CNC machining key facts: what the numbers actually mean
A working reference for design engineers and buyers who need to judge a machined part before it is quoted. It covers how metal is removed, where tolerance and finish limits sit, which geometry forces a 5-axis setup, and what inspection should look like at the end.
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How CNC machining removes metal
CNC machining is subtractive. A rotating cutter is driven along a programmed path through a solid block, and the shape left behind is the part. The program comes from a CAD model converted into toolpaths, so the geometry in the model sets what the machine can reach. Everything else — tolerance, finish, cycle time — follows from tool diameter, spindle speed, feed rate and how rigidly the workpiece is held.
The cutter leaves marks. A larger stepover or a faster feed leaves visible scallops that you can measure as surface roughness. A Ø12 mm end mill can clear material quickly, but it cannot enter a 6 mm internal corner; the tool radius is always left in the corner unless you change tools or change the design. This single fact explains most of the pushback you get on a quote.
Heat is the other limit. Aluminium 6061-T6 cuts cleanly at high spindle speeds and will hold ±0.005 mm on a stable setup. Titanium Ti-6Al-4V and Inconel transfer heat into the tool instead of the chip, so feeds and depths drop, tool life shortens, and thin walls start to move. Parts in those alloys cost more because the machine occupies the spindle longer, not because the material itself is rare.
Hardness and fixturing decide the rest. A part with no flat face to clamp cannot be held without soft jaws or a fixture, and a fixture is real cost. When we review a model for DFM, we look for a clamping face, a wall thickness above roughly 1 mm, and features that a standard cutter can reach before we quote a tolerance.
CNC machining key facts about tolerance and fit
Tolerance is the total allowed variation on a dimension. A drawing callout of ±0.005 mm means the finished feature must land inside a 0.010 mm band, which is about one fifth of a human hair. That band is achievable on a well-held aluminium part, and it is not achievable on a long slender part that flexes under cutting force. Tolerance is a property of the setup, not a property of the machine alone.
Standard machined work usually sits at ±0.05 mm or looser. Tightening a callout from ±0.1 mm to ±0.01 mm does not just slow the cut; it adds in-process measurement, temperature stability and possibly a second operation. If a feature only needs to locate a bolt, a loose tolerance is correct. If it establishes an axis for a bearing, it needs the tight one, and the cost is justified.
Datums matter as much as the number. A ±0.01 mm callout referenced to a face that is itself not flat is ambiguous. Put the datum on a machined surface, keep the stack short, and avoid dimensioning a tight feature across a soft or thin section. We inspect to the datum stated on the drawing, and a well-defined datum removes argument at first article.
On our equipment, ±0.005 mm is the working precision limit on suitable geometry, and we report it as ±0.0002 in for imperial drawings. Surface finish runs from Ra 0.2–0.8 μm for fine work, Ra 0.8–1.6 μm for most functional surfaces, and Ra 1.6–3.2 μm as-machined. Finish and tolerance should be called out separately, because a part can hold a tight dimension with a rough surface and vice versa.
When 3-axis is enough and when 5-axis is required
A 3-axis machine moves the tool in X, Y and Z while the part stays still. One setup machines one face. If a part has features on five faces, a 3-axis route needs multiple setups, and every re-clamp introduces a small position error. That error is often larger than the tolerance you asked for, which is the practical reason to move up in axis count.
A 5-axis machine tilts and rotates the tool or the table so the cutter can approach the part from an angle. Undercuts, deep pockets with curved floors, angled holes and blended surfaces can be cut in one setup. For a part with complex geometry, 5-axis usually improves accuracy more than it improves speed, because the reference stays the same from the first cut to the last.
There is a cost boundary. Simple prismatic parts with holes and slots are cheaper on 3-axis, and putting them on a 5-axis center wastes spindle time. Short-run or one-off parts benefit most from 5-axis because the setup cost is spread over few pieces. High-volume simple parts often move to casting or stamping once the design is frozen.
Our shop runs 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm, with a Ø400 mm rotary table for round work. That spread matters because it lets us route a job to the machine that fits it rather than forcing every part through the most expensive option.
Feature limits that decide whether a part can be machined
Internal corners carry the tool radius. A pocket with a 2 mm corner radius needs a cutter of 4 mm diameter or less, which limits depth and stiffness. Deep narrow slots are the hardest case: tool length grows, deflection grows, and the achievable finish drops. If a corner radius is not functional, opening it up to the next standard cutter size is the cheapest change a designer can make.
Aspect ratio is the second constraint. A wall 0.5 mm thick and 40 mm tall will chatter and may distort after the vise is released. We would rather machine it with light passes and support material behind it, which is slower and costs more. Where possible, keep wall thickness above 1 mm and add ribs instead of going thinner.
Holes have their own rules. A drilled hole is not a reamed hole. If a bore needs to hold ±0.01 mm and a specific finish, it will be drilled undersize and reamed or bored, and that is a separate operation. Threads below M2 and features narrower than 1 mm are possible in some materials but are fragile to inspect and easy to damage in handling.
Text and marking have a floor too. Laser marking and engraving go down to 1.5 mm minimum character height. Below that, legibility depends on the material and the finish, and we will say so rather than accept a callout that cannot be verified. On any drawing, dimensions you can measure are the ones that get controlled.
