Section 13. qxd

helix angles
of the flutes vary from
10 to 45
. The standard 
point angle
is 118
. There are a number of 
drill
grinders
on the market designed to give the proper angles. The point
may be ground either in the 
standard
or the 
crankshaft
geometry. The
drill geometry for high-speed steel twist drills for a variety of work-
piece materials is given in Table 13.4.6.
Among the common 
types of drills
(Fig. 13.4.14) are the combined
drill and countersink or 
center drill,
a short drill used to center shafts
before squaring and turning: the 
step drill,
with two or more diameters;
the 
spade drill
which has a removable tip or bit clamped in a holder on
the drill shank, used for large and deep holes; the 
trepanning tool
used
to cut a core from a piece of metal instead of reducing all the metal
1
⁄
16
1
⁄
2
1
⁄
32
1
⁄
4
1
⁄
64
3
⁄
4
1
⁄
8
1
⁄
16
1
⁄
32
1
⁄
2
1
⁄
64
1
⁄
4
1
⁄
64
1
⁄
64
Fig. 13.4.13
Straight shank twist drill.
Table 13.4.6
Recommended Drill Geometry for High-Speed Steel Twist Drillls
Point 
Lip relief 
Chisel edge
Helix
Material
angle, deg
angle, deg
angle, deg
angle, deg
Point grind
Aluminum alloys
90–118
12–15
125–135
24–48
Standard
Magnesium alloys
70–118
12–15
120–135
30–45
Standard
Copper alloys
118
12–15
125–135
10–30
Standard
Steels
118
10–15
125–135
24–32
Standard
High-strength steels
118–135
7–10
125–135
24–32
Crankshaft
Stainless steels, low-strength
118
10–12
125–135
24–32
Standard
Stainless steels, high-strength
118–135
7–10
120–130
24–32
Crankshaft
High-temperature alloys
118–135
9–12
125–135
15–30
Crankshaft
Refractory alloys
118
7–10
125–135
24–32
Standard
Titanium alloys
118–135
7–10
125–135
15–32
Crankshaft
Cast irons
118
8–12
125–135
24–32
Standard
Plastics
60–90
7
120–135
29
Standard
S
OURCE
: “Machining Data Handbook,” published by the Machinability Data Center, Metcut Research Associates Inc.
Section_13.qxd 10/05/06 10:32 Page 13-60

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REAMING
13-61
REAMING
A
reamer
is a multiple-cutting edge tool used to enlarge or finish holes,
and to provide accurate dimensions as well as good finish. Reamers are
of two types: (1) rose and (2) fluted.
The 
rose reamer
is a heavy-bodied tool with end cutting edges. It is
used to remove considerable metal and to true up a hole preparatory to
flute reaming. It is similar to the three- and four-fluted drills. Wide
cylindrical lands are provided back of the flute edges.
Fluted reamers
cut principally on the periphery and remove only 0.004
to 0.008 in (0.1 to 0.2 mm) on the bore. Very narrow cylindrical margins
are provided back of the flute edges, 0.012 to 0.015 in (0.3 to 0.4 mm)
wide for machine-finish reaming and 0.004 to 0.006 in (0.1 to 0.15 mm)
for hand reaming, to provide free cutting of the edges due to the slight
body taper and also to pilot the reamer in the hole. The hole to be flute- or
finish-reamed should be true. A rake of 5
is recommended for most ream-
ing operations. A reamer may be straight or helically fluted. The latter pro-
vides much smoother cutting and gives a better finish.