Gauges in Engineering
Gauges are inspection tools used in manufacturing to check whether a part’s dimensions are within specified tolerances. Unlike measuring instruments (like vernier calipers), gauges do not give numerical readings—they simply tell whether a part is acceptable (“Go”) or not acceptable (“No-Go”).
Two common types of limit gauges are:
1. Plug Gauge
Definition:
A plug gauge is a cylindrical gauge used to check the internal dimensions of a part, such as the diameter of a hole.
It consists of two ends:
- GO End: This end checks the maximum material condition, meaning it must enter the hole if the hole is large enough.
- NO-GO End: This end checks the minimum material condition, meaning it must not enter the hole if the hole is too large.
Sketch Description
- Draw a horizontal rod with two different diameter ends.
- Label the smaller diameter end as GO End.
- Label the slightly larger end as NO-GO End.
- Label the handle in between.
📝 Caption: Plug Gauge – for checking hole diameter using GO/NO-GO method
Example Use:
If the acceptable hole diameter is 20.00 mm ± 0.02 mm:
- GO plug = 19.98 mm
- NO-GO plug = 20.02 mm
If the GO end fits into the hole and the NO-GO end does not, the hole is within tolerance.
Applications:
- Inspection of holes in gears, bushings, engine blocks, etc.
- Mass production where quick checking is needed.
2. Snap Gauge
Definition:
A snap gauge is a U-shaped gauge used to check the external dimensions of a part, like the diameter of a shaft.
It also has two settings:
- GO Jaw: The larger jaw that the shaft must fit through.
- NO-GO Jaw: The smaller jaw that the shaft must not fit through.
Sketch Description
- Draw a U-shaped frame with two pairs of measuring jaws.
- Label the outer (wider) opening as GO Jaw.
- Label the inner (narrower) opening as NO-GO Jaw.
- Label the handle or grip area.
📝 Caption: Snap Gauge – for checking external diameter using GO/NO-GO method
Example Use:
If the shaft diameter tolerance is 25.00 mm ± 0.03 mm:
- GO opening = 25.03 mm (shaft must pass through)
- NO-GO opening = 24.97 mm (shaft must not pass through)
If the shaft passes GO and doesn’t pass NO-GO, it is acceptable.
Applications:
- Checking shaft sizes in automotive and aerospace components.
- Used directly on production line for high-speed inspection.
Comparison Table:
| Feature | Plug Gauge | Snap Gauge |
|---|---|---|
| Checks | Internal dimensions (holes) | External dimensions (shafts) |
| Shape | Cylindrical with double ends | U-shaped with GO and NO-GO jaws |
| GO Function | Should enter the hole | Part should pass through |
| NO-GO Function | Should not enter the hole | Part should not pass through |
| Common Use | Hole diameter verification | Shaft diameter verification |
Advantages of Plug and Snap Gauges:
- Fast inspection – ideal for mass production.
- No need for skilled labor.
- Highly accurate and reliable for dimensional checks.
Conclusion:
Plug and Snap Gauges are essential tools in quality control. They provide quick and precise checks for holes and shafts respectively. Though they do not measure exact values, they are very efficient for pass/fail inspection during manufacturing.