Unit 11 Machining Specifications and Drawing Notes 203
common terms symbolically or with alphabetic
characters and should be used on drawings in lieu
of words when feasible. In former practice, the local
note for two holes may have specified
.250
DRILL,
TWO PLACES
and the word DEEP, if used, would
be after the value instead of the deep symbol in
front of the value.
Some hole notes contain the term TYP. This
stands for typical. When this term appears in the
specification, the note applies to all similar features
on the print, unless otherwise noted.
Slotted holes are dimensioned various ways,
as discussed in the unit on dimensioning. Usually,
a slotted hole is dimensioned by diameter, with a
center-to-center linear dimension, but the radius is
indicated as
FULL R
or just simply R, Figure 11-5.
Reamed holes are created with a machine
tool called a ream or reamer. This tool primarily
applies to metal manufacturing and creates a very
true, smooth, and accurately sized hole. To create a
reamed hole, the hole is initially created as a drill hole
slightly smaller (.010† to .025†) than the finished size.
Then, the hole is reamed to the specified finished
size. The specification for a reamed hole, if required,
is shown in Figure 11-6. The drilled hole size may
be omitted, but if the process to create the hole is
critical, it may also be shown in the specification.
Counterbores and Countersinks
Counterbored holes have been cylindrically
enlarged on the one or both ends to form a recessed
flat shoulder. A counterbore is often used to recess
a bolt or machine screw head below the surface of
the part. A counterbore specification is shown in
Figure 11-7. Current standards provide a symbol
for counterbore and depth. The counterbore
diameter, the through hole, and the depth of the
counterbore are often given in one note that points
to the outermost circle representing a plan view
of the hole. However, separate notes can be used.
Although discouraged,
C’BORE
may be used as the
abbreviation in the callout instead of the symbol.
Spotfaced holes are simply very shallow
counterbores. The spotface provides a flat surface on
rough stock for the purpose of a bearing or seating
surface (for a bolt head, nut, etc.). See Figure 11-8. The
Figure 11-4.
These callouts use standard symbols for number of
places, diameter, depth, counterbore, and countersink.
Figure 11-5.
For a slotted hole, the diameter is usually given along
with the center-to-center linear distance. The radius is
simply indicated as
R
or
FULL
R.
Figure 11-6.
If the specification of the manufacturing processes
is deemed necessary, a drill and ream note may be
used. In former practice, drawings often specified the
manufacturing process.