill in 6061 with a light finishing pass reaches Ra 0.8–1.6 μm. Polishing or a fine finishing pass can reach Ra 0.2–0.8 μm on the same material. As-machined surfaces usually land at Ra 1.6–3.2 μm, which is fine for brackets and housings but not for sealing faces.

Tool wear is the hidden variable. A carbide insert loses its edge gradually, and the first sign is usually a change in chip color or a rise in spindle load, not a visible defect. In-process monitoring catches that drift before parts go out of tolerance.

GreatLight holds ±0.005 mm (±0.0002 in) on metal parts and inspects 100% of them before shipment. Raw material certificates, in-process checks and final reports are available on request. That discipline matters more on titanium and Inconel, where a worn tool can scrap a part that took hours to reach its final shape.

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  Heat leaves with the chipIf chips come off blue, the cut is too hot.
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  Finish rangeRa 1.6–3.2 μm as-machined, Ra 0.2–0.8 μm with a finishing pass.
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  Wear is gradualSpindle load and chip color warn you before size drifts.

Start with the feature, not the machine. A part with rotational symmetry and no cross-holes belongs on a lathe. A prismatic housing with pockets on three faces suits a 4-axis mill. A contoured surface with undercuts wants 5-axis. A thin sheet bracket is usually laser cut and formed, not milled from solid, because milling it wastes material and time.

Volume changes the answer too. One prototype is machined from billet because no tooling cost is involved. At a few thousand parts a year, die casting or vacuum casting can beat machining on unit cost. Between those ends, CNC machining stays competitive because the setup cost is low and the tolerance is tight.

Material availability affects lead time. Common grades like 6061-T6 and 304 are usually in stock. Inconel or a specialty titanium grade may need to be ordered, which pushes the schedule out before any chip is cut. Share the material spec early so the shop can confirm stock.

At GreatLight there is no minimum order quantity. We quote and run a free DFM analysis within 12 hours, start production within 24 hours of approval, and ship parts in 3–5 days. Uploads stay confidential and an NDA is available on request.

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  Feature firstLet the geometry pick the machine, not the other way around.
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  Volume mattersMachining wins from one part to a few thousand per year.
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  Check stock earlyExotic alloys add lead time before cutting starts.