Understanding Dial Bore Gauge Graduations

A dial bore gauge is a precision instrument used to measure the diameter of a bore or hole. It is particularly useful for checking the internal dimensions of cylinders, engine blocks, and other parts where precise measurements are needed.

The main component of a dial bore gauge is a dial indicator attached to a measuring head. The measuring head has contact points that expand or contract based on the size of the bore it is placed inside. This movement is then amplified and shown on the dial indicator.

Why Graduations are Clockwise and Anti-Clockwise

The dial indicator on a dial bore gauge is typically set to zero at a nominal dimension, often using a setting ring or a known standard. When the gauge is inserted into the bore, the needle on the dial moves. The direction and amount of movement indicate how the measured bore diameter compares to the nominal dimension.

• If the bore is larger than the nominal dimension, the contact points move inward, causing the dial needle to move in one direction (e.g., clockwise, indicating a positive reading or larger size).
• If the bore is smaller than the nominal dimension, the contact points move outward slightly (as the centralizer and feeler make contact first), causing the dial needle to move in the opposite direction (e.g., anti-clockwise, indicating a negative reading or smaller size).

The graduations on the dial are marked in both clockwise and anti-clockwise directions specifically to show this difference from the set zero point. This difference from the nominal or set size is known as the deviation.

Analyzing the Options

Let's look at why 'Deviation' is the correct answer and why the other options are not what the clockwise and anti-clockwise graduations primarily measure:

• Deviation: This refers to the difference between a measured size and a nominal or target size. The dial bore gauge, once set to zero at a standard, shows exactly how much larger or smaller (the deviation) the bore is compared to that standard. The clockwise and anti-clockwise markings represent positive and negative deviations, respectively.
• Tolerance: Tolerance is the permissible variation in size or form allowed for a feature. While a dial bore gauge is used to check if a bore is within its specified tolerance, the graduations themselves show the *measured deviation* from the nominal size, not the tolerance limits themselves. You interpret the measured deviation to see if it falls within the tolerance range.
• Allowance: Allowance is the intentional difference in dimensions between mating parts to achieve a desired fit (e.g., clearance fit or interference fit). This is a design concept, not something directly measured by the gauge's dial graduations showing movement from zero.
• Dimension: While a dial bore gauge measures a dimension (the bore diameter), the clockwise and anti-clockwise graduations represent the *difference* from a set dimension, not the absolute dimension itself directly from zero on the dial. The absolute dimension is the set dimension plus or minus the measured deviation.

Therefore, the graduations in both directions are designed to show the deviation from a set nominal size.

Revision Table: Dial Bore Gauge Terms

Additional Information on Dial Bore Gauge Usage

Using a dial bore gauge requires careful technique to ensure accurate measurement. Key steps include:

• Selecting the correct contact point and extension rod combination for the bore size.
• Setting the gauge to zero using a master setting ring or a micrometer set to the nominal size.
• Inserting the gauge into the bore at an angle and rocking it gently to find the minimum reading, which represents the true diameter.
• Reading the deviation shown on the dial indicator.
• Adding or subtracting this deviation from the master size to get the actual bore size.

Understanding that the dial shows deviation is crucial for correctly interpreting the measurements taken by a dial bore gauge.