Machinery's Handbook, 31st Edition
784 CALIPERS, VERNIER AND DIAL Construction Requirements.— Materials: The beam and all measuring surfaces having direct contact with the work shall be made of high grade tool steel. The measuring faces may be made of Tool Steel with Carbide inserts. All materials shall be sound and free of injurious porosity, soft spots, hard spots and other defects. All parts subject to wear, breakage or distortion shall be replaceable. Main-scale Member (Beam): The main-scale member, or beam, of the Type 1 caliper shall be graduated as defined herein for each design variant. The main-scale shall be of a cross sectional area as to provide stability without undue weight when the instrument is used to make measurements at the maximum range. The measuring face and slide locating edge shall be straight within 0.0003 inch for each linear foot of the beam length. The total tolerance for the instrument (if over 12 inch) shall not be accumulated in any 12 inch increment and all deviation in straightness shall be evenly distributed along the entire length of the instrument. Slide: The slide or moveable jaw shall be machined to receive the main-scale beam and shall have a fit and surface finish so as to allow the slide to move through the entire length of the beam without lateral shake or bind when not locked into position. The locating side of the slot shall be square with the measuring plane within 0.0003 inch along the full length of the contact surface. The opposite side of the moveable jaw shall be fitted with a nib or knife edge inside measuring jaw and a locking screw. Vernier Plate: The vernier plate shall either overlap or be flush with the edge of the main scale. The overlap of the main scale shall be no less than 1/64 inch and straight with 0.002 inch in the full length. The vernier plate shall be beveled to prevent parallax error. If the scale is installed flush along the edge of the main scale, the edge of the Vernier Plate shall slide easily along the main scale. The edges of both the main scale and the vernier plate shall extend fully to the edges. The Plate shall be held in place with two screws through elongated mounting holes to allow no less than 0.010 inch longitudinal adjustment. Measuring Contacts: The measuring contacts shall have a surface finish of 8 m in Ra or better and have a hardness value of no less than 60 to 65 RC (Rockwell C scale). Scale and Graduations.— All calipers regardless of Type or Class will have a main scale and a Vernier scale or dial. The vernier scale shall be a direct reading scale so that when aligned with the zero point of the data scale, the first division of the vernier scale will appear below the first mark on the main scale. All vernier scale graduation spacing is a precise fraction smaller than the marks on the main scale. All graduations shall be machine cut or etched between 0.003 to 0.006 in width and from 0.001 to 0.003 inch in depth and shall not vary in width on the main scale or the vernier scale by more than 0.001 inch. The edges of the graduations shall be straight and not vary more than 0.0002 inch throughout the full length of the graduations along the full length of the in- strument. The graduations shall be sharp and well defined and contrast with the surround - ing area through chemical etching or other similar methods so that the graduations are distinct and easily readable to enable the use of the instrument under normal working conditions. The graduations will be parallel to each other and square to the locating edge of the main scale within 0.0004 inch. The vernier scale graduations shall be parallel and square to the reading edge of the vernier plate or scale within 0.0002 inch. The index of both the main and vernier scales will be located referencing the measuring contacts planes so as to be direct reading. Inch Graduated Instruments.— Main Scale: The inch graduated instruments will be graduated to read in thousandths of an inch increments. Each inch of the main scale shall be represented with a graduation mark representing the inch incremental value. Each inch segment shall be subdivided into ten increments of 0.100 inch. Each 0.100 inch increment shall be further divided into 4 increments of 0.025 inch each for 100 divisions per inch instruments or two divisions of 0.050 inch for 50 divisions per inch instruments. Each graduation of the main scale within the first foot shall be accurately located to the refer - ence zero index within 0.0003 inch. With each additional foot length of the main scale and
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