Page 667 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

64 8 Chapter 23 Machining Processes: Turning and Hole Making

EXAMPLE 23.4 Material-removal Rate and Torque in Drilling
A hole is being drilled in a block of magnesium alloy Referring to Table 21.2, let’s take an average unit
with a 10-mm drill bit at a feed of 0.2 mm/rev and power of 0.5 W-s/mm3 for magnesium alloys. The
with the spindle running at N = 800 rpm. Calculate power required is then
the material-removal rate and the torque on the drill.
Power = (210)(0.5) = 105 W.
Solution The material-removal rate is calculated Power is the product of the torque on the drill and
from Eq. (23.3): the rotational speed, which in this case is
(800)(2'rr)/60 = 83.8 radians per second. Noting
that W = ]/s and] = N ni, we find that
-
(1f)(10)2
MRR = --Z- (0.2)(800)
105
T- 83.8 - 1.25 N~m.
= 12,570 mm3/min = 210 mm3/s.

23.5.4 Drill Materials and Sizes
Drills usually are made of high-speed steels (M1, M7, and M10) and solid carbides
or with carbide tips (typically made of K20 (C2) carbide), like those shown in
Fig. 23.21c and d. Drills are now commonly coated with titanium nitride or
titanium carbonitride for increased wear resistance (Section 22.5 ). Polycrystalline-
diamond-coated drills are used for producing fastener holes in fiber-reinforced plas-
tics. Because of their high wear resistance, several thousand holes can be drilled with
little damage to the material.
Although there are continued developments, standard twist-drill sizes consist
basically of the following series:
° Numerical: No. 97 (0.0059 in.) to No. 1 (0.228 in.)
° Letter: A (0.234 in.) to Z (0.413 in.)
° Fractional: Straight shank from é to lé in. (in é -in. increments) to 1% in.
(ini -in. increments), and larger drills in larger increments. Taper shank from§
to 1; in. (in é -in. increments) to 3.5 in. (in % -in. increments)
° Millimeter: From 0.05 mm (0.002 in.) in increments of 0.01 mm.
Note: 1 in. = 25.4 mm.

23.5.5 Drilling Practice
Drills and similar hole-making tools usually are held in drill chucks, which may be
tightened with or Without keys. Special chucks and collets with various quick-
change features that do not require stopping the spindle are available for use on
production machinery.
Because it does not have a centering action, a drill tends to “walk” on the
workpiece surface at the beginning of the operation. This problem is particularly
severe with small-diameter long drills and can lead to failure. To start a hole prop-
erly, the drill should be guided, using fixtures (such as a bushing) to keep it from
deflecting laterally. A small starting hole can be made with a center drill (usually
with a point angle of 60°), or the drill point may be ground to an S shape (helical or
spiral point). This shape has a self-centering characteristic-thus eliminating the
need for center drilling-and produces accurate holes with improved drill life. These
factors are particularly important in automated production with CNC machines, in
which the usual practice is to use a spot drill. To keep the drill more centered, the
point angles of the spot drill and of the drill are matched. Other alternatives for