How CNC Machining Works: From CAD File to Inspected Part
This page walks through how CNC machining works at the machine level: how a 3D model becomes toolpaths, how G-code drives the axes, and how the part is held, cut, and measured. Written for engineers and buyers who need to judge whether a design is machinable and what drives cycle time.
In this article
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What matters most
How CNC machining works as a subtractive process
CNC machining starts with a block of material and removes what is not the part. A rotating cutting tool, held in a spindle, moves along programmed paths while the workpiece stays clamped to a table or chuck. The shape comes from the difference between the two.
The machine does not decide anything on its own. A CAM programmer studies the model, picks tools, sets stepovers and stepdowns, and posts the result as G-code. The controller then executes that code at speeds and feeds the programmer chose. Change the code and you change the part.
That is the whole loop: model, toolpath, code, cut, measure. Each stage can lose accuracy. A perfect model with a loose setup still produces a bad part, and a rigid setup cannot save a toolpath that leaves material behind.
Compared with manual machining, the difference is repeatability rather than capability. A skilled operator can turn a good part by hand. CNC machining works the same way on part one and part ten thousand, provided the tool wears predictably and the fixture does not move.
- 1Rotating tool, fixed partMilling spins the cutter; turning spins the workpiece instead.
- 2Code before cutEvery motion is defined before the spindle starts.
- 3Repeatability over skillThe same program gives the same result on every cycle.
What file formats does CNC machining require?
Send a solid model, not a drawing alone. STEP (.step, .stp) is the safest choice because it carries exact B-rep geometry. IGES works for older CAD seats, and Parasolid or native SolidWorks files are fine when the shop uses the same platform.
STL and OBJ are mesh formats. They approximate curved surfaces with triangles, so a hole that should be Ø10 mm may measure slightly off and toolpaths can show faceting. If STL is all you have, expect the programmer to rebuild or repair the mesh, which adds a day of prep.
Native CAD files also expose the feature tree, which helps the programmer understand design intent. A bare solid still works, but if a tight tolerance only matters on one bore, say so in a note. The programmer will spend the extra passes there and leave the rest at general tolerance.
Include the 2D drawing when tolerances, surface finishes, or threads are not obvious from the solid. A STEP file with no notes will be cut to general tolerances, typically ±0.1 mm on milled features, which is often not what the designer had in mind.
How G-code and the controller drive the axes
G-code is a list of commands. G0 moves at rapid speed to a position, G1 moves at a programmed feed rate, G2 and G3 cut arcs, and M-codes handle spindle on, coolant, and tool changes. Each line gives coordinates and a feed. The controller reads one block ahead and interpolates the motion.
On a 3-axis mill, the cutter moves in X, Y, and Z. A 4-axis machine adds rotation about one axis, usually A, so features on four sides of a part can be cut in one setup. A 5-axis machine adds a second rotary axis, letting the tool tilt and reach undercuts and angled faces.
Simultaneous 5-axis means all five axes move at once along a curved path. That is what lets a shop cut an impeller or a contoured aerospace bracket without repositioning the part. Indexed 5-axis is simpler: rotate to an angle, lock, then cut in three axes.
The controller also runs the feedback loop. Encoders on each axis report actual position many times per second, and the drive corrects any error. This is why CNC machining holds ±0.005 mm on a good machine. Thermal growth, tool wear, and fixture movement are the errors the loop cannot see.
Workholding: how the part is held without distorting it
A vise is the default for prismatic parts up to about 400 mm. Clamp on a solid section, not on a thin wall, and support the bottom face fully. A part that rocks in the vise will cut oversize on one side and undersize on the other.
For thin plates, vacuum chucks or low-profile clamps spread the load. Soft jaws machined to the part profile are common for second operations because they grip the finished surface without marking it. For round parts, a 3-jaw chuck or a collet block keeps runout low.
Large parts need a different approach. A 4,000 × 400 × 150 mm travel machine can take long extrusions or frame rails, but the fixture has to be equally long and stiff. Bolt the part to a tooling plate with tabs, then cut the tabs off in a finishing pass.
Roughing forces push the part away from the tool. If the setup allows any lift or slide, the first pass will be shallow and the last pass will be deep. Check clamp torque and re-touch off the tool after the first roughing cut.
Feeds, speeds, and why chatter appears
Cutting speed is surface speed at the tool edge, given in m/min. Feed is the advance per tooth, in mm/tooth. Multiply feed per tooth by tooth count and spindle rpm to get the table feed in mm/min. Get these three numbers right and the chips come off clean.
Aluminum 6061 runs fast: 300–500 m/min surface speed with a 2-flute or 3-flute carbide end mill, coolant or air blast. Stainless 304 runs at a quarter of that, around 80–120 m/min, with heavier feed per tooth and plenty of coolant. Titanium TC4 is slower still and needs low surface speed to keep heat out of the edge.
Chatter is vibration at a frequency set by the tool, the holder, and the part. It shows up as a rippled wall and a loud whine. The usual fixes are shorter tool overhang, a stiffer holder, a lighter radial depth of cut, or a different spindle speed to move off the resonant point.
Climb milling, where the cutter tooth enters at maximum chip thickness, is standard on CNC machines with ball screws. It gives better surface finish and longer tool life than conventional milling on most materials. Keep the radial engagement around 5–10% of cutter diameter for finishing passes.
