Checking fixtures usually include stop and stop method, coloring method, light gap method and indicator table method.
1. Pass and stop method: Use the pass end and stop end of the gauge to control the size of the workpiece so that it does not exceed the tolerance zone. For example, when measuring the aperture, if the through end of the smooth plug gauge passes but the stop end does not, the aperture is qualified. Gauges detected by the stop and stop method are also called limit gauges (Figure 1). Common limit gauges include threaded plug gauges, threaded ring gauges and calipers.
2. Coloring method: apply a thin layer of suitable pigment (such as Prussian blue or lead red powder) on the working surface of the gauge, and then grind the surface of the gauge and the surface to be measured together. The size of the colored area and the uneven distribution of the measured surface indicate its error. For example, use a taper gauge to check the taper hole of the machine tool spindle, and use a flat ruler to check the straightness of the machine tool rail.
3. Light gap method: make the measured surface contact the measuring surface of the gauge, and place a light source behind or use natural light. When the gap is small to a certain extent, the transmitted light becomes colored light due to the phenomenon of optical diffraction. When the gap is 0.5 microns, the transmitted light can be seen. The size of the gap can be judged by the color of the transmitted light. The size of the gap and the degree of unevenness indicate the size, shape, or position error of the measured size. For example, use a ruler to check the straightness, and use a square and a flat plate to check the verticality.
4. Indication method: Compare the precise geometry of the gauge with the measured geometry, and use a dial gauge or micrometer to indicate the error of the measured geometry. For example, use a flat plate and a micrometer to measure the straightness of a ruled workpiece; use a sine gauge, a flat plate and a micrometer to measure the angle.
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