Engineers are often presented with this as a binary: additive or subtractive. It rarely is.
CNC machining has been the standard for high-precision, high-strength parts for decades — a subtractive process that carves function out of a solid billet. Industrial Multi Jet Fusion has changed the calculation, building complex, functional nylon parts faster and frequently cheaper. Neither replaces the other. What follows is how to tell which one your part wants.
How the two processes actually work
MJF, in short
A thin layer of nylon powder is spread across the build platform. A carriage sweeps over, depositing a fusing agent on the cross-section of the part and a detailing agent at the edges to keep definition crisp. Infrared lamps pass over the bed and fuse the marked areas. Repeat, several thousand times.
The consequence that matters: complexity is close to free. A simple block and an intricate lattice take much the same time and cost, because the machine is processing layers rather than features.
CNC, in short
A design becomes G-code, which tells the machine how to move. The workpiece is held stationary, or rotated on a 5-axis machine, while spinning tools shave material away until the part appears.
The consequence that matters: you pay for what you remove and the time it takes to remove it. Complexity costs money, and every feature needs tool access.
The technical comparison
| Consideration |
MJF |
CNC machining |
| Complex geometry |
Internal channels, undercuts and lattices at no extra cost |
Limited by tool access; some features need splitting and joining |
| Materials |
Nylon PA 12, PA 11 and TPU |
Metals and the full engineering plastics range |
| Strength ceiling |
Tough engineering nylon, isotropic in all axes |
Full billet properties — 100% dense, matching the raw material data |
| Tolerance |
Suits assemblies and repeat batches; critical features post-machined |
Microns where it matters — H7 bores, bearing fits, sealing faces |
| Surface finish |
Uniform matte grain; improves with vapour smoothing
|
Smooth and sharp-edged straight off the machine |
| Cost driver |
Part volume — complexity is free |
Material removed plus machining time |
| Setup |
Effectively none — nest and build |
CAM programming, fixturing and machine setup |
| Batch of 50–500 plastic parts |
Nests hundreds per build |
Each part machined individually |
Geometry: MJF takes it
If the design uses generative or topology-optimised geometry — material removed wherever it is not doing work — MJF is the better answer. No tooling, no cutting head to fit in. CNC is bounded by what a tool can reach: a curved internal channel cannot be machined without splitting the part and joining it afterwards, and a true square internal corner is not possible at all with a round cutter.
Material and load: CNC takes it
If you need the thermal conductivity of copper, the hardness of steel, or a specific aluminium or stainless specification, machining is the answer, and it handles high-performance plastics that powder-bed processes do not run. MJF nylons are genuinely tough — they go into drones, automotive interiors and orthotics — but they will not match the yield strength or temperature capability of machined metal, and nobody is served by pretending otherwise.
Precision: CNC takes it, with a caveat
CNC holds tolerances a powder-bed process cannot, and produces crisp edges and genuinely flat sealing faces. MJF has a granular matte texture and wider tolerances that vary with part size, geometry, wall thickness and orientation — realistic per-process figures are in the design guidelines. The caveat is that MJF parts can be post-machined at the few features that need precision, which is usually cheaper than machining the whole part.
Cost: it depends on complexity and quantity
This is the crux. In CNC you pay for waste and for time, so a part needing five hours in the machine is expensive whatever it weighs. In MJF you pay for the volume of the part, and the complexity comes along free. A simple prismatic block is often cheaper machined. An intricate consolidated housing is almost always cheaper printed.
Which process fits your part?
| Choose MJF when |
Choose CNC when |
| The part has internal channels, lattices or organic geometry |
You strictly need metal, or a plastic MJF does not run |
| You want to consolidate an assembly of several parts into one |
The part has critical fits — H7 holes, bearing surfaces, sealing faces |
| You need a batch of 50 to 500 plastic parts quickly |
You need a smooth aesthetic metal finish or a conductive surface |
| Nylon properties suit the service environment |
The part is safety-critical or heavily loaded |
| Weight matters and you want low density |
The part is blocky and simple — machining is often cheaper |
The hybrid route
You often do not have to choose. A common workflow is to print the part in MJF to capture the geometry that would be painful to machine, then machine only the features that need precision — boring out a bearing bore, facing a sealing surface, adding threaded inserts where clamp load demands metal threads.
That gives you the design freedom of additive with the tolerance of machining, and it is usually cheaper than either extreme. It is also, in practice, what a lot of well-designed production parts turn out to be.
What we would tell you
We run additive in-house and work with an approved UK machining network, which means we have no particular incentive to push you either way. Send the CAD, the load case and the quantity, and we will tell you which process we would use and why — including when the answer is "machine it".
Upload through the instant quote for additive pricing on up to 100,000 parts, or talk to an engineer on 024 77360 144.
Frequently asked questions
Is MJF cheaper than CNC machining?
It depends almost entirely on geometry and quantity. MJF prices on part volume and gives you complexity for nothing, so intricate or consolidated parts are usually cheaper printed. CNC prices on material removed and machining time, so a simple prismatic block is often cheaper machined. For batches of 50 to 500 plastic parts, MJF nesting usually wins comfortably.
Are MJF parts as strong as CNC machined parts?
Not compared with machined metal, and we would not claim otherwise. A machined billet part is 100% dense with the full published properties of the raw material. MJF nylons are tough, impact-resistant and isotropic, which makes them genuinely suitable for functional end-use parts — but they will not match the yield strength or temperature ceiling of aluminium or steel. Compare published figures on the
material datasheets.
Can MJF hold tight tolerances?
Powder-bed tolerances are wider than machined ones, and they vary with part size, geometry, wall thickness and build orientation — the
design guidelines set out realistic figures. Where a specific feature needs precision, the usual approach is to design in an allowance and post-machine that feature, which gets you a tight fit without machining the whole part.
What can MJF make that CNC cannot?
Anything a cutting tool cannot reach. Curved internal channels, enclosed voids, lattice structures and true square internal corners are all straightforward on a powder bed and either impossible or require splitting and rejoining on a machine. Consolidating several machined components into one printed part is often the biggest single saving available.
What materials can MJF print?
Nylon
PA 12, bio-based
PA 11 Gen2 and
TPU 01 elastomer. If the application demands metal, PEEK or acetal, machining is the route — for large or high-temperature polymer parts, fibre-reinforced
FDM grades cover ground MJF does not.
How long does MJF take compared with CNC?
MJF runs to 5 to 7 working days as standard, with no tooling, fixturing or CAM programming in front of it. Machining lead times depend on the shop's queue, the setup involved and the complexity of the part. Where the timeline is tight,
SLS and
FDM also offer 1 to 3 working days on express.
Can you machine a 3D printed part?
Yes, and it is often the right answer. Printing captures the awkward geometry, then machining brings the critical features into tolerance — bearing bores, sealing faces, datum surfaces. Threaded inserts get fitted at the same stage where clamp load or repeat assembly demands a metal thread.
