DRILLING JIGS, GUIDES, OR FIXTURES.--When a large number of pieces are
to be drilled alike, as in the case when work is done to special gauges,
special chucking devices called jigs, or fixtures, are employed to guide
the drill, and insure that the holes shall be pierced accurately in the
required location, and test pieces or gauges are provided to test the
work from time to time to insure that errors have not arisen by reason
of the wear of these drill-guiding devices.
[Illustration: Fig. 1699.]
[Illustration: Fig. 1700.]
Suppose, for example, that we have a link, such as in Fig. 1699, and
that we require to have the holes throughout a large number of them of
equal diameter at each end and the same distance apart, and if we could
prevent the wear of the tools, and so continue to produce any number of
links all exactly alike, we could provide a simple test gauge, such as
shown in the figure, making it pass the proper distance apart, and of a
diameter to fit the holes; but as we cannot prevent wear to the tools we
must fix a limit to which such wear may be permitted to occur, and
having reached that point they must be restored and corrected. We must
at the same time possess means of testing in what direction the wear has
induced error. Let it be assumed that the bore at A should be 1/2 inch
and that at B 3/8 inch in diameter, that their distance from centre is
to be, say, six inches, and that either bore may vary in diameter to the
amount of 1/1000 inch, while the distance from centre to centre of the
bores may also vary 1/1000 inch. Now let it be noted that if one piece
be made 1/2000 inch too short, and another 1/2000 inch too long we have
reached the extent of the limit, there being 1/1000 inch difference
between them, although neither piece varies more than 1/2000 inch from
the standard. Similarly in the bore diameters, if the bore, say at A, is
1/2000 inch too large in one piece and 1/2000 too small in another,
there is a difference of 1/1000 between them, although each varies only
the 1/2000 inch from the standard. In making test gauges for the holes,
therefore, we must consider in what direction the tool will wear; thus,
suppose that the finishing reamer for the holes is made when new to the
standard diameter, and it can only wear smaller, hence a plug gauge of
the standard diameter and 1/1000 inch smaller would serve thus, as so
long as the smaller one will go in the limit of wear is not reached;
when it will not go in sufficiently easily the reamer must be restored
to fit the standard gauge. On the other hand, the reamer when new may be
made 1/1000 inch above the standard size and restored when it has worn
down to the standard size. In this case the bore diameter is still
within the limit as long as the small gauge will enter; but when it fits
too tight the reamer must be restored to the large plug gauge, the forms
of these gauges being shown in Fig. 1700.
[Illustration: Fig. 1701.]
[Illustration: Fig. 1702.]
In Figs. 1701 and 1702 we have a jig or fixture for holding the link
during the drilling process. It consists of two parts, C and D, between
which the link is held by the screws E and F. The two hubs, G and H, are
provided with hardened steel bushes, I and J, which are pierced with
holes to receive and guide the drilling tool or reamer, and it is
evident that in time the bore of these bushes will wear, and if they
wear on one side more than on another they may wear longer or shorter
between the centres or axis; hence we require gauges such as shown in
Fig. 1703, one being longer between centres and the other shorter, in
each case to the amount of the prescribed limit. In this case, so long
as the holes are kept within the prescribed limit of diameter, the
distance apart of the two holes will be within the limit so long as
neither of the limit gauges will enter; and when they will enter the
bushes I J must be restored.
It is to be remarked, however, that the variation in the diameter of the
holes affects these standards, since if the holes are made sufficiently
large either gauge would enter, although the axis of the holes and of
the pins on the gauge might be the proper distance apart; hence the
gauging for length depends to some degree upon the degree of accuracy in
gauging for diameter.
[Illustration: Fig. 1703.]
Referring now to the construction of the jig, or fixture for drilling
the link shown in Figs. 1701 and 1702: the base piece is provided with
two short hubs, R and S, upon which the link is to sit, and it is
obvious that these hubs must be faced off true with the bottom face of
the base, while the link must also be faced so that it will be level,
and not be bent or sprung when clamped by the screws E F. It is obvious
that the hubs R and S may be omitted, and the link be flat on the base
plate; but this would not be apt to hold the link so steadily, and
greater care would be required to keep the surface clean. It is also
obvious that in the form of jig shown there is a tendency of the screws
E and F to bend the piece D; but in the case of small pieces, as, say,
not exceeding 8 inches long, piece D may be made strong enough to resist
the screw pressure without bending. If, however, the link were, say, 18
inches long, it would be preferable to have projections in place of the
hubs R, S, and to let these projections extend some distance along each
end of the link, using four holding screws, and clamping the piece D on
the inside of the hubs H G. To facilitate the rapid insertion and
removal of the link into and from the jig cap-piece, D is pivoted on
screw F, while a slot V is cut at the other end, so that when the two
screws E, F are loosened, the cap-piece D may be swung out of the way
without entirely removing it.
[Illustration: Fig. 1704.]
[Illustration: Fig. 1705.]
[Illustration: Fig. 1706.]
[Illustration: Fig. 1707.]
In Fig. 1704 we have a link in which a hole is to be bored at one end at
a certain distance from a pin at the other, and the fixture, or jig for
drilling, is shown in the sectional view, Fig. 1705, the side view, Fig.
1706, and the top view, Fig. 1707. It is obvious that the pin P and the
face W of the link must be made true, and that a hardened steel bush may
be placed in the hub to receive the pin P. The screw E binds one end of
the cap D, and eye-bolts with thumb-nuts F bind the other, these bolts
being pivoted at their lower ends, and passing through slots in D, so
that as soon as nuts F are loosened, their bolts may be swung out clear
of the cap, which may be swung on one side from the pin N as a pivot.
[Illustration: Fig. 1708.]
[Illustration: Fig. 1709.]
In Fig. 1708 we have a piece containing three holes, which are to be
drilled in a certain position with regard to each other, and with regard
to the face A. This brings us to the consideration that in all cases the
work must be chucked or held true by the faces to which it is necessary
that the holes must be true, and as in this case it is the face A, the
jig must be made to hold the piece true by A, the construction being as
in Fig. 1709, which represents a top view, and a sectional side view.
The upper plate D carries three hardened steel bushes, A, B, and C, to
receive the drilling tools, and thus determine that the holes shall be
drilled at their proper positions with relation to each other, and is
provided with a face N, against which the face (A, Fig. 1708) may be
secured by the screw H, and thus determine the positions of the holes
with, regard to that face. At E, F, and G are eye-bolts for clamping the
work between the cap and the base plate, which is made large so that it
may lie steadily on the table of the drilling machine. When the nuts E,
F, and G and the screw H are loosened the cap D may be lifted off and
the work removed.
If the holes are required to be made very exact in their positions with
relation to one edge, as well as to the face A of the work, two screws K
would be required, one binding the cap against the lug M of the base,
and the other binding the edge of the work against the same lug.
The usefulness of jigs, or fixtures, is mainly confined to small work in
which a great many duplicate pieces are to be made, and their designing
calls for a great deal of close study and ingenuity. They can obviously
be applied to all kinds of small work, and as a general principle the
holes and pins of the work are taken as the prime points from which the
work is to be held.
