Short Answer:

Tolerance in mechanical engineering means the allowed variation in the size of a part. It shows how much bigger or smaller a part can be from its exact measurement and still work properly. Tolerance is given in millimeters or micrometers and is written along with the part’s dimensions.

Tolerance is very important in fitting because it ensures that parts fit together correctly, even when there are small differences in their sizes. Without proper tolerance, parts may be too tight or too loose, leading to problems in assembly, performance, or safety of the machine.

Detailed Explanation:

Tolerance and its importance in fitting

In the real world, it is impossible to make every part with exact size. There will always be small differences due to tool wear, machine vibration, material properties, or temperature changes. That is why engineers use tolerance, which defines the acceptable limit of variation in a part’s dimension.

For example, if the size of a shaft is given as 20 mm ± 0.05 mm, it means the actual shaft can be anywhere between 19.95 mm and 20.05 mm, and it will still be acceptable. This small range is called tolerance.

In fitting work, where parts must join or move properly, having the right tolerance is very important. It helps avoid problems like tight fits, gaps, friction, or part failure.

Types of tolerance

There are mainly three types of tolerance used in engineering:

1. Unilateral Tolerance
   • The variation is only in one direction (either above or below the basic size).
   • Example: 20 mm +0.02 / –0.00
2. Bilateral Tolerance
   • The variation is allowed on both sides of the basic size.
   • Example: 20 mm ± 0.05
3. Limit Tolerance
   • The dimension is given directly with upper and lower limits.
   • Example: 19.95 mm to 20.05 mm

These tolerances are selected based on the type of fit needed in the assembly—whether a clearance fit, interference fit, or transition fit.

Why tolerance is important in fitting

1. Ensures proper fit between parts
   Tolerance allows parts like shafts and holes to be manufactured within acceptable limits so that they can fit together perfectly.
2. Allows smooth motion or tight locking
   Some parts need to slide (like bearings), while others need to stay fixed (like gear shafts). Proper tolerance controls how loose or tight the fit will be.
3. Reduces assembly problems
   If parts are made without tolerance, they might not fit at all, or need rework. Tolerance reduces the chance of rejection and saves time.
4. Controls cost and manufacturing effort
   Making a part with zero tolerance (exact size) is very costly and difficult. Tolerance allows easy manufacturing without losing function.
5. Improves interchangeability
   When many parts are made in bulk, proper tolerance ensures that any part can be assembled with another matching part, even if made separately.
6. Helps in quality control
   Tolerance gives a clear range for inspection. Parts outside the tolerance limit are rejected to maintain quality and safety.

Example of tolerance in fitting

Imagine a shaft and hole assembly. The hole is supposed to be 20.00 mm, and the shaft is 19.98 mm. If the hole is made slightly smaller (19.95 mm) and the shaft is slightly bigger (20.02 mm), they will not fit. But with proper tolerance, these small changes are allowed and still work correctly.

This shows how important it is to define tolerance in all engineering drawings and during fitting work.

Conclusion

Tolerance is a key concept in mechanical fitting that allows small variations in the size of parts. It ensures that even with minor differences, parts will still work together as expected. Without tolerance, fitting would become difficult, time-consuming, and costly. By using proper tolerance, engineers make sure that machines are easy to assemble, work smoothly, and are reliable over time.