Bore and ID Gages

Bore gages and ID gages are designed for dimensional measurement or assessment of the internal diameter of components.  Bore gages and ID gages are available that employ variable or fixed mechanical, electronic or pneumatic technologies. Specialized bore gages have the capability to measure the degree of roundness (lobes), taper or internal steps, grooves or serrations.

Mechanical gages use comparison or the physical movement and displacement of a gaging element (e.g., spindle, slide, stem) to determine the dimensions of a part or feature.  Micrometers, calipers, indicators, plug gages, ring gages or snap gages are examples of mechanical gages.

Electronic gages use LVDT, capacitance, inductive or other electronic probes to sense the distance or displacement of a contact or stylus.  Mechanical gages such as micrometers, plug gages, and snap gages may employ an integral electronic probe in addition to the mechanical gaging elements.

Pneumatic gages or gaging systems use the changes in flow or pressure in air nozzles or inlets internally located in air plugs, probes, rings or snaps or other pneumatic gages.  Pneumatic comparators, digital readouts, analog amplifiers, columns or flowmeter / rotameter tubes are used to display air gaging dimensional data.

Specific types of bore gages and ID gages include internal calipers, slot gages, indicating bore gages, and 3-point bore gages.

Calipers use a precise slide movement for inside, outside, depth or step measurements. While calipers do not typically provide the precision of micrometers, they provide a versatile and broad range of measurement capabilities: inside (ID), outside (OD), depth, step, thickness and length. Shop tool, spring-type or firm joint calipers consist of two legs with a scissor action and are usually used for comparative gaging or measurement transfer; although some spring-type calipers have dial indicators.

Slot gages are expanding collet type gages used for comparative measurement of small holes.  The gage is expanded in the hole and then removed and measured with a micrometer or other external (OD) gage. Alternately, the hole or slot gage can be set and used to check if a hole is above or below a specific size or tolerance.

Indicating bore gages are gaging devices for comparative measurements where the linear movement of a spindle or plunger is amplified and displayed on a dial, column or digital display.  Typically, indicators have a lower discrimination (

0.001" to 0.0001") and greater range (

+/- 1.000" to +/- 0.050" total) compared to comparators.  The level or precision is sufficient for measurement of precision ground parts and for the calibration of other gages.

3-point bore gages have three contact points mounted on arms that expand out from a central point.  3-point bore gages can detect lobe or out-of-roundness conditions, which is an advantage over 2-point ID gages.  These gages usually have dial or digital displays.

• Ins and Outs of ID/OD Comparative Gaging
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