What Is Tolerance?
Tolerance is the allowed range of a dimension. No machine cuts a part to a perfect size. Tolerance tells the shop how close they need to get.
A dimension of 1.000" with a tolerance of ±0.005" means the part can be 0.995" to 1.005". Any size in that range passes inspection.
Tighter tolerance means less room for error. The machine must run slower and take lighter cuts. This raises cost.
Three Tolerance Levels
Standard: ±0.005" (±0.127 mm)
This is the default for most CNC shops. If your drawing has no tolerance listed, most shops use ±0.005". It works for clearance holes, mounting surfaces, and general outlines.
Tight: ±0.002" (±0.050 mm)
Use this for parts that fit together. Bearing bores, locating pins, and sliding fits need this level. The machine takes extra passes. The operator checks dimensions more often.
Precision: ±0.001" (±0.025 mm)
This is the tightest level for standard CNC milling. It requires slow feeds, sharp tools, and careful setup. Use it only for critical mating surfaces. Aerospace and medical parts often need this level.
| Level | Tolerance | Metric | Cost Impact | Common Use |
|---|---|---|---|---|
| Standard | ±0.005" | ±0.127 mm | 1x (baseline) | General features, clearance holes |
| Tight | ±0.002" | ±0.050 mm | 1.5–2x | Bearing bores, locating features |
| Precision | ±0.001" | ±0.025 mm | 2–3x | Aerospace, medical, press fits |
| Ultra-precision | ±0.0005" | ±0.013 mm | 3–5x | Optics, semiconductor, sealing surfaces |
Cost Impact
Tolerance is the biggest cost driver in CNC machining. A part with all dimensions at ±0.005" costs far less than the same part at ±0.001".
Here is why tight tolerances cost more:
- Slower cutting. The machine takes lighter cuts at lower feeds.
- More passes. Rough, semi-finish, and finish passes are needed.
- Better tools. Tighter tolerance needs sharper, more expensive tools.
- More inspection. Each tight feature must be measured and logged.
- Higher scrap risk. More parts fail inspection. Scrap cost gets built in.
On most parts, only 20% of features need tight tolerance. The other 80% work fine at standard. Apply tight tolerances only where they matter. This can cut your part cost by 30–50%.
When to Use Each Level
Use Standard (±0.005") For:
- Clearance holes for bolts
- Outer profiles and cosmetic edges
- Mounting surfaces (non-critical)
- Slot widths for wire routing
Use Tight (±0.002") For:
- Bearing bores and seats
- Dowel pin holes
- Sliding fits between mating parts
- Locating features on fixtures
Use Precision (±0.001") For:
- Press-fit bores
- Seal grooves (O-ring glands)
- Aerospace mating surfaces
- Medical device interfaces
Common Feature Tolerances
| Feature | Typical Tolerance | Notes |
|---|---|---|
| Drilled holes | ±0.002" | Reaming gives ±0.0005" |
| Milled pockets | ±0.003"–0.005" | Depends on depth and tool reach |
| Turned diameters | ±0.001" | Lathes hold diameter well |
| Thread pitch | Class 2B (standard) | Class 3B for tight fits |
| Surface flatness | 0.002" over 6" | Grinding gives 0.0005" over 6" |
| Hole position | ±0.005" true position | Tighter needs GD&T callout |
Tips to Save Money
- Use a general tolerance block. Put ±0.005" in your title block. Only call out tighter tolerances where needed.
- Tolerance only what matters. Ask: "Will the part fail if this feature is off by 0.005"?" If not, leave it at standard.
- Use bilateral tolerances. A tolerance of ±0.002" is easier to hit than +0.000/-0.004".
- Avoid tolerance stacking. Dimension from a single datum. This prevents errors from adding up.
- Talk to your machinist. They can tell you which tolerances are easy and which are hard for your specific part.
Do not put ±0.001" on every dimension. This forces the shop to machine every surface slowly. Your part will cost 2–3x more than it needs to. Only apply tight tolerances to features that truly require them.
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Upload CAD for Instant QuoteFrequently Asked Questions
What is standard CNC tolerance?
Standard CNC tolerance is ±0.005" (±0.127 mm). Most shops hold this on every feature unless you ask for tighter. It works for clearance holes, mounting surfaces, and general outlines.
How tight can CNC machining hold?
CNC milling can hold ±0.001" on most features. CNC grinding and lapping can reach ±0.0002". The tighter you go, the more it costs. Most parts only need a few features held to tight tolerances.
Does tighter tolerance cost more?
Yes. Going from ±0.005" to ±0.001" can double or triple the cost of a feature. The machine runs slower, needs more passes, and requires inspection. Only apply tight tolerances where the part truly needs them.
What tolerance should I use for press fits?
Press fits need ±0.0005" or tighter. The hole is sized slightly smaller than the pin or shaft. Common classes include FN1 (light press) and FN2 (medium press). Always specify the fit class on your drawing.
Do I need to tolerance every dimension?
No. Use a general tolerance block like ±0.005" for all dimensions unless noted. Then only call out tighter tolerances on the few features that need them. This approach saves time and money.
