CNC milling vs 3D printing: which suits which part

Two opposite principles

Computer-controlled milling (CNC) starts with a solid block of metal or plastic, and a rotating tool removes material until the finished part is left. It is classic machining. With FDM 3D printing it is the other way round: the part is added from filament layer by layer.

All the other differences follow from this. A cutter has to be able to reach the spot, so it cannot make complex cavities or internal channels. Printing has no such limit, but a layered part is weaker than a solid block.

Where each technology is better

• Accuracy. CNC reaches hundredths of a millimetre. With FDM expect tolerances of around a tenth of a millimetre, so precise fits (bearings, threads) are handled by machining afterwards or by a gap in the design.
• Materials. CNC works aluminium, steel, brass and engineering plastics. We only print PLA and PETG, so we cannot make metal parts.
• Shape. Lattices, cavities and organic shapes are the territory of printing. Sharp inside corners and deep narrow pockets are a problem for a cutter.
• Price and time. CNC needs programming and fixturing, so for a single part it often does not pay off. Printing needs none of that, so it is cheaper for one-offs and prototypes.

What this means for your order

If a part is meant for light mechanical load, a cover, a holder, a template or a prototype, printing in PLA or PETG is usually enough, and you see the price and lead time straight after uploading your model. If a part carries large forces, rubs against metal or needs a precise fit, it belongs on a CNC machine, and you can print it as a pattern to check dimensions.

The two can also be combined: a printed part for the shape, with metal inserts and threads bought elsewhere, or heat-set inserts in a hole that you design into the model. We do not offer inserting the nuts as a service.

Designing for printing if the part will be machined afterwards

A combination is often worth it. The shape is printed with an allowance of 0.3 to 0.5 mm where a precise hole or face is to go later, and that is then drilled or faced afterwards. This uses the freedom of printing and the accuracy of machining. You arrange the machining separately, as we do not do it.

The opposite case is a functional model: if you want to check that a cabinet, holder or cover fits in the assembly, a printed pattern costs a fraction of a milled part and you get it in days. If a part is too big for one piece, see splitting a model.