Method of Expressing Tolerance as per BIS
In engineering manufacturing, it is not possible to produce parts with exact dimensions every time. Therefore, tolerances are specified to allow small variations in size. According to BIS (Bureau of Indian Standards), tolerances are expressed using a standard system of limits and fits. This system ensures interchangeability and proper functioning of machine components.
Expression of Tolerance
Tolerance is the difference between the maximum and minimum permissible sizes of a component.
Tolerance = Upper Limit – Lower Limit
According to BIS standards, tolerances are expressed using letters and numbers. The letter represents the position of the tolerance zone relative to the basic size, while the number indicates the tolerance grade.
Example: 40 H7
- 40 – Basic size (in mm)
- H – Position of tolerance zone
- 7 – Tolerance grade
Fits
A fit is the relationship between two mating parts, usually a hole and a shaft. The type of fit depends on the difference between the hole size and the shaft size.
Types of Fits
1. Clearance Fit
In a clearance fit, the hole is always larger than the shaft. This allows free movement between the parts.
Applications:
- Running shafts
- Sliding machine parts
- Rotating components
Hole Size |--------| Shaft Size |----|
2. Transition Fit
In a transition fit, the hole and shaft sizes are nearly equal. The assembly may have either a small clearance or slight interference.
Applications:
- Machine tool assemblies
- Couplings
- Precision mechanical parts
Hole Size |------| Shaft Size |------|
3. Interference Fit
In an interference fit, the shaft is always larger than the hole. This produces a tight joint that usually requires force or heating to assemble.
Applications:
- Permanent assemblies
- Press fits
- Gear and bearing mounting
Hole Size |----| Shaft Size |------|
Vernier Height Gauge
A vernier height gauge is a precision measuring instrument used to measure vertical dimensions and mark lines on workpieces. It is generally used along with a surface plate for accurate measurement and layout work.
Material
Vernier height gauges are usually made from hardened steel or stainless steel to ensure durability and resistance to wear.
Construction
The instrument consists of a heavy base that rests on the surface plate and a vertical column with a graduated scale. A movable slider carrying a vernier scale and scriber moves along the column to measure or mark heights.
Main Parts
- Base
- Main Scale
- Vernier Scale
- Slider
- Scriber
- Fine Adjustment Screw
- Locking Screw
Graduations
Metric System
- Main scale graduated in millimeters.
- Least count usually 0.02 mm.
English System
- Main scale graduated in inches.
- Least count usually 0.001 inch.
Uses
- Measuring vertical heights of components.
- Marking lines on workpieces.
- Inspection and layout work.
- Checking step heights or depth differences.
Care and Maintenance
- Always use the gauge on a clean surface plate.
- Keep the base and scriber clean.
- Apply oil to prevent rust.
- Avoid dropping the instrument.
- Store it safely in a protective case.
Conclusion
The BIS system of tolerances and fits ensures accuracy and interchangeability of engineering components. Understanding different types of fits and proper use of precision instruments such as the vernier height gauge helps maintain high quality and precision in engineering manufacturing.