3D printing or CNC machining: which should you choose for your parts?

3D printing builds a part layer by layer, by adding material. CNC machining starts from a block or a bar and removes material with cutting tools. These two approaches create different strengths and limits; seven criteria help you decide.

1. Quantity

3D printing has almost no start-up costs: no tooling, quick file preparation. The price of the first part is close to that of the tenth. In machining, programming and setup are fixed costs that are diluted as quantity grows (see CNC machining prices). Printing is therefore advantageous for a single part or a few copies, machining for larger batches. The tipping point has no universal value: it depends on size, geometry and technology. Ask for prices for several quantities. In SLS or MJF, filling a single build with many parts brings down the unit cost.

2. Geometry

Printing allows internal channels, lattice structures, organic shapes and the merging of several parts into one. Machining has to reach every surface with a tool: no closed cavities, internal radii dictated by the cutter, limited deep pockets. Printing has its own constraints too: supports, overhangs, minimum thicknesses, removal of powder or resin.

3. Material

Machining covers a very wide range of metals and engineering plastics, from semi-finished products with known, certifiable properties. Printing offers a narrower palette, and the properties depend on the process, the orientation of the part and the post-processing.

4. Tolerances

Machining routinely holds tight tolerances on functional surfaces. As printed, parts show wider deviations, in the order of a tenth to a few tenths of a millimetre depending on the technology, size and material. Critical bearing seats, bores and threads are then finished by machining. See machining tolerances.

5. Surface finish

A machined surface is smooth and regular. A printed part retains the mark of its process: layer lines in FDM, a slightly powdery grain in SLS or MJF, surface roughness and support marks in metal. Post-processing (sanding, shot peening, dyeing, painting, polishing) improves the appearance.

6. Lead time

For a single part, printing is often faster: no CAM program, no material to procure. It does, however, depend on print time, which can run to several hours for a large part, and on post-processing. A simple machined part can be just as fast; a complex part that takes a long time to program is better printed.

7. Cost

Printing is charged according to machine time (often linked to the height of the part or the volume occupied in the build chamber), the material consumed, supports and post-processing. Machining is charged according to cycle time, the whole raw stock and fixed costs. A compact, solid part is often cheaper machined, a hollow, complex part sometimes cheaper printed. Only a quote settles it.

The ISO/ASTM 52900 standard classifies additive manufacturing processes into seven families. In practice, five technologies cover most of the requests sent to 3D printing suppliers.

FDM (fused deposition modelling)

A molten thermoplastic filament is deposited layer by layer: PLA, PETG, ABS, ASA, nylon, polycarbonate, sometimes fibre-reinforced. An economical technology, suited to large parts and functional prototypes. Limits: visible layer lines, lower strength between layers, moderate accuracy.

SLA and DLP (stereolithography)

A photosensitive resin is cured by laser or projector. Excellent level of detail and smooth surface: appearance parts, models, small precise parts. Limits: resins that can be brittle, UV ageing, post-processing (washing, post-curing).

SLS (selective laser sintering)

A laser sinters polyamide powder (PA12, PA11…). The powder supports the part: no supports, complex geometries, many parts per build. Robust, functional parts, with a slightly powdery grain.

MJF (Multi Jet Fusion)

A technology developed by HP: polyamide powder is fused using agents and infrared energy. Comparable to SLS in its uses, with good mechanical consistency and high productivity on small runs. Parts are typically grey.

Metal: SLM and DMLS

A laser melts a metal powder (316L stainless steel, 17-4PH, AlSi10Mg aluminium, Ti6Al4V titanium, nickel alloys such as Inconel). These designations are similar. Geometries impossible to machine: cooling channels, lightweighting. Limits: high cost, supports to be removed, residual stresses (stress-relief heat treatment is frequent), rough as-built surface and seats to be machined.

Favour 3D printing if…

• you need a single part or a few prototypes quickly;
• the geometry is complex (internal channels, lattices, single-piece parts);
• the part is customised or part of a very small run;
• tolerances are moderate or limited to a few surfaces that will be finished afterwards;
• you want to compare several design variants before freezing the definition.

Favour CNC machining if…

• you require tight tolerances and a good surface finish on functional surfaces;
• the material must be specific, certifiable or high-strength;
• the part is compact and solid, or produced in a regular series;
• you want consistent, repeatable mechanical properties;
• the part is subject to high stresses, fatigue or temperature, and its performance must be guaranteed.

The choice is not always binary. Several combinations take advantage of the strengths of each process:

• Printing followed by machining. The part is printed, then the bearing seats, bores, threads and mounting faces are machined to size. This is standard practice in metal printing and frequent in polymers.
• Printed prototype, machined or moulded series. Printing validates the shape and assembly at lower cost, then series production moves to a more economical process: see from prototype to series production.
• Printed jigs and fixtures. Inspection fixtures, drilling or positioning jigs and printed holding clamps support machined production.
• Hybrid assembly. A machined base, rigid and precise, receives a complex printed element, lighter or more elaborate.

For these cases, the file must specify the functional surfaces, those that will be machined afterwards and the tolerances expected: a neutral 3D file and a dimensioned drawing avoid misunderstandings.

If you are undecided, the most effective approach is to compare concrete offers. In your request, describe the function of the part, the quantity, the material you want and the critical tolerances, and ask for a price for each process under consideration. On Meaflow, posting a request is free: several suppliers can send you their quote, which you compare, then you discuss through the messaging system with those who interest you. Your drawings are only visible to logged-in members.

For a reliable quote, state in the request:

• the function of the part and its environment (temperature, humidity, UV, contact with chemicals);
• the quantity, with alternative price breaks, and the lead time you want;
• the target material or the properties you are looking for;
• the critical tolerances, surface finish and any colour;
• the need for finishing (sanding, painting, treatment) and machining after printing.

Also discover the suppliers in CNC machining and 3D printing, or the prototyping page if what you need is a first part to validate.