CNC Tolerances Explained: Practical Guide to GD&T for 2026
What does ±0.005 mm really mean? Engineer’s practical guide to CNC tolerances, tolerance stack-up, GD&T basics, how to specify and how to inspect — without inflating cost.
Specifying tolerances is the single biggest lever you have over CNC part cost. Specify too loose and the part doesn’t function. Specify too tight everywhere and you triple the price for no benefit. This guide explains what tolerances mean in real machining terms, how GD&T helps, and how to get them right.
What a tolerance really is
A tolerance is the allowable variation in a dimension. If a drawing says “25.00 ±0.05 mm”, the part is acceptable when the measured dimension falls between 24.95 and 25.05 mm.
Three things determine the achievable tolerance on any given feature:
- Machine capability. A 20-year-old manual mill holds ±0.1 mm at best. A modern Mazak 5-axis cell holds ±0.005 mm comfortably. The machine sets the floor.
- Material behaviour. Aluminium machines stably; titanium springs back; thin-walled stainless deflects under cutter pressure. Material doubles or halves the achievable tolerance.
- Feature geometry. Tolerances on a 5 mm feature near the chuck are easy. Tolerances on a 200 mm-long thin wall are hard. Geometry can make the same nominal dimension 10× harder to hold.
What CNC can actually hold
| Feature type | Standard CNC | Precision CNC | High-end (with care) |
|---|---|---|---|
| External dimensions | ±0.10 mm | ±0.025 mm | ±0.005 mm |
| Hole diameter | ±0.05 mm | ±0.013 mm | ±0.005 mm |
| Threaded holes (pitch) | 6H class fit | 5H class fit | Custom |
| Surface finish (Ra) | 3.2 µm | 0.8 µm | 0.4 µm |
| Flatness (over 100 mm) | 0.05 mm | 0.013 mm | 0.005 mm |
| Parallelism (over 100 mm) | 0.05 mm | 0.013 mm | 0.005 mm |
| Concentricity / true position | 0.05 mm | 0.025 mm | 0.013 mm |
| Angle accuracy | ±0.5° | ±0.1° | ±0.05° |
Tolerance stack-up — the silent cost driver
When several toleranced features have to interact, their individual tolerances add up. This is “tolerance stack-up” — and ignoring it leads to assemblies that fail QA even though every individual part is within spec.
Worked example: a stack of three identical washers, each toleranced ±0.05 mm thick:
Per-washer tolerance
Each washer can be 0.05 mm too thin or 0.05 mm too thick. Range = 0.10 mm per washer.
Worst-case stack
Three washers at the worst end: 3 × 0.05 = 0.15 mm thinner OR 0.15 mm thicker than nominal. Total range: 0.30 mm.
Statistical stack (RSS)
In practice not every washer is at the extreme. Root-Sum-Square gives more realistic ±0.087 mm at 3-sigma.
Design implication
If your assembly needs to fit in a 25 ±0.10 mm slot, 75 ±0.30 mm worst-case won’t fit. You must either tighten individual tolerances OR widen the slot OR redesign.
GD&T (Geometric Dimensioning and Tolerancing) in 5 minutes
GD&T is a symbolic language (ASME Y14.5 / ISO 1101) for specifying not just dimensions but the geometric relationships between features. It’s how aerospace, automotive and medical drawings communicate what really matters.
| Symbol | Name | Controls |
|---|---|---|
| — | Straightness | How straight a line/axis is |
| ◯ (circle) | Circularity / roundness | How round a cross-section is |
| ⌭ | Cylindricity | 3D roundness over the length of a cylinder |
| ▱ (parallelogram) | Flatness | How flat a surface is |
| ∥ | Parallelism | Surface parallel to a datum |
| ⊥ | Perpendicularity | Surface perpendicular to a datum |
| ∠ | Angularity | Surface at a specific angle to a datum |
| ⌖ | True position | Where a feature is, relative to datums |
| ◎ | Concentricity | Whether two cylinders share an axis |
| ⌭ R | Runout | Combined error during rotation |
| Σ | Profile | Allowable variation of a curved surface |
Two key concepts make GD&T more powerful than “plus-minus” tolerancing:
Datums
- A datum (A, B, C…) is a feature you reference everything else from.
- Establishes a coordinate system on the part.
- Without datums, “perpendicular” is ambiguous — perpendicular to what?
Bonus tolerance
- GD&T allows extra tolerance when a feature is at maximum material condition (MMC).
- A hole at its smallest allowed size has more “bonus” positional tolerance.
- Lets the shop produce in-spec parts that “plus-minus” alone would reject.
How to specify tolerances on a drawing
Use a title-block default
Top right of the drawing: “General tolerance: ISO 2768-mK” (or ASME equivalent). Now most dimensions don’t need explicit tolerances.
Tolerance only what matters
Mating dimensions, sealing faces, bearing seats, datums. Leave decorative or non-functional features at the default.
Use the tightest tolerance only where required
A typical optimised drawing has 3–5 features at ±0.025 mm and the rest at ±0.1 mm default.
Specify surface finish where it matters
Use the standard finish symbol (✓ with Ra value) on the surfaces that need it. “Ra 0.8” on a sealing face; rest defaults.
