Sheet Metal Tolerance Guide
Cutting, bending, and forming — what to expect
Sheet metal fabrication combines multiple operations (cutting, bending, punching, welding), and each adds tolerance stack-up. Understanding per-operation tolerances helps you design parts that fit together reliably without over-specifying — which drives up cost and lead time.
Overview
Laser cutting is the most precise operation (±0.05 mm). Bending introduces the most variation (±0.10–0.25 mm per bend). Welded assemblies stack all tolerances together. Smart design sequences critical features in as few operations as possible.
Tolerance Specifications
| Feature | Standard | Precision | Notes |
|---|---|---|---|
| Laser Cut Dimensions | ±0.10 mm | ±0.05 mm | Fiber lasers are most precise. CO2 lasers slightly wider kerf. Thickness affects precision. |
| Punched Hole Position | ±0.10 mm | ±0.05 mm | CNC turret punch. Position accuracy depends on sheet size and number of hits. |
| Bend Angle | ±0.5° | ±0.25° | CNC press brake with back gauge. Springback varies by material and grain direction. |
| Bend Position (Bend Line) | ±0.15 mm | ±0.08 mm | Measured from nearest feature to bend line. More bends = more stack-up. |
| Hole-to-Bend Distance | ±0.25 mm | ±0.15 mm | Combination of cut and bend tolerance. Keep holes away from bends (min 2× material thickness). |
| Flatness (Before Bending) | 0.5 mm per 300 mm | 0.25 mm per 300 mm | Sheet stock has inherent flatness variation. Leveling helps but adds cost. |
| Welded Assembly | ±0.5 mm | ±0.25 mm | Heat distortion is the main factor. Fixture welding and sequence control improve results. |
| Hardware Insert Position | ±0.15 mm | ±0.08 mm | PEM inserts pressed after cutting. Position inherits cut tolerance. |
| Overall Size (After Bending) | ±0.25 mm per bend | ±0.15 mm per bend | Each bend adds its tolerance to overall size. 4 bends = ±1.0 mm standard. |
Key Considerations
Minimize Bend Count
Each bend adds ±0.15–0.25 mm. Reducing from 6 bends to 4 can tighten overall tolerance by 0.5 mm without changing specs.
Self-Locating Features
Design tabs, slots, and interlocking features so parts align during welding. This removes dependence on fixture accuracy.
Critical Dimensions from One Operation
If two holes must be precisely spaced, ensure they're punched or laser-cut in the same operation — not one before and one after bending.
Slots > Holes for Assembly
Use slots (elongated holes) where possible to absorb tolerance stack-up in multi-part assemblies.
💰 Cost Impact of Tolerances
Precision sheet metal requires slow cutting speeds, per-part inspection, and sometimes secondary machining on bends — adding 25–40% to fabrication cost.
Standard tolerances are achievable on any modern CNC laser + press brake setup. No premium.
For assemblies with multiple bends, design clearance holes (0.5 mm oversize) instead of specifying tight position tolerances. Saves significant cost and still assembles perfectly.
⚠️ Common Tolerance Mistakes
- ⚠ Specifying ±0.1 mm on overall dimensions of a part with 4+ bends — the tolerance stack-up makes this physically impossible without machining.
- ⚠ Placing critical hole patterns near bend lines — holes deform if closer than 2× material thickness to a bend.
- ⚠ Ignoring grain direction effects on bend quality — bending parallel to grain causes cracking in certain alloys.
- ⚠ Over-tolerancing cosmetic panels that will be painted — paint adds 0.05–0.10 mm to dimensions.
- ⚠ Expecting weldment flatness without calling out stress-relief or straightening operations.
💡 Pro Tips
- ▸ Use bend relief cuts (small slots at bend terminations) to prevent tearing and improve bend consistency.
- ▸ For high-volume sheet metal, progressive dies hold tighter tolerances than individual press brake operations — but tooling cost is higher.
- ▸ Our sheet metal factories in Vietnam operate fiber lasers (Trumpf, Bystronic) that match Western precision at 40–50% lower cost.
- ▸ Request a first-article with CMM bend angle measurements to validate tooling setup before production.
- ▸ Specify the K-factor or bend allowance in your drawing to avoid manufacturing interpretation differences.
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