What Can CNC Machines Make?

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  What can CNC machines make: part families that fit the process
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  Milling, turning and 5-axis: which process makes which shape
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  Size limits: how big a part a CNC machine can make
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  Materials and the machinability trade-off
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  Tolerance and surface finish: what the machine can hold
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  When CNC machining is the right call, and when it is not
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  Which CNC process makes which part

The honest answer is that a CNC machine makes anything you can hold rigidly and reach with a spinning cutter. That covers brackets, housings, manifolds, impellers, gear blanks, valve bodies, heat sinks, lens rings and bone plates. It also covers mold inserts, jigs and fixtures, and one-off prototype shells.

The limits are geometric, not creative. A deep internal pocket with a sharp square corner needs a cutter radius, so you get a fillet. A hole on the side of a tall boss may need a right-angle head or a second setup. A thin wall under 0.5 mm will chatter unless you support it or slow the finishing pass.

This page is written for design engineers and sourcing people who need to decide early. Read it to sort your part into prismatic, rotational or freeform, then pick a machine class before you send drawings out.

Every part falls into one of three shape families. Prismatic parts have flat faces and drilled holes, so 3-axis milling handles them. Rotational parts are turned on a lathe. Freeform parts have curved, non-prismatic surfaces and usually need 4-axis or 5-axis work to avoid extra fixtures.

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  PrismaticPlates, brackets, housings, covers. 3-axis or 4-axis milling.
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  RotationalShafts, bushings, fittings, pins. CNC turning, often with live tooling.
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  FreeformImpellers, turbine blades, organic housings. 5-axis simultaneous cutting.
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  HybridTurned body with milled flats or cross-holes. Mill-turn centers.

CNC milling spins a multi-flute cutter and moves it along X, Y and Z. It removes material from a block or plate, so it suits pockets, slots, profiles and threaded holes. A 3-axis mill is the workhorse for flat-sided parts. Add a fourth axis and you can index the part to machine four faces without re-clamping.

CNC turning spins the workpiece against a single-point tool. It is the fastest way to make round parts such as shafts, spacers and hydraulic fittings. Live tooling on a mill-turn center adds cross-drilling and milling in the same cycle, which removes a second setup and the position error that comes with it.

Simultaneous 5-axis machining moves the tool and the table at the same time. That lets a short, stiff cutter reach undercuts and blend complex surfaces in one setup. Deep cavities and contoured impeller blades often cannot be cut any other way without leaving witness marks or tool marks.

Pick the process by shape first, then by tolerance and volume. A round part with a tight runout is a turning job. A boxy part with true-position holes is a milling job. A part with both, in a hard alloy, is usually a mill-turn job.

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  3-axis millingFlat faces, pockets, slots. Best cost per part for simple geometry.
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  4-axis millingAdded rotary indexing for multi-face parts and cylindrical features.
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  5-axis machiningUndercuts, organic surfaces, single-setup accuracy on complex parts.
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  Mill-turnOne cycle for turned bodies with cross-holes and milled flats.

Work envelope decides the upper bound. A compact 3-axis machine with 500 × 500 × 450 mm travels makes small plates, brackets and mold inserts. A large gantry-style machine with 4,000 × 400 × 150 mm travels handles long structural rails, extrusion profiles and frame members.

Part size is not only about travel. A 4,000 mm part needs a machine bed that supports it without sag, and a spindle that can reach the middle without overhang. Long thin parts also need stress relief before finishing, otherwise they bow after the clamps come off.

On the small end, a Ø400 mm rotary table sets the practical limit for 4-axis work on a round part. Below roughly 3 mm in feature size, tool deflection and runout start to dominate, so micro-features need small-diameter tooling, high spindle speed and light depth of cut.

If your part is larger than the envelope, it can often be split into bolted or welded sub-assemblies and machined in sections. That is a design decision, not a machining one, so raise it before the drawing is frozen.

