Squares and Surface Plates Unit 8

Objectives The machinist’s combination square Three types of solid and adjustable squares Two types of surface plates

Square Used for layout, inspection and setup Manufactured to various degrees of accuracy Range from semiprecision to precision squares Precision squares are hardened and accurately ground

Machinist's Combination Square Used for quick check of 90 and 45 degree angles and measure of length Part of combination set that includes square head, center head, bevel protractor and graduated grooved rule

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Precision Square Used chiefly for inspection and setup purposes Hardened and accurately ground Must be handled carefully to preserve accuracy Great variety manufactured but all variations of either solid square or adjustable square

Beveled-Edge Square Better quality standard squares used in inspection Beveled edge allows blade to make line contact with work More accurate check Work is square (90 degrees) if both sides touch surface of work

Toolmaker's Surface Plate Square Provides convenient method of checking work for squareness on surface plate One-piece construction Little chance of any inaccuracy developing

Cylindrical Square Master squares which other squares checked Consists of thick-walled alloy steel cylinder Hardened, ground and lapped Outside diameter nearly true cylinder Ends ground and lapped square with axis Ends recessed and notched to decrease inaccuracy from dust Setting carefully on clean surface plate and rotate to force dust and dirt into end notches

Cylindrical Square Another type of cylindrical square is the direct-reading cylindrical square (Fig. 8-6), which indicates directly the amount that the part is out of square. One end of the cylinder is lapped square with the axis, while the other end is ground and lapped slightly out of square. The circumference is etched with several series of dots that form elliptical curved lines. Each curve is numbered at the top to indicate the amount, in ten-thousandths of an inch (.0001), that the workpiece is out of square over the length of the square. Absolute squareness, or zero deviation, is indicated by an etched, vertical dotted line on the square. When used, the square is carefully placed in contact with the work and turned until no light is seen between it and the part being inspected. The uppermost curved line in contact with the work is noted and followed to the top, where the number shows the amount the work is out of square. This square may also be used as a conventional cylindrical square if the opposite end, which is ground and lapped square with the axis, is used. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Adjustable Squares Not as accurate as good solid square Used where impossible to use fixed square Diemaker's square Used to check clearance angle on dies Blade adjusted to angle of workpiece by blade-adjusting screw, then angle checked with protractor Direct-reading type indicates angle of blade

Adjustable Micrometer Square Used to check part for squareness accurately When a piece of work is being checked and light shows between the blade and the work, turn the micrometer head until the full length of the blade, which may be tilted, touches the work. The amount the part is out of square may be read from the micrometer head. When the micrometer head is set at zero, the blade is perfectly square with the beam. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Straightedges Used to check surfaces for flatness and act as guide for scribing long, straight lines in layout work Rectangular bars of hardened and accurately ground steel Edges flat and parallel Plain or beveled edges Generally made of cast iron with ribs

Surface Plates Rigid block of granite or cast iron Flat surface used as reference plane for layout Generally have three-point suspension to prevent rocking when mounted on uneven surface Two types Cast-iron plates Granite surface plates

Surface Plates Cast-iron plates Granite surface plates Well ribbed and high strength Good wear-resistance qualities After machined, surface scraped by hand to flat Operation long and cost high Granite surface plates Manufactured from gray, pink, or black granite Several degrees of accuracy Extremely flat finishes produced by lapping

Advantages of Granite Plates Not appreciably affected by temperature change Will not burr, therefore, accuracy not impaired Nonmagnetic Rustproof Abrasives will not embed themselves as easily in the surface

Care of Surface Plates Keep surface plates clean at all times Clean occasionally with solvent to remove film Protect with wooden cover when not in use Use parallels whenever possible to prevent damage to plates by rough parts or castings Remove burrs from workpiece before placing it on plate

Care of Surface Plates Slide heavy parts onto the plate rather than place them directly on the plate Remove all burrs from cast-iron plates by honing When not in regular use, cover cast-iron plates with thin film of oil to prevent rusting Center punching or prick punching layout lines should not be done on a surface plate