
138 Fundamentals of Machining Processes
where
l
a
=
cot χ
where
l is the length of the drilled hole (cutting length) in mm
l
o
is the amount of overrun, usually 2–3 mm
l
a
is the tool approach distance in mm
For machining blind holes, l
o
= 0. Hence,
ma
5.3.6 Dimensional Accuracy
The accuracy and shape of drilled holes is much more important than sur-
face quality. There are errors (Figure 5.21) that occur to various degrees as a
result of
N
χ
S
L
l
o
l
a
l
FIGURE 5.20
Elements of drilling tool travel.