Understanding CNC Machining Tolerances and Surface Finish
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Introduction
Two of the most common questions engineers ask when sourcing CNC machined parts are: "What tolerances can you hold?" and "What surface finish will I get?" The answers aren't always straightforward — they depend on material, geometry, and process selection.
Specifying unnecessarily tight tolerances or premium surface finishes is one of the fastest ways to inflate manufacturing costs. This guide helps you understand what's achievable, when precision actually matters, and how to communicate your requirements clearly.
1. CNC Machining Tolerances Explained
What Is a Tolerance?
A tolerance defines the acceptable range of deviation from a nominal dimension. For example, a 10mm shaft with a tolerance of ±0.05mm means the actual part can measure anywhere from 9.95mm to 10.05mm and still pass inspection.
Standard CNC Tolerance Levels
| Tolerance Grade | Range (Metric) | Range (Imperial) | Typical Application |
|---|---|---|---|
| **General** | ±0.125mm | ±0.005" | Non-critical dimensions, clearance holes |
| **Standard** | ±0.05mm | ±0.002" | Most machined features, general fits |
| **Precision** | ±0.025mm | ±0.001" | Bearing fits, mating surfaces, dowel holes |
| **High Precision** | ±0.01mm | ±0.0004" | Aerospace, medical, optical components |
| **Ultra Precision** | ±0.005mm | ±0.0002" | Gauge blocks, precision spindles (special process) |
The Cost of Tight Tolerances
Every time you halve the tolerance band, manufacturing cost can double or triple:
| Tolerance | Relative Cost | Why |
|---|---|---|
| ±0.125mm | 1× (baseline) | Standard machining, any shop |
| ±0.05mm | 1.5× | More inspection, slower feeds |
| ±0.025mm | 2.5× | Specialized fixturing, temperature control |
| ±0.01mm | 5×+ | Climate-controlled environment, specialized equipment |
| ±0.005mm | 10×+ | Grinding/EDM required, extensive inspection |
Rule of thumb: Only specify tight tolerances on features that actually need them — mating surfaces, bearing seats, locating features. Use general tolerances for everything else.
2. GD&T Quick Reference
Geometric Dimensioning and Tolerancing (GD&T) goes beyond simple ± dimensions to control form, orientation, and position.
Common GD&T Symbols for CNC Machining
| Symbol | Name | What It Controls |
|---|---|---|
| ⊥ | Perpendicularity | 90° relationship between features |
| ∥ | Parallelism | Parallel relationship between surfaces |
| ◎ | Concentricity | Shared center axis of cylindrical features |
| ⌖ | Position | True position of features (holes, slots) |
| ○ | Circularity | Roundness of cylindrical features |
| ▱ | Flatness | Flatness of a surface |
| ⟂ | Profile | Surface contour control |
GD&T Best Practices for CNC Parts
- **Use datums**: Always define primary, secondary, and tertiary datums
- **Avoid over-constraining**: Don’t apply GD&T to features that don’t need it
- **Consider inspection**: Can your GD&T callouts actually be measured?
- **Position over ±**: Use true position for hole locations instead of ± dimensions
3. Surface Finish (Roughness) Guide
Surface finish is measured in Ra (Roughness Average), typically in micrometers (μm) or microinches (μin).
Common Ra Values for CNC Machining
| Ra (μm) | Ra (μin) | Process | Appearance | Typical Use |
|---|---|---|---|---|
| 6.3 | 250 | Rough machining | Visible tool marks | Non-critical surfaces |
| 3.2 | 125 | Standard machining | Fine tool marks visible | General machined surfaces |
| 1.6 | 63 | Fine machining | Smooth, minor marks | Sealing surfaces, bearing seats |
| 0.8 | 32 | Precision machining | Very smooth | Hydraulic seals, precision fits |
| 0.4 | 16 | Grinding/Polishing | Mirror-like | Optical, medical implants |
| 0.2 | 8 | Super-finishing | Mirror | Precision gauges, optics |
Surface Finish by Material
Different materials achieve different finishes with the same process:
| Material | As-Machined Ra | Best Achievable Ra |
|---|---|---|
| Aluminum 6061 | 1.6-3.2 μm | 0.2 μm (polished) |
| Stainless Steel 304 | 1.6-3.2 μm | 0.4 μm |
| Brass C360 | 0.8-1.6 μm | 0.2 μm |
| POM (Delrin) | 0.8-1.6 μm | 0.4 μm |
| PEEK | 1.6-3.2 μm | 0.8 μm |
| Titanium | 1.6-3.2 μm | 0.4 μm |
4. Surface Finish Options Beyond Machining
Anodizing (Aluminum)
- **Type II**: Decorative, 5-25μm thickness, multiple colors
- **Type III (Hardcoat)**: 25-150μm, wear-resistant, dark gray/black
- **Effect on dimensions**: Adds approximately 50% of coating thickness
Plating
- **Zinc plating**: Corrosion protection for steel, 5-25μm
- **Nickel plating**: Wear + corrosion resistance, 10-50μm
- **Chrome plating**: Decorative or hard chrome for wear surfaces
Coating
- **Powder coating**: 60-120μm, durable colored finish for steel/aluminum
- **Painting**: Wet paint, thinner than powder coat
Passivation (Stainless Steel)
- Chemical treatment that removes free iron, improving corrosion resistance
- No dimensional change
Bead Blasting
- Creates uniform matte finish
- Often used before anodizing
- Removes minor tool marks
5. How to Specify Tolerances and Finishes
On Your Drawing
- **Title block**: Define general tolerances (e.g., “Unless specified: ±0.125mm”)
- **Critical dimensions**: Call out specific tolerances only where needed
- **Surface finish symbol**: Use the check mark symbol with Ra value
- **GD&T frame**: Add feature control frames for geometric requirements
In Your RFQ
Include a clear note like:
> "General tolerance: ±0.125mm. Critical dimensions marked with specific tolerances on drawing. Surface finish: 3.2μm Ra as-machined except where noted."
Pro Tip: Use a Tolerance Table
Create a simple table on your drawing:
| Feature Type | Tolerance |
|---|---|
| General dimensions | ±0.125mm |
| Bearing bores | H7 (+0.025/-0) |
| Dowel holes | ±0.01mm |
| Threaded holes | ±0.25mm (position) |
| Surface finish (general) | Ra 3.2μm |
| Surface finish (sealing) | Ra 1.6μm |
6. Common Mistakes to Avoid
- **Specifying Ra 0.8 everywhere** — Adds unnecessary cost. Only seal faces and bearing journals need it.
- **Ignoring material limits** — Some plastics can’t achieve the same finish as metals.
- **Forgetting post-processing effects** — Anodizing adds thickness; specify if dimension is pre- or post-finish.
- **Over-tolerancing** — ±0.005mm on a clearance hole for an M6 bolt is wasteful.
- **No inspection method** — If you specify it, make sure it can be measured.
Get Your Parts Right the First Time
At Leland CNC, our engineers review every drawing and flag tolerance/finish issues before machining begins. We'll tell you if a specified tolerance is unnecessary (saving cost) or if a finish won't work with your chosen material.
Related Articles
- CNC Milling vs CNC Turning: Which Process to Choose?
- How to Design Parts for CNC Machining (DFM Guide)
- Anodizing vs Powder Coating vs Plating: Surface Finishes Compared
Published: July 2026 | Leland CNC — Precision Manufacturing Since 2009