blades. See Figure 6-13. This tool should not be
used where great precision is required. A double
square has a standard graduated blade, a beveled
blade, and a drill grinding blade.
The beveled blade has one angled edge for
measuring octagons, and another angle for
checking hexagons. A drill grinding blade also has
two angled edges. One end is beveled to 59° for
drill sharpening. The other end is beveled to 41°
and is used to check the cutting angles of machine
screw countersinks. Both ends are graduated for
measuring the length of the cutting lips, to ensure
the cutting tools are sharpened on center.
6.2.7 Combination Set
The combination set, shown in Figure 6-14,
can be adapted to perform a wide variety of
layout operations. A complete combination set
consists of a hardened blade (a No. 4 graduated
rule), a square head, a center head, and a bevel
protractor. The blade fits all three heads.
The square head has a standard 90° edge and
a 45° edge. This makes it possible for the tool to
be used as both a standard square, Figure 6-15,
and a miter square. By projecting the graduated
blade a given distance below the edge, the tool
can also be used as a depth gage. The spirit level
built into one edge of the square head allows the
tool to be used as a simple level.
The center head has two extending edges
forming a 90° angle. With the rule properly
inserted, the center head can be used to quickly
locate the center of round stock. See Figure 6-16.
The protractor head can be rotated through
180° and is graduated accordingly. The head can
be locked to the blade with a locking nut, making
Chapter 6 Layout Work 101
Figure 6-12.
The hardened steel square has true right angles, both
inside and outside, and is designed for laying out
lines that must be highly accurate. (L. S. Starrett Co.)
Figure 6-13.
The double square uses interchangeable blades,
making it more practical than a fixed blade square
for many jobs. It has a graduated blade, a beveled
blade, and a drill grinding blade.
Figure 6-14.
A combination set is designed for many different
types of layout work.
Figure 6-15.
The rule and square head of the combination set can
be used to check the squareness of two machined
surfaces.