Abrasive drill

An abrasive drill of small diameter is provided with a cored portion that is eccentric, i.e., lacking a substantial concentric portion, relative to the longitudinal axis of the drill. The core, regardless of shape, is located so that during drill rotation all of the surface of a workpiece within the outline of the end of the drill is abraded by some part of the rotating end of the drill. The drill composition comprises abrasive particles in a less abrasive matrix, and the core is not hollow but comprises relatively non-abrasive material. The drill is formed by deposition of matrix material entraining abrasive particles onto a taut filamentary mandrel. The mandrel preferably comprises one or more round filaments, either straight or twisted together, alongside the longitudinal axis of the drill, and may be metallic, such as copper wire or may be non-metallic, such as nylon or other synthetic polymeric textile fiber composition. The drill diameter may be on the order of a millimeter or so or smaller.

This invention relates to abrasive drills of small diameter. 
Conventional core drills are hollow cylindrical members having abrasive or 
cutting members variously distributed on or in the tubular structure. The 
core reduces the lateral drilling thickness and permits the injection of 
coolant or lubricant, resulting in increased drill life and improved 
economy of operations. However, in small sizes the complementary rodlike 
portion being removed from the workpiece often has insufficient structural 
integrity and breaks into pieces large enough to jam the drill, thereby 
forfeiting the advantages of core drilling. 
A primary object of the present invention is provision of a small-diameter 
of "micro" abrasive drill with a cored portion that extends its usability 
to smaller dimensions than feasible in conventional core drills. 
Another object of this invention is provision of a cored abrasive drill 
that disintegrates the entire portion of material being removed from a 
workpiece by means of such drill. 
A further object of the present invention is a method of forming such cored 
drills. 
Other objects of the present invention, together with means and methods for 
attaining the various objects, will be apparent from the following 
description and the accompanying diagrams, which are presently by way of 
example rather than limitation. All views are larger than actual size.

