CNC Machining 101: Getting Started
This CNC machining 101 getting started guide explains what happens between your CAD file and a finished metal part, and which numbers you should expect. It is written for design engineers, mechanical drafters and buyers releasing a first machined part. By the end you can pick a process, set tolerances that do not waste money, and read a quote without guessing.
In this article
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Key takeaways
How CNC machining 101 getting started actually works
CNC means computer numerical control. A machine tool reads a list of coordinates and moves a spindle or a turret to those coordinates. The cutter spins, the table or the chuck indexes, and material leaves the workpiece as chips. Nothing is cast or squeezed into shape. The finished part is the stock minus everything the tool touched.
The chain has four links. Design produces a 3D solid, usually STEP or IGES. CAM software reads that solid and generates toolpaths. A post-processor converts toolpaths into G-code for a specific machine. The controller executes the G-code, and a probe or a CMM checks the result against the drawing.
That chain matters because errors compound. A model with an open surface can produce a toolpath that gouges a wall. A toolpath with the wrong stepover leaves visible scallops. A post-processor set for the wrong control can output an arc the machine cannot read. Most first-part problems trace back to the front of the chain, not the machine.
GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, with 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The largest travel is 4,000 × 400 × 150 mm. A Ø400 mm rotary table handles round parts that need work on more than one face.
- 1Stock removalStart from bar, plate or billet; the cutter takes away what the part does not need.
- 2Rigidity winsA short, thick cutter deflects less, so it holds tolerance and leaves a better finish.
- 3One setup, one datumEvery extra setup adds a re-clamping error you have to budget for.
- 4Heat moves metalRoughing heats the part; let it cool before the finishing pass.
Milling, turning and 5-axis: which one fits your part
Milling cuts with a rotating multi-flute cutter while the workpiece stays clamped. It suits pockets, slots, flats and holes on prismatic parts. Three axes cover most brackets and housings. Add a fourth axis and you can machine four faces without re-clamping, which is usually cheaper than a second setup.
Turning spins the workpiece against a single-point insert. It suits shafts, bushings, fittings and anything with a dominant axis of revolution. Diameters hold tight on a lathe; length-to-diameter ratios above about 6:1 start to need a tailstock or a steady rest, or the part whips.
Five-axis machining tilts the tool or the table so the cutter reaches undercuts, deep cavities and contoured surfaces in one setup. It is the right answer for impellers, turbine blades and complex aerospace housings. It is the wrong answer for a flat plate with four holes, where it only adds programming time.
Mill-turn centers do both in one cycle. A part that is turned on the outside and milled on a flange can come off complete, which removes a re-clamp and a second fixture. For small lots this often beats running two separate machines.
- 13-axis millFlat plates, brackets, simple housings. Fastest to program and quote.
- 24-axis millParts needing access to faces around a single axis of rotation.
- 35-axis millUndercuts, compound angles, contoured surfaces. One setup, more programming.
- 4CNC latheRound parts with a clear centerline. Best diameter control per dollar.
What tolerance and surface finish you can actually get
Tolerance is the allowed deviation from the drawing dimension. GreatLight holds ±0.005 mm (±0.0002 in) on critical features when the part and setup allow it. That is not a default. It is a capability that costs cycle time, because the operator has to take lighter cuts, measure more often and control temperature.
Surface finish is measured as Ra, the arithmetic mean roughness. As-machined surfaces land around Ra 1.6–3.2 μm. A well-tuned finishing pass reaches Ra 0.8–1.6 μm. Fine finishing with a small stepover and a sharp insert reaches Ra 0.2–0.8 μm. Below that you are into grinding, lapping or polishing, which is a different operation.
Tolerance and finish interact. A wall machined to ±0.005 mm usually has a fine finish because the same light cuts produce both. But a fine finish does not guarantee tight tolerance. Cosmetic polishing can round an edge and pull a dimension out of spec while the surface looks better.
The practical rule: tolerance only the features that touch something else. Put a clearance hole at ±0.1 mm and save the ±0.005 mm budget for the bore that presses onto a bearing. Drawings that tolerance every dimension equally are the most common reason a simple part quotes high.
- 1Call out the fitSpecify the mating part or the fit class, not just a plus-minus number.
- 2Datums firstA clean datum scheme lets the inspector measure what you actually meant.
- 3Ra on the facePut finish callouts on sealing and sliding surfaces only.
- 4Sharp cornersInternal sharp corners need a smaller cutter, which means more passes.
Material choice changes the cutting parameters
Aluminum is the default for prototypes. 6061 and 6061-T6 cut fast, hold a good finish and take anodizing well. 7075 is stronger and machines cleanly but costs more and anodizes to a darker tone. 2024 has better fatigue behavior and poorer corrosion resistance, so it usually gets a coating.
Stainless steels fight back. 303 is the free-machining grade and is the easiest to run. 304 and 316 gum up on light cuts and work-harden if the tool rubs instead of cutting, so the feed has to stay aggressive. 17-4PH (SUS630) machines in the annealed state and then ages to high strength, which is useful for shafts and valve parts.
Steels like 1018, 1045, 4130, 4140 and 4340 cover the range from mild to high-strength. They cut predictably and are cheap, but they rust, so plan a finish. Tool steel is for molds and dies where wear resistance matters more than machinability.
Titanium and Inconel are the hard cases. Ti-6Al-4V (TC4) has low thermal conductivity, so heat stays in the cutter and tool life drops. Inconel work-hardens hard and needs rigid setups and low surface speeds. Both are machinable, but expect longer cycle times and plan the finishing strategy before you quote.
