Section 13. qxd

gun drill,
run at a high speed under a light feed,
and used to drill small long holes; the 
core drill
used to bore out cored
holes; the 
oil-hole drill,
having holes or tubes in its body through which
oil is forced to the cutting lips; 
three-
and 
four-fluted drills,
used to
enlarge holes after a leader hole has been cored, punched, or drilled
with a two-fluted drill; 
twist drills
made from flat high-speed steel or
drop-forged to desired shape and then twisted. Drills are also made of
solid carbide or of high-speed steel with an insert of carbide to form the
chisel edge and both cutting edges. They are used primarily for drilling
abrasive or very hard materials.
Drilling Recommendations
The most common tool material for
drills is high-speed steel M1, M7, and M10. General recommendations
for speeds and feeds in drilling a variety of materials are given in 
Table 13.4.7. Hole depth is also a factor in selecting drilling parameters.
A general 
troubleshooting guide
for drilling is given in Table 13.4.8.
Table 13.4.8
General Troubleshooting Guide 
for Drilling Operations 
Problem
Probable causes
Drill breakage
Dull drill; drill seizing in hole because of chips
clogging flutes; feed too high; lip relief angle
too small
Excessive drill wear
Cutting speed too high; ineffective cutting fluid;
rake angle too high; drill burned and strength
lost when sharpened
Tapered hole
Drill misaligned or bent; lips not equal; web not
central
Oversize hole
Same as above; machine spindle loose; chisel
edge not central; side pressure on workpiece
Poor hole surface finish
Dull drill; ineffective cutting fluid; welding of
workpiece material on drill margin; improperly
ground drill; improper alignment
Section_13.qxd 10/05/06 10:32 Page 13-61

---