Drilling fixtures may, however, be applied with great advantage to work
of considerable size in cases where a number of duplicate parts are to
be made, an example of this kind being given in the fixtures for
drilling the bolt holes, &c., in locomotive cylinders.
[Illustration: Fig. 1710.]
For drilling the cylinder covers and the tapping holes in the cylinder,
the following device or fixture is employed: The flanges of the cylinder
covers are turned all of one diameter, and a ring is made, the inside
diameter of which is, say, an inch smaller than the bore of the
cylinder; and its outside diameter is, say, an inch larger than the
diameter of the cover. On the outside of the ring is a projecting flange
which fits on the cover, as in Fig. 1710, _a_ being the cylinder cover,
and _b_ _b_ a section of the ring, which is provided with holes, the
positions in the ring of which correspond with the required positions of
the holes in the cover and cylinder; the diameter of these holes (in the
ring, or template, as it is termed) is at least one quarter inch larger
than the clearing holes in the cylinder are required to be. Into the
holes of the template are fitted two bushes, one having in its centre a
hole of the size necessary for the tapping drill, the other a hole the
size of the clearing drill; both these bushes are provided with a handle
by which to lift them in and out of the template, as shown in Fig. 1711,
and both are hardened to prevent the drill cutting them, or the borings
of the drill from gradually wearing their holes larger. The operation is
to place the cover on the cylinder and the template upon the cover, and
to clamp them together, taking care that both cover and template are in
their proper positions, the latter having a flat place or deep line
across a segment of its circumference, which is placed in line with the
part cut away on the inside of the cover to give free ingress to the
steam, and the cover being placed in the cylinder so that the part so
cut away will be opposite to the port in the cylinder, by which means
the holes in the covers will all stand in the same relative position to
any definite part of the cylinder, as, say, to the top or bottom, or to
the steam port, which is sometimes of great importance (so as to enable
the wrench to be applied to some particular nut, and prevent the latter
from coming into contact with a projecting part of the frame or other
obstacle): the positions of the cylinder, cover, template, and bush,
when placed as described, being such as shown in Fig. 1712, _a_ _a_
being the cylinder, B the steam port, C the cylinder cover, D the
template, and E the bush placed in position. The bush E having a hole in
it of the size of the clearance hole, is the one first used, the drill
(the clearance size) is passed through the bush, which guides it while
it drills through the cover, and the point cuts a countersink in the
cylinder face. The clearing holes are drilled all round the cover, and
the bush, having the tapping size hole in it, is then brought into
requisition, the tapping drill being placed in the drilling machine, and
the tapping holes drilled in the cylinder flange, the bush serving as a
guide to the drill, as shown in Fig. 1712, thus causing the holes in the
cover and those in the cylinder to be quite true with each other. A
similar template and bush is provided for drilling the holes in the
steam chest face on the cylinder, and in the steam chest itself. While,
however, the cylinder is in position to have the holes for the steam
chest studs drilled, the cylinder ports may be cut as follows:--
[Illustration: Fig. 1711.]
[Illustration: Fig. 1712.]
[Illustration: Fig. 1713.]
The holes in the steam chest face of the cylinder being drilled and
tapped, a false face or plate is bolted thereon, which plate is provided
with false ports or slots, about three-eighths of an inch wider and
three-fourths of an inch longer than the finished width and length of
the steam ports in the cylinder (which excess in width and length is to
allow for the thickness of the die). Into these false ports or slots is
fitted a die to slide (a good fit) from end to end of the slots. Through
this die is a hole, the diameter of which is that of the required
finished width of the steam ports of the cylinder; the whole appliance,
when in position to commence the operation of cutting out the cylinder
ports, being as illustrated in Fig. 1713, _a_ _a_ being the cylinder, B
B the false plate, C the sliding die, and D D the slots or false ports
into which the die C fits. Into the hole of the die C is fitted a
reamer, with cutting edges on its end face and running about an inch up
its sides, terminating in the plain round parallel body of the reamer,
whose length is rather more than the depth of the die C. The operation
is to place the reamer into the drilling machine, taking care that it
runs true. Place the die in one end of the port, as shown in Fig. 1713,
and then wind the reamer down through the die so that it will cut its
way through the port of the cylinder at one end; the spindle driving the
drill is then wound along. The reamer thus carries the die with it, the
slot in the false face acting as a guide to the die.
In the case of the exhaust port, only one side is cut out at a time. It
is obvious that, in order to perform the above operation, the drilling
machine must either have a sliding head or a sliding table, the sliding
head being preferable.
[Illustration: Fig. 1714.]
The end of the slot at which the die must be placed when the reamer is
wound down through the die and cylinder port, that is to say, the end of
the port at which the operation of cutting it must be commenced, depends
solely on which side of the port in the cylinder requires most metal to
be cut off, since the reamer, or cutter, as it may be more properly
termed, must cut underneath the heaviest cut, so that the heaviest cut
will be forcing the reamer back, as shown in Fig. 1714, _a_ being a
sectional view of the cutter, B the hole cast in the cylinder for the
port, _c_ the side of the port having the most cut taken off, D the
direction in which the cutter _a_ revolves, and the arrow E the
direction in which the cutter _a_ is travelling up to its cut. If the
side F of the port were the one requiring the most to be cut off, the
cutter _a_ would require to commence at the end F, and to then travel in
the direction of the arrow G. The reason for the necessity of observing
these conditions, as to the depth of cut and direction of cutter travel,
is that the pressure of the cut upon the reamer is in a direction to
force the reamer forward and into its cut on one side, and backward and
away from its cut on the other side, the side having the most cut
exerting the most pressure. If, therefore, the cutter is fed in such a
direction that this pressure is the one tending to force the cutter
forward, the cutter will spring forward a trifle, the teeth of the
cutter taking, in consequence, a deep cut, and, springing more as the
cut deepens, terminate in a pressure which breaks the teeth out of the
cutter.
If, however, the side exerting the most pressure upon the reamer is
always made the one forcing the cutter back, as shown in Fig. 1714, by
reason of the direction in which the cutter is travelled to its cut, the
reamer, in springing away from the undue pressure, will also spring away
from its cut, and will not, therefore, rip in or break, as in the former
case.
[Illustration: Fig. 1715.]
In cutting out the exhaust port, only one side, in consequence of its
extreme width, may be cut at one operation; hence there are two of the
slots D, Fig. 1713, provided in the false plate or template for the
exhaust port. The cutter _a_ must, in this case, perform its cut so that
the pressure of the cut is in a direction to force the cutter backwards
from its cut. The time required to cut out the ports of an ordinary
locomotive cylinder, by the above appliance, is thirty minutes, the
operation making them as true, parallel, and square as can possibly be
desired.
DRILLS AND CUTTERS FOR DRILLING MACHINES.--In the drilling machine, as
in the lathe, the twist drill is the best tool that can be used for all
ordinary work, since it produces the best work with the least skill, and
is the cheapest in the end. As, however, the twist drill has been fully
discussed with reference to its use upon lathe work, it is unnecessary
to refer to it again more than to say that it possesses even greater
advantages when used in the drilling machine than it does when used in
the lathe; because as the drill stands vertical the flat drill will not
relieve itself of the cuttings, and in deep holes must be occasionally
withdrawn from the hole in order to permit the cuttings to be extracted,
an operation that often consumes more time than is required for the
cutting duty. Furthermore, as flat drills rarely run true they place
excessive wear upon the drilling machine spindle, causing it to wear
loose in its bearings, which is a great detriment to the machine.
