Always keep a straightedge in a box when it is not
in use. Some straightedges are marked with two arrows,
one near each end, which indicate balance points. When
a box is not provided, place resting pads on a flat surface
in a storage area where no damage to the straightedge
will occur from other tools. Then, place the straightedge
so the two balance points sit on the resting pads.
MACHINIST'S SQUARE.--The most common
Figure 1-23.--Straightedge.
type of machinist's square has a hardened steel blade
securely attached to a beam. The steel blade is NOT
graduated. (See fig. 1-24.) This instrument is very
useful in checking right angles and in setting up work
The size of machinist's squares ranges from 1 1/2 to
36 inches in blade length. You should take the same
care of machinist's squares, in storage and use, as you
do with a micrometer.
SINE BAR.--A sine bar (fig. 1-25) is a precision
tool used to establish angles that require extremely close
accuracy. When used in conjunction with a surface
plate and gauge blocks, angles are accurate to 1 minute
Figure 1-24.--Machinist's square.
(1/60). The sine bar is used to measure angles on work
and to lay out an angle on work to be machined, or work
may be mounted directly to the sine bar for machining.
The cylindrical rolls and the parallel bar, which make up
the sine bar, are all precision ground and accurately
positioned to permit such close measurements. Be sure
to repair any scratches, nicks, or other damage before
you use the sine bar, and take care in using and storing
the sine bar. Instructions on using the sine bar are
included in chapter 2.
PARALLEL BLOCKS.--Parallel blocks
(fig. 1-26) are hardened, ground steel bars that are
used to lay out work or set up work for machining.
The surfaces of the parallel block are all either
parallel or perpendicular, as appropriate, and can be
used to position work in a variety of setups with
Figure 1-25.--Sine bars.
28.319
Figure 1-26.--Parallel blocks.
1-19