Introduction
Every dimension on a mechanical drawing carries a tolerance — the permissible range of variation from the nominal value. Setting tolerances correctly is one of the most cost-sensitive decisions a designer makes. Tolerances that are unnecessarily tight drive up manufacturing cost; tolerances that are too loose cause functional failures.
Why Tolerances Exist
No manufacturing process produces perfectly sized parts. Tool wear, thermal expansion, material variation — all introduce deviation from the nominal dimension. Tolerances define the acceptable deviation range and give the manufacturer a clear pass/fail criterion.
Types of Tolerance
Dimensional Tolerance
The permitted variation in a linear or angular dimension.
- Bilateral: φ50 ±0.05 → acceptable range 49.95–50.05 mm
- Unilateral: φ50 +0.05/−0.02 → acceptable range 49.98–50.05 mm
Geometric Tolerance (GD&T)
Controls form, orientation, location, and runout — characteristics that dimensional tolerance alone cannot fully define. Common symbols: flatness, cylindricity, parallelism, perpendicularity, true position, circular runout. Use GD&T when a dimension being “in size” is not sufficient to ensure function.
Fits Between Mating Parts
When a shaft fits inside a hole, the relationship between their tolerances determines the fit type. ISO 286 defines a system of tolerance grades (IT grades) and fundamental deviations:
| Fit Type | Characteristic | Typical Use | Example (ISO) |
|---|---|---|---|
| Clearance fit | Shaft smaller than hole; always a gap | Rotating shafts, bearings | H7/g6 |
| Transition fit | May have small clearance or interference | Location pins, light press fits | H7/k6 |
| Interference fit | Shaft larger than hole; press or shrink fit | Gear/shaft joints, fixed assemblies | H7/p6 |
How to Set Tolerances: Function-First Approach
The correct sequence:
- Determine the functional requirement — what must this dimension control?
- Determine the maximum allowable variation before function is compromised
- Set the tolerance to match that requirement
- Verify the tolerance is achievable with the planned manufacturing process
Never tighten a tolerance just to feel safe. Every tightening of tolerance increases machining cost — sometimes significantly.
Achievable Tolerances by Process
| Process | Typical Achievable Tolerance |
|---|---|
| Turning / milling | ±0.05 mm (standard), ±0.02 mm (careful) |
| Grinding | ±0.005–0.01 mm |
| Lapping / honing | ±0.001 mm and below |
Specifying a tolerance tighter than the process can achieve reliably results in high scrap rates or a request from the machine shop to relax the requirement.
FAQ
Q. What does H7/g6 mean on a drawing?
A. H7 is the hole tolerance (capital letter = hole side, 7 = IT grade); g6 is the shaft tolerance (lowercase = shaft side, 6 = IT grade). This combination produces a clearance fit. H7 is the most common hole tolerance in general engineering; the shaft tolerance letter determines the fit type.
Q. I don’t know what tolerance to use. What should I do?
A. Start from function: what is the consequence if the dimension is 0.1 mm off? 0.5 mm? Use that analysis to set the boundary. Then check whether the process can achieve it. When uncertain, ask your manufacturing team — they often know better than the design standard what is achievable at your specific facility.
Q. When should I use GD&T instead of coordinate tolerances?
A. Use GD&T when the shape, orientation, or location of a feature matters beyond its size — for example, a face that must be flat for a seal, or a bolt pattern where position (not just size) is critical to assembly. GD&T communicates design intent more precisely than ± tolerances alone.
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