CHAPTER V
TYPES OF DRILLS AND DRILL SOCKETS
It should become obvious that the first cost of a drill is likely to be a very unimportant matter, as the ultimate criterion of the price of the tool is the number of holes which can be drilled before it is worn out. Such being the case, the wise purchasing agent will always decide upon the type of drill
which the experience of men in the manufacturing departments of his company has shown to be capable of giving the greatest amount of service per unit cost.
Two-fluted Twist Drills.
By far the majority of drilling operations are performed with either the milled twist drill or the so-called flat-twisted drill. illustrated at B, Both of these types have certain features to commend them to the consideration of the average machine shop manager. It is generally conceded that greater accuracy is secured through drilling with the milled twist drill, but to offset
this feature, the flat-twisted drill has about 60 per cent more chip clearance in the flutes. This reduces the amount of power required to drive the drill, and also makes it less liable that the drill will be broken. To offset the claim for greater accuracy of holes drilled with milled twist drills, the claim is made that, as most holes have to be reamed before their accuracy can be depended upon, it is economical to take advantage of the lower price of the flat-twisted drill. These drills are made by twisting a hot flat steel bar to the required form.
Three- and Four-fluted Drills.
— In drilling large holes, it will sometimes be found that owing to an insufficient amount of power in the drilling machine on which the operation is to be performed, or for some other reason, a hole of the required size cannot be drilled at one operation. In such cases, a small hole is drilled, after which this hole is drilled out to the required size at a second operation. For drilling out or enlarging the hole, use is generally made of either a three- or four-fluted twist drill . These drills are not adapted for drilling holes from solid metal, but are used only for enlarging a hole already drilled.Straight-fluted and Flat Drills.
Straight-fluted Drill
— While drilling brass and thin sheet metal, trouble is sometimes experienced through the rake of the cutting edges causing the drill to " catch " or " dig in." To overcome this trouble, profitable use may be made of a straight-fluted drill. By having two straight flutes in this type of drill, the rake angle of
the cutting edges is eliminated, which is found advantageous for certain classes of drilling for the reasons just mentioned.
Flat Drills.
There are shown two types of drills which are employed for the same general classes of service for which the straight-fluted drill is used, although the form of the two latter types is quite different from that of the drill with straight flutes.The type known as a flat " track drill '' and has a shank of suitable form for use in a blacksmith's drill-ing machine; Another type of flat drill is made by milling away from J to f of the material at opposite sides of a piece of round steel and providing two cutting edges in the manner shown.
Such a drill may be employed for drilling brass or thin sheet metal, although it does not find very general application at the present time.
The Teat Drill.
— The drill shown at H, Fig. i, is generally known as a " teat " drill. This type of drill is employed fordrilling shallow holes where it is required to have a flat bottom in the hole, or the teat drill may be used for drilling out the bottom of a hole produced with a regular twist drill in order to produce a flat bottom.
Center Drill and Countersink.
The familiar combination center drill and countersink is used for producing centers in which twist drills may be started, for centering work ready to be set up on a lathe, etc. This tool finds such general application in the machine shop that it requires no further discussion.
Oil Tube Drills.
— For drilling rather deep holes, it may befound that trouble will be experienced in getting the required
quantity of oil or cutting compound to the point of the drill,
due to tendency of the chips to carry the fluid back with them
before it reaches the bottom of the hole. To overcome this
difficulty, a practice is made of using what are known as " oil
tube " drills which are provided with ducts through which the
lubricant can be carried right to the cutting point, and after
doing its work the lubricant and chips escape through the flutes
of the drill in the usual manner. At/, Fig. i, there is shown the
most generally used type of oil tube drill which will be seen to
have two oil ducts runnning through the metal between the two
flutes in the drill.
Hollow and Rifle Barrel Drills.
— At K and L, Fig. i , there
are shown types of drills which are intended for drilling deeper
holes than could be handled with the type of drill shown at /.
The drill point at K is known as a " hollow " drill and is gen-
erally used in a lathe where the work revolves and the drill
remains stationary. There is a hole extending lengthwise
through the shank to connect the grooves with a tubular shank
of the required length for the depth of hole that is to be ma-
chined in the' work. The drill point is threaded and fitted into
this tube. Drill point L is part of what is known as a "rifle
barrel " drill. This steel point will be seen to have a crescent-
shaped duct through which oil is forced at a pressure of about
800 pounds per square inch in order that it may wash the chips
back through the straight flute at the side of the drill. This
steel drill point is secured to the end of a tube which is flattened
at one side in order that oil may flow through the tube and
escape through a channel outside the tube which corresponds
in shape to the single flute in the drill point. It will be ap-
parent that this type of drill has one cutting lip and that the
clearance surface at the point of the drill is backed oflf to pro-
vide the necessary clearance for the cutting edge.
The Cannon Drill.
