Most Basic Knowledge of CNC Processing
This page explains what actually happens inside a CNC machine: how metal is cut, what the axes do, how tolerance and surface finish are held, and where the process stops working. It is written for design engineers and buyers who need to judge whether a part suits CNC machining before they release a drawing.
Most basic knowledge of CNC processing: what happens at the tool edge
Every CNC process removes material with a wedge-shaped tool that is harder than the workpiece. The tool is pushed into the metal at a set feed per tooth, and the metal ahead of the edge shears off as a chip. Heat leaves mostly with that chip, not with the part. When chip evacuation is poor, heat stays in the cut and the edge wears fast.
Three numbers control the cut: cutting speed in surface meters per minute, feed per tooth, and depth of cut. Push any one too far and the edge chips. Pull all three back too far and the tool rubs instead of cutting, which work-hardens stainless and smears aluminium. The useful window between those two failures is narrow on titanium and wide on 6061.
Chip shape tells you which side of the window you are on. Long curled chips on aluminium mean the feed is healthy. Fine powder or a blue discoloration on steel means heat is building and the feed or the coolant needs to change before the edge fails.
A CNC machine does not decide any of this by itself. The programmer sets the numbers, the operator watches the chips, and the tool wears a little on every part. That slow wear is why a run of 500 parts drifts unless offsets are adjusted during the run.
How many axes a part really needs
A three-axis mill moves the tool along X, Y and Z while the part stays clamped. It cuts any face the tool can reach from above. Pockets, slots, flats and drilled holes on one side of a part are all three-axis work, and it is the fastest and cheapest way to make them.
A four-axis machine adds rotation about one axis, usually A. The part turns while the tool stays in place, so features on four sides can be cut without re-chucking. Long shafts with cross holes, and cylindrical parts with milled flats, are typical four-axis work.
A five-axis machine adds a second rotary axis, so the tool can approach the part from nearly any direction. That matters for contoured surfaces such as impellers, turbine blades and undersides that a straight tool cannot reach. It also cuts deep pockets with a short, stiff tool instead of a long one.
The second rotary axis is not free. Five-axis machines cost more per hour, need more setup thought, and the programmer has to control the tool axis to avoid a collision. Choose five axes when the geometry demands it, not because it sounds better on a quote.
Where tolerance and surface finish come from
Tolerance and finish are two separate targets, and they come from different parts of the process. Tolerance is about where the surface sits. Finish is about how smooth that surface is. A part can hold a tight tolerance with a rough finish, or a smooth finish with a loose tolerance.
Tolerance is set by the machine, the fixture and the temperature. A rigid setup holds size; a part that moves under clamping force does not. Thermal growth moves a 500 mm aluminium part several hundredths of a millimeter as the shop warms up through the day, which is why tight work is measured in a controlled room.
Finish is set by the tool nose radius, the feed, and the stability of the cut. A small nose radius and a light feed leave a finer scallop. Chatter overrides all of that: a vibrating tool leaves marks far rougher than the feed would suggest, no matter how slow the pass.
GreatLight holds ±0.005 mm on turned and milled features when the drawing calls for it, with surface finish from Ra 0.2–0.8 μm on fine work up to Ra 1.6–3.2 μm as-machined. Every part is inspected before shipment, with reports on request.
What changes when the part gets big
Size changes the physics. A 4,000 mm part sags under its own weight, so the fixture has to support it along its length, not just at the ends. A heavy workpiece also stores more energy in the cut, so any vibration grows instead of dying out.
Reaching the feature is the second problem. A long tool deflects, and deflection shows up as taper in a deep bore or a wall that is thicker at the bottom. Machining both sides in one setup beats re-fixturing, because every re-clamp adds a new datum error.
GreatLight runs 16 simultaneous five-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, with a maximum processing size of 4,000 mm and a Ø400 mm rotary table. Long parts up to 4,000 × 400 × 150 mm travel through the large machines.
