How to CNC Process Metal: 7 Steps From Drawing to Finished Part

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  What matters before you release a metal part
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  Reading the drawing before any metal is cut
• 3
  Choosing the setup: 3-axis, 4-axis, 5-axis or mill-turn
• 4
  When CNC processing metal is the wrong answer
• 5
  Step by step: from raw bar to inspected metal part
• 6
  Which route fits your metal part

Every job starts with a drawing review, not with a machine. We look at the material callout, the tightest tolerance on the sheet, and the surface finish notes. Those three items decide the machine, the number of setups and the inspection plan. A part in 6061-T6 with ±0.05 mm on everything is a different job from the same part in 17-4PH with two bores at ±0.005 mm.

Material choice comes first because it sets the cutting conditions. Aluminium 6061, 2024, 5052, 6082 and 7075 cut fast and hold a good finish. Stainless 303 and 304 gum up at low speed, so we run higher feed per tooth and lighter radial engagement. Titanium TC4 and Inconel need low surface speed, rigid tooling and patience. Copper and brass cut cleanly but move with heat, so roughing and finishing are separated.

The second question is functional. Which surfaces touch another part? Which holes carry a bearing, a pin or a seal? Those are the features that need the tight tolerance, and they should be called out on the drawing. Painting a ±0.005 mm band across the whole part adds cost without adding function. Engineers who mark only the critical features usually get a lower quote and a shorter lead time.

We return a DFM analysis with the quotation, normally within 12 hours. It lists the features that will be hard to hold, the corners that a standard end mill cannot reach, and any wall thin enough to chatter. You can accept the changes or keep the design as drawn. Either way you know the risk before the first cut.

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  Call out only functional tolerancesBlanket tight tolerance doubles cost with no gain.
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  Check corner radiiAn internal corner needs a cutter radius; a sharp corner needs EDM.
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  Note the datum schemeDatums that flip with the part create stack-up.
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  State the finishRa 1.6–3.2 μm as-machined is standard; Ra 0.2–0.8 μm needs extra passes.

The setup decision is about reach, not about machine prestige. A part with all features open from the top and one side is a 3-axis job. Add a fourth face and a 4-axis mill with an indexer keeps the part in one workholding position. Add compound angles, deep pockets on several faces or a contoured surface, and a 5-axis center removes the refixtures that would otherwise stack up error.

GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm. Travels run from 500 × 500 × 450 mm on the compact machines up to 4,000 × 400 × 150 mm on the large gantry, with a Ø400 mm rotary table available for round parts.

Mill-turn matters when the part is mostly round but has milled flats, cross holes or slots. Turning and milling in one cycle keeps concentricity between the bore and the outer diameter, which a two-machine route cannot match. For a shaft with a keyway and an off-axis port, mill-turn usually removes one setup and one inspection step.

Do not specify 5-axis because it sounds better. Simultaneous 5-axis programming takes longer, and the machine time is higher. Use it when the geometry genuinely needs continuous tool axis change, such as an impeller, a turbine blade root or a port with a compound angle. Otherwise the cheaper 3-axis route with a well-made fixture gives the same part.

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  3-axisPrismatic parts, features from one or two directions.
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  4-axisParts needing three or four faces in one index.
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  5-axisCompound angles, contoured surfaces, deep multi-face pockets.
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  Mill-turnRound parts with milled features and tight concentricity.

CNC process metal wins on tolerance, material choice and geometry freedom. It loses on unit cost at high volume. If a part will run 50,000 pieces a year in a simple shape, die casting or stamping will beat machining on price per piece. Machining is still the right choice for the first article, the bridge tooling and any feature that needs to stay within ±0.005 mm after casting.

Very thin, very large and very light parts can also be a poor fit. A 4,000 mm part with 0.8 mm walls will move during and after cutting, no matter how careful the passes are. In that case a fabricated sheet metal assembly or a casting with a machined interface is more stable and cheaper.

Deep small holes are another limit. A hole 20 times its diameter deep in stainless usually needs drilling from both ends or EDM, and the straightness will still drift. Tell us the depth-to-diameter ratio at quoting time and we will say whether drilling or EDM makes sense.

Finally, consider the finish. If the part needs a mirror polish over a large freeform surface, hand polishing hours can exceed machining hours. Bead blasting, tumbling or a specified Ra band often reaches the function at a fraction of the cost.

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  High volume, simple shapeDie casting or stamping beats machining on piece price.
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  Very thin wallsUnder about 1 mm on a large part, distortion is likely.
• 3
  Deep small holesRatios above 10:1 need a different process or a two-side approach.
• 4
  Mirror freeform surfacesHand polishing time can exceed cutting time.