CNC Machining
CNC machining is a subtractive process: computer-controlled cutting tools remove material from a solid block until only the part remains. For prototypes and small batches it usually means CNC milling, where a spinning cutter follows programmed toolpaths through a clamped block of metal such as aluminium or steel.
Updated 28 Sep 2026 · 5 min read · By Wyntek
How CNC milling works
A CNC mill clamps the workpiece, usually in a vise or a fixture, and moves it and a rotating cutter relative to each other along programmed paths. CAM software turns the CAD model into those toolpaths, and the machine's controller runs them. Roughing passes remove most of the stock quickly, then finishing passes cut the final surfaces.
A 3-axis mill moves the cutter in X, Y, and Z, with the tool always pointing down at the part. A 5-axis mill adds two rotary axes that tilt or turn the part or the spindle, so the cutter can reach more faces without the part being clamped again.
Setups
A setup is one way of clamping the part in the machine. On a 3-axis mill the cutter only reaches faces that point up at the spindle, so a part with features on its top and bottom needs at least two setups, and features on the sides can need more. Between setups the part is unclamped, turned, clamped again, and located again.
Each setup adds handling time and affects how accurately features relate to each other. Features cut in the same setup are positioned by the machine alone. Features cut in different setups also depend on how precisely the part was located again, so put features that must line up closely, such as a bore and the holes around it, where one setup can reach them all.
Tool access
A milling cutter is a spinning cylinder on the end of a spindle, and it can only cut surfaces it can reach in a straight line. Standard cutters cannot reach undercuts, internal passages, or faces hidden behind other features.
Depth is limited by stiffness. A cutter's deflection grows with the cube of its overhang, so a tool that sticks out twice as far bends about eight times as much under the same load. Deflection shows up as tapered walls, chatter marks, and lost accuracy, which is why deep, narrow pockets are slow and costly to cut. Keep pockets as shallow and as wide as the design allows, and avoid undercuts unless the function needs them.
Internal corner radii
Because the cutter is round, every vertical inside corner of a pocket or slot is left with a radius at least equal to the cutter's radius. A perfectly sharp internal corner cannot be milled.
Make each internal corner radius slightly larger than the radius of the tool that will cut it. When the two are equal, the cutter meets extra material, a jump in engagement, and a sudden change of direction in the corner, which causes chatter and a poor finish. A tool smaller than the corner can sweep round it instead. Larger radii also let the shop use a bigger, stiffer cutter, which matters most in deep pockets.
If a square-cornered part has to sit in a milled pocket, add relief cuts, often called dog-bones, that extend past each corner so the mating corner clears.
Tolerances
A tolerance is how far a dimension may vary from its nominal size. Drawings usually set one general tolerance for every dimension without its own, often by citing ISO 2768-1 and one of its classes: f (fine), m (medium), c (coarse), or v (very coarse).
| Nominal size | Fine (f) | Medium (m) | Coarse (c) |
|---|---|---|---|
| 0.5 to 3 mm | ±0.05 mm | ±0.1 mm | ±0.2 mm |
| Over 3 to 6 mm | ±0.05 mm | ±0.1 mm | ±0.3 mm |
| Over 6 to 30 mm | ±0.1 mm | ±0.2 mm | ±0.5 mm |
| Over 30 to 120 mm | ±0.15 mm | ±0.3 mm | ±0.8 mm |
| Over 120 to 400 mm | ±0.2 mm | ±0.5 mm | ±1.2 mm |
Tighter tolerances take slower finishing passes, more careful setups, and more inspection, so they cost more. Put them only on the features that need them, such as bores for bearings or pins, mating faces, and hole positions, and leave the rest at the general tolerance.
When to choose CNC machining
Choose machining when the part must be metal, hold tight tolerances, carry threads or flat datum faces, or behave like the final production part under heat and load. Choose 3D printing when the shape has internal channels or organic forms a cutter cannot reach, or when a plastic prototype is enough to answer the question.
At Wyntek
Wyntek machines aluminium and steel on Pro plans and up, from 1 to 100 parts per job. Upload a STEP file, then choose the material and a tolerance class: ±0.2 mm (±0.008 in) standard, ±0.1 mm (±0.004 in) tight, or ±0.05 mm (±0.002 in) precision for suitable features. You see the price in US dollars before you submit, and every part is scanned and inspected before it ships.
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