Stock removal is the third issue. A large block can lose most of its mass as chips, and that releases internal stress. The part then bows after machining. Rough it, let it rest, then finish it, rather than cutting to final size in one pass.
How material choice changes the cut
Aluminium 6061 and 7075 cut fast and hold a good finish, which makes them the default for prototypes and housings. 7075 is stronger and machines cleanly, but it is less weldable and costs more. Both are easy to anodize, so they suit visible parts.
Stainless 304 and 316 need lower surface speed and a steady feed to avoid work hardening. Once the surface hardens, the next pass rubs instead of cutting and the edge fails. 17-4PH adds strength and is common in medical and aerospace work.
Titanium Ti-6Al-4V and Inconel are the hard cases. They conduct heat poorly, so the edge runs hot, and they work-harden quickly. Tool life is short and cycle time is long. Use them when the service temperature or strength demands it, not by default.
Plastics behave differently again. POM and PEEK cut cleanly but melt if the feed is too light, and they move with temperature far more than metals. ABS and PC are common for enclosures where the load is low. Carbon fibre needs diamond tooling and dust control.
Which setup fits the part
Pick the row that matches the geometry, not the one that sounds most capable.
| Part feature | Best setup | Why |
|---|---|---|
| Flat plate, holes on one face | 3-axis mill | Single approach, lowest cost per hour |
| Shaft with cross holes | 4-axis mill | Rotation avoids a second chucking |
| Impeller or blade contour | 5-axis center | Tool reaches compound angles in one setup |
| Large frame over 1 m | 5-axis or mill-turn | Fixture and travel decide the machine |
| Turned part with milled flats | Mill-turn center | One setup holds concentricity |
| Thin wall under 1 mm | 3-axis, light passes | Less force, easier to support |
The honest trade-off
If the part is prismatic with features on two or three faces, run it on a three-axis or four-axis machine and spend the saving on inspection. Go to five axes only when the surface is contoured, the tool cannot reach the feature, or one setup is the only way to hold the tolerance.
Common questions
How tight a tolerance can CNC machining hold in normal production?
On a rigid setup with a controlled temperature, ±0.005 mm is realistic on turned and milled features. That is not automatic for every feature on every part.
Long bores, thin walls and deep pockets lose accuracy because the tool deflects. Tell us which dimensions carry the fit, and we will plan the setup around those instead of tightening the whole drawing.
Does a smoother surface always cost more?
It costs more time, so it costs more money. A finer finish needs a smaller stepover or a slower feed, and sometimes a separate finishing pass with a different tool.
Ask for the finish the function needs. A sealing face may need Ra 0.8 μm; a bracket that bolts to a frame is fine at Ra 3.2 μm.
What is the smallest feature you can machine?
Small end mills get fragile fast. A slot or pocket narrower than about 1 mm needs a very small tool, light depth of cut and a slower feed, and the tool may break during the run.
Laser marking goes down to 1.5 mm character height if the goal is identification rather than structure.
Why does my part move after the first machining pass?
Rolled or cast stock carries internal stress. When you cut one side away, the balance changes and the part bows.
The usual fix is a roughing pass, a rest, then a finishing pass. On large parts we may rough, let the part relax overnight, then finish.
Can you machine a single prototype and then a production run?
Yes. There is no minimum order quantity, so the same process can start from one part and scale to 10,000 or more.
Keeping the same setup and tooling between the prototype and the run is what keeps the dimensions consistent.
How do you protect a drawing that is not public yet?
Uploads are handled as confidential. We can sign an NDA before any file is shared, and the quote and DFM analysis come back within 12 hours.
Production can start within 24 hours of an approved order, and parts normally ship in 3–5 days.
Send the drawing, get a real answer
Upload your files and an engineer will review the geometry, the tolerance callouts and the material before quoting.
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More machining notes
We publish setup notes, tooling trials and inspection data from the factory floor.
