204 Print Reading for Industry
depth is often omitted. The machinist understands
to simply “break off” the rough surface at a depth of
roughly .062† or less. A new ASME standard symbol
introduced in 2009 features an “SF” inside the
counterbore bracket symbol as a way of specifying
the counterbore is a spotface. As with most new
symbols, it may not be present on older prints.
Countersunk holes have a cone-shaped
enlargement on the end. These holes are usually
used to provide a seat for conical screw heads and
rivets. Current standards provide a symbol for
countersinking holes. See Figure 11-9. Older practices
specified countersink with
C’SINK
and this may
still be found on some drawings. A countersunk
hole is often drawn with an included angle of 90fl,
even if the angle is 82fl. There are different ways to
specify the size of a countersink. Usually the
included angle is called out in the local note, which
is determined by the bit. The machinist also then
needs a specification for the larger diameter of the
rim or the depth of the countersink.
When a larger hole has a conical transition
to a smaller hole, this is called a counterdrill. The
transition is usually shown at the same angle as
a drill bit tip (120fl). Dimensioning the angle is
considered optional. The depth of the counterdrill
is considered to be the portion of the larger hole
that is full diameter. See Figure 11-10.
Holes are sometimes drilled and countersunk
for the purpose of holding the part between lathe
centers or in a machining fixture. These are called
machining centers and have no other function
than providing a place to hold the part. Machining
centers can be indicated by local note or callout.
Machining center surfaces can also be identified as
datum surfaces to establish a datum axis.
Common Machining Operations
There are various machining operations that
can be performed on a part. In the past, these
operations were performed on manual machining
equipment. Today, in many cases the machining
process has been automated with CNC equipment.
This section covers various machining processes. The
processes are the same whether performed by CNC
equipment or on manual machining equipment.
.750 THRU
1.000
.375
3/4! DIA THROUGH
1! C'BORE
3/8! DEEP
Current Standard Past Practice
Figure 11-7.
The figure on the left shows the current practice for specifying a counterbore hole. The figure on the right shows the
older practice.
.750 THRU
1.000
SPOTFACE
Figure 11-8.
The specification for a spotface is shown here.