- 1Plastics tooPOM, PEEK, PC and ABS machine well; PEEK holds tight tolerance at high cost.
- 2Thermal growthAluminum expands about twice as much as steel per degree, so measure at room temperature.
- 3Chips tell youShort, curled chips mean the feed is right. Fine dust means the tool is rubbing.
- 4Finish follows materialHarder alloys usually need a coating before they need a polishing step.
How the part gets checked before it ships
Inspection starts before cutting. Raw material certificates confirm the grade you specified, because a 6061 bar that is actually 6063 will anodize to a different color. In-process checks catch a drifting dimension while the part is still clamped and can still be corrected.
Final inspection happens on every part before shipment. Critical features are measured on a CMM or with a height gauge and pin gauges, depending on the geometry. Reports are available on request. GreatLight publishes a 99.99% qualification rate, which reflects parts that pass inspection the first time.
For regulated work the paperwork matters as much as the metal. GreatLight holds ISO 9001:2015 for quality management, IATF 16949:2016 for automotive, ISO 13485:2016 for medical devices and ISO 27001:2022 for information security. Those certificates decide whether a part is even eligible for your program.
Confidentiality is part of the flow. Uploads stay secure, and an NDA is available on request before you send drawings. If your design is not public, get the agreement signed first, then upload.
- 1Material certsAsk for them when the alloy grade is a functional requirement.
- 2First articleFor a new program, inspect the first part fully before the run continues.
- 3TraceabilityLot numbers link a finished part back to the heat of material it came from.
- 4NDA firstSign before upload if the geometry is sensitive.
Step by step: from file to first part
A normal first article at GreatLight moves through these six stages.
- 1Send the model and drawingSTEP or IGES for geometry, PDF for tolerances, datums and finish callouts. Include the material grade and quantity.
- 2Read the DFM feedbackQuotation and free DFM analysis come back within 12 hours. Thin walls, deep pockets and tight corners get flagged here.
- 3Fix the model before cuttingLoosen tolerance on non-critical features, add a corner radius to deep pockets, and confirm the datum scheme.
- 4Approve and startProduction can start within 24 hours of approval. No minimum order quantity, from one prototype to 10,000+ parts.
- 5Inspect the first articleThe first part is measured against the drawing before the rest of the run continues.
- 6Ship with paperworkParts ship in 3–5 days with 100% inspection before shipment and reports on request.
Which process and tolerance to pick
Match the part geometry to the process, then set tolerance only where it is needed.
| Part type | Best process | Typical tolerance | Surface finish |
|---|---|---|---|
| Flat plate with holes | 3-axis mill | ±0.05 mm | Ra 1.6–3.2 μm |
| Bracket, four faces | 4-axis mill | ±0.025 mm | Ra 1.6–3.2 μm |
| Shaft or bushing | CNC lathe | ±0.01 mm | Ra 0.8–1.6 μm |
| Impeller or blade | 5-axis mill | ±0.005 mm | Ra 0.8–1.6 μm |
| Turned OD plus milled flange | Mill-turn center | ±0.01 mm | Ra 0.8–1.6 μm |
| Sealing face | Mill plus fine finish | ±0.005 mm | Ra 0.2–0.8 μm |
| Housing over 1,000 mm | Large-travel mill | ±0.05 mm | Ra 1.6–3.2 μm |
The practical verdict
If your part is prismatic and simple, choose a 3-axis mill and tolerance only the mating features. If it has undercuts, compound angles or contoured surfaces, choose 5-axis and accept the extra programming. If it is round, choose turning. Do not buy precision you cannot measure.
CNC machining 101 getting started: common questions
Do I need a 3D model, or is a 2D drawing enough?
A 3D solid is the working file. CAM software generates toolpaths from surfaces, so a 2D drawing alone means someone has to model the part first, which adds cost and a chance of misreading your intent.
The drawing still matters. It carries tolerances, datums, surface finish callouts and notes that a bare STEP file cannot express.
What is the smallest internal corner radius I can use?
The corner radius equals the cutter radius. A 6 mm end mill leaves a 3 mm corner. If the drawing calls for a 1 mm corner, the shop must switch to a 2 mm cutter, which is slower and more prone to breaking.
Where the corner is not functional, open it up. A larger radius usually removes a finishing pass and lowers the price.
How deep can a pocket be relative to its width?
A common limit is about 3:1 depth to width with a standard end mill. Beyond that, the cutter deflects and the walls taper, so the shop has to step down in smaller increments or use a longer, more expensive tool.
If the design needs a 6:1 pocket, expect longer cycle times and check whether the feature can be reached from the other side instead.
Does anodizing change the dimensions?
Yes. Anodic coatings grow both into and out of the surface, so a Type II coating typically adds a few micrometres per side. Hardcoat is thicker. If a bore has a tight fit, mask it or cut it undersize before coating.
Tell the shop which surfaces are cosmetic and which are functional so the masking plan matches your intent.
What quantity makes CNC cheaper than molding?
CNC needs no tooling, so the first part and the hundredth part cost roughly the same per unit. Injection molding needs a mold, which only pays off at higher volumes.
For prototypes and bridge production, CNC wins. For a stable design at high volume, compare a molded quote against the machined one before committing.
Can you machine a part from a material not on your list?
Often yes, if the stock is available in a machinable form and the geometry suits the process. Send the grade and the condition, such as annealed or pre-hardened.
Some grades machine poorly or need heat treatment after cutting. The DFM review will say so before production starts.
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