1. Squares and Surface Plates
Unit 8

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

3. 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

4. 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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6. 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

7. 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

8. 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

9. 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

10. 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.
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11. 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

12. 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.
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13. 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

14. 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

15. 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

16. 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

17. 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

18. 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