CNC Machining Tolerances: ISO 2768 vs GD&T and What You Can Actually Hold
Short answer: a note like "ISO 2768-mK" on your drawing is loose — it allows ±0.1 mm on small features and ±0.5 mm on 100 mm parts, plus form tolerances that most engineers never think about. Real production CNC machining holds ±0.01 mm comfortably and ±0.005 mm on critical features with CMM verification. General tolerances describe leftovers; GD&T describes what has to work. If a hole must align with a mating part, call it out explicitly or you are trusting luck.
Tolerance language confuses more engineers than any other part of a drawing. The funny thing is the vocabulary is small. Once you understand ISO 2768 general tolerances and the handful of GD&T symbols that matter, you can read any drawing and — more usefully — write drawings that machine shops quote honestly and hit on the first batch.
What Does ISO 2768-mK Mean on a Drawing?
ISO 2768 is the shorthand that says "dimensions without a specific tolerance callout follow these limits." It has two parts. ISO 2768-1 covers linear and angular dimensions, with tolerance classes f (fine), m (medium), c (coarse), and v (very coarse). ISO 2768-2 covers geometrical tolerances — flatness, straightness, perpendicularity, symmetry, runout — with classes H, K, and L.
So "ISO 2768-mK" reads: linear and angular dimensions per class m, geometrical tolerances per class K. It is the most common note on real production drawings, and it is not tight:
| Nominal size range (mm) | Class f (±mm) | Class m (±mm) | Class c (±mm) | Class v (±mm) |
|---|---|---|---|---|
| 0.5 to 3 | 0.05 | 0.1 | 0.2 | 0.5 |
| 3 to 6 | 0.05 | 0.1 | 0.2 | 0.5 |
| 6 to 30 | 0.1 | 0.2 | 0.5 | 1.0 |
| 30 to 120 | 0.15 | 0.3 | 0.8 | 1.5 |
| 120 to 400 | 0.2 | 0.5 | 1.2 | 2.5 |
| 400 to 1000 | 0.3 | 0.8 | 2.0 | 4.0 |
Read that middle column: an m-class part of 100 mm length may be off by ±0.3 mm. For most housings and brackets that is fine. For a bearing bore, a locating pin, or anything that mates with another component, it is not — which is why parts that must actually fit each other carry explicit callouts or GD&T instead of relying on the title block note.
What Are the GD&T Symbols You Actually Need?
GD&T (Geometric Dimensioning and Tolerancing) is a symbol language for controlling form, orientation, and position. You do not need all of it. A handful of characteristics covers 90% of machined part requirements:
| Symbol | Controls | What it means in practice | Typical achievable on CNC |
|---|---|---|---|
| Straightness | Form of a line | Surface must not bow or wave along its length | 0.01 mm easy, tighter with grinding |
| Flatness | Form of a surface | Surface must sit in two parallel planes a set distance apart | 0.02 mm over 100 mm typical |
| Circularity | Form of a circle | Roundness of a bore or shaft cross-section | 0.01 mm |
| Cylindricity | Form of a cylinder | Roundness plus straightness along a bore or shaft | 0.01–0.02 mm |
| Perpendicularity | Orientation | Surface or hole square to a datum | 0.02 mm typical |
| Parallelism | Orientation | Two surfaces or axes stay parallel | 0.02 mm typical |
| Position | Location | True position of a hole center relative to datums | ±0.01–0.02 mm |
| Runout | Composite | Total wobble of a rotating feature | 0.02 mm typical |
The core idea: a position or perpendicularity callout ties a feature to a datum, so the part is inspected the same way it is assembled. That is the whole point — GD&T exists to communicate function, not to decorate the drawing.
What Can CNC Machining Actually Hold in Production?
Machines are better than most people believe. A rigid CNC mill or lathe in a temperature-controlled shop repeats within a few microns on a good day. The practical limits come from fixturing, tool wear, material movement, and measurement — not the machine's positioning spec.
| Tolerance demand | What a good shop holds | Where it gets hard |
|---|---|---|
| ±0.1 mm | Trivially, all features | Nowhere |
| ±0.05 mm | Standard, every feature | Nothing special needed |
| ±0.01 mm | Routinely, on defined features | Thin walls, long parts, soft materials |
| ±0.005 mm | Production capability, CMM verified | Material stability, temperature, fixture rigidity |
| ±0.002 mm and tighter | Grinding, honing, or lapping territory | Process changes, long lead time, high cost |
Our production line holds ±0.005 mm on critical features in stable materials like aluminum and steel, and every batch ships with a dimensional inspection report so the number is checkable, not claimable. It is the same production floor covered in our CNC machining services in China guide, where turning, milling, and precision work share one plant. Parts built to that standard are exactly the kind of CNC precision components we make every week.
How Do Tighter Tolerances Change Cost and Lead Time?
Tolerance is bought with time. Cutting to ±0.1 mm means one pass, full speed. Cutting to ±0.005 mm means slower feeds, lighter cuts, more tool changes, more measurement stops, and occasional scrapped parts when material moves. The jump from ±0.1 mm to ±0.01 mm can add 30–80% to machining time; below ±0.005 mm the curve goes nearly vertical because you leave ordinary CNC and enter grinding territory.
