Gear Tooth Vernier

When using this type of measuring instrument, it is important to be aware of certain factors that can affect its accuracy. As the gear tooth caliper measures the tooth thickness at a point referenced from the OD of the gear, it is important that the OD is kept within control. A 0.003" (0.076mm) change in the OD radius will make a 0.002" (0.050mm) change in the tooth thickness.
i.e., (Sin(a*2))*((OD Theoretical-OD Actual)/2) = 0.002"

The difference between the the theoretical OD and the actual OD should be divided by 2 to acquire the amount radially, then if it is plus material it is added to the chordal addendum on the vertical scale and if it is minus material it is subtracted from the chordal addendum on the vertical scale. This will then compensate for the OD discrepancy on that particular gear.
Any burrs on the tips of the teeth will act like an enlarged OD and also give an inaccurate size. Remove all burrs before measuring.

Note: As with all inspection instruments and gauges, gear tooth verniers should be regularly checked for calibration and included in a 'calibration scheme'. Due to the nature of attaining a 'feel', the jaws can wear over a period of time, to allay this wear they are often carbide tipped. Each scale has a plate, held by two screws to allow for correctional adjustment.


	Gear Tooth Vernier

The size of a gear can be measured in a variety of ways using many different types of instrumentation. These can be hand tools or mechanical/electronic machines that can be operated by manual or automatic means. The gear tooth vernier or caliper falls in the 'hand - mechanical' category. One of the benefits of using a caliper is that the gear does not have to be removed from the machine but a certain amount of 'feel' is required for accurate results. The caliper consists of two adjustable verniers, that reference two diameters on the gear and provide a measurement.
	Vertical Scale: This is set to the Chordal Addendum (h_a) of the gear tooth. The foot (pink), when placed against the OD of the gear, will position the ends of the two jaws at a point on the Reference Diameter (d). Horizontal Scale: This is used to measure the Chordal Thickness (S_n) of the gear tooth. It can either be a) pre-set to the chordal thickness and used as a gauge to confirm tooth size, or b) the caliper is placed in position on the gear tooth and this scale adjusted until the correct 'feel' is obtained and the chordal thickness size read off the scale.
When using this type of measuring instrument, it is important to be aware of certain factors that can affect its accuracy. As the gear tooth caliper measures the tooth thickness at a point referenced from the OD of the gear, it is important that the OD is kept within control. A 0.003" (0.076mm) change in the OD radius will make a 0.002" (0.050mm) change in the tooth thickness. i.e., (Sin(a2))((OD Theoretical-OD Actual)/2) = 0.002" The difference between the the theoretical OD and the actual OD should be divided by 2 to acquire the amount radially, then if it is plus material it is added to the chordal addendum on the vertical scale and if it is minus material it is subtracted from the chordal addendum on the vertical scale. This will then compensate for the OD discrepancy on that particular gear. Any burrs on the tips of the teeth will act like an enlarged OD and also give an inaccurate size. Remove all burrs before measuring. Note: As with all inspection instruments and gauges, gear tooth verniers should be regularly checked for calibration and included in a 'calibration scheme'. Due to the nature of attaining a 'feel', the jaws can wear over a period of time, to allay this wear they are often carbide tipped. Each scale has a plate, held by two screws to allow for correctional adjustment.

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