Fig. 1715 represents a piece of work that can be readily drilled with a
twist drill but not with a flat one, such work being very advantageous
in cutting out keyways. All that is necessary is to drill the three
holes B first, and if the drill runs true and the work is properly held
and the drill fed slowly while run at a quick speed the operation may be
readily performed.
The speeds and feeds for twist drills are given in connection with the
use of the drill in the lathe, but it may be remarked here that more
duty may be obtained by hand than by automatically feeding a drill,
because in hand feeding the resistance of the feed motion indicates the
amount of pressure on the drill, and the feed may be increased when the
conditions (such as soft metal) permits, and reduced for hard spots or
places, thus preserving the drill. Furthermore, the dulling of the drill
edges becomes more plainly perceptible under hand feeding.
The commercial sizes of both taper and straight shank twist drills are
as follows:--
--------+-------++-------+-------++-------+-------++--------+-------
Dia- |Length.|| Dia- |Length.|| Dia- |Length.|| Dia- |Length.
meter. | ||meter. | ||meter. | ||meter. |
--------+-------++-------+-------++-------+-------++--------+-------
1/4 | 6-1/8 || 25/32| 9-7/8 ||1-5/16 |14-1/4 ||1-27/32 |16-3/8
9/32 | 6-1/4 || 13/16|10 ||1-11/32|14-3/8 ||1-7/8 |16-1/2
5/16 | 6-3/8 || 27/32|10-1/4 ||1-3/8 |14-1/2 ||1-29/32 |16-1/2
11/32 | 6-1/2 || 7/8 |10-1/2 ||1-13/32|14-5/8 ||1-15/32 |16-1/2
3/8 | 6-3/4 || 29/32|10-5/8 ||1-7/16 |14-3/4 ||1-31/32 |16-1/2
13/32 | 7 || 15/16|10-3/4 ||1-15/32|14-7/8 ||2 |16-1/2
7/16 | 7-1/4 || 31/32|10-7/8 ||1-1/2 |15 ||2-1/32 |16-1/2
15/32 | 7-1/2 ||1 |11 ||1-17/32|15-1/8 ||2-1/16 |17
1/2 | 7-3/4 ||1-1/32 |11-1/8 ||1-9/16 |15-1/4 ||2-1/8 |17
17/32 | 8 ||1-1/16 |11-1/4 ||1-19/32|15-3/8 ||2-3/16 |17
9/16 | 8-1/4 ||1-3/32 |11-1/2 ||1-5/8 |15-1/2 ||2-1/4 |17-1/2
19/32 | 8-1/2 ||1-1/8 |11-3/4 ||1-21/32|15-5/8 ||2-5/16 |17-1/2
5/8 | 8-3/4 ||1-5/32 |11-7/8 ||1-11/16|15-3/4 ||2-3/8 |18
21/32 | 9 ||1-3/16 |12 ||1-23/32|15-7/8 ||2-7/16 |18-1/2
11/16 | 9-1/4 ||1-7/32 |12-1/8 ||1-3/4 |16 ||2-1/2 |19
23/32 | 9-1/2 ||1-1/4 |12-1/2 ||1-25/32|16-1/8 || |
3/4 | 9-3/4 ||1-9/32 |14-1/8 ||1-13/16|16-1/4 || |
--------+-------++-------+-------++-------+-------++--------+-------
Twist drills are also made to the Stubs wire gauge as follows:--
+-----------------+---------++-----------------+--------+
|Numbers by gauge.| Length. ||Numbers by gauge.| Length.|
+-----------------+---------++-----------------+--------+
| 1 to 5 | 4 || 31 to 35 | 2-5/8 |
| 6 " 10 | 3-11/16 || 36 " 40 | 2-7/16 |
| 11 " 15 | 3-1/2 || 41 " 45 | 2-1/4 |
| 16 " 20 | 3-1/4 || 46 " 50 | 2-1/16 |
| 21 " 25 | 3-1/16 || 51 " 60 | 1-3/4 |
| 26 " 30 | 2-13/16 || 61 " 70 | 1-1/2 |
+-----------------+---------++-----------------+--------+
[Illustration: Fig. 1716.]
Fig. 1716 represents the flat drill, which has three cutting edges, A,
B, and C. The only advantages possessed by the flat drill are that it
will stand rougher usage than the twist drill, and may be fed faster,
while it can be more easily made. Furthermore, when the work is
unusually hard the flat drill can be conveniently shaped and tempered to
suit the conditions.
The drill is flattened out and tapered thinnest at the point C. The side
edges that form the diameter of the drill are for rough work given
clearance, but for finer work are made nearly cylindrical, as in the
figure.
The flattening serves two purposes: first, it reduces the point of the
drill down to its proper thinness, enabling it to enter the metal of the
work easily, and secondly, it enables the cuttings to pass upward and
find egress at the top of the hole being drilled.
The cutting edges are formed by grinding the end facets at an angle as
shown, and this angle varies from 5° for drilling hard metal, such as
steel, to 20° for soft metal, such as brass or copper.
[Illustration: Fig. 1717.]
The angle of one cutting edge to the other varies from 45° for steel to
about 35° or 40° for soft metals. The object of these two variations of
angles is that in hard metal the strain and abrasion is greatest and the
cutting edge is stronger with the lesser degree of angle, while in
drilling the softer metals the strain being less the cutting edge need
not be so strong and the angles may be made more acute, which enables
the drill to enter the metal more easily. The most imperfect cutting
edge in a drill is that running diagonally across the point, as denoted
by A in Fig. 1717, because it is less acute than the other cutting
edges, but this becomes more acute and, therefore, more effective, as
the angles of the facets forming it are increased as denoted by the
dotted lines in the figure. It is obvious, however, that the more acute
these angles the weaker the cutting edge, hence an angle of about 5° is
that usually employed.
It is an advantage to make the cutting edge at A, Fig. 1717, as short as
possible, which may be done by keeping the drill point thin; but if too
thin it will be apt either to break or to operate in jumps (especially
upon brass), drilling a hole that is a polygon instead of a true circle.
The cutting edges should not only stand at an equal degree of angle to
the axial line of the drill, but should be of equal lengths, so that the
point of the drill will be in line with the axial line of the drill. If
the drill runs true the point will then be in the axial line of
rotation, and the diameter of hole drilled will be equal to the diameter
of the drill.
If, however, one cutting edge is longer than the other the hole drilled
will be larger than is due to the diameter of the drill.
[Illustration: Fig. 1718.]
Suppose, for example, the drill to be ground as in Fig. 1718, the
cutting edge F being the longest and at the least angle, then the point
G of the drill, when clear of the work, will naturally revolve in a
circle around the axial line H of the drill's rotation. But when the
drilling begins, the point of the drill meets the metal first and
naturally endeavours to become the centre of rotation, drilling a
straight conical recess, the work moving around with the point of the
drill. If the work is prevented from moving, either the drill will
spring or bend, the point of the drill remaining (at first) the centre
of rotation at that end of the drill, or else the recess cut by the
drill will be as in the figure, and the hole will be larger in diameter
than the drill.
