Measuring toolsWhen accurate measurement of the part is required, the vernier calipers, micrometers and slide gauges are used. When it is not so important to have an accurate measurement of the part, metal rules, inside calipers and outside calipers will do. Steel rules server for determining the length and sometimes the depth of the part to be measured. Steel rules are graduated in millimeters, but in the USA and England they are graduated in thirty-seconds and sixty-fourths of an inch. The outside calipers are used for determining the internal diameters of the part. Its construction is almost similar to that of the outside caliper measurements of the part. The outside and inside caliper measurements may be read by placing the legs of the calipers on a rule and placing one leg and the end of the rule against a flat surface. Vernier calipers can be used for measuring both external sizes of a part. By using vernier calipers measurements up to 001 may be determined. Vernier calipers consist of a primary steel scale and four jaws. Jaws are integral with the scale. Jawsattached to the frame sliding along the scale. The frame may be clamped on the scale in any position by means of an adjusting screw. External surfaces may be measured with jaws, while internal surfaces are measured with jaws. Тhe depth of recesses is measured with a depth rod. A vernier on the frame serves for measuring fractional parts of a millimeter. The micrometer is an instrument for precise measurement of length and thickness of a part to one ten thousandth of an inch. The principle of operation is similar to that of vernire calipers. A graduated thimble serves as the primary scale. The scale on the barrel of the micrometer is used for the vernier reading. The part to be measured is placed between the anvil and micrometer screw, which is called a “movable spindle”. By rotating the thimble, the spindle is removed or approached to the anvil. By rotating the ratchet thimble the spindle is moved and thus the partis pressed to the anvil. Thereupon the rotation of the ratchet thimble is discontinued, the micrometer opening is fixed by the lock nut and the reading is taken. The micrometer reading is the sum total of the barrel divisions, the thimble divisions with respect to the axial line on the barrel, and the vernier reading. The dial indicator is a gauge with a graduated dial and an indicator pointer connected to a test point by a system of levers. Any movement of the test point is magnified by the indicator pointer. The dial indicator is used to check the shape of a part, the precision of its machining, as well as for checking the accuracy of cuttingmachines. Thread plug gauges or internal gauges, being of “go” and “no-go” type, are generally used for testing threads or tapped holes. Depth gauges are used for measuring the depth of grooves and holes. Their principle of operation is similar to that of vernier calipers. A primary scale sliding in a frame may be locked in any position by a screw. The depth of a groove or a hole is measured by means of the primary scale and a vernier. Conical surfaces are measured with control gauges. The universal angle gauge consists of a base with a primary scale attached to the base. A quadrant with a vernier may be moved along the arc of the base. A triangle may be attached to the quadrant by means of a holder. In its turn a detachable rule is attached to the triangle. The triangle and the detachable rule can be moved along the face of the quadrant. Angles can be measured within the range of 0 to 320 by means of the universal angle gauge.
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