That cost lands in two places: unit price and lead time. A tightly toleranced batch takes longer on the machine and longer in inspection. The practical rule: tolerance only the features that interface with something else, and let general tolerances handle the rest. If your design needs a ±0.005 mm bore, fine — it will be machined and CMM-checked. If it has a cosmetic pocket that does not matter, say ±0.2 mm and keep the price down. Our CNC machining cost guide shows exactly how each tolerance class moves the quote.
How Should You Call Out Tolerances on a Machined Part Drawing?
Write the drawing the way you would explain the part to a machinist over the phone. Start with the ISO 2768-mK title block note for everything ordinary. Then add explicit ± tolerances or GD&T only where function demands it: mating bores, locating features, press-fit diameters, datum surfaces, hole patterns that must align.
Three habits prevent most tolerance headaches. First, pick datums that match assembly — if a bracket bolts to a frame, datum off the bolt face, not a random edge. Second, remember tolerance stack-up: three features each at ±0.1 mm can land 0.3 mm away from where you wanted, which is why position callouts to a single datum beat chains of ± dimensions. Third, avoid the "tight everything" reflex — every unnecessary micron raises the quote, and our quote checklist will show you where. When in doubt, send us the drawing: turning parts and milled parts alike get quoted at your real tolerances within 12 hours, and if a callout only drives cost up, we will say so.
Email sc@bquq.com or WhatsApp +86 137 1315 7787 with your PDF/DXF/STEP file. An engineer reviews it and replies with price, lead time and DFM notes on working days.
Which CNC process fits your part? (Decision tree)
| If your part... | Choose | Why |
|---|---|---|
| Is round or has turned features OD/ID/threads | CNC turning | One-setup turning of round features |
| Is prismatic with pockets, flats or slots | CNC milling (3/4-axis) | Multi-face access |
| Is under 32 mm diameter and long (L/D over 5) | Swiss-type turning | Guide bushing supports the cut |
| Has complex 3D contours on several faces | 5-axis machining | Fewer setups, better accuracy |
| Needs tighter than +/-0.005 mm or a fine finish | Add grinding or lapping | Machining alone reaches its limit |
| Runs in thousands and the shape is simple | Consider stamping instead | Lower unit cost at volume |
Frequently Asked Questions
Is ISO 2768-mK good enough for my machined parts?
A: For most housings, brackets, and non-mating features, yes. Class m allows ±0.1 to ±0.5 mm depending on size. Any feature that must fit, align, or seal with another part needs an explicit callout or GD&T on top.
What is the difference between a tolerance and GD&T?
A: A ± tolerance controls a single dimension. GD&T controls the relationship between features — position of a hole to a datum, flatness of a sealing face, perpendicularity of a bore to a surface. Use GD&T when assembly function depends on more than individual sizes.
What is the tightest tolerance CNC machining can hold?
A: In production, ±0.005 mm on critical features is the practical boundary for CNC with CMM verification. Beyond that you need grinding, honing, or lapping, which changes the process and the price significantly.
Why does my machined part cost more when I add tolerances?
A: Tighter tolerance means slower machining, more setups, more inspection, and more scrap risk. Going from ±0.1 mm to ±0.01 mm can add 30–80% to machining time. Tolerance only the features that need it.
Does a tighter tolerance on the drawing guarantee better parts?
A: No. It guarantees a higher quote. Quality comes from process: rigid machines, correct tooling, and inspection. A loose drawing at a disciplined shop beats a tight drawing at a careless one — that is why we CMM-check and ship reports with every batch.
Related Articles
- How Much Does CNC Machining Cost? Factory Pricing Factors (2026 Guide) — CNC machining cost explained by a China source factory: material, tolerance, quantity, finishing, setup. Real price ranges and a quote checklist. Get a quote in 12 hours.
- CNC Machining Services in China: Capabilities, Tolerances & Lead Times — CNC machining services from a Dongguan source factory: ±0.005 mm capability, ISO9001, common materials and finishes, real lead times. Quote in 12 hours.
Data Sources and Verification
Tolerances, cycle times and price ranges in this guide come from BQUQ production records at our Dongguan plant, where CNC machining (±0.005 mm), stamping, custom springs and heat sinks run under one roof. BQUQ is an ISO 9001:2015 certified factory; the certificate and batch inspection reports are available on request with every quotation.
Related Resources
- About BQUQ: an ISO9001-certified source factory in Dongguan running four production lines under one roof.
- CNC products and services: browse the CNC turning parts, CNC milling parts, and CNC precision components we machine every day.
- Industry trends: manufacturing, material market, and sourcing analysis for buyers.
- Technical articles: engineering guides on CNC, heat sinks, springs, and stamping — more where this one came from.
- FAQ hub: quick answers on CNC, stamping, springs, and heat sinks.
- Case studies: real parts and real numbers from projects we engineered and delivered.
- Contact us: send your drawing and get a quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