[Illustration: Fig. 1719.]
[Illustration: Fig. 1720.]
If, however, the drill is ground as shown in Fig. 1719, the edge E being
nearest to a right angle to the axial line H of the drill, the drilling
will be performed as shown in the figure, the edge E cutting the cone L,
the edge F serving simply to enlarge the hole drilled by E. Here, again,
if the work is held so that it cannot move, the point of the drill will
revolve in a circle, and in either case, so soon as the point of the
drill emerges the diameter of the hole drilled will decrease, the
finished hole being conical as shown in Fig. 1720 at A.
It may be remarked that the eye of the workman is (for rough work, such
as tapping or clearing holes) sufficient guide to enable the grinding of
the drill true enough to partly avoid the conditions shown in these two
figures (in which the errors are magnified for clearness of
illustration), because when the want of truth is less in amount than the
thickness of the drill point, the centre of motion of the drill point
when the drill has entered the work to its full diameter becomes neither
at the point of the drill nor in the centre of its diameter, but
intermediate between the two.
[Illustration: Fig. 1721.]
[Illustration: Fig. 1722.]
Thus, in Fig. 1721, A is the centre of the diameter of the drill, but
the cutting edge C being shorter than D throws the point of the drill
towards E, hence the extra pressure of D on the incline of the recess it
cuts, over the like pressure exerted by C tends to throw the centre of
rotation towards E, the natural endeavor of the drill point to press
into the centre of the recess acting in the same direction. This is in
part resisted by the strength of the drill, hence the centre of rotation
is intermediate as at B in figure. The dotted circle is drawn from the
axial line of the drill as a centre, while the full circle is drawn from
B as a centre. The result of this would be that the point of the drill
would perform more duty than is due to its thickness, and the recess cut
would have a flat place at the bottom, as shown in Fig. 1722 at O. This,
from the want of keenness of the cutting edge running diagonally across
the drill point, would cause the drill to cut badly and require more
power to drive and feed.
[Illustration: Fig. 1723.]
The edges at the flat end of the drill, as at A, A in Fig. 1723, should
have a little clearance back from the cutting edge though they may be
left the full circle as, at A, A, but in any event they should not have
clearance sufficient to form them as at B, B, Fig. 1723, because in that
case the side edges C, C would cut the sides of the hole. In large
drills, especially, it is necessary that the edges have but little
clearance, and that the form of the clearance be as shown in Fig. 1044,
with reference to twist drills. When no edge clearance whatever is given
the edges act to a certain extent as guides to the drill, but if the
drill is not ground quite true this induces a great deal of friction
between the edges of the drill and the side of the hole.
In any case of improper grinding the power required to drive the drill
will be increased, because of the improper friction induced between the
sides of the drill and the walls of the hole.
For use on steel, wrought iron, and cast iron the lip drill shown in
Fig. 1724 is a very efficient tool. It is similar to the flat drill but
has its cutting edge bent forward. It possesses the keenness of the
twist drill and the strength of the flat drill, but as in the case of
all drills whose diameters are restored by forging and hand grinding, it
is suitable for the rougher classes of work only, and requires great
care in order to have it run true and keep both cutting edges in action.
It is sometimes attempted to give a greater cutting angle to a flat
drill by grinding a recess in the front face, as at A in Fig. 1725, but
this is a poor expedient.
Fig. 1726 represents what is known as the tit drill. It is employed to
flatten the bottoms of holes, and has a tit T which serves to steady it.
The edges A, B of this drill may be turned true and left without
clearance, which will also serve to steady the drill. The tit T should
be tapered towards the point, as shown, which will enable it to feed
more easily and cut more freely. The speed of the drill must be as slow
again as for the ordinary flat drill, and not more than one-third as
fast as the twist drill.
To enable a drill to start a hole in the intended location the
centre-punch recess in the centre of that location should be large
enough in diameter at the top to admit the point of the drill, that is
to say, the recess should not be less in diameter at the top than the
thickness of the drill point.
[Illustration: Fig. 1724.]
[Illustration: Fig. 1725.]
[Illustration: Fig. 1726.]
[Illustration: Fig. 1727.]
If the drill does not enter true the alteration is effected as shown in
Fig. 1727, in which A represents the work, B a circle of the size of the
hole to be drilled, and C the recess cut by the drill, while D is a
recess cut with a round-nosed chisel, which recess will cause the drill
to run over in that direction.
[Illustration: Fig. 1728.]
It is a good plan when the hole requires to be very correctly located to
strike two circles, as shown in Fig. 1728, and to define them with
centre-punch marks so that the cuttings and oil shall not erase them, as
is apt to be the case with lines only. The outer circle is of the size
of hole to be drilled, the inner one serves merely as a guide to true
the drilling by.
If the work is to be clamped to the work table an alteration in the
location of the recess cut by the drill point may be made by moving the
work. In this case the point of the drill may be fed up so as to enter
into and press against the centre-punch mark made in the centre of the
location of the hole to be drilled, which, if the drill runs true will
set the work true enough to clamp it by. The alteration to the recess
cut by the drill when first starting to bring the hole in its true
position should be made as soon as a want of truth is discernible,
because the shallower the recess the more easily the alteration may be
made.
Sometimes a small hole is drilled as true to location as may be, and
tested, any error discovered being corrected by a file; a larger drill
is then used and the location again tested, and so on; in this way great
precision of location may be obtained.
The more acute angle the cutting edges form one to the other, or in
other words, the longer the cutting edges are in a drill of a given
diameter, the more readily the drill will move over if one side of the
recess be cut out as in Fig. 1727, and from some experiments made by
Messrs. William Sellers and Co., it was determined that if the angle of
one cutting edge to the other was more than 104° the drill would cease
to move over.
In drilling wrought iron or the commoner qualities of steel the drill
should be liberally supplied with either water or oil, but soapy water
is better than pure. This keeps the drill cool and keeps the cutting
edge clean, whereas otherwise the cuttings under a coarse feed are apt
to stick fast to the drill point if the speed of the drill is great.
Furthermore, under excessive duty the drill is apt to become heated and
softened.
For cast steel oil is preferable, or if the steel be very hard it will
cut best dry under a slow speed and heavy pressure.
For brass and cast iron the drill should run dry, otherwise the cuttings
clog and jam in the hole. When the drill squeaks either the cutting edge
is dulled and the drill requires regrinding, or else the cuttings have
jammed in the hole, and either defect should be remedied at once.
As soon as the point of the drill emerges through the work the feed
should be lessened, otherwise the drill is apt to force through the
weakened metal and become locked, which will very often either break or
twist the drill. This may be accomplished when there is any end play to
the drilling machine spindle by operating the feed motion in a direction
to relieve the feed as soon as the point of the drill has emerged
through the bottom of the hole, thus permitting the weight of the
spindle to feed the drill. In a drilling machine, however, in which the
weight of the spindle is counterbalanced, the feed may be simply reduced
while the drill is passing through the bottom of the hole.
Drills for work of ordinary hardness are tempered to an orange purple,
but if the metal to be cut is very hard a straw color is preferable, or
the drill may be left as hard as it leaves the water; that is to say
hardened, but not tempered. In these cases the speed of the drill must
be reduced.
