Technical
Metal Parts Tolerance Guide: ISO 2768, GD&T Standards and How to Avoid Costly Mistakes
Master ISO 2768 tolerance classes, GD&T fundamentals, and the 5 most expensive tolerance mistakes engineers make — and how to avoid them.
Tolerances are the silent cost driver in metal parts manufacturing. A single overly-tight tolerance on a non-functional surface can double your part cost — and many engineers don’t realize it until the quote arrives. This guide explains everything you need to know about specifying tolerances correctly.
1. What Is ISO 2768? The Global Standard for General Tolerances
ISO 2768 is the international standard that defines general tolerances for linear and angular dimensions when individual tolerances are not specified on the drawing. If your drawing says a dimension is “50mm” without a ± value, ISO 2768 fills in the allowed deviation.
| Tolerance Class | Designation | 0.5–3mm | 3–6mm | 6–30mm | 30–120mm | Typical Application |
|---|---|---|---|---|---|---|
| Fine | f | ±0.05 | ±0.05 | ±0.1 | ±0.15 | Precision instruments, aerospace |
| Medium | m | ±0.1 | ±0.1 | ±0.2 | ±0.3 | General mechanical engineering (most common) |
| Coarse | c | ±0.2 | ±0.3 | ±0.5 | ±0.8 | Stamped parts, weldments, low-precision |
| Very Coarse | v | — | ±0.5 | ±1.0 | ±1.5 | Structural steel, non-critical |
For most CNC-machined parts, specifying ISO 2768-m (medium) with critical features individually toleranced at ±0.05mm is the cost-optimal approach. Over-specifying to ISO 2768-f globally can increase costs by 30–50% without meaningful functional benefit.
2. GD&T Basics: What Every Engineer Needs to Know
Geometric Dimensioning and Tolerancing (GD&T) goes beyond simple ± dimensions to control form, orientation, location, and runout. Here are the most commonly used GD&T symbols in metal parts manufacturing:
| Symbol | Name | What It Controls | Example Application | Typical Value |
|---|---|---|---|---|
| ⟂ | Perpendicularity | 90° relationship between features | Boss relative to mounting face | 0.05mm |
| ◎ | Concentricity | Coaxial alignment | Bearing bore to shaft | 0.03mm |
| ⌖ | Position | True position of features | Hole pattern location | 0.1mm @ MMC |
| ∥ | Parallelism | Parallel relationship | Guide rail surfaces | 0.03mm |
| ⌓ | Flatness | Surface deviation from plane | Sealing surfaces | 0.05mm |
| ↗ | Runout | Circular/Total deviation during rotation | Shafts, pulleys | 0.03mm |
3. The 5 Most Expensive Tolerance Mistakes (And How to Avoid Them)
Mistake #1: Over-Tolerancing Everything
Applying ±0.01mm to every dimension on a drawing is the fastest way to double your machining cost. CNC shops charge more for tight tolerances because they require slower feeds, specialized tooling, temperature-controlled environments, and 100% inspection. Fix: Reserve tight tolerances only for functional interfaces — bearing fits, seal grooves, alignment features. Everything else can use ISO 2768-m.
Mistake #2: Ignoring Material Behavior
Different materials hold tolerances differently. Aluminum warps more than steel during machining. Plastics expand with temperature. Brass is gummy and can deflect tools. Fix: Account for material properties when specifying tolerances. Stainless steel can hold ±0.01mm; aluminum typically needs ±0.02mm minimum for reliable production.
Mistake #3: Not Specifying Datum References
A position tolerance without a datum reference is meaningless. Your manufacturer will guess — and their guess may not match your assembly requirements. Fix: Always define a complete datum reference frame (typically 3 datums: primary, secondary, tertiary) for GD&T callouts.
Mistake #4: Conflicting Tolerances
Specifying both a tight dimensional tolerance (+0.01mm) and a loose geometric tolerance (flatness 0.5mm) on the same feature creates confusion and unnecessary inspection cost. Fix: Review your drawings for contradictions. If a surface is flat within 0.05mm, its dimensional tolerance should logically be at least ±0.025mm.
Mistake #5: No Inspection Plan
Sending drawings with ±0.005mm tolerances but no inspection plan means you might pay for CMM inspection the manufacturer didn’t perform — or worse, receive parts that don’t fit because critical tolerances weren’t verified. Fix: Include a simple inspection plan: which dimensions are critical (100% inspect), which are important (sample inspect), and which are reference-only.
4. Practical Tolerance Guidelines by Process
| Process | Typical Tolerance | Achievable (Premium) | Cost Impact of Tight Tol. |
|---|---|---|---|
| CNC Milling | ±0.05mm | ±0.005mm | +30–100% |
| CNC Turning | ±0.025mm | ±0.003mm | +25–80% |
| Laser Cutting | ±0.1mm | ±0.05mm | +15–30% |
| Sheet Metal Bending | ±0.3mm | ±0.1mm | +20–40% |
| Investment Casting | ±0.3mm | ±0.1mm | +40–100% |
| Die Casting | ±0.1mm | ±0.05mm | +30–70% |
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