— In general respects the deep-hole drillshown at M is of the same design as that shown at L, except
that the cutting edge is " stepped " instead of straight, in order
to provide for breaking up the chips into such a form that they
will be more readily cleared from the hole. This is known as a
" cannon " drill, and is adapted for the drilling of larger sized
holes than are cut by the drill point shown at L. In the case
of the last three types of drill points shown in this illustration,
the work revolves and the drill remains stationary, a method that
produces greater accuracy.
Spiral and Rake Angles of Twist Drills.
— In addition to thestandard dimensions of twist drills which have been given in
connection with instructions for drill grinding, the following
angles and dimensions are fairly standard: At the point of the
drill, the spiral angle of the flute makes an angle of 25 degrees
with the center line of the drill, and this angle is gradually
decreased to 20 degrees at the end of the flute adjacent to the
shank, the necessity for this change in angle being to increase
the chip clearance to compensate for the thickening of the web
of the drill. Drills are not made of the same diameter from
end to end, but decrease from the point toward the shank by
an amount varying from 0.00025 to 0.0015 inch per inch of
length, according to the diameter of the drill. The body of
the drill is not made exactly round, it being general practice to
back off or relieve the land of the drill from a Short distance
behind the leading edge of each flute. The thickness of the web
of the drill also increases from the point to the shank and thcf
standard rake angle for the cutting edges is 3I degrees. In a
drill made by the Detroit Twist Drill Co;, the rake angle is
increased to 4^ degrees and the spiral angle of the flutes is 32
degrees at the point of the drill and 27 degrees at the ends of the
flutes adjacent to the shank, thus representing an increase of 7
degrees in each case, from the angles which are recognized as
representative of standard practice.
Drill Shanks.
Just as the milled twist drill and the flat-twisted drill are the two types with which a great majority of
drilling operations are performed, so the taper shank or the
straight shank are the two types of shanks that are most com-
monly used. At the same time, there are a number of different
types of drill shanks which have certain points of merit in which
the reader will be mterested. In Figs. 2 and 3 are shown illus-
trations of a number of different types of drill shanks, and the
advantages of these will be very briefly discussed. Starting
with the taper shank, the reader's attention is called to the
fact that drills are made in a great variety of sizes to meet
the requirements of various classes of work, but to simplify
the problems connected with drill manufacture, there is not
the same diversity in the sizes of drill shanks. There are six
sizes of drill shanks with Morse tapers running from Nos. i to
6, inclusive. The ranges of drill sizes for each of these shanks
are shown in Table i.
Next to the taper shank, as regards the extent to which it is
used, comes the straight shank, and drills with shanks of this
type are used in various types of drill chucks. One of the
chief claims made for this type is that its use avoids the pos-
sibility, of trouble through twisting off the tang at the end of a
taper shank drill. At C, Fig. 2, there is shown what is known
as the " double-grooved " or Graham shank which is machined
with two parallel grooves of uniform depth and angle. These
grooves correspond to the shape of jaws of the chuck in which
th^ shank is held, thus preventing all possibility of the shank
slipping. With this form of shank it is possible to hold with
one chuck a complete range of sizes of drills or other tools, and
to tighten or release the chuck without the use of wrenches,
drifts, or other tools. A somewhat similar shank is shown at
Z), this being adapted for use in what is known as a " black-
smith's " drill press. Here it will be apparent that there is a
flat milled on the shank of the drill to provide a positive drive.
A flat-twisted drill of the type made by the Celfor Tool Co. is
shown at E and this drill has a shank particularly adapted for
use in connection with the drill chuck manufactured by this
company. Used in connection with the Celfor chuck, this
type of shank provides a positive drive and its form is exception-
ally well adapted to requirements which arise in connection with
manufacturing flat-twisted drills. A disadvantage of this type
of shank, however, is that it must be used in connection with a
special chuck. At F there is shown a drill with a square shank
adapted for use in a ratchet. Both this type of shank and the
shank for blacksmiths' drills shown at D are used to a very
slight extent in manufacturing plants, their chief field being
in the jobbing shop and on repair work, etc.
Drill Sockets.
Mention has already been made of the six
different Morse taper drill shanks that are in general use. The
spindle of a drilling machine is bored with a taper socket to fit
the size of shank covering the range of drill sizes which will be
most generally used upon the drilling machine in question. At
the same time, the occasion will frequently arise for using a drill
with a shank of some size other than the one corresponding to
the taper socket in the drilling machine spindle. To meet the
requirements of such cases, use is made of the familiar drill
sockets which have an inside taper hole corresponding to the
shank of the drill it is required to use, and a taper on the out-
side which corresponds to the socket bored in the drilling ma-
chine spindle. Examples of such sockets are shown at G, H,
and /, Fig. 2, these being for what are known as " short " shank
drills; and, in addition to these sockets, most manufacturers also
make a series of sockets of the same types, but adapted for
use in connection with ** long '' shank drills. The socket shown
at / is provided with a shank which is left rough so that it may
be machined to fit the spindle of the machine tool in which the
socket will be used. Tables 2 and 3 give the ranges of combina-
tions of inside and outside tapers in which sockets of the types
shown at G and H are made. At / there is shown a special
form of split socket or sleeve which provides for using a drill
with a grooved shank of the form shown at C in a drilling
machine equipped with the familiar Morse taper socket in the
spindle. It will at once be apparent that the outside of this
sleeve is of the usual taper shank form, while two jaws on the
inside of the sleeve enter the grooves milled in the shank of
the drill.