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  Compact envelope500 × 500 × 450 mm and 500 × 310 × 200 mm travels.
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  Medium envelope750 × 1,150 × 550 mm and 600 × 600 × 600 mm travels.
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  Large envelopeUp to 4,000 mm maximum processing size on gantry machines.
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  Rotary limitØ400 mm rotary table for indexed 4-axis work.

Aluminum 6061 and 7075 cut fast and hold a good finish, which is why they cover most prototype and production parts. Stainless 303 and 304 machine cleanly but work-harden if the feed is too light. Titanium Ti-6Al-4V and Inconel need low surface speed, rigid setups and plenty of coolant.

Plastics behave differently. POM and ABS cut easily but can melt or burr if the spindle runs too fast. PEEK and carbon fiber are abrasive, so tool life drops and the cutter needs a sharp edge and a controlled feed. Carbon fiber also calls for dust extraction.

The machinability index is a useful first check. It compares a material to free-cutting steel, where a higher number means easier cutting. A low index is not a reason to avoid a material; it is a signal to expect slower cycle times, more tool changes and a different setup plan.

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  Aluminum6061-T6, 7075, 2024. Fast cutting, good finish, low weight.
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  Stainless and steel303, 304, 316L, 17-4PH, 4140. Watch work-hardening and heat.
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  Titanium and alloysTi-6Al-4V, Inconel, magnesium. Low speed, high rigidity, coolant.
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  PlasticsPOM, PEEK, PC, ABS, carbon fiber. Control heat and dust.

Our machines hold ±0.005 mm (±0.0002 in) on critical features, measured after the part has settled at room temperature. That number applies to a defined feature, not to every dimension on the drawing. Chasing it everywhere raises cost with no functional gain.

Surface finish is set by the finishing pass. As-machined surfaces sit around Ra 1.6–3.2 μm. A careful finishing pass reaches Ra 0.8–1.6 μm. Fine finishes down to Ra 0.2–0.8 μm need a smaller step-over, a sharper tool and more time, so reserve them for sealing faces and bearing seats.

Tolerance and finish interact. A tight bore with a rough finish may still leak. A smooth surface on a loose dimension still fits badly. Decide which dimensions carry function, then let the rest run at general tolerance.

Inspection backs this up. We check raw material, monitor in-process and inspect 100% before shipment, with reports available on request. Datum strategy matters as much as the machine, so call out your datums on the drawing.

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  As-machinedRa 1.6–3.2 μm. General surfaces, non-sealing faces.
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  High finishRa 0.8–1.6 μm. Mating faces, sliding surfaces.
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  Fine finishRa 0.2–0.8 μm. Seals, bearing bores, optical seats.
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  Tolerance±0.005 mm on functional features, 100% inspected.

CNC machining wins when you need tight tolerance, a real engineering material and a part that must survive load. It also wins for low to mid volume, because there is no tooling cost. We run from one prototype to 10,000+ part runs with no minimum order quantity, so the first article and the production batch come off the same process.

It is a poor fit for thin-walled hollow shells, large panels with simple bends, or parts where the geometry is mostly cosmetic. Sheet metal fabrication, die casting or 3D printing usually beat milling on cost and speed for those shapes. A good shop will tell you so instead of quoting anyway.

Cycle time is the main cost driver, not material alone. A part with many setups, deep pockets or a hard alloy costs more than its size suggests. Simplifying a fillet, opening a pocket or relaxing a non-functional tolerance can cut cycle time without hurting the part.

Prototyping and production share the same inspection standard. That is what makes the transition from a one-off sample to a 10,000-part run predictable.

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  Good fitTight tolerance, load-bearing parts, real alloys, low to mid volume.
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  Poor fitThin hollow shells, simple bent panels, purely cosmetic shapes.
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  Cost leverFewer setups, shallower pockets, relaxed non-functional tolerances.
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  AlternativeSheet metal, die casting, vacuum casting or 3D printing.