Key considerations when choosing CNC and 3D printing for new parts

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  What matters most when choosing CNC and 3D printing
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  CNC vs 3D printing: decision table for new parts
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  Start with tolerance when choosing CNC and 3D printing
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  Wall thickness, holes and threads: where printed parts fail
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  Material choice and what it costs you later
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  Unit count, lead time and MOQ when choosing CNC and 3D printing
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  Supplier checks: certification, inspection and confidentiality
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  Step by step: choosing CNC and 3D printing for a new part

The first question is not which machine is cheaper. It is what the drawing actually demands. If a bore mates with a bearing or a shaft seals against an O-ring, the tolerance band is tight and the surface must be smooth. That points to CNC. If the part is a cover, a jig body or a housing that only holds a PCB, the fit is loose and a printed part will do the job.

Write the real tolerance on the drawing. Engineers often mark ±0.05 mm across an entire part when only two features need it. That single habit can double the price, because the shop has to slow the cut, add a semi-finish pass and inspect more features. Mark the tight dimensions and leave the rest at general tolerance.

Geometry sets the second limit. CNC cutters reach a feature from one direction, and a long tool deflects. Deep pockets narrower than 4× the cutter diameter, sharp internal corners and undercuts all push cost up. Printing has no cutter to reach around, so curved internal channels and lattice structures come free. The trade is surface finish and repeatability.

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  Tight fitsBearing bores, seal grooves and dowel holes: machine them.
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  Loose fitsCovers, brackets and cable guides: print them first.
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  Sharp cornersAdd a corner radius at least equal to the cutter radius.

Printed parts fail at thin walls and small holes far more often than at the outer skin. On FDM, keep walls at 1.0 mm or thicker, and use a multiple of the nozzle diameter so the slicer lays full beads. Below that, the wall prints as a single weak strand and snaps under hand torque. SLA holds 0.6 mm walls, but thin resin sections curl during post-cure.

Holes are the next trap. A printed Ø3 mm hole usually comes out undersized by 0.1–0.2 mm because of shrink and the slicer's path compensation. If the hole carries a screw that threads into plastic, that is fine. If it locates a pin, drill it after printing or plan for CNC.

Threads deserve a rule of thumb. Printed threads below M6 strip easily. Model a plain hole and tap it, or use a heat-set insert. Machined threads from M1.6 upward hold torque and survive vibration, which matters on automotive and robotics parts. For any thread that will be assembled more than a few times, machine it.

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  FDM walls1.0 mm minimum; 1.6 mm is safer for load-bearing bosses.
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  SLA walls0.6 mm minimum; brace tall thin sections.
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  Small holesAdd 0.15 mm to the modeled diameter, then ream if needed.
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  ThreadsPrint above M6 or tap after printing; machine below that.

Material narrows the choice quickly. If the part sees 150 °C, UV or fuel exposure, most photopolymer resins are out. PA12 and PEEK print, but PEEK needs a heated chamber and the price per part climbs fast. Aluminium 6061-T6 and 316L stainless are common, well understood and machine cleanly, which keeps quotes predictable.

Think about the next step, not just the prototype. A printed housing that later becomes die cast needs draft angles and a wall thickness the casting can fill. A machined bracket that later becomes sheet metal needs bend radii. Designing the prototype for the production process saves a full redesign round.

Surface finish is part of the material decision. Anodizing needs aluminium, and hardcoat anodizing adds wear resistance on sliding surfaces. Electroless nickel suits steel and copper alloys. Printed parts can be sanded and painted, but the layer lines remain visible on flat faces unless you spend time on finishing.

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  High temperatureAbove 120 °C, move from resin to PEEK or metal.
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  Wear surfacesHardcoat anodizing on aluminium, or hardened steel.
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  Production pathDesign the prototype for the process that will make the 10,000th part.

Printing wins on one to five units, especially with complex geometry. There is no setup, no fixture and no programming time. The part exists the next day. That speed is real value during a design review, even if the part is not the final material.

CNC wins from roughly twenty units upward, and the crossover moves lower as geometry gets simpler. Setup and programming are one-time costs. Once the fixture is on the table and the program is proven, the twentieth part costs far less than the first. For a simple bracket, machining twenty units can beat printing them.

MOQ is where small teams get stuck. Some shops quote a minimum of 50 or 100 pieces, which blocks a pilot build. GreatLight runs no minimum order quantity, from one prototype to 10,000+ part runs, so a single bracket and a full production lot go through the same quote process. Quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours. Parts ship in 3–5 days.

Lead time has two parts: the quote and the build. Ask which one you are waiting on. A shop that quotes in two days and machines in three is slower than one that quotes in half a day and machines in five.

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  1–5 unitsPrint unless the material is the blocker.
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  20+ unitsMachine, and check the setup cost amortizes.
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  No MOQOne prototype and a 10,000-part run use the same process.

Certification is a filter, not a guarantee. ISO 9001:2015 covers the quality system. IATF 16949:2016 matters for automotive programs. ISO 13485:2016 applies to medical devices, and ISO 27001:2022 covers information security if you are sending CAD files for regulated products. Ask which certificate applies to the plant that will run your part, not the group.

Inspection depth separates shops. A supplier that measures the first article and the last article is not the same as one that inspects every part. GreatLight runs raw material checks, in-process monitoring and a 100% inspection before shipment, with reports on request. For a new part with no history, ask for the report on the first run.

Confidentiality is a real requirement for unreleased products. Uploads are secure and confidential, and an NDA is available on request. If your legal team needs a signed agreement before drawings leave the building, sort that out before the quote, not after.

Finally, check the machine list against your part size. A shop with a 4,000 mm maximum processing size can handle long rails and frames. A shop with compact travels cannot, no matter what the website claims.

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  Match the certIATF for automotive, ISO 13485 for medical, ISO 27001 for data.
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  Inspection levelConfirm whether it is first-article or 100% before shipment.
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  NDA timingSign it before drawings are uploaded.
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  Size fitCompare part envelope against real machine travels.