To assist a drill in taking hold of hard metal it is an excellent plan
to jag the surface of the metal with a chisel which will often start the
drill to its cut when all other means have failed. It is obvious from
previous remarks that the harder the drill the less the angle of the end
facets.
In cases of extreme hardness two drills may with advantage be used
intermittently upon the same hole; one of these should have its cutting
edges ground at a more acute angle one to the other than is the case
with the other drill, thus the cutting edge will be lessened in length
while the drill will retain the strength due to its diameter, so that a
maximum of pressure may be placed upon it. When one drill has cut deep
enough to bring its full length of cutting edge into action, it may be
removed and the other drill employed, and so on.
The drill (for hard steel) should be kept dry until it has begun to cut,
when a very little oil may be employed, but for chilled cast iron it
should be kept dry.
Small work to be drilled while resting upon a horizontal table may
generally be held by hand, and need not therefore be secured in a chuck
or to the table, because the pressure of the drill forces the work
surface to the table, creating sufficient friction to hold the work from
rotating with the drill. For large holes, however, the work may be
secured in chucks or by bolts and plates as described for lathe and
planer work, or held in a vice.
The following table for the sizes of tapping holes is that issued by the
Morse Twist Drill and Machine Co. In reply to a communication upon the
subject that company states. "If in our estimate the necessary diameter
of a tap drill to give a full thread comes nearest to a 1/64 inch
measurement, we give the size of the drill in 64ths of an inch. If
nearest to a 32nd size of drill we give the drill size in 32nds of an
inch."
In the following table are given the sizes of tapping drills, to give
full threads, the diameters being practically but not decimally
correct:--
-------+-----------------+-----------------------
Dia- | Number | Drill for
meter | threads | [V]-thread.
of tap.| to inch. |
-------+-----------------+-----------------------
1/4 |16 18 20| 5/32 5/32 11/64
9/32 |16 18 20| 3/16 13/64 13/64
5/16 |16 18 --| 7/32 15/64 --
11/32|16 18 --| 1/4 17/64 --
3/8 |14 16 18| 1/4 9/32 9/32
13/32|14 16 18| 19/64 21/64 21/64
7/16 |14 16 --| 21/64 11/32 --
15/32|14 16 --| 23/64 3/8 --
1/2 |12 13 14| 3/8 25/64 25/64
17/32|12 13 14| 13/32 27/64 27/64
9/16 |12 14 --| 7/16 29/64 --
19/32|12 14 --| 15/32 31/64 --
5/8 |10 11 12| 15/32 1/2 1/2
21/32|10 11 12| 1/2 17/32 17/32
11/16|11 12 --| 9/16 9/16 --
23/32|11 12 --| 19/32 19/32 --
3/4 |10 11 12| 19/32 5/8 5/8
25/32|10 11 12| 5/8 21/32 21/32
13/16|10 -- --| 21/32 -- --
27/32|10 -- --| 11/16 -- --
7/8 | 9 10 --| 45/64 23/32 --
29/32| 9 10 --| 47/64 3/4 --
15/16| 9 -- --| 49/64 -- --
21/32| 9 -- --| 51/64 -- --
1 | 8 -- --| 13/16 -- --
1-1/32 | 8 -- --| 53/64 -- --
1-1/16 | 8 -- --| 55/64 -- --
1-3/32 | 8 -- --| 57/64 -- --
1-1/8 | 7 8 --| 29/32 15/16 --
1-5/32 | 7 8 --| 15/16 31/32 --
1-3/16 | 7 8 --| 31/32 1 --
1-7/32 | 7 8 --|1 1-1/32 --
1-1/4 | 7 -- --|1-1/32 -- --
1-9/32 | 7 -- --|1-1/16 -- --
1-5/16 | 7 -- --|1-3/32 -- --
1-11/32| 7 -- --|1-1/8 -- --
1-3/8 | 6 -- --|1-1/8 -- --
1-13/32| 6 -- --|1-5/32 -- --
1-7/16 | 6 -- --|1-5/32 -- --
1-15/32| 6 -- --|1-3/16 -- --
1-1/2 | 6 -- --|1-15/64 -- --
1-17/32| 6 -- --|1-9/32 -- --
1-9/16 | 6 -- --|1-9/32 -- --
1-19/32| 6 -- --|1-5/16 -- --
1-5/8 | 5 5-1/2 --|1-9/32 1-5/16 --
1-21/32| 5 5-1/2 --|1-5/16 1-11/32 --
1-11/16| 5 5-1/2 --|1-11/32 1-3/8 --
1-23/32| 5 5-1/2 --|1-3/8 1-13/32 --
1-3/4 | 5 -- --|1-13/32 -- --
1-25/32| 5 -- --|1-7/16 -- --
1-13/16| 5 -- --|1-15/32 -- --
1-27/32| 5 -- --|1-1/2 -- --
1-7/8 | 4-1/2 5 --|1-17/32 1-17/32 --
1-29/32| 4-1/2 5 --|1-9/16 1-9/16 --
1-15/16| 4-1/2 5 --|1-19/32 1-19/32 --
1-31/32| 4-1/2 5 --|1-5/8 1-5/8 --
2 | 4-1/2 -- --|1-21/32 -- --
-------+-----------------+-----------------------
-------+---------------------+--------------------
Dia- | Drill for U.S.S. | Drill for
meter | thread. | Whitworth
of tap.| | thread.
-------+---------------------+--------------------
1/4 | -- -- 3/16| -- -- 3/16
9/32 | -- -- -- | -- -- --
5/16 | -- 1/4 -- | -- 15/64 --
11/32| -- -- -- | -- -- --
3/8 | -- 9/32 -- | -- 9/32 --
13/32| -- -- -- | -- -- --
7/16 | 11/32 -- -- | 11/32 -- --
15/32| -- -- -- | -- -- --
1/2 | -- 13/32 -- | 3/8 -- --
17/32| -- -- -- | -- -- --
9/16 | 7/16 -- -- | -- -- --
19/32| -- -- -- | -- -- --
5/8 | -- 1/2 -- | -- 1/2 --
21/32| -- -- -- | -- -- --
11/16| -- -- -- | -- -- --
23/32| -- -- -- | -- -- --
3/4 | 5/8 -- -- | 5/8 -- --
25/32| -- -- -- | -- -- --
13/16| -- -- -- | -- -- --
27/32| -- -- -- | -- -- --
7/8 | 23/32 -- -- | 23/32 -- --
29/32| -- -- -- | -- -- --
15/16| -- -- -- | -- -- --
21/32| -- -- -- | -- -- --
1 | 27/32 -- -- | 27/32 -- --
1-1/32 | -- -- -- | -- -- --
1-1/16 | -- -- -- | -- -- --
1-3/32 | -- -- -- | -- -- --
1-1/8 | 15/16 -- -- | 15/16 -- --
1-5/32 | -- -- -- | -- -- --
1-3/16 | -- -- -- | -- -- --
1-7/32 | -- -- -- | -- -- --
1-1/4 |1-1/16 -- -- |1-1/16 -- --
1-9/32 | -- -- -- | -- -- --
1-5/16 | -- -- -- | -- -- --
1-11/32| -- -- -- | -- -- --
1-3/8 |1-5/32 -- -- |1-5/32 -- --
1-13/32| -- -- -- | -- -- --
1-7/16 | -- -- -- | -- -- --
1-15/32| -- -- -- | -- -- --
1-1/2 |1-9/32 -- -- |1-9/32 -- --
1-17/32| -- -- -- | -- -- --
1-9/16 | -- -- -- | -- -- --
1-19/32| -- -- -- | -- -- --
1-5/8 | -- 1-3/8 -- |1-23/64 -- --
1-21/32| -- -- -- | -- -- --
1-11/16| -- -- -- | -- -- --
1-23/32| -- -- -- | -- -- --
1-3/4 |1-1/2 -- -- |1-1/2 -- --
1-25/32| -- -- -- | -- -- --
1-13/16| -- -- -- | -- -- --
1-27/32| -- -- -- | -- -- --
1-7/8 | -- 1-5/8 -- |1-37/64 -- --
1-29/32| -- -- -- | -- -- --
1-15/16| -- -- -- | -- -- --
1-31/32| -- -- -- | -- -- --
2 |1-23/32 -- -- |1-45/64 -- --
-------+---------------------+--------------------
To drive all drills by placing them directly in the socket of the
drilling machine spindle would necessitate that all the drills should
have their shanks to fit the drilling machine socket. This would involve
a great deal of extra labor in making the drills, because the socket in
the machine spindle must be large enough to fit the size of shank that
will be strong enough to drive the largest drill used in the machine,
hence the small drills would require to be forged down from steel equal
to the full diameter of the shank of the largest drill. To obviate this
difficulty the sockets already described with reference to drilling in
the lathe are used.
