CNC加工公差を理解する
公差とは、製造された特徴に対して許容される寸法変動の範囲です。 If a shaft is designed at 25.000 mm with a tolerance of ±0.01 mm, any shaft measuring between 24.990 mm and 25.010 mm is acceptable. Tolerances exist because perfect accuracy is physically impossible — and unnecessary: most parts function perfectly within a defined range.
Tighter tolerances cost more — not because precision is inherently expensive, but because it requires slower feed rates, additional finishing passes, higher-grade tooling, and more time on CMM inspection. A ±0.005 mm hole might require a finish boring operation and 100% CMM inspection where a ±0.05 mm hole is inspected by sampling. Specify only what function demands.
The table below covers Ginwate's standard, tight and precision tolerance capabilities across our main processes. Where no tolerance is called out on a drawing, we default to ISO 2768-m (medium) for linear dimensions and ISO 2768-K for geometric tolerances.
特徴別の公差能力
主要な特徴タイプ全体の標準・精密公差範囲。 Values for aluminum unless otherwise noted.
| Feature | Standard | Tight | Precision |
|---|---|---|---|
| General Linear Dimensions | ±0.1 mm | ±0.01 mm | ±0.005 mm |
| Hole Diameter (milled) | ±0.05 mm | ±0.01 mm | ±0.005 mm |
| Shaft / Pin Diameter (turned) | ±0.05 mm | ±0.005 mm | ±0.003 mm |
| Bore Diameter (turned) | ±0.05 mm | ±0.005 mm | ±0.003 mm |
| Swiss CNC Diameter | ±0.01 mm | ±0.005 mm | ±0.003 mm |
| 5-Axis Compound Features | ±0.02 mm | ±0.005 mm | ±0.001 mm |
| Flatness | 0.1 mm / 100mm | 0.02 mm / 100mm | 0.005 mm / 100mm |
| Parallelism | 0.1 mm | 0.02 mm | 0.005 mm |
| Perpendicularity | 0.1 mm | 0.02 mm | 0.01 mm |
| Concentricity / Runout | 0.05 mm TIR | 0.01 mm TIR | 0.005 mm TIR |
| True Position (milled hole) | ±0.1 mm dia. | ±0.02 mm dia. | ±0.01 mm dia. |
| Thread Pitch Diameter | 6H / 6g class | 5H / 5g class | 4H / 4g class |
| Surface Finish Ra | 1.6–3.2 μm | 0.8 μm | 0.4 μm |
| Surface Finish Rz | 12.5 μm | 6.3 μm | 1.6 μm |
| Angular Tolerance | ±0.5° | ±0.1° | ±0.05° |
Precision tolerances require DFM review, additional inspection and may increase lead time. Availability depends on material, part geometry and feature size. Contact us to confirm.
ISO公差等級
ISO 2768は線寸法の一般公差を定義します。高い精度等級はコストが高く、リードタイムが延びます。
表面粗さ（Ra値）
Ra（算術平均粗さ）は最も一般的な表面仕上げ指定です。標準Ra 0.4μm、精密研削でRa 0.2μmを達成します。
幾何公差（GD&T）
GD&Tは線寸法を超えた設計意図を伝えるための言語を提供します。
Referenced to a machined datum face. Use 5-axis for compound angle perpendicularity.
Requires precision datum registration. Achievable on grinding to 0.002 mm.
Affected by fixturing spring and material stress relief. Grinding recommended for tight flatness.
Measured in CMM. Achievable through cylindrical grinding after turning.
Combined roundness + straightness. Grinding post-turning for <0.005 mm.
Use coordinate boring or jig boring for ≤0.02 mm. 5-axis reduces repositioning error.
Measured during turning or on CMM. Grinding after turning for <0.005 mm TIR.
Full surface runout. Requires accurate live-centre between-centres turning.
5-axis machining and tilted-head boring for tight angular features.
Complex surface profiles verified on CMM. 5-axis recommended for bilateral tolerance.
All GD&T measurements performed on Zeiss CMM with Calypso measurement software. First-article inspection reports available with FAIR documentation.
材料別の達成可能公差
Material properties — thermal expansion, machinability, and springback — affect achievable tolerances. Here are the tightest tolerances we routinely hold for each material.
| Material | Tightest Achievable | Notes |
|---|---|---|
| Aluminum (6061, 7075) | ±0.003 mm | Best machinability. Standard tight tolerances routinely achieved. |
| Brass (C360) | ±0.003 mm | Free-machining. Excellent dimensional stability. |
| Stainless Steel 303 | ±0.005 mm | Good machinability vs other SS grades. Work-hardening limited. |
| Stainless Steel 316L | ±0.005 mm | Work-hardens. Requires sharp tools and controlled feeds. |
| Titanium Ti-6Al-4V | ±0.005 mm | Low thermal conductivity increases tool wear. Achievable with correct tooling. |
| 4140 Alloy Steel | ±0.005 mm | Heat treat distortion possible — machine after final heat treatment where possible. |
| PEEK | ±0.01 mm | Plastic creep under clamping load. Requires careful fixturing strategy. |
| Delrin (POM) | ±0.01 mm | Absorbs moisture — allow for environmental dimensional change in tight-fit applications. |
公差設計のヒント
これらの原則を適用することでコストとリードタイムを削減し、機能要件を満たす部品を確保できます。
- Apply tolerances only where functionally required — tighter-than-necessary tolerances increase cost significantly (±0.01 mm may cost 2–5× more than ±0.1 mm for the same feature).
- Use GD&T rather than coordinate tolerances for mating features — true position controls location far better than bilateral +/- dimensions on X and Y separately.
- Provide a datum hierarchy on your 2D drawing — without clear datums, our engineers cannot guarantee that measured dimensions will match design intent.
- Specify surface finish only on functional surfaces — 'all surfaces Ra 0.8' adds unnecessary cost and lead time. Leave non-functional faces as machined.
- Design features to be accessible from one setup direction where possible — repositioning introduces additional error. 5-axis helps when this is unavoidable.
- Consider material spring-back for thin-walled parts — aluminum walls <2 mm and titanium walls <1.5 mm can deflect during cutting, effectively moving the surface from its nominal position.
- Thread callouts must specify form, class and depth — 'M8 thread' alone is ambiguous. Use 'M8×1.25 – 6H × 20mm deep' to be complete.
- For press or interference fits, specify the fit class (H7/p6, H7/n6) and let us calculate the actual dimension limits — ISO 286 fit notation is unambiguous and internationally understood.
検査レポート対応可能
- First Article Inspection Report (FAIR)
- CMM Dimensional Report (Zeiss)
- Surface Roughness Certificate (Ra/Rz)
- Material Test Report (MTR / Mill Cert)
- PPAP Level 1–3 Documentation
- AS9102 First Article Inspection
