CNC machining and 3D printing: a guide

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  How CNC machining forms a part
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  How 3D printing builds a part
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  Where each process wins and where it does not
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  Materials, finishes and what we see in practice
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  Step by step: choosing a process for a real part
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  CNC machining and 3D printing at a glance

CNC machining starts with a solid block, bar or casting and cuts material away. A rotating cutter or a single-point tool follows toolpaths generated from CAD and CAM software, and the machine's ballscrews and servos hold the cutter on that path. What is left at the end is the part. Nothing about the shape depends on a mold, a nozzle or a support structure.

The process is subtractive, so the geometry has to be reachable by a tool. A 3-axis mill cuts from one direction and needs open access to every face it touches. A 4-axis machine adds rotation about one axis; a 5-axis machine tilts and rotates the tool or the table together, so undercut walls, deep pockets and angled holes can often be finished in a single setup. Fewer setups means fewer repositioning errors and tighter position tolerance between features.

Heat is the main physical limit. Cutting metal converts energy into heat in the shear zone, and that heat moves into the chip, the tool and the workpiece. Too much heat at the cutting edge dulls carbide quickly; too much heat in the workpiece moves the part as it cools. Coolant, feed rate and depth of cut are how machinists keep that balance.

Rigidity sets the other boundary. A thin wall or a long, unsupported section will deflect under cutting force, spring back, and leave chatter marks. This shows up as a surface that reads Ra 3.2 μm when the drawing asked for Ra 0.8–1.6 μm. On our 5-axis centers we work up to a 4,000 mm maximum processing size, and tooling reach, not table size, is usually the first thing that runs out.

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  Subtractive from solid stockBar, plate, block or casting becomes the finished part.
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  Tool access drives designUndercuts and deep pockets favor 4-axis or 5-axis work.
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  Heat and rigidity set limitsThin walls deflect; heavy cuts build heat.

3D printing adds material instead of removing it. A slicer takes the solid model, cuts it into layers, and the machine deposits or cures material one layer at a time. FDM extrudes a thermoplastic filament through a heated nozzle. SLA and DLP cure liquid resin with a light source. SLS and metal systems fuse powder with a laser, layer by layer, in a build chamber.

The layer height is the resolution you feel on the surface. A 0.2 mm layer leaves visible stair-stepping on curved faces; a 0.05 mm resin layer is much smoother but takes longer to build. Printing direction also sets anisotropy: the bond between layers is weaker than the material inside a layer, so a part printed flat and a part printed standing up can have very different strength along the same axis.

Support structures are the cost of overhangs. FDM and SLA need supports under any face that overhangs beyond roughly 45°, and those supports have to be cut away and sanded. SLS does not need them because the surrounding powder holds the part, which is why it handles nested geometry and internal channels that would be awkward to machine.

Post-processing decides how the part performs. As-printed resin is brittle and continues to cure under UV light, so it is usually washed and post-cured. Metal printed parts go through debinding, sintering and often hot isostatic pressing before they carry real load. Skipping those steps is the most common reason a printed metal bracket fails early.

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  Layer height = surface finish0.05 mm resin prints smooth; 0.2 mm FDM shows steps.
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  Build direction = strengthLayer bonds are the weak plane in FDM and SLA parts.
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  Post-processing carries loadSintering and HIP decide metal part properties.

Tolerance is the clearest split. Milled and turned metal parts hold ±0.005 mm (±0.0002 in) on critical features at GreatLight, and surface finish can reach Ra 0.2–0.8 μm after fine machining. FDM typically holds ±0.5 mm on a good day, and SLA improves that to roughly ±0.1 mm on small parts. If a bore needs to take a press-fit bearing, that is a machining job.

Mechanical property is the second split. Machined 6061-T6, 7075 or 17-4PH is fully dense and heat-treated to a known temper. Printed plastic is porous between layers and printed metal carries residual porosity unless it is sintered and pressed properly. For a part that sees fatigue cycling, high pressure or a safety factor, subtractive wins on a well-understood basis.

Cost follows quantity. Machining has almost no setup cost beyond programming and fixturing, so one prototype and ten thousand parts use the same basic tooling. Printing has low setup cost too, but build time scales with part volume and machine hours, so unit cost falls more slowly with quantity. Printing stays competitive when the geometry is complex and the volume is low.

Some geometry only prints. Internal conformal cooling channels, lattice infill and hollow sections that follow a curved surface are natural for additive and painful or impossible for a cutter. That is why the two processes often meet in the same program: a printed prototype to check fit and feel, then a machined version in the final alloy. Our custom 3D printing service and 5-axis machining both run in-house, so a design review can compare both routes before tooling is cut.

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  Tight tolerance goes to machining±0.005 mm and Ra 0.2–0.8 μm on metal.
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  Fatigue and pressure go to machiningFully dense, predictable temper.
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  Complex internal geometry goes to printingLattices and conformal channels.

The material list decides most of the argument. We machine 6061-T6, 7075, 304 and 316L stainless, 17-4PH, 4140, TC4 titanium, Inconel and magnesium AZ31B. Plastics cover ABS, PC, POM, PA, PEEK and carbon fibre. Printing covers resins, ABS, PC and POM for prototypes, plus metal powder routes when the geometry justifies the cost. If your part has to sit in a hydraulic valve body or a hot engine bay, the printable shortlist is small.

Finishing is where the two diverge again. Machined parts take anodizing in clear, colour, hardcoat or conductive form, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling and polishing. Laser marking holds a minimum character height of 1.5 mm. Printed parts take dye, primer and paint, and most structural finishing steps do not apply.

Inspection is the last gate. We run raw material checks, in-process monitoring and a final inspection, with reports on request, and inspect 100% of parts before shipment. That matters more for printing than for machining, because layer defects and incomplete fusion hide inside the part rather than on the surface.

The pattern we see most often: a printed prototype for form and fit, then a machined version of the same envelope for test and production. It is a fast route, and it catches interference that a screen never shows. It also forces the design team to think about wall thickness and tool reach about two weeks earlier than they otherwise would.

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  Metal choice is broad on the millAluminium, stainless, steel, titanium, Inconel, magnesium.
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  Finish options are deeper on machined partsAnodizing, plating, powder coat, laser marking.
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  Inspection covers the whole lot100% inspection before shipment; reports on request.