The employment of these sockets preserves the truth of the bore of the
drilling machine spindle by greatly diminishing the necessity to insert
and remove the shank from the drill spindle, because each socket
carrying several sizes of drills (as given with reference to lathe work)
the sockets require less frequent changing.
[Illustration: Fig. 1729.]
Drill shanks are sometimes made parallel, with a flat place as at A in
Fig. 1729, to receive the pressure of the set-screw by which it is
driven. To enable the shank to run true it must be a close fit to the
socket and should be about five diameters long. The objection to this
form is that the pressure of the set-screw tends to force the drill out
of true, as does also the wear of the socket bore.
These objections will obviously be diminished in proportion as the drill
shank is made a tight fit to the socket, and to effect this and still
enable the drill to be easily inserted and removed from the socket, the
drill shank may be first made a tight fit to the socket bore, and then
eased away on the half circumference on the side of the flat place,
leaving it to fit on the other half circumference which is shown below
the dotted line B in the end view in the figure. The set-screw is also
objectionable, since it requires the use of a wrench, and is in the way
and liable to catch the operator's clothing.
There is, however, one advantage in employing a set-screw for twist
drills, inasmuch as that, on account of the front rake on a twist drill,
there is a strong tendency for the drill, as soon as the point emerges
through the work, to run forward into the work and by ripping in become
locked. This is very apt to be the case if there is any end play in the
driving spindle, because the pressure of the cut forces the spindle back
from the cut; but so soon as the drill point emerges and the pressure is
reduced, the weight of the spindle acting in concert with the front rake
on the drill causes the spindle to drop, taking up the lost motion in
the opposite direction. In addition to this the work will from the same
cause lift and run up the drill, often causing an increase in the duty
sufficient to break the drill.
If the spindle has no lost motion and the work is bolted or fastened to
the table or in a chuck, the drill if it has a taper shank only will
sometimes run forward and slip loose in the driving socket. This,
however, may be obviated by feeding the drill very slowly after its
point emerges through the work.
Yet another form in which the cylindrical shanks of drills have been
driven is shown in Fig. 1730. The shank is provided with a longitudinal
groove turning at a right angle; at its termination the socket is
provided with a screw whose point projects and fits into the shank
groove. The drill is inserted and turned to the right, the end of the
screw driving the drill and preventing it from coming out or running
forward.
Flat drills are usually provided with a square taper shank such as shown
in Fig. 1730, an average amount of taper being 1-1/4 inches per foot.
There are several disadvantages in the use of a square shank.
1st. It is difficult to forge the drill true and straight with the
shank.
2nd. It is difficult to make the square socket true with the axial line
of the machine spindle, and concentric with the same from end to end.
3rd. It is difficult to fit the shank of the drill to the socket and
have its square sides true with the axial line of the drill.
4th. It is an expensive form of shank to fit. It is a necessity,
however, when the cutting duty is very heavy, as in the case of stocks
carrying cutters for holes of large diameter.
In order to properly fit a square shank to a socket it should be pressed
into the socket by hand only, and pressed laterally in the direction of
each side of the square. If there is no lateral movement the shank is a
fit, and the spindle may be revolved to see if the drill runs true, as
it should do if the body of the drill is true with the shank (and this
must always be the case to obtain correct results). The drill must be
tried for running true at each end of the cylindrical body of the drill,
which, being true with the square shank, may be taken as the standard of
truth in grinding the drill, so that supposing the hole in the driving
spindle to be true and the drill shank to be properly fitted, the drill
will run true whichever way inserted. If the body of the drill runs out
of true it will cause a great deal of friction by rubbing and forcing
the cuttings against the sides of holes, especially if the clearance be
small or the hole a deep one.
[Illustration: Fig. 1730.]
In fitting the shank, the fitting or bearing marks will show most
correctly when the shank is driven very lightly home, for if driven in
too firmly the bearing marks will extend too far in consequence of the
elasticity of the metal. If the hole in the spindle is not true with the
axial line of the spindle, or if the sides of the hole are not a true
square or are not equidistant from the axial line of the spindle, the
drill must be fitted with one side of its square shank always placed to
the same side of the square in the socket, and these two sides must
therefore be marked so as to denote how to insert the drill without
having to try it in the socket. Usually a centre-punch mark, as at E,
Fig. 1731, is made on the drill and another on the collar as at _f_.
To enable the extraction of the drill from the socket the latter is
provided with a slot, shown in figure at C, the slot passing through the
spindle and the end of the drill protruding into the slot, so that a key
driven into the slot will force the drill from the socket. The key
employed for this purpose should be of some soft metal, as brass or hard
composition brass, so that the key shall not condense or press the metal
of the keyway, and after the key is inserted it should be lightly tapped
with a hammer, travelling in the direction of the line of the spindle
and not driven through the keyway.
The drill should not be given a blow or tap to loose it in the spindle,
as this is sure in time to make its socket hole out of true.
[Illustration: Fig. 1731.]
[Illustration: Fig. 1732.]
The thread shown on the end of the drill spindle in figure is to receive
chucks for holding and driving drills.
[Illustration: Fig. 1733.]
The various forms of small drill chucks illustrated in connection with
the subject of lathe chucks are equally suitable for driving drills in
the drilling machine.
Fig. 1732, however, represents an excellent three-jawed chuck for
driving drills, the bite being very narrow and holding the drill with
great firmness.
Fig. 1733 represents a two-jawed drill chuck in which the screws
operate a pair of dies for gripping parallel shank drills, the screws
being operated independently.
