What is Tolerance?
Tolerance is the permissible limit or limits of variation in a physical dimension of a manufactured part. It is the difference between the maximum and minimum acceptable limits of a dimension. Tolerance is always allowed because perfect accuracy is practically impossible and very expensive to achieve in mass production.
In simple terms:
Tolerance defines how much variation is acceptable in the size of a part without affecting its functionality.
Basic Terms:
- Nominal Size: The ideal or theoretical size of a part (also called the design size).
- Upper Limit: The maximum permissible size of the part.
- Lower Limit: The minimum permissible size of the part.
- Tolerance: Upper Limit – Lower Limit
Example:
If a shaft has a diameter specified as 30 mm ± 0.05 mm,
Then the Upper Limit = 30 + 0.05 = 30.05 mm
And the Lower Limit = 30 – 0.05 = 29.95 mm
Hence, Tolerance = 30.05 – 29.95 = 0.10 mm
Types of Tolerance
There are mainly two types of dimensional tolerance:
1. Unilateral Tolerance
In unilateral tolerance, the variation is allowed only in one direction (either positive or negative) from the nominal size.
Example:
If the size of a hole is specified as 20 mm +0.02 mm / –0.00 mm,
then the actual hole can be between 20.00 mm and 20.02 mm only.
This means:
- Upper Limit = 20.02 mm
- Lower Limit = 20.00 mm
- Tolerance = 0.02 mm (only above nominal size)
Diagram Description:
Draw a horizontal line marked as 20.00 mm (Nominal Size)
Draw a mark at 20.02 mm (Upper Limit)
No lower deviation; hence only one arrow going upward labeled as +0.02 mm
Caption: “Unilateral Tolerance”
Uses of Unilateral Tolerance:
- Mostly used in hole and shaft systems.
- Helps in easy gauging and inspection.
- Reduces machining time as allowance is given in one direction only.
2. Bilateral Tolerance
In bilateral tolerance, the variation is allowed in both directions (positive and negative) from the nominal size.
Example:
If a dimension is given as 50 mm ± 0.05 mm,
then the part size can vary between 49.95 mm and 50.05 mm.
This means:
- Upper Limit = 50.05 mm
- Lower Limit = 49.95 mm
- Tolerance = 0.10 mm (±0.05 mm in both directions)
Diagram Description:
Draw a horizontal line at 50.00 mm (Nominal Size)
Mark both 49.95 mm and 50.05 mm on either side
Two arrows: one going up (+0.05 mm), one going down (–0.05 mm)
Caption: “Bilateral Tolerance”
Uses of Bilateral Tolerance:
- Common in general machining and component manufacturing.
- Suitable when equal deviation is acceptable on both sides of the nominal size.
- Allows greater flexibility in manufacturing.
Comparison Table:
| Feature | Unilateral Tolerance | Bilateral Tolerance |
|---|---|---|
| Variation direction | In one direction only | In both directions |
| Example | 20 +0.02 / –0.00 mm | 20 ± 0.02 mm |
| Manufacturing Ease | Easier to inspect | Balanced deviation is possible |
| Cost | Lower | Slightly higher due to complexity |
| Common Use | Holes and shafts | General parts |
Conclusion:
Understanding tolerance is crucial in mechanical design and manufacturing. It ensures that even with small dimensional variations, the parts will still function properly and fit together. Depending on the product design and its functional requirements, either unilateral or bilateral tolerance is used.