Material choice changes the process, not just the price
Aluminium is the default for prototypes and most functional parts. Grades 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 are all stocked or sourced. The free-machining tempers cut fast and hold tolerance well. 7075 gives higher strength at the cost of more tool wear. Anodizing works well on most of these grades, though the color match shifts between alloys.
Stainless and steel cover the load-bearing work. Grades 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH are common, along with 1018, 1045, 4130, 4140, 4340, A36 and tool steel. 304 and 316 work-harden, so feeds must stay aggressive. 17-4PH machines well in the annealed condition and then heat treats to high strength, which is why it appears on pump and valve parts.
Titanium and nickel alloys are the slow ones. TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B / AZ91D are available, and each changes the cutting strategy. Inconel needs low surface speed and rigid setups. Magnesium needs care with chips and coolant. Copper and brass grades such as C101, C110, beryllium copper, C27400, C28000 and C36000 cut easily and are often chosen for conductivity or wear.
Plastics follow different rules. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre all machine, but they expand with heat and can chip at the exit. PEEK and carbon fibre need sharp tooling and slow feeds. Finishing options include anodizing in clear, color, hardcoat and conductive types, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing.
What a realistic process timeline looks like
A CNC job moves through six stages: DFM review, quotation, material procurement, programming, machining and inspection. The DFM review is where cost is won or lost. A note about a corner radius or a tolerance that cannot be measured can save a second operation. We return a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours of approval.
Programming converts the model into toolpaths and a setup sheet. The programmer chooses the workholding, selects cutters from what is on the shelf, and decides the order of operations. Changing a design after programming usually means reprogramming. That is why a design freeze before the first cut is worth more than a rushed start.
Machining runs in shifts across the three plants. Parts ship in 3–5 days for typical work. Historical late-delivery probability sits below 2%. That number is not a promise on any single order, but it tells you the schedule is built with some margin rather than none.
Inspection closes the loop. Every part is checked before shipment: raw material verification, in-process monitoring and final inspection, with reports on request. On a tight-tolerance job, the inspection method is agreed before cutting so that both sides measure the same feature the same way. Certification to ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022 covers quality, automotive, medical and information security scopes.
CNC machining key facts at a glance
Figures reflect GreatLight shop capability and standard practice.
| Item | Typical range | Engineering meaning |
|---|---|---|
| Tolerance | ±0.05 mm standard, ±0.005 mm tight | Tight calls need rigid setups and inspection |
| Surface finish | Ra 1.6–3.2 μm as-machined | Fine finish starts at Ra 0.8 μm |
| Fine finish | Ra 0.2–0.8 μm | Adds a finishing pass and time |
| Max part size | 4,000 × 400 × 150 mm | Larger work may need a different process |
| Min wall | About 1 mm | Thinner walls flex and chatter |
| Corner radius | Half the cutter diameter | Small radius needs a small cutter |
| Marking | 1.5 mm min character height | Below that, legibility is not assured |
| Lead time | Ships in 3–5 days | Quote and DFM back within 12 hours |
| Order size | No MOQ | From one prototype to 10,000+ parts |
The short version
If the part is simple, prismatic and forgiving, ask for 3-axis machining and a ±0.05 mm tolerance; if it has features on several faces, thin walls or an axis-critical bore, pay for 5-axis and a controlled finish. Tightening a callout you cannot measure adds cost and buys nothing.
Questions engineers ask after the basics
How tight a tolerance can CNC machining actually hold?
On suitable geometry with stable fixturing, ±0.005 mm is achievable on our equipment, and we also write it as ±0.0002 in.
The limit is set by part stiffness, not by the machine alone. Long thin parts, unsupported walls and features far from a datum will not hold that band no matter how the program is written.
Do I need 5-axis machining for a part with angled holes?
Not always. A single angled face can be cut on a 3-axis machine with an angled fixture, and that is often cheaper.
5-axis becomes the better route when the part has features on several faces, blended surfaces or undercuts that would otherwise need three or four re-clamps.
What surface finish should I specify?
Call out only what the function needs. Ra 1.6–3.2 μm is normal as-machined work; Ra 0.8–1.6 μm suits most sliding and sealing surfaces.
Ra 0.2–0.8 μm is available but adds a finishing pass. A cosmetically fine surface on a hidden face is cost with no benefit.
How are uploads and drawings handled?
Uploads are secure and confidential. We can work under an NDA on request before any file is shared.
We hold ISO 27001:2022 for information security, which is the certification that covers how data is stored and accessed.
Can you machine one prototype and then scale to production?
Yes. There is no minimum order quantity, so a single prototype and a 10,000+ part run go through the same quoting route.
The useful step is a design freeze before the production run, because a change after programming means reprogramming and re-fixturing.
What information do you need to quote accurately?
A 3D model or a fully dimensioned 2D drawing, the material and temper, the tolerance and finish callouts, and the quantity.
If a datum or a tight feature is ambiguous, tell us how the part is used. The function usually decides the tolerance faster than a stack of notes.
Put these facts against your part
Send a model or drawing and we will return a quotation with a free DFM analysis within 12 hours, including notes on any callout that cannot be measured.
12-hour quote100% inspectionNo MOQ
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