Quick Guide to Precision Measuring Instruments

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PG17Quick Guide to Measurementa'aGuide plateWorkpiecea'aAnvilGuide plateContact pointWorkpiece04535405109876ThimbleOuter sleeveHow to Read the Scale(1) Outer sleeve 35 mm(2) Thimble 0.015 mm Reading 35.015 mmGraduation 0.005mm(2)(1)Changes in measured values at different measuring pointsWhen Holtest is used, the measured value differs between measurement across the anvil and the measurement only at the tip of the anvil due to the product mechanism.Adjust the start point under the same condition before measurement.When you use the tip of the anvil for measurement, adjust the start point for using the tip of the anvil.45051DIV. 0.005mm45WorkpieceAdjustment ofstart point45051DIV. 0.005mm45Measurement error due to temperature variation of Tubular Inside MicrometersHeat transfer from the operator to the micrometer should be minimized to avoid any signicant measuring error due to temperature difference between the workpiece and micrometer. If the micrometer is held directly by hand when measuring, use gloves or hold the heat-insulator (if tted).aaAiry and Bessel PointsWhen a length standard bar or inside micrometer lies horizontally, supported as simply as possible at two points, it bends under its own weight into a shape that depends on the spacing of those points. There are two distances between the points that control this deformation in useful ways, as shown below. The ends of a bar (or micrometer) can be made exactly horizontal by spacing the two supports symmetrically as shown above. These points are known as the ‘Airy Points’ and are commonly used to ensure that the ends of a length bar are parallel to one another, so that the length is well dened.The change in length of a bar (or micrometer) due to bending can be minimized by spacing the two supports symmetrically as shown above. These points are known as the ‘Bessel Points’ and may be useful when using a long inside micrometer.Bore Gages●Mitutoyo bore gages for small holes feature contact elements with a large curvature so they can be easily positioned for measuring the true diameter (in the direction a-a’) of a hole. The true diameter is the minimum value seen on the dial indicator while rocking the bore gage as indicated by the arrow.●The spring-loaded guide plate on a Mitutoyo two-point bore gage automatically ensures radial alignment so that only an axial rocking movement is needed to nd the minimum reading (true diameter).If the Tubular Inside Micrometer is misaligned in the axial or radial direction by an offset distance X when a measurement is taken, as in Figures 1 and 2, then that measurement will be in error as shown in the graph below (constructed from the formulae given above). The error is positive for axial misalignment and negative for radial misalignment.Misalignment ErrorsFigure 1Figure 2:Inside diameter to be measuredL:Length measured with axial offset XX:Offset in axial direction △:Error in measurement △:L−=√2+X2 −:Inside diameter to be measuredL:Length measured with radial offset XX:Offset in radial direction △:Error in measurement △:L−=√2–X2 −XXLL109876543210.100.090.080.070.060.050.040.030.020.01ℓ=200mmℓ=500mmℓ=1000mmError (positive for axial, and negative for radial, misalignment) (mm)Misalignment (offset) of one end of micrometer (mm)Airy points (a 0.577)Bessel points (a 0.559)