3D Printing Starter Guide: 5 Essentials You Can't Ignore

Printing wins on internal geometry. Conformal cooling channels, lattice blocks, merged sub-assemblies that would need five separate parts in machining. When the shape is the hard part, print it. Tooling cost is near zero and the first article arrives fast.

Machining wins on load path. A printed aluminium bracket can look correct and still fail in fatigue, because layer bonds and microporosity act as crack starters. Wrought billet is homogeneous in all three axes, so a 6061-T6 part that passed its static test also passes its vibration test.

Use them in sequence, not as rivals. Iterate shape, fit and assembly on the printer while the design is still moving, then switch to precision CNC machining the moment performance has to be validated. GreatLight runs both processes under one roof, so the handoff does not need a new supplier, a new DFM cycle or a new set of tolerances.

A rule that saves money: print anything that touches only air or cable routing. Machine anything that carries torque, preload or a fatigue cycle. That single line removes most of the arguments inside a design review.

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  PrintInternal channels, lattices, merged assemblies, early fit checks.
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  MachineLoad paths, bearing seats, sealing faces, threaded joints.
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  BothPrint the shape, machine the critical interface.

A datasheet that says "nylon" tells you almost nothing. PA12 printed by SLS, PA6 reinforced with carbon fibre and PA6 extruded into a machined block behave like three different materials. Strength, elongation and moisture uptake all move with the process, not just the polymer name.

The same trap exists in metal. A printed 316L part and a machined 316L part share a grade name and very little else. Printed metal carries porosity and a directional grain, so its fatigue limit is lower than the wrought bar stock we machine every day.

For anything structural, we ask what the part must do rather than what it must be made of. Load, temperature, chemical exposure, wear surface, weight budget. From those answers, a machined 6061-T6, 7075 or 17-4PH part is often cheaper than it looks, because it needs no post-processing to reach its final properties.

Plastics still have a place. POM and PEEK machined parts handle sliding contact and heat far better than most printed polymers, and ABS, PC or carbon fibre stock gives you a real engineering plastic at prototype quantity.

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  Check the process, not the gradeSLS, FDM and machined stock differ in strength and anisotropy.
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  Watch moisturePA and ABS absorb water and change dimension over weeks.
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  Match finish to functionSliding and sealing surfaces rarely print well.

Printed parts are not isotropic. Strength differs along X, Y and Z because each layer is a bond, not a continuous grain. A bracket printed standing up can delaminate at a fraction of the load that the same bracket printed on its side carries.

Orientation is a design decision, not a slicing afterthought. If the tensile load runs across layers, rotate the part on the bed or add a machined feature to take that load. Where a printed boss meets a machined shaft, the joint should be designed so the load enters the metal.

Geometry also has process limits that CAD hides. Sharp internal corners concentrate stress in a printed part more than in a machined one, because the bond line follows the corner. A 0.5 mm fillet costs nothing to add and removes a common failure point.

Draft, minimum feature size and unsupported overhangs all change between processes. Send the model with the load direction marked, and the orientation choice becomes a conversation instead of a guess.

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  Mark the load axisA note on the drawing is worth more than a tolerance block.
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  Add fillets earlyCorners are where printed parts crack first.
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  Design the metal insertLet machined features carry threads and bearings.

A printed part comes off the machine with layer lines, support scars and an open surface. For a form study that is fine. For a mating face, a seal groove or a bearing bore, it is not, and no amount of sanding will turn a rough surface into a tolerance.

This is the stage where schedules slip. Support removal, sanding, priming, coating and re-measuring add days, and each step can move a dimension. A part that measured correctly on the bed may not assemble after finishing.

Machined surfaces arrive at a defined value. We work to Ra 0.2–0.8 μm for fine finishes, Ra 0.8–1.6 μm for general high-finish work, and Ra 1.6–3.2 μm as-machined. Because the geometry is cut, not grown, the finish and the dimension come from the same setup.

A practical middle path: print the body, machine the interfaces. Bore, face and thread the surfaces that must be precise, and leave the organic geometry printed. You keep the design freedom and remove the assembly risk.

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  Name the functional surfaceOnly the sealing, sliding and locating faces need fine finish.
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  Budget the finishing timeSupport removal and coating are real schedule items.
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  Machine the interfacesBores, faces and threads are cheap to cut after printing.

The unit price on a quote is the smallest number in the comparison. Add the finishing steps, the reprints after a failed fit, the inspection time, and the engineering hours spent chasing a dimension that will not repeat.

Printed parts hide cost in iteration. Each change is cheap to print but expensive to re-qualify, because the mechanical properties shift with orientation and process settings. A stable machined process gives you the same part next month, from the same drawing, with the same inspection report.

Volume changes the answer again. There is no minimum order quantity here, so a single prototype and a 10,000-part run both go through the same quotation route. At low volume the processes are close. As volume climbs, the machined or cast route pulls ahead because setup is paid once.

The cheapest path is usually a hybrid. Print for the iterations that will be thrown away, machine the version that ships. GreatLight quotes that mix with a free DFM analysis within 12 hours, and production can start within 24 hours once the drawing is frozen.

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  Count the iterationsReprints and re-qualification are part of the price.
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  Count the finishingPost-processing is often 20–40% of a printed part's lead time.
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  Count the riskA stable process is worth more than a low unit price.