_BORING AND DRILLING._
Boring, as distinguished from drilling, consists in turning out
annular holes to true dimensions, while the term drilling is
applied to perforating or sinking holes in solid material. In
boring, tools are guided by axial support independent of the
bearing of their edges on the material, while in drilling, the
cutting edges are guided and supported mainly from their contact
with and bearing on the material drilled.
Owing to this difference in the manner of guiding and supporting
the cutting edges, and the advantages of an axial support for tools
in boring, it becomes an operation by which the most accurate
dimensions are attainable, while drilling is a comparatively
imperfect operation; yet the ordinary conditions of machine fitting
are such that nearly all small holes can be drilled with sufficient
accuracy.
Boring may be called internal turning, differing from external
turning, because of the tools performing the cutting movement,
and in the cut being made on concave instead of convex surfaces;
otherwise there is a close analogy between the operations of
turning and boring. Boring is to some extent performed on lathes,
either with boring bars or by what is termed chuck-boring, in the
latter the material is revolved and the tools are stationary.
Boring may be divided into three operations as follows:
chuck-boring on lathes; bar-boring, when a boring bar runs
on points or centres, and is supported at the ends only; and
bar-boring when a bar is supported in and fed through fixed
bearings. The principles are different in these operations, each
one being applicable to certain kinds of work. A workman who can
distinguish between these plans of boring, and can always determine
from the nature of a certain work which is the best to adopt, has
acquired considerable knowledge of fitting operations.
Chuck-boring is employed in three cases; for holes of shallow
depth, taper holes, and holes that are screw-threaded. As pieces
are overhung in lathe-boring there is not sufficient rigidity
neither of the lathe spindle nor of the tools to admit of deep
boring. The tools being guided in a straight line, and capable of
acting at any angle to the axis of rotation, the facilities for
making tapered holes are complete; and as the tools are stationary,
and may be instantly adjusted, the same conditions answer for
cutting internal screw-threads; an operation corresponding to
cutting external screws, except that the cross motions of the tool
slide are reversed.
The second plan of boring by means of a bar mounted on points or
centres is one by which the greatest accuracy is attainable;
it is like chuck-boring a lathe operation, and one for which no
better machine than a lathe has been devised, at least for the
smaller kinds of work. It is a problem whether in ordinary machine
fitting there is not a gain by performing all boring in this
manner whenever the rigidity of boring bars is sufficient without
auxiliary supports, and when the bars can pass through the work.
Machines arranged for this kind of boring can be employed in
turning or boring as occasion may require.
When a tool is guided by turning on points, the movement is
perfect, and the straightness or parallelism of holes bored in this
manner is dependent only on the truth of the carriage movement.
This plan of boring is employed for small steam cylinders,
cylindrical valve seats, and in cases where accuracy is essential.
The third plan of boring with bars resting in bearings is more
extensively practised, and has the largest range of adaptation. A
feature of this plan of boring is that the form of the boring-bar,
or any imperfection in its bearings, is communicated to the work;
a want of straightness in the bar makes tapering holes. This, of
course, applies to cases where a bar is fed through fixed bearings
placed at one or both ends of a hole to be bored. If a boring-bar
is bent, or out of truth between its bearings, the diameter of the
hole being governed by the extreme sweep of the cutters is untrue
to the same extent, because as the cutters move along and come
nearer to the bearings, the bar runs with more truth, forming a
tapering hole diminishing toward the rests or bearings. The same
rule applies to some extent in chuck-boring, the form of the lathe
spindle being communicated to holes bored; but lathe spindles are
presumed to be quite perfect compared with boring bars.
The prevailing custom of casting machine frames in one piece, or
in as few pieces as possible, leads to a great deal of bar-boring,
most of which can be performed accurately enough by boring bars
supported in and fed through bearings. By setting up temporary
bearings to support boring-bars, and improvising means of driving
and feeding, most of the boring on machine frames can be performed
on floors or sole plates and independent of boring machines and
lathes. There are but few cases in which the importance of studying
the principles of tool action is more clearly demonstrated than
in this matter of boring; even long practical experience seldom
leads to a thorough understanding of the various problems which it
involves.
Drilling differs in principle from almost every other operation
in metal cutting. The tools, instead of being held and directed
by guides or spindles, are supported mainly by the bearing of the
cutting edges against the material.
A common angular-pointed drill is capable of withstanding a greater
amount of strain upon its edges, and rougher use than any other
cutting implement employed in machine fitting. The rigid support
which the edges receive, and the tendency to press them to the
centre, instead of to tear them away as with other tools, allows
drills to be used when they are imperfectly shaped, improperly
tempered, and even when the cutting edges are of unequal length.
Most of the difficulties which formerly pertained to drilling are
now removed by machine-made drills which are manufactured and
sold as an article of trade. Such drills do not require dressing
and tempering or fitting to size after they are in use, make true
holes, are more rigid than common solid shank drills, and will
drill to a considerable depth without clogging.
A drilling machine, adapted to the usual requirements of a machine
fitting establishment, consists essentially of a spindle arranged
to be driven at various speeds, with a movement for feeding the
drills; a firm table set at right angles to the spindle, and
arranged with a vertical adjustment to or from the spindle, and a
compound adjustment in a horizontal plane. The simplicity of the
mechanism required to operate drilling tools is such that it has
permitted various modifications, such as column drills, radial
drills, suspended drills, horizontal drills, bracket drills,
multiple drills, and others.
Drilling, more than any other operation in metal cutting, requires
the sense of feeling, and is farther from such conditions as admit
of power feeding. The speed at which a drill may cut without
heating or breaking is dependent upon the manner in which it
is ground and the nature of the material drilled, the working
conditions may change at any moment as the drilling progresses;
so that hand feed is most suitable. Drilling machines arranged
with power feed for boring should have some means of permanently
disengaging the feeding mechanism to prevent its use in ordinary
drilling.
I am well aware how far this opinion is at variance with practice,
especially in England; yet careful observation in a workshop will
prove that power feed in ordinary drilling effects no saving of
time or expense.
(1.) What is the difference between boring and drilling?--(2.)
Why will drills endure more severe use than other tools?--(3.)
Why is hand feeding best suited for drills?--(4.) What is the
difference between boring with a bar supported on centres and one
fed through journal bearings?