How Is CNC Machining Different To 3D Printing?

• 1
  Key takeaways
• 2
  How CNC machining different 3D printing processes work
• 3
  Tolerance, surface finish and fit
• 4
  Cost per part at different volumes
• 5
  When each process is the right call
• 6
  Design rules that change between the processes
• 7
  Step by step: choosing the process
• 8
  CNC machining different 3D printing: comparison table

CNC machining is subtractive. A solid block of aluminium, stainless or titanium goes onto a vise or fixture, and rotating cutters remove everything that is not the part. Because the workpiece is solid metal throughout the cut, the finished part keeps the wrought properties of the mill certificate. Nothing is sintered, melted or bonded a second time.

3D printing is additive. Material is deposited or fused layer by layer, usually 20 µm to 100 µm thick. Metal printing uses laser or electron beam melting of powder; polymer printing uses extrusion or resin curing. Each layer bonds to the previous one, and that bond line becomes the weak plane of the part.

The practical consequence is the direction of strength. A machined 6061-T6 bracket has the same yield strength in every direction. A printed bracket is typically 20–50% weaker across layer lines, and the number depends on laser power, scan strategy and powder quality, not just the alloy name.

So the first question is not which process is better. It is whether the part can tolerate directional strength and a coarser tolerance band. If yes, printing may be faster and cheaper. If no, the part belongs on a mill.

GreatLight runs both processes under one roof. That matters because a printed prototype often needs a machined interface, a bored bearing seat or a faced gasket surface before it can be tested honestly.

Tolerance is where the two processes separate most clearly. Our CNC machines hold ±0.005 mm on critical features, with routine shop work at ±0.01 mm. Metal 3D printing normally holds ±0.1 mm, and thin walls or tall parts drift further. A printed part that must mate with a machined one usually needs a machined allowance of 0.3–0.5 mm on every mating surface.

Surface finish follows the same pattern. As-machined surfaces land at Ra 1.6–3.2 μm, and fine finishing reaches Ra 0.2–0.8 μm. Printed surfaces sit much rougher. Powder bed parts typically start around Ra 8–15 μm and show stair-stepping on curved faces and angled overhangs.

Stair-stepping is geometry dependent. A curved face printed with a 50 µm layer shows visible bands. The same face on a 3-axis mill comes off smooth because the cutter follows the curve continuously. If the part is a visible housing or a sealing face, plan on post-processing.

Some features simply cannot be printed accurately without support removal damage. Deep bores, sharp internal corners and fine threads are all easier to cut than to print. If a drawing calls for a Ø6 H7 bore, cut it.

For a single bracket, printing can be cheaper because there is no fixture, no CAM programming and no material blank to buy. That advantage shrinks fast as quantity rises. Printing time scales almost linearly with part count, so ten printed parts take roughly ten times as long as one.

CNC cost is front-loaded. Programming, workholding and first-article inspection happen once. After that, cycle time per part is fixed, and each additional part costs only material plus machine time. On a 5-axis centre with a Ø400 mm rotary table, a small bracket may run 4–8 minutes per piece.

The crossover usually sits between 5 and 20 parts, depending on geometry and material. Simple flat parts cross earlier. Complex parts with many setups cross later, because printing avoids the setup cost entirely.

Material cost also runs the other way. A machined part buys a full billet and turns much of it into chips. For titanium or Inconel, that scrap is expensive. Printing uses only the powder or wire the part needs, which is why printed titanium can compete on material cost even when it loses on time.

There is no minimum order quantity here. We machine from one prototype to runs of 10,000 or more, so the crossover point is a real decision rather than a supplier constraint.

Choose CNC machining when the part carries load, seals against something, rotates, or must hold a tolerance over temperature. Automotive transmission components, engine blocks, hydraulic manifolds and custom fixtures all fall here. The same applies to anything needing wear resistance or thermal stability.

Choose 3D printing when the part is a form-and-fit check, a jig that sees low force, a ducting shape, or a design that would be impossible to cut. Internal cooling channels that curve through a block are a classic print-only feature. So are lattice structures and hollow shells with trapped geometry.

Many projects use both. Print the housing to check fit and ergonomics in a week. Machine the load-bearing insert, the bearing seat and the sealing face. Assemble and test. This sequence catches design errors before tooling money is spent.

Watch for the parts that look printable but are not. A thin-wall aluminium cover with a precision bolt circle will distort after printing and still need machining, so printing bought nothing. A polymer part that sees 80 °C in service may creep. Neither failure shows up in a CAD review.

For aerospace and medical work, material traceability usually decides the question. Machined parts come with mill certificates tied to a heat number. Printed parts need powder lot records and process logs, which is a heavier paperwork chain for the same drawing.

Machining rewards simple access. A cutter has to reach the feature, so deep pockets want a corner radius at least equal to the tool radius, and no feature should sit behind an undercut unless you have 5-axis capability. Standard practice is a corner radius of 1.5–3 mm for a 6 mm cutter.

Printing rewards self-supporting geometry. Overhangs beyond 45° from vertical need support, and support leaves witness marks. Minimum feature size sits around 0.4–0.5 mm for most metal systems. Horizontal holes print oval, so a Ø8 mm hole should be modelled at Ø7.6 mm and drilled to size afterward.

Wall thickness is a shared constraint but the numbers differ. Machined walls below 0.8 mm tend to chatter and deflect. Printed walls below 0.5 mm may not build at all. If a design calls for a 0.6 mm wall in aluminium, the drawing needs a second look.

Threads are the clearest split. Cut threads are strong and inspectable with a gauge. Printed threads are weak and rarely pass a go/no-go check. Model printed holes undersize and tap or machine them.

Both processes need a datum strategy. On a machined part, pick a face and two holes and hold everything from them. On a printed part, the build plate is your only reliable datum, so orient the part with its most critical face flat on the plate.