A Beginners Guide to CNC Basics
This beginners guide to CNC basics is written for engineers and buyers who need to read a drawing, pick a process, and judge what a shop can actually hold. It covers how metal is removed, what the common machine types do, where tolerance and finish limits sit, and when CNC is the wrong call.
Key takeaways
What CNC removes, and what it cannot
CNC stands for computer numerical control. A CAM programmer converts a 3D model into toolpaths, and the machine follows those coordinates to move a spinning cutter through the stock. Chips come off. The part that remains is whatever the cutter left behind. That single fact explains most of the rules that follow.
Subtraction sets the boundary conditions. You cannot machine a closed internal cavity without a hole to reach it. A sharp inside corner cannot be cut by a round tool, so drawings need a corner radius at least as large as the smallest cutter that can reach that depth. Deep pockets need a cutter long enough to reach the floor, and a long cutter deflects. That deflection shows up as taper on the wall.
Additive processes build up. CNC cuts down. For a part that needs an internal lattice, thin walls, or a hollow shell with no access, 3D printing is usually the better route. For a part that needs load-bearing threads, a press fit, or a sealing face, machining wins because the material stays solid and the surface is cut, not sintered.
- 1Tool radius ruleThe smallest inside corner equals the radius of the cutter that can reach it.
- 2Depth-to-diameterPast roughly 4:1, a cutter needs a reduced stepdown or a larger tool.
- 3Access ruleEvery cut surface must be reachable by a tool holder, not just the cutter tip.
3-axis, 4-axis, and 5-axis: what each one buys you
A 3-axis mill moves X, Y, and Z. The tool always points straight down. This is the workhorse for plates, brackets, housings, and anything you can reach from the top, plus a second setup from the side or bottom. Most flat parts should be quoted as 3-axis. It is the fastest and cheapest option when the geometry allows it.
A 4-axis mill adds rotation around one axis, usually A. The part turns while the tool cuts. This lets you machine a cylinder, a shaft with flats, or four faces of a prism without re-fixturing. It removes one setup and the alignment error that comes with it.
A 5-axis machine adds two rotary axes, so the tool can tilt. The payoff is access. Undercuts, angled holes, and contoured surfaces can be cut in a single setup. On a part with five sides of features, 5-axis can replace three or four separate 3-axis setups. Fewer setups means tighter true position between features, because every feature comes off the same datum. GreatLight runs 16 simultaneous 5-axis machining centers alongside 27 three-axis machines and 12 four-axis mills, so the setup count is chosen by geometry, not by what is free.
Reading tolerance and surface finish on a drawing
Tolerance is the allowed deviation from the nominal size. A drawing that says Ø25.00 ±0.05 mm is a normal machining call. A drawing that says Ø25.000 ±0.005 mm is a different job. The tighter band needs a finishing pass, a sharp tool, temperature control, and more inspection time. It also needs a machinist who checks the cutter before the last pass, not after.
Surface finish is measured in Ra, the arithmetic mean roughness in micrometres. As-machined faces usually land around Ra 1.6–3.2 μm with a standard end mill. A finer stepover and a finishing cutter bring it to Ra 0.8–1.6 μm. Below that, you are into Ra 0.2–0.8 μm territory, which calls for a specific toolpath and often a secondary operation.
Do not put a blanket tolerance on the whole drawing. Call out the tight dimension and leave the rest at the title-block default. A shop reads a tight tolerance as a machining requirement on every face it touches, and prices accordingly. One tight bore on a loose plate is cheap. A whole plate at ±0.005 mm is not.
- 1Datum firstA tolerance without a datum has no defined measurement direction.
- 2Stack-upsTolerances add along a chain; a 0.01 mm error can become 0.05 mm at the far end.
- 3Finish is localA sealing face may need Ra 0.4 μm while the rest runs as-machined.
How material choice changes the cut
Aluminium 6061-T6 is the default for prototypes and most housings. It cuts fast, holds a good finish, and takes anodizing well. 7075 is stronger but gummier, so it wants sharper tools and a lighter stepover. 2024 machines well but corrodes without a coating.
Stainless 303 is the free-machining grade. 304 and 316 gum up more, work-harden under a dull cutter, and need a rigid setup with steady feed. 17-4PH (SUS630) cuts cleanly in the annealed state and can be aged afterward, which is useful for parts that need strength without distortion during machining.
Titanium TC4 (Ti-6Al-4V) and Inconel sit at the other end. They generate heat at the cutting edge, so the tool needs high pressure coolant and a conservative feed. Tool wear is fast. That translates to longer cycle times and a different price per part. Plastics like POM and PEEK cut easily but move with temperature, so a tight plastic tolerance often needs a cool-down before the final measurement.
Choosing a machine type by part geometry
Pick the lowest axis count that still reaches every feature.
| Part shape | Best setup | Why |
|---|---|---|
| Flat plate, holes from one side | 3-axis | One setup, no rotation needed |
| Shaft with flats and slots | 4-axis | Rotates while cutting, no re-fixture |
| Housing with side ports | 4-axis or 3-axis, two setups | Side access without full contouring |
| Impeller or blade contour | 5-axis simultaneous | Tool tilts to follow the surface |
| Undercut or angled deep hole | 5-axis | Straight tool cannot reach it |
| Five-sided prismatic block | 5-axis | Replaces four setups with one |
| Turned diameter plus milled flats | Mill-turn | Turning and milling in one program |
| Long weldment, 4,000 mm | Large-travel 3-axis | Fits the 4,000 × 400 × 150 mm envelope |
When CNC is the wrong answer
For a hollow shell with internal features, a lattice, or a one-off cosmetic model, 3D printing is faster and cheaper. For a load-bearing part with threads, seals, or a press fit, choose CNC. If the part is a thin-walled shell in high volume, look at die casting or vacuum casting before machining.
Frequently asked questions
What file formats does a CNC shop need?
A STEP or IGES file is the standard for 3D geometry. A 2D PDF with the drawing and tolerances is still required, because the model does not carry datums or finish callouts.
STL files work for simple shapes but lose precision on curved faces. If you only have an STL, expect the shop to ask questions about critical dimensions.
How long does a first article take?
For a simple 3-axis part, cutting can often start within 24 hours of a released drawing, and parts ship in 3–5 days. Complex 5-axis work with a custom fixture takes longer because the fixture itself has to be made.
Quotation and DFM analysis come back within 12 hours, so the review loop is usually the first thing that decides the schedule.
Is there a minimum order quantity?
No. One prototype and a 10,000-part run both go through the same process. The difference is fixture design and whether a second or third setup is worth hard tooling.
For low volumes, the setup cost is spread over few parts, so the per-unit price is higher. That is normal, not a penalty.
What tolerance should I put on a prototype drawing?
Start with the title-block default, usually ±0.1 mm for metal. Tighten only the features that mate with something else, such as a bearing bore, a dowel hole, or a sealing groove.
A prototype at ±0.05 mm is usually enough to prove fit and function. Save ±0.005 mm for production parts where the stack-up demands it.
How do I keep my design confidential?
Uploads are handled as secure and confidential. A non-disclosure agreement is available on request before files are shared.
If your program requires it, ask for the NDA first and send the model afterward.
Why does my quote go up when I add a chamfer callout?
A chamfer on an accessible edge is a few seconds of toolpath. A chamfer on an internal edge behind a wall may need a different tool, a different toolpath, or a second setup.
The cost is not the chamfer. It is the access.
Send a drawing, get a real answer
Upload a STEP file and a 2D drawing. We return a quotation and a DFM review within 12 hours, with the tolerance and setup notes you need to finalize the design.
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