3D Printing vs CNC Machining

Everything else follows from one fact: printing adds material, machining takes it away. Hold on to that and the trade-offs become obvious rather than something to memorise.

Geometry: printing wins, and it is not close

A cutting tool has to physically reach every surface it makes. That rules out sealed internal cavities, deep narrow pockets, undercuts without a second setup, and organic shapes without an expensive multi-axis machine. Every one of those adds fixturing, setups and cost.

A printer does not care. An internal channel that snakes through a block costs the same as a solid block minus the plastic. Lattices, hollows and curves are free. If your part is complicated, printing is often not just cheaper — it is the only sensible route.

The mirror image is also true. A simple flat plate with four holes is trivial to machine and slow to print, because the printer has to build every millimetre of it.

Accuracy: machining wins, and it is not close

Realistic FDM accuracy is around ±0.2 mm on a small feature, drifting on larger parts. A CNC mill works an order of magnitude tighter without trying, and it produces a surface you can measure against.

That gap decides a lot:

• Bearing seats, precision bores, mating faces — machining.
• A precision thread that carries load — machining, or a metal insert in a printed part. See can you 3D print threads.
• A bracket that holds a camera roughly where you want it — printing, easily.

Our tolerances and fit guide has the actual numbers. A useful middle route: print the part, then drill the two holes that must be exact. You get machining accuracy on the features that need it and printing economics on everything else.

Material

Machining takes whatever you can buy as a billet — aluminium, steel, brass, acetal, nylon, acrylic. Fully dense, isotropic, with published properties you can design against.

FDM takes what comes on a spool. That is a real but narrower shelf: PLA, PETG, ABS, ASA, TPU, nylon, and carbon-fibre-filled versions of some of those. See what we print. And a printed part is never fully isotropic — the layer welds are the weak direction, as covered in print orientation and strength.

We do not machine metal. If your part needs to be metal, we will tell you that and you should go to a machine shop.

Cost and waste

Machining bills setup, fixturing, programming and tool time. A one-off is disproportionately expensive because the setup cost lands on a single part. It also cuts most of the billet into swarf — you are paying for material that ends up in a bin.

Printing has near-zero setup and uses roughly the plastic that ends up in the part, plus supports. That makes one-offs and short runs cheap, which is why prototyping is almost always printed.

The quick decision

Print it if: the geometry is complicated, the quantity is low, ±0.2 mm is fine, plastic is fine, and you want it this week.

Machine it if: the tolerances are tight, it must be metal, the surface must be smooth as-made, it carries a serious load, or the shape is simple and prismatic.

Where printing is the wrong process

Plainly:

• Anything safety-critical in a load path. Vehicle steering, brakes, suspension, lifting or climbing gear. Metal, machined, every time.
• Precision fits and gauges. Printed plastic moves with temperature and humidity. It is not a reference surface.
• Sustained high heat or pressure. Layer lines are a leak path and plastics creep.
• Fine threads under real load. Print the body, tap it or fit a heat-set insert — the plastic threads alone will strip.
• Mirror or machined-looking surfaces as-printed. Achievable with sanding and paint, but that is labour you should price honestly.

Not sure which you need?

Describe the part, what it does and what it fits. If the answer is "this wants machining", we would rather say so than take the order. If it is a printing job, get an estimate or upload the file and we will tell you how we would orient and build it.