Add datums for GD&T-controlled features
Pick the most-stable, most-machined surface as datum A. Usually a large flat face. Datum B and C are perpendicular to A.
How tolerances are verified
| Tool | Best for | Tolerance reach |
|---|---|---|
| Steel rule | Rough check during machining | ±0.5 mm |
| Vernier / digital caliper | General-purpose checking | ±0.05 mm |
| Micrometer | External diameters, thicknesses | ±0.005 mm |
| Bore gauge | Internal diameters | ±0.005 mm |
| Pin gauge / plug gauge | Hole sizes (go / no-go) | IT class fit |
| Height gauge with indicator | Heights, perpendicularity | ±0.01 mm |
| Surface plate + indicator | Flatness, parallelism over 100s of mm | ±0.005 mm |
| CMM (Coordinate Measuring Machine) | Complex features, GD&T verification, FAI | ±0.002 mm |
| Optical comparator | Profiles, threads, sharp corners | ±0.005 mm |
| Surface roughness tester | Ra, Rz measurements | 0.01 µm |
Cost impact of tightening tolerances
Indicative cost multiplier vs the same feature at default ±0.10 mm:
| Tolerance | Cost multiplier | Why |
|---|---|---|
| ±0.10 mm (default) | 1.0× | Standard cycle, hand-held inspection. |
| ±0.05 mm | 1.2× | Slightly slower cuts, may need micrometer. |
| ±0.025 mm | 1.5× | Quality CNC machine, micrometer or bore gauge inspection. |
| ±0.013 mm | 2× | Modern 5-axis or grinding machine, CMM verification. |
| ±0.005 mm | 3–5× | Top-tier machine, climate-controlled cell, full CMM, sometimes hand finishing. |
| ±0.002 mm | 10×+ | Hand lapping, jig grinding, hours of inspection per part. |
Common mistakes to avoid
- Tightening every dimension to ±0.005 mm. The classic rookie mistake. Triples cost for no functional benefit.
- No general-tolerance call-out. Without ISO 2768 in the title block, every dimension becomes ambiguous and the shop will quote conservatively.
- Toleranced angles smaller than ±0.5°. Most CNC mills handle ±0.5° easily. Tighter angles often require fixturing or grinding.
- Demanding mirror surface (Ra 0.05) on functional surfaces. Polishing adds significant cost and lead time. Use Ra 0.8 unless optical or sealing function actually requires better.
- Overlapping datum schemes. If A is the bottom and B is the side, don’t also reference the bottom as B somewhere else. Pick a clean A-B-C scheme and stick with it.
- Forgetting MMC modifiers. True position with no modifier is the strictest interpretation. Adding Ⓜ (MMC) gives the shop legitimate bonus tolerance — use it where appropriate.
- Tolerancing what can’t be measured. If the only inspection tool that can verify the spec costs $250k, expect to pay for that inspection.
Frequently Asked Questions
- 0.005 mm is roughly 1/10 the thickness of a human hair. You can’t see it, you can’t feel it with your finger, and you can’t measure it with a hand-held caliper. It requires a calibrated micrometer at minimum, ideally a CMM. This is why it’s expensive.
- Plus-minus is fine for non-mating consumer parts. Use GD&T when: function depends on relationships between features (concentric shafts, perpendicular faces), parts mate to assemblies, or the customer is in regulated industries (aerospace, medical, automotive). GD&T is more precise but takes more drawing skill.
- ISO 2768 is the international standard for “general tolerances” — sane defaults for any dimension you don’t explicitly tolerance. Class “m” (medium) is the most common: ±0.1 mm for 0–6 mm features, scaling up for larger ones. Almost every CNC drawing benefits from a 2768-mK call-out in the title block.
- Yes, on the right material and geometry. We have multiple 5-axis cells and a temperature-controlled CMM. For aerospace and medical work we routinely hit ±0.005 mm with 100% verification. Cost is 3–5× a standard ±0.025 mm part.
- Hand tools (calipers, micrometers) are cheap and fast for individual dimensions in production batches. CMM is required for true position, multi-feature relationships, free-form surfaces, and FAI documentation. Most jobs use both — micrometers for in-process checks, CMM for sample inspection.
- A documented inspection of the very first part produced, verifying every dimension and feature on the drawing matches the CAD model. AS9102-compliant FAI is required for aerospace orders. JLYPT issues FAI reports as a standard option — specify at quote time.
- For specific features yes — ground bearing seats, lapped sealing faces, jig-bored holes can reach ±0.002 mm. But these are specialty operations adding significant cost and lead time. If you genuinely need this precision for production, talk to us early via the contact form.
What does ±0.005 mm really feel like?
Should I always specify GD&T or is plus-minus enough?
What is ISO 2768 and should I use it?
Can JLYPT hit ±0.005 mm consistently?
How do I choose between micrometer and CMM inspection?
What is a First Article Inspection (FAI)?
Can I get tolerances tighter than ±0.005 mm?
About the author
JLYPT Engineering Team
Senior CNC Application Engineers
Our application engineering team brings 15+ years of combined experience producing precision components for aerospace, medical, robotics and industrial automation customers.
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