In other forms of similar chucks the bite is a [V] recess parallel to
the chuck axis, the only difference between a drill chuck for a drilling
machine and one for a lathe being that for the former the jaws do not
require outside bites nor to be reversible.
[Illustration: Fig. 1734.]
Holes that are to be made parallel, straight, cylindrically true in the
drilling machine, are finished by the reamer as already described with
reference to lathe work, and it is found as in lathe work that in order
that a reamer may finish holes to the same diameter, it is necessary
that it take the same depth of finishing cut in each case, an end that
is best obtained by the use of three reamers, the first taking out the
irregularities of the drilled hole, and the second preparing it for the
light finishing cut to be taken by the third.
All the remarks made upon the reamer when considered with reference to
lathe work apply equally to its use in the drilling machine.
Another tool for taking a very light cut to smooth out a hole and cut it
to exact size is the shell reamer shown in Fig. 1734, which fits on a
taper mandrel through which passes a square key fitting into the square
slot shown in the shell reamer.
[Illustration: Fig. 1735.]
Reamers may be driven by drill chucks, but when very true and parallel
work is required, and the holes are made true before using the reamer,
it is preferable to drive them by a socket that permits of their moving
laterally. Especially is this the case with rose-bits. Fig. 1735, which
is taken from _The American Machinist_, represents a socket of this
kind, being pivoted at its driving or shank end, and supported at the
other by two small spiral springs. The effect is that if the socket does
not run quite true the reamer is permitted to adjust itself straight and
true in the hole being reamed, instead of rubbing and binding against
its walls, which would tend to enlarge its mouth and therefore impair
its parallelism.
Cotter drills, slotting drills, or keyway drills, three names
designating the same tool, are employed to cut out keyways, mortises, or
slots.
Fig. 1736 represents a common form of cotter or keyway drill, the
cutting edges being at A, A, and clearance being given by grinding the
curve as denoted by the line C. In some cases a stock S and two
detachable bits or cutters C, C, are used as in Fig. 1737, the bits
being simple tools secured in slots in the stock by set-screws, and thus
being adjustable for width so that they may be used to cut keyways of
different widths.
The feed of keyway drills should be light, and especial care must be
taken where two spindles are used, to keep them in line, or otherwise
the keyway will not come fair, as is shown in Fig. 1738, where the half
drilled from side A and that drilled from side B are shown not to come
fair at their point of junction C. This is more apt to occur when a deep
keyway is drilled one half from each side. Hence in such a case great
care must be exercised in setting the work true, because the labor in
filing out such a keyway is both tedious and expensive.
[Illustration: Fig. 1736.]
[Illustration: Fig. 1737.]
In producing holes of above or about two inches in diameter, cutters
such as shown in Fig. 1739 may be employed. A is a stock carrying a
cutter B secured in place by a key C. Holes are first drilled to receive
the pin D, which serves as a guide to steady the stock. The amount of
cutting duty is obviously confined to the production of the holes to
receive the pin and the metal removed from the groove cut by the
cutters, so that at completion of the cutter duty there comes from the
work a ferrule or annular ring that has been cut out of the work.
[Illustration: Fig. 1738.]
For use on wrought iron or steel the front faces of the cutters may be
given rake as denoted by the dotted line at E, and smooth and more rapid
duty may be obtained if the cutter be set back, as in Fig. 1740, the
cutting edge being about in a line with line A, in which case the front
face may be hollowed out as at B, and take a good cut without the
digging in and jumping that is apt to occur in large holes if the cutter
is not thus set back. The larger the diameter of the work the greater
the necessity of setting the cutting edge back, thus in Fig. 1741 the
cutter is to be used to cut a large circle out of a plate P, as, say, a
man-hole in a boiler sheet. The cutter C is carried in a bar B secured
in the stock A by a screw, and unless the cutter is set well back it is
liable to dip into the work and break.
[Illustration: Fig. 1739.]
[Illustration: Fig. 1740.]
[Illustration: Fig. 1741.]
It is obvious that the pin E in the figure must be long enough to pass
into the hole in the plate before the cutter meets the plate surface and
begins to cut, so that the pin shall act as a guide to steady the
cutter, and also that in all cutters or cutter driving stocks the shank
must be either of large diameter or else made square, in order to be
able to drive the cut at the increased leverage over that in drilling.
[Illustration: Fig. 1742.]
In these forms of tube plate cutters it is necessary to drill a hole to
receive the pin D. But this necessity may be removed by means of a
cutter, such as shown in Fig. 1742, which is given simply as a
representative of a class of such cutters. A is a cutter stock having
the two cutters B B fitted in slots and bolted to it. C is a spiral
spring inserted in a hole in A and pressing upon the pin D, which has a
conical point. The work is provided with a deep centre-punch mark
denoting the centre of the hole to be cut. The point of D projects
slightly beyond the cutting edges of the cutters, and as it enters the
centre-punch mark in the work it forms a guide point to steady the
cutters as they rotate. As the cutters are fed to their cut, the pin D
simply compresses the spiral spring C and passes further up the cutter
stock. Thus the point of D serves instead of a hole and pin guide.
[Illustration: Fig. 1743.]
A simple form of adjustable cutter is shown in Figs. 1743 and 1744. It
consists of a stock A A with the shank B, made tapering to fit the
socket of a boring or drilling machine. Through the body of the stock is
a keyway or slot, in which is placed the cutter C, provided in the
centre of the upper edge with a notch or recess. Into this slot fits the
end of the piece D, which is pivoted upon the pin E. The radial edge of
D has female worm teeth upon it. F is a worm screw in gear with the
radial edge of D. Upon the outer end of F is a square projection to
receive a handle, and it is obvious that by revolving the screw F, the
cutter C will be moved through the slot in the stock, and hence the size
of the circle which the cutter will describe in a revolution of the
stock A may be determined by operating the screw F. Thus the tool is
adjustable for different sizes of work, while it is rigidly held to any
size without any tendency whatever either to slip or alter its form. The
pin G is not an absolutely necessary part of the tool, but it is a
valuable addition, as it steadies the tool. This is necessary when the
spindle of the machine in which it is used has play in the bearings,
which is very often the case with boring and drilling machines. The use
of G is to act as a guide fixed in the table upon which the work is
held, to prevent the tool from springing away from the cut, and hence
enabling it to do much smoother work. It is usual to make the width of
the cutter C to suit some piece of work of which there is a large
quantity to do, because when the cutter is in the centre of the stock
both edges may perform cutting duty; in which case the tool can be fed
to the cut twice as fast as when the cutter is used for an increased
diameter, and one cutting edge only is operative. The tool may be put
between the lathe centres and revolved, the work being fastened to the
lathe saddle. In this way it is exceedingly useful in cutting out plain
cores in half-core boxes.
[Illustration: Fig. 1744.]
In addition to its value as an adjustable boring tool this device may be
used to cut out sweeps and curves, and is especially adapted to cutting
those of double eyes. This operation is shown in Fig. 1744, in which D
is the double eye, A is the tool stock, F is the adjusting screw, and C
is the cutter. The circular ends of connecting rod strips and other
similar work also fall within the province of this tool, and in the case
of such work upon rods too long to be revolved this is an important
item, as such work has now to be relegated to that slowest and most
unhandy of all machine tools, the slotting machine.
