What CNC Machining Gives a Prototype That Printing Cannot
Additive processes build a part layer by layer; CNC machining removes material from solid stock. For a prototype, that difference decides what the part can prove.
- Material truth. A machined part is made of the actual alloy the design specifies — 6061-T6, 7075, 304 or 316 stainless, POM, PEEK. A printed part is made of a facsimile, and behaves like one.
- Real mechanical behaviour. Threads hold, walls carry load, fasteners torque to spec. Machined prototypes survive the handling a printed part would deform under.
- Dimensional honesty. Machining holds tolerances in the range a production part will need, so a fit result means something about production rather than about the prototype process.
- Surface for real finishing. Anodising, bead blasting and painting behave on machined substrate the way they will on the production part.
The trade-off is honest and worth stating: machining is usually slower per part and more expensive at high quantity. Below a few hundred pieces it is normally the faster route to a trustworthy answer.
Machines, and the Accuracy Each One Holds
| Machine | Type | Travel | Documented accuracy | What it is used for |
|---|---|---|---|---|
| Mazak Variaxis i-600 (2) | 5-axis machining centre | 600 × 400 × 350 mm | ±0.005 mm | Complex 3D surfaces in one setup, reducing fixture error |
| Haas VF-2 (2) | 3-axis vertical machining centre | 762 × 406 × 508 mm | ±0.005 mm | Prismatic parts, pockets, plates, general milled geometry |
| Makino PS105 | Vertical machining centre | 1050 × 550 × 460 mm | ±0.02 mm | Larger parts where the envelope matters more than the tightest tolerance |
| Haas ST-10 (2) | CNC lathe | φ200 × 500 mm | ±0.01 mm | Turned shafts, bushings, threaded and concentric parts |
| Sodick AQ537L | Wire EDM | 370 × 300 × 300 mm | ±0.001 mm | Hardened material, sharp internal corners, fine slots |
| Sodick AD30L (2) | Die-sinking EDM | 450 × 300 × 300 mm | ±0.003 mm | Cavities and ribs a rotating cutter cannot reach |
When a drawing calls for ±0.001 mm, the honest question is which feature needs it. Wire EDM holds that figure on a swept profile; a milled 3D surface does not. Matching the tolerance to the process is what makes a quote achievable rather than optimistic.
Where CNC Prototyping Fails, and How to Avoid It
- Sharp internal corners. Every cutter has a radius. A corner tighter than that radius needs EDM, which adds an operation. Increasing the corner radius is usually free; adding EDM is not.
- Deep narrow pockets. Tool length-to-diameter ratio limits reach. Beyond roughly 4:1 the cutter deflects and the wall finish suffers. Deep slots often need EDM or a design change.
- Blanket tolerances. A drawing that tightens every dimension costs more than one that tightens only the functional features, and it does not make the part better.
- Thin walls. Walls below about 0.8 mm deflect under cutting force, chatter, and move after the last pass. Uniform wall thickness is the single most reliable design rule.
- Undefined datums. A dimension referenced to a datum that does not exist on the finished part cannot be inspected, and will be argued about rather than measured.
- Features that cannot be reached in one setup. Each additional setup adds error, because the part is re-located. Five-axis work exists to remove setups, not to show off.
What the Quote Depends On
Four things move a CNC prototype quote, and only one of them is cutting time:
- Number of setups. A part requiring five setups costs more than the same geometry reached in two, regardless of cycle time.
- Material. Aluminium cuts quickly. Stainless and tool steel cut slowly and wear tooling. An unusual grade or temper adds procurement time.
- Tolerance and inspection. A CMM dimensional report is a scheduled operation. First-article inspection adds time in exchange for evidence.
- Finishing. Anodising and painting add separate queue time, and can move a dimension, which is why finished parts are re-checked.
Verification
Parts are checked on dedicated metrology equipment rather than on the machine that cut them:
- Zeiss Contura G2 CMM — ±0.001 mm, 700 × 700 × 600 mm envelope, for full dimensional reports and first-article inspection.
- Mitutoyo Surftest SJ-210 — surface roughness Ra 0.005 to 40 μm.
- TESA Micro-Hite 600 — height and step measurement, 0 to 600 mm.
ISO 9001:2015, product qualification ratio above 99%. Of 50+ staff, 2 are dedicated inspectors alongside 6 engineers and 4 designers.
What to Send
- 3D CAD of the current revision, plus the revision number
- 2D drawing or PMI marking datums, critical dimensions and tolerances
- Which dimensions are functional-critical versus reference
- Material and heat treatment, where specified
- Whether the part must fit an existing assembly, with mating-part CAD if so
- The inspection report format required, if any