In general, the objects of the present invention are accomplished via a 
cylindrical drill having a cored portion that is eccentric, i.e., lacking 
a substantial concentric portion, relative to the longitudinal axis of the 
drill. The drill composition comprises abrasive particles in a less 
abrasive matrix, and the cored portion comprises relatively non-abrasive 
material. 
The drill is formed by deposition of matrix material entraining abrasive 
particles onto a taut filamentary mandrel, which preferably comprises one 
or more round filaments, either straight or twisted together, alongside 
(e.g., tangential to) the longitudinal axis of the drill. The filaments 
may be metallic, such as copper wire, or may be non-metallic, such as 
nylon or other synthetic polymeric textile fiber composition. The 
deposition step forms the cylindrical drill outline to the desired 
diameter, which may be on the order of a millimeter or so or 10ths 
thereof. 
FIGS. 1, 2, and 3 show in end elevation three alternative embodiments of 
cored drill, according to this invention, characterized by core outlines 
of one, two, and three circles, respectively, being the transverse 
cross-sectional outlines of the respective cores. Each such outline is 
tangential to the longitudinal axis of the drill, and the second and third 
have corresponding two-lobe and three-lobe configurations. Thus, FIG. 1 
shows an end (round) of drill 10 having as an eccentric core mandrel 
simple cylindrical filament 11 tangential to the longitudinal axis of the 
drill (in this view, the center of the end). FIG. 2 shows an end of drill 
20 having as a core mandrel a pair of simple cylindrical filaments 21, 22 
mutually tangential to one another and to the drill axis. FIG.. 3 shows an 
end of drill 30 having as a core mandrel three simple cylindrical 
filaments 31, 32, 33 mutually tangential to one another and to the drill 
axis. The drills are shown stippled to suggest their content of abrasive 
particles. 
FIG. 4 shows drill 10 of FIG. 1 in perspective, with cored portion 11 shown 
in broken lines (inside) as a straight cylinder tangential to the drill 
axis. 
FIGS. 5a and 5b show two variants of the drill of FIG. 2, in which the 
cylinders of the cored portion (shown in broken lines inside) are, 
respectively, straight and twisted around one another while remaining 
tangential to the drill axis. Thus, in FIG. 5a, composite two-lobed core 
24 is straight, while in FIG. 5b similar two-lobed core 25 is helical. 
FIGS. 6a and 6b show two variants of the drill of FIG. 3, in which the 
cylinders of the cored portion (shown in broken lines inside) are, 
respectively, straight and twisted together while remaining tangential to 
the drill axis. Thus, in FIG. 6a, composite three-lobed core 34 is 
straight, while in FIG. 6b similar three-lobed core 35 is helical. 
Although plural round filaments have been disclosed in forming multi-lobed 
mandrels, it is apparent that single non-round filaments of similar 
overall shape could be used instead, being produced by drawing of metal 
through suitably shaped dies or extrusion of fiber-forming compositions 
through correspondingly shaped spinnerets. Examples of a two-lobed 
filament and a three-lobed filament useful as such mandrels appear end-on 
in FIGS. 7 and 8, respectively. Thus, in FIG. 7, filament 50 has left lobe 
portion 51 and right lobe portion 52 joined by intervening web portion 53. 
Similarly, in FIG. 8, filament 60 has three lobes 61, 62, and 63 at 120 
degrees of arc from one another joined by intervening web portion 64. 
Although the respective web portions are concentric with the drill axis 
they are so small relative to the core lobes as to be insubstantial, 
whereupon drills with such cores will not leave undisintegrated portions 
of a workpiece but will abrade essentially all the surface thereof within 
the outline of the rotating end of such drill. 
In summary, the drills of this invention are made possible by first 
selecting, arranging, and tautening the mandrel filament(s). Thus, if more 
than one such filament is to be used as the mandrel, they are juxtaposed 
laterally to one another and optionally twisted, depending upon whether an 
"a" type or "b" type of core is desired. Suitable filament diameter is 
from about one-twentieth to one-third the desired diameter of the 
resulting drill. The filament(s) may be tautened by heating, stretching 
till taut, and cooling under tension. Metallic filaments may be 
resistance-heated by passing electrical current therethrough. 
Then, the mandrel receives abrasive particles entrained in an electrically 
conductive matrix material to form the drill (to the desired diameter). 
Electroforming is readily carried out by deposition from a nickel solution 
in which the abrasive particles are suspended or at least circulating. As 
an alternative to cathodic deposition, electroless chemical reduction may 
be employed. Such techniques are well known in the art, and as their 
details need be merely conventional they will not be specified here. When 
electrically non-conductive mandrel materials are employed they can be 
coated with graphite or conductive adhesive materials to assist in the 
deposition process. 
With a single off-axial cylindrical mandrel, access of abrasive thereto 
will be graduated circumferentially to locate the core eccentrically as 
desired in a resulting cylindrical (round cross-section) drill. When 
plural filamentary or single multi-lobed mandrels are employed, uniform 
access of matrix material and abrasive particles to the mandrel is 
permitted, with the desired circular cross-section facilitated otherwise, 
as by rotation of the mandrel, not an uncommon technique. 
The abrasive particles may be diamond, cubic boron nitride, alumina, 
tungsten carbide, or other recognized abrasives, such as oxides, carbides, 
or nitrides. Diamond is preferred because of its superior hardness and 
ready availability in desired size range. Cubic boron nitride is also very 
hard and, being more resistant to high temperatures, is useful in drilling 
metals, such as tool steel. The drills of this invention are most useful 
in drilling non-metals, usually hard brittle materials, such as ceramics 
and gemstones (both artificial and natural), which are difficult to drill 
by other means and methods and which tend to crumble when abraded. Thus, 
an insubstantial concentricity of core in such drills ensures that such 
materials being drilled do not leave any central undrilled fragments, for 
lateral abrasion by the cusplike portions of the drill body between the 
respective lobes of the core crumbles the axial fragments. 
A suitable range of particle size is from about a ten-thousandth to a half 
of the drill diameter. Thus, for a drill 1 or 2 millimeters in diameter, 
such particles could range from about one-tenth to 1000 microns in 
"diameter" and preferably from about 30 to 300 microns. Practical 
diameters for "micro" drills of this invention are on the order of a 
millimeter or so or even 10ths of a millimeter. 
Although certain embodiments of this invention have been illustrated and 
described, other modifications may be made, as by adding, combining, or 
subdividing parts or steps, or by substituting equivalents, while 
retaining advantages and benefits of the invention, which itself is 
defined in the following claims.