Where to Get a Part CNC Machined

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  What actually decides where to get a part CNC machined
• 2
  Machine capability and the geometry it can hold
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  Tolerance, finish and how they are verified
• 4
  Materials, finishes and where shops draw the line
• 5
  Matching the part to the process

The question is not which shop is biggest. It is which shop can hold your tolerances on your material, at your quantity, and prove it. A bracket at ±0.1 mm and an implant housing at ±0.005 mm do not need the same supplier. Start by reading your own drawing: tolerance, surface finish, material, and how many parts you need. Those four numbers narrow the field faster than any supplier list.

Machining capability is a chain, not a single number. Machine geometry sets the size limit. Spindle and tooling set the finish. Metrology sets the tolerance you can trust. A shop with a 4,000 mm travel machine but no coordinate measuring machine cannot verify a long part. Ask what measures the part, not just what cuts it.

Volume changes the answer too. One prototype and a 10,000 part run rarely go to the same place. Prototype shops optimize for setup speed and no minimum order quantity. Production shops optimize for fixture cost spread over many parts. Sending a one-off to a high-volume shop usually means a slow quote and a high price.

Finally, communication is a technical variable. If the shop returns a DFM note pointing at a thin wall or an unreachable corner, that is engineering. If it only returns a price, you carry all the risk. For a part cnc machined from a fresh design, the DFM feedback is often worth more than a small price difference.

Axis count decides which faces you can reach in one setup. A three-axis mill cuts from one direction. A four-axis mill adds a rotary table, so you machine around the part. A five-axis center tilts the tool or the table, which lets you reach undercuts and compound angles without re-fixturing. Every extra setup adds stack-up error, so fewer setups usually means tighter true position.

Size and travel matter as much as axis count. A large gantry with 4,000 × 400 × 150 mm travel suits long structural parts, but it will not hold the same finish as a compact 500 × 500 × 450 mm machine cutting a small housing. Match the machine envelope to the part envelope. Cutting a 40 mm part on a 4 m machine wastes rigidity.

Turning and mill-turn cover round parts. A mill-turn center machines a shaft and then mills a flat or a cross-hole without moving the part to a second machine. That protects concentricity between the turned diameter and the milled feature. If your part has both, ask whether it can be done in one operation.

Five-axis is not automatically better. For a simple prismatic part, a three-axis machine with a good fixture is faster and cheaper. Five-axis pays off when the part has organic surfaces, deep pockets with angled walls, or features on several faces that must share one datum. Choose the process from the geometry, not the brochure.

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  3-axisPrismatic parts, one accessible face, simple fixtures.
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  4-axisRound or wrapped features, holes on multiple sides.
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  5-axisCompound angles, undercuts, organic surfaces, one-setup datums.
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  Mill-turnShafts with milled flats or cross-holes, tight concentricity.

A tolerance claim is only as good as the measurement behind it. ±0.005 mm is a real number, but it applies to specific features on specific materials with controlled temperature. On a 300 mm aluminium part, thermal expansion alone can move the dimension more than the tolerance. Ask which features carry the tight tolerance and how they are checked.

Surface finish and tolerance pull against each other. A Ra 0.8–1.6 μm finish is a normal machined finish. Ra 0.2–0.8 μm needs finer feeds, sharper tooling and often a finishing pass, which adds time. If only a sealing face needs the fine finish, say so on the drawing. Specifying the whole part at Ra 0.2 raises the price for no functional gain.

Inspection should be planned, not promised. A serious shop checks raw material on arrival, monitors dimensions during the run, and does a final inspection before shipment. Reports are available on request. For a first article, ask for the actual measured values on the tight features, not a pass or fail stamp.

Watch for tolerances that fight each other. A tight bore and a tight shaft with a loose position callout may still assemble poorly. Geometric callouts like true position and perpendicularity describe the relationship between features, and they matter more than any single size. If the drawing is silent, the shop will guess.

Material range tells you how broad the shop really is. Aluminium 6061 and 7075 cut easily and are common. Stainless 316L and 17-4PH work-harden, so they need the right speeds and sharp tooling. Titanium Ti-6Al-4V and Inconel generate heat and wear tools fast, and not every shop keeps the tooling or the coolant for them. Ask directly about the alloy you need.

Plastics behave differently from metals. POM and ABS machine cleanly. PEEK needs higher temperatures and can distort if the shop treats it like aluminium. Carbon fibre reinforced plastic wears cutters and needs diamond tooling for a clean edge. A shop that lists PEEK but has never held a tolerance on it will struggle with your part.

Finishing is a separate step with its own lead time. Anodizing, plating, powder coating and black oxide all change the surface and sometimes the dimension. Hard anodize builds a layer that can move a tight fit. Laser marking needs a minimum character height of 1.5 mm to stay legible. Plan the finish before the machine runs, not after.

One practical rule: send the shop the material and finish together with the drawing. If the shop proposes a material swap to hit a tolerance or a lead time, ask what changes in the part. Sometimes the swap is fine. Sometimes it changes wear, weight or corrosion behavior. You decide, not the shop.