Understanding Gauges and Material Conditions

Gauges are inspection tools used to check if a manufactured part is within its specified size limits. They are often used for rapid inspection on the production floor.

There are primarily two types of limit gauges:

• GO gauge: Checks the maximum material condition (MMC). It must fit or pass over the feature.
• NOGO gauge: Checks the minimum material condition (LMC). It must not fit or pass over the feature.

Material Conditions Explained

Material conditions refer to the size of a feature relative to its tolerance range:

• Maximum Material Condition (MMC): This is the condition where a feature contains the maximum amount of material within its size tolerance. For an external feature (like a shaft), this is the upper limit of size. For an internal feature (like a hole), this is the lower limit of size.
• Minimum Material Condition (LMC): This is the condition where a feature contains the minimum amount of material within its size tolerance. For an external feature (like a shaft), this is the lower limit of size. For an internal feature (like a hole), this is the upper limit of size.

Let's summarize material conditions for holes and shafts:

NOGO Gauge for a Hole

The NOGO gauge is designed to check the LMC of the feature it is inspecting. According to Taylor's Principle of gauging, the NOGO gauge should check only one size, which is the limit corresponding to the minimum material condition.

For a hole, the minimum material condition (LMC) occurs when the hole is at its largest possible size within the tolerance range. This largest size is the upper limit of size for the hole.

Therefore, the NOGO gauge for a hole is designed to have a size equal to the upper limit of size of the hole. If the hole is correctly manufactured, its size will be at or below this upper limit, and the NOGO gauge will not enter or pass through the hole. If the hole is larger than the upper limit (i.e., outside its tolerance and beyond LMC), the NOGO gauge would incorrectly pass through, indicating a defect.

Why Upper Limit for NOGO Gauge (Hole)?

A hole's size can vary between its lower limit and its upper limit. The material condition changes as the size changes:

• Size at Lower Limit: This is the smallest hole size, meaning maximum material is present. This corresponds to MMC.
• Size at Upper Limit: This is the largest hole size, meaning minimum material is present. This corresponds to LMC.

The NOGO gauge checks if the hole is too large. A hole is too large if its size exceeds the upper limit. Thus, the NOGO gauge must be made to the size of the upper limit of the hole to check this maximum allowable size (which is the minimum material condition).

In summary, the NOGO gauge for a hole checks the hole's size at the minimum material condition, which corresponds to the upper limit of size of the hole.

Revision Table: Key Concepts in Gauging

Additional Information on Limit Gauges

Limit gauges, like GO and NOGO gauges, are attribute gauges. They don't measure the exact size but indicate whether the part's size is within the acceptable limits (between the lower and upper limits). This makes them fast for inspection but they don't provide information about how close the size is to the limit.

Taylor's Principle of Gauging states that the GO gauge should check all related elements of the feature simultaneously and should be a complete functional gauge replicating the mating part at its MMC. The NOGO gauge, on the other hand, should check only one dimension at the LMC and should be a single-size gauge.

For a hole with a tolerance, defined by a lower limit (LL) and an upper limit (UL):

• GO gauge size = LL (This checks if the hole is not too small, i.e., at or above its MMC size).
• NOGO gauge size = UL (This checks if the hole is not too large, i.e., at or below its LMC size).

The question specifically asks for the minimum material condition for the hole that the NOGO gauge checks. For a hole, the minimum material condition is when the hole is largest, which is its upper limit of size.