For use in connection with high-speed steel drills where a
strong positive drive is required, the Morse Twist Drill & Ma-
chine Co. makes sockets of the form shown dXK, Fig. 2, which
have a clutch to engage a corresponding clutch member between
the taper shank and the body of the drill which is shown below
the socket at Ki, The drill shank has no tang; therefore, no
dependence is placed upon this method of driving and there is
no danger of trouble from broken tangs. Another method of
overcoming trouble from this source is shown in the case of
drill shanks L and Af , Fig. 2, both of which are adapted for use
in the type of socket shown at N and N^ Fig. 3, drills and
sockets of this type being made by the Pratt & Whitney Co. The
socket is of the usual type, except that there is a steel stud that
engages the side of the groove in the drill shank and forces the
shank back into the socket. In this way, the socket is sure to
seat itself properly, so that the maximum friction drive is secured,
and this is supplemented by the positive driving furnished by the
steel stud. As no reliance is placed upon a tang on the drill
shank, there is no danger of the shank being twisted off under
the most severe conditions of service for which high-speed steel
drills are used. At 0, Fig. 3, is shown a drill socket made by the
Rich Tool Co., of Chicago, which is tapped to receive the
threaded end of the drill shown at Oi. A socket which is made
by the Morse Twist Drill & Machine Co., and known as the
** Andrews " patent drill socket, is shown at P. This socket
is fitted with a key sliding in a radial slot in the holding head.
The key bears upon an inclined seat in the shank of the drill
and is forced to its seat by a cap fitting over the holding head.
Turning this cap by hand in one direction holds the drill firmly
in place, while turning it in the opposite direction releases the
grip so that the drill may be readily removed from the socket.
Methods of Utilizing Drills with Broken Tangs. — While
going about among machine shops in which the management
prides itself upon taking advantage of all possibilities of reducing
production costs, it is not uncommon to see a considerable
accimiulation of twist drills lying in some corner of the tool-
room waiting until such a time as they can be disposed of for
their junk value. The " business end " of such drills will
often be found in perfect condition, but they have been dis-
carded because the tang has been twisted ojff the shank, making
it impossible to drive the drill in the usual form of socket.
The old saying that " prevention is better than cure " holds
particularly true in this case, and a little care exercised by
drilling machine operators would often be the means of saving
the twisting of tangs off many drills. Despite a somewhat
general opinion to the contrary, the tang on a taper shank is not
responsible for furnishing anything like the entire driving power.
Where a shank is properly seated in its socket, friction between
the shank and socket will exert a very powerful influence in
driving the drill. To take advantage of this friction drive,
however, it is quite necessary for the socket to be perfectly
clean before the drill shank is pushed into it. If a small chip
is clinging to the inside of the socket when the drill shank is '
pushed into place, this chip will prevent the shank from coming
into contact with the socket so that advantage may be taken of
the friction drive, and, as a result, the entire strain will come
upon the tang at the end of the shank with a consequent in-
crease in the probability that this tang will be twisted off.
Several methods have been devised for the utilization of drills
on which the tang has been twisted off from the shank. At Q,
Fig. 3, there is shown a socket corresponding to the usual type
of socket, except that it is split. Even where the tang has
been twisted ojff a drill, a socket of this kind will provide a
sufficiently tight grip on the drill shank to afford the necessary
driving power. At R is shown what is known as a " tang gage.''
Where the tang has been twisted off a drill shank, this gage is
used to mark the outline of a new tang after which the shank is
ground down to the outline laid out with this gage, so that the
drill can once more be driven in the usual manner. The '* wear-
ever " drill socket shown at S is made by Scully- Jones & Co.
It has a flat on the inside of the socket to drive drills from which
the tang has been twisted off, a corresponding flat being ground
on the shank of the drill to engage this flat in the socket. The
flange at the bottom of this socket furnishes additional strength
and prevents the socket from spreading. The " use-em-up "
drill socket shown at T is made by the American Specialty Co.,
and reference to this socket will make it apparent that pro-
vision is made for using a drill with a broken tang by simply
grinding a flat on the side of the drill shank. The socket shown
at U has nothing to do with the utilization of drills on which
the tang has been twisted off from the shank. This type of
socket is for use in connection with oil tube drills. The collar
on the socket is held stationary by the supply pipe that connects
with a nipple through which oil is delivered to the tubes in the
drill that carry it direct to the cutting point.
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