Finishing options after the cut
As-machined surfaces sit around Ra 1.6–3.2 μm and show visible tool marks. That is acceptable for brackets, housings, and internal parts where finish does not affect function. A finishing pass with a smaller stepover gets to Ra 0.8–1.6 μm without changing the geometry.
Bead blasting evens out the appearance and removes light burrs. Tumbling does the same for small parts in volume. Brushing gives a directional grain, and polishing reaches Ra 0.2–0.8 μm for optical or sealing surfaces.
Anodizing adds a hard, corrosion-resistant oxide layer on aluminum. Type II is decorative and clear or colored; hardcoat builds a thicker layer for wear surfaces. Electroless nickel, zinc plating, and black oxide cover steel and copper alloys.
Laser marking handles part numbers and logos, with a minimum character height of 1.5 mm. Mark after finishing, not before, or the coating will bury the text. Keep marked areas away from sealing faces.
Step by step: how a part moves through the shop
- 11. Review the model and drawingCheck units, material, tolerances, and finish notes. Flag any feature a standard end mill cannot reach, such as a deep pocket with sharp internal corners or a hole with a high depth-to-diameter ratio.
- 22. Run DFM and set the processDecide stock size, number of setups, and which features go on which operation. Typical stock allowance is 0.5–1.0 mm per side on milled faces. Quotation and DFM analysis come back within 12 hours.
- 33. Plan tools and toolpathsPick the largest cutter the geometry allows, then smaller tools for corners. Rough with a 50–70% stepover, finish with 5–10%. Use rest machining to clear material the big cutter left behind.
- 44. Post G-code and set the work offsetPost the program for the specific machine and control. Touch off X, Y, and Z, set the work offset, and verify tool lengths with a presetter or on-machine probe before the first cut.
- 55. Cut the first articleRun with the rapid override down and the feed override at 50% for the first few passes. Listen for chatter and watch chip color. Aluminum chips should be bright; blue or brown means too much heat.
- 66. Inspect and adjustMeasure critical features with calipers, micrometers, or a CMM. If a bore is 0.02 mm under, adjust cutter compensation rather than re-cutting the whole part. Record offsets for the run.
- 77. Run production and inspectCut the remaining parts with in-process checks every few pieces. Final inspection covers dimensions, threads, and surface finish. Parts ship in 3–5 days, with 100% inspection before shipment.
Which process fits the part?
Use this to pick a method before requesting a quote.
| Method | Best for | Typical tolerance | Watch out for |
|---|---|---|---|
| 3-axis milling | Prismatic parts, flat faces, open pockets | ±0.05 mm | Features on five sides need extra setups |
| 4-axis milling | Shafts, long parts, four-sided features | ±0.02 mm | Rotary alignment adds setup time |
| 5-axis simultaneous | Contoured surfaces, undercuts, impellers | ±0.005 mm | Programming time is higher |
| CNC turning | Round parts, threads, bores on axis | ±0.01 mm | Off-axis holes need a second op |
| Mill-turn | Round parts with milled flats or slots | ±0.01 mm | Limited to parts that fit the bar or chuck |
| Manual turning | One-off simple round parts | ±0.1 mm | Not repeatable across a run |
If the part has to be right on the first run
Send the STEP file and drawing with tolerances marked. You get DFM feedback and a quote within 12 hours, and the same programmer who reviews it stays on the job through inspection.
Common questions
How long does CNC machining take for a prototype?
For simple parts, production can start within 24 hours of an approved program and parts typically ship in 3–5 days. Complex 5-axis work with multiple setups adds time for programming and fixturing.
The main variables are feature count, tolerance, and how many setups the part needs. A part that can be cut in one setup moves much faster than one that needs four.
What tolerance can CNC machining hold?
GreatLight holds ±0.005 mm (±0.0002 in) on critical features under controlled conditions. General milled features usually run at ±0.1 mm unless the drawing says otherwise.
Tight tolerances apply to specific features, not the whole part. Marking one bore at ±0.005 mm costs far less than marking every dimension that tight.
Which materials are easiest to machine?
Aluminum 6061 and 6061-T6 cut fast and hold good finish. Brass C36000 and stainless 303 are also free-machining grades.
Stainless 304, 316, titanium TC4, and Inconel cut much slower and wear tools faster. PEEK and POM machine well; ABS and PC can gum up without the right feeds.
Is CNC machining good for high volume?
CNC holds up well into the thousands when the part geometry suits it, and there is no minimum order quantity. From one prototype to a 10,000+ part run is normal here.
Above that, die casting or injection molding often wins on unit cost. CNC is usually the right call when tolerance is tight or the design is still changing.
What happens to the scrap material?
Chips are separated by alloy, cleaned, and sold back for melting. Aluminum is kept apart from steel and stainless because mixed chips have little value.
Credits from scrap reduce net material cost. On aluminum parts, chip value can offset a noticeable share of the raw stock price.
Do you sign an NDA?
Yes. Uploads are secure and confidential, and an NDA is available on request before files are shared.
Send the NDA first if your program requires it. We can review and return it before any drawing reaches the shop floor.
Ready to quote a machined part?
Upload your model and get a quote with DFM notes in 12 hours. No minimum order, tolerances to ±0.005 mm, 100% inspection before shipment.
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