It is obvious that any of the ordinary forms of cutter may be used in
this stock.
[Illustration: Fig. 1745.]
For enlarging a hole for a certain distance the counterbore is employed.
Fig. 1745 represents a counterbore or pin drill, in which the pin is cut
like a reamer, so as to ream the hole and insure that the pin shall fit
accurately. The sides are left with but little clearance and with a dull
edge, so that they will not cut, the cutting edges being at _e_, _c_ and
the clearance on the end faces.
[Illustration: Fig. 1746.]
For counterboring small holes or for facing the metal around their ends,
the form of counterbore shown in Fig. 1746 is employed. The pin must be
an accurate fit to the hole, and to capacitate one tool for various
sizes of holes the bit is made interchangeable. The stock has a flat
place on it to receive the pressure of the screw that secures the
counterbore, and the end of the stock is reduced in diameter, so that
the counterbore comes against a shoulder and cannot push up the stock
from the pressure of the feed; the end of the counterbore is bored to
receive the tit pin, thus making it permissible to exchange the pin, and
use various sizes in the same counterbore.
[Illustration: Fig. 1747.]
[Illustration: Fig. 1748.]
Twist drills for use in wood work are given a conical point, as was
shown with reference to lathe drills, and when the holes are to be
countersunk, an attachment, such as shown in Fig. 1747, may be used. It
is a split and threaded taper, so that by operating the nut in one
direction it may be locked to the drill, while by operating it in the
other it will be loosened, and may be adjusted to any required distance
from the point of the drill, as shown in Fig. 1748.
[Illustration: Fig. 1749.]
For larger sizes of holes a stock and cutter, such as shown in Fig.
1749, may be employed, receiving a facing of counterboring cutter such
as A, or a countersink bit such as B, and the bit may be made to suit
various sizes of holes by making its diameter suitable for the smallest
size of hole the tool is intended for, and putting ferrules to bring it
up to size for larger diameters.
The cutters are fastened into the stock by a small key or wedge, as
shown. By having the cutter a separate piece from the stock, the cutting
edges may be ground with greater facility, while one stock may serve for
various sizes of cutters. The slot in the stock should be made to have
an amount of taper equal to that given to the key, so that all the
cutters may be made parallel in their widths or depths, and thus be more
easily made, while at the same time the upper edge will serve as a guide
to grind the cutting edges parallel to, and thus insure that they shall
stand at a right angle to the axis of the stock, and that both will
therefore take an equal share of the cutting duty.
When cutters of this kind are used to enlarge holes of large diameter it
is necessary that the pin be long enough to pass down into a bushing
provided in the table of the machine, and thus steady the bar or stock
at that end.
For coning the mouths of holes the countersink is employed, being
provided with a pin, as shown in Fig. 1750; and it is obvious that the
pin may be provided with bushings or ferrules. The smaller sizes of
countersinks are sometimes made as in Fig. 1751, the coned end being
filed away slightly below the axis so as to give clearance to the
cutting edge.
[Illustration: Fig. 1750.]
[Illustration: Fig. 1751.]
[Illustration: Fig. 1752.]
Fig. 1752 refers to a device for drilling square holes. The chuck for
driving the drill is so constructed as to permit to the drill a certain
amount of lateral motion, which is rendered necessary by the peculiar
movement of the cutting edges of the drill which does not rotate on a
fixed central point, but diverges laterally to a degree proportional to
the size of the hole. For the chuck the upper part of the cavity of a
metal cylinder is bored out so as to fit on the driving spindle. Below
this bore a square recess is made, and below this latter and coming well
within the diameter of the square recess, is a circular hole passing
through the end of the chuck. The drill holder or socket is in a
separate piece, the bottom portion of which is provided with a square or
round recess for holding the drill shanks, and is held firmly in its
socket by means of a set-screw. The upper part of the socket consists
first of a screw (Fig. 1752) at S; secondly, of a squared shoulder B;
thirdly, of a cylindrical shoulder D, and the circular part E, the drill
shank being inserted at H. N is a nut holding the drill socket in the
chuck. The socket being inserted in the chuck, the loose square collar
C, which has an oblong rectangular slot in it, is put in, passing over
the squared part of the socket. The nut N is then screwed up, bringing
the face of E up to the face of the chuck, but not binding C, because C
is thinner than the recess in which it lies. When this is done the
socket will readily move in a horizontal plane to such a distance as the
play between the two sides of the loose collar C and two of the sides of
the recess will permit, while in the other direction it will move in a
horizontal plane such distance as the play between the two sides of the
square shoulder of the socket and the ends of the rectangular slot in
the loose collar C will permit. The amount of this horizontal motion is
varied to suit the size of the square hole to be drilled. Near to the
lower end or cutting edges of the drill, there is fixed above the work a
metal guide plate F having a square hole of the size requiring to be
drilled. The drill is made three-sided, as shown, the dimensions of the
three sides being such that the distance from the base to the apex of
the triangle is the same as the length of the sides of the hole to be
drilled. The drill may then be rotated through F as a guide, when it
will drill a square hole.
The method of operation is as follows: The three-sided drill being fixed
in the self-adjusting chuck, the guide bar with the square guide hole
therein rigidly fixed above the point in the work where it is required
to drill, the drilling spindle carrying the chuck drill is made to
revolve, and is screwed or pressed downwards, upon which the drill works
downwards through the square guide hole, and drills holes similar in
size and form to that in the guide. The triangular drill for drilling
dead square holes may also be used without the self-adjusting drill
chuck in any ordinary chuck, when the substance operated upon is not
very heavy nor stationary; then, instead of the lateral movement of the
drill, such lateral movement will be communicated by the drill to the
substance operated upon.
In making oblong dead square-cornered holes, either the substance to be
operated upon must be allowed to move in one direction more than
another, or the hole in the guide plate must be made to the shape
required, and the drill chuck made to give the drill greater play in one
direction.
[Illustration: Fig. 1753.]
The boring bars and cutters employed in drilling and boring machines are
usually solid bars having fixed cutters, the bars feeding to the cut.
[Illustration: Fig. 1754.]
[Illustration: Fig. 1755.]
[Illustration: Fig. 1756.]
Figs. 1753, 1754, 1755, and 1756, however, represent a bar having a
device for boring tapers in a drilling or boring machine. It consists of
a sleeve A fixed to the bar S, and having a slideway at an angle to the
bar axis. In this slideway is a slide carrying the cutting tool and
having at its upper end a feed screw with a star feed. Fig. 1753 shows
the device without, and Fig. 1754 with, the boring bar. A is a sleeve
having ribs B to provide the slideway C for the slide D carrying the
cutting-tool T. The feed screw F is furnished with the star G between
two lugs H K. A stationary pin bolted upon the work catches one arm of
the star at each revolution of the bar, and thus puts on the feed. To
take up the wear of the tool-carrying slide, a gib M and set-screws P
are provided, and to clamp the device to the boring-bar it is split at Q
and furnished with screws R. The boring-bar S, furthermore, has a collar
at the top and a nut N at the bottom. The tool, it will be observed, can
be closely held and guided, the degree of taper of the hole bored being
governed by the angle of the slideway C to the axis of the sleeve.