twist drill which has come into such universal use, has
superseded the old, flat, forged drill which, for so many years,
held without rival the first position as a tool for producing
circular holes in metal. For the needs of its day, it
served its purpose well.
The advancements along mechanical lines demanded a better tool
for this work, however, and the twist drill resulted, brought
out in practically the same form as now used, the principal
recent improvements being mostly in slight changes in forms, and
its more accurate production due to improved methods of
The flat drill, as used for metal work, is generally of the form
shown in Fig. 28. It is made from round stock, is forged
thin at the lips, and ground as shown in figure, with three
cutting edges - A, B and C.
This is a very accommodating sort of a tool, being capable of
producing a number of holes of different diameters, yet,
approximately, the width of the drill. The disadvantage of
this adjustability, however, lies in the fact that the size of
hole wanted cannot ordinarily be produced.
The flat drill has no lands, as that part of the twist drill
between flutes is called, to steady and guide it in the work.
Consequently, the hole drilled will usually not be round, and
should be the point of the drill strike the side of a small blow
hole or a soft spot in the metal being drilled-as frequently
happens in working cast metal-the point will drift towards this
spot, thus making a hole that is neither round not straight.
This is shown in Fig. 29.
In order to drill holes approximately to size with the flat
drill, it is necessary that the cutting lips be most carefully
ground. The angle of the lips with the axis of the drill must be
equal, otherwise one cutting edge will perform all the work, and
will dull quickly, due to this double duty.
The pressure on the
cutting lip will crowd the point, causing it to revolve in a
small circle about the center of revolution. This will cause the
other flute to cut slightly at its outer end, thus producing a
hold of larger diameter than the width of flat. This is shown is
The cutting lips should be of equal length with their
intersection in the axis of rotation of the drill. If one lip is
longer than the other, the diameter of the hole drilled will
depend on the length of this long lip, as it will rotate about
C, its central axis, as shown in Fig. 31.
In case the intersection of the lips does not fall on the axis
of the drill, the one li is thereby made longer than the other,
and the hole drilled will again be large, as the tool will
spring an amount sufficient to allow it to revolve about C
instead of its true axis, and the length of the long flute,
again determines the diameter of the hole drilled, as shown in
The first cost of the flat drill is small, and the results
obtained by its use usually poor. Its only advantage lies in the
fact that it can readily be forged and tempered to do work on
extremely hard metals.
The flat drill, ground thin and tempered
hard, is a valuable tool for drilled hard steel or chilled iron,
as it will in that form take hold of metal that the twist drill
will not touch. It also makes a convenient extension drill, as
it can be readily formed on the end of a long bar of steel.
The flat drill is not adapted to the drilling of deep holes, as
it does not free itself of chips. It is largely used for
roughing out cored holes, preparatory to boring, which work is
very destructive, due to scale and sand, to the land clearances
of twist drills. When so used in a lathe, the drill is held
against the dead center and fed forward by the rail screw, the
About 1860, twist drills, having milled flutes, were first
placed on the market. Previous to this date, however, drills
with flutes produced by filing and twisting of the flat stock
had been used to a limited extent.
In Fig. 33 is shown a taper shank twist drill. A A are the
flutes, B B are the lands, C-the metal between the flutes-the
web, D D the lips, E the shank and F the tang. The center or
grinding line is the fine line running along the bottom of each
flute, and serves as a guide to the lips in grinding so their
intersection will fall in the center of the drill.