Quick Guide to Precision Measuring Instruments

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PG58Up slopeDown slope77˚ or less87˚ or lessδ3: Software processing. To measure a workpiece contour that involves a large displacement in the vertical direction with high accuracy, one of these compensation methods needs to be implemented.Traceable AngleThe maximum angle at which a stylus can trace upwards or downwardsalong the contour of a workpiece, in the stylus travel direction, is referred to as the traceable angle. A one-sided sharp stylus with a tip angle of 12° (as in the above gure) can trace a maximum 77° of up slope and a maximum 87° of down slope. For a conical stylus (30° cone), the traceable angle is smaller. An up slope with an angle of 77° or less overall may actually include an angle of more than 77° due to the effect of surface roughness. Surface roughness also affects the measuring force.For model CV-3200/4500, the same type of stylus (SPH-71: one-sided sharp stylus with a tip angle of 12°) can trace a maximum 77° of up slope and a maximum 83° of down slope.Overload Safety CutoutIf an excessive force (overload) is exerted on the stylus tip due, perhaps, to the tip encountering a too-steep slope on a workpiece feature, or a burr, etc., a safety device automatically stops operation and sounds an alarm buzzer. This type of instrument is commonly equipped with separate safety devices for the tracing direction (X axis) load and vertical direction (Y axis) load.For model CV-3200/4500, a safety device functions if the arm comes off the detector mount.Simple or Complex Arm GuidanceIn the case of a simple pivoted arm, the locus that the stylus tip traces during vertical movement (Z direction) is a circular arc that results in an unwanted offset in X, for which compensation has to be made. The larger the arc movement, the larger is the unwanted X displacement (δ) that has to be compensated. (See gure, lower left.) The alternative is to use a complex mechanical linkage arrangement to obtain a linear transla-tion locus in Z, and therefore avoid the need to compensate in X.Z axis Measurement MethodsThough the X axis measurement method commonly adopted is by means of a digital scale, the Z axis measurement divides into analog methods (using a differential transformer, etc.) and digital scale methods. Analog methods vary in Z axis resolution depending on the measurement magnication and measuring range. Digital scale methods have xed resolution. Generally, a digital scale method provides higher accuracy than ananalog method.Compensating for Stylus Tip RadiusA recorded prole represents the locus of the center of the ball tip roll-ing on a workpiece surface. (A typical radius is 0.025mm.) Obviously this is not the same as the true surface prole so, in order to obtain an accurate prole record, it is necessary to compensate for the effect of the tip radius through data processing.If a prole is read from the recorder through a template or scale, it isnecessary to compensate for the stylus tip radius beforehand accordingto the applied measurement magnication.AccuracyAs the detector units of the X and Z axes incorporate scales, the mag-nication accuracy is displayed not as a percentage but as the linear displacement accuracy for each axis.Compensating for Arm RotationThe stylus is carried on a pivoted arm so it rotates as the surface is traced and the contact tip does not track purely in the Z direction. Therefore it is necessary to apply compensation in the X direction to ensure accuracy. There are three methods of compensating for arm rotation. 1: Mechanical compensationδ: Unwanted displacement in X to be compensatedrrrRecorded profileStylusWorkpiece contourr: Stylus tip radiusδFulcrumStylusMeasuring arm2: Electrical compensationContracer (Contour Measuring Instruments)