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Chapter 10: D. Surface Plate Tools and Techniques 93
30°
15°
Total angle 45°
15°
30°
Total angle 15°
Figure 10.5 Addition and subtraction of angle blocks.
Angle Blocks
Angle blocks are simply hardened blocks that are lapped to precise angles and can
be wrung together like gage blocks. Unlike gage blocks, angle blocks can be added
or subtracted as shown in Figure 10.5.
Part II.D.2
Protractors
Protractors are used to measure angles. A simple protractor consists of a graduated
head (in degrees) along a semicircle. A blade is connected to the center pin. By
rotating the blade on the pin, any angle from zero to 180 o can be measured.
Combination Sets. Combination protractor and depth gages combine a movable
graduated depth gage with a protractor head.
The combination set consists of a center head, protractor, and square with a 45°
surface, all of which are used individually in conjunction with a steel rule. The
heads are mounted on the rule and clamped in any position along its length by
means of a lock screw. The center head is used to scribe bisecting diameters of
the end of a cylindrical piece to locate the center of the piece. The protractor reads
directly in degrees. Both the square head and the protractor may contain a small
spirit level.
A bevel protractor utilizes a vernier scale to show angles as small as five
minutes.
The universal bevel protractor is widely used for angle measurement in many
applications. The word universal refers to the capacity of the instrument to be
adaptable to a great variety of work configurations and angular interrelations
(Farago and Curtis 1994).
Dividing Heads
Mechanical dividing heads are often employed for the circular measurement of
angular spacing. The optical dividing head performs the same function but more
precisely.