Metal cutting tool

A metal cutting tool is disclosed which includes a cylindrical cutting body (12) having an outer cylindrical surface (13) approximating the size of the hole desired and an inner wall (14) which tapers towards a base (15). The cutter body further includes a notch (17) for removal of cutting chips which has a socket disposed on the leading edge thereof. A carbide cutting tip (26) is disposed in the socket with the cutting tip being substantially pyramidically shaped, having a blunt cutting edge (35) extending above the cutter body. The metal cutting tool further includes a pilot bit (36) extending axially through the center of the cutter body, with the pilot including a short fluted (37) section which extends above the top of the cutting tip and provides a pilot hole for guiding the cutting tip during hole cutting. The metal cutting tool is extremely durable and is particularly suited for use with a hand held drill, providing a minimum of resistance during cutting, reducing tip binding or breakage and thereby providing smooth holes in sheet metal at field locations.

TECHNICAL FIELD 
This invention relates to metal cutting tools and more particularly to 
trepanning tools for cutting holes in sheet metal using a hand held drill. 
BACKGROUND 
Tools for cutting holes in various materials such as sheet metal generally 
fall into two types, those which operate by cutting or removing the entire 
interior area of a circle and those which operate by cutting only the 
circumferential area of a circle thereby producing an interior circular 
plug during the cutting operation. A conventional fluted drill is an 
example of the former type of tool. Such tools have the disadvantage of 
requiring substantially more cutting activity in order to produce the 
desired hole. Over a certain diameter, it is much more efficient and 
generally quicker to utilize a tool which cuts a thin circumferential 
circle for removing the metal as a plug. Such tools are generally known as 
trepanning tools. Tools of this type require substantially less cutting to 
achieve a desired hole and therefore provide large diameter holes quickly 
and with substantially less energy consumption. 
The use of trepanning tools which cut only the circumferential area of 
circles to provide large diameter holes is widely known. Such tools 
generally comprise an elongated tubular cutting head having cutting blades 
or tips secured thereto which engage a work piece to shave or cut portions 
thereof to define the circumference of a circle upon rotation of the tool. 
Accordingly, as cutting of a work piece is effected, a center plug is 
produced which is received in the center area of the elongated tubular 
cutting head. Various ejector devices are commonly used with such tools to 
remove the plug from the tool, such as spring loaded arms. Tools of this 
type have been available for cutting holes having diameters ranging from 
approximately less than an inch to holes having extremely large diameters 
for example up to 10 inches or more. Such large diameter trepanning tools 
are generally associated with lathe type machines which are fixedly 
mounted for producing holes in an automated fashion. Generally, such 
trepanning tools are therefore rigidly aligned with the material to be cut 
which is usually also clamped or fixed to prevent movement. While such 
applications are well known in the art, there has been a continuing effort 
to produce tools suitable for use in a hand held fashion which will 
provide a relatively precise and smooth hole in thin sheet metal for use, 
for example, in field construction such as by an electrician in cutting 
holes in electrical boxes. In addition, it is desirable to cut holes in 
other types of sheet metal such as stainless steel, which is finding more 
use in home and building construction. 
Various efforts have been made to produce trepanning tools which can be 
effective in cutting holes in sheet metal in diameters from approximately 
1/2 inch up to 2 inches. However, such efforts have been generally 
unsatisfactory, as it is difficult to align hand held tools for producing 
smooth round holes. In addition to the loss of rigidity, hand held drills 
have torque and RPM limits considerably less than those available in a 
fixed machine structure. Consequently, a trepanning tool for hand held 
applications must produce holes with a minimum of resistance to preclude 
stalling or binding. 
Most of the tools produced to date have suffered from rapid wear which 
requires frequent replacement of either the entire tool or of the cutting 
tips attached thereto. For example, in U.S. Pat. No. 4,490,080 to Kezran, 
there is disclosed a hole cutting tool which includes removable cutting 
tips. Kezran relies on means which retain the tips in slots comprising a 
retaining wedge which engages the leading edge of the slot and engages the 
leading edge of the tip to urge it against the trailing edge of the slot 
so that the cutting tip is wedged in position with the angle of the wedge 
and the angle of the trailing edge of the slot preventing outward movement 
of the tip. While such a trepanning tool may be effective in various 
applications, where field use is complicated it is difficult to provide 
for tip replacement as such tips are small and easily lost. Should a 
trepanning tool with dull tips be used, it would require additional torque 
and undergo accelerated wear, and produce a ragged hole due to binding of 
the tips in the sheet metal. 
Generally, where multi-tip tools are used, wear may be uneven, causing 
binding to occur through misalignment of one or more tips within the hole 
Such binding is frustrating for the tool operator, and difficult to 
control with a hand held driving device, requiring additional manual force 
to overcome; Such a problem is additionally time consuming and often 
produces a ragged hole. Consequently, the search continues for a 
trepanning tool which is substantially wear resistant, and capable of 
producing smooth holes without binding in field applications using a hand 
held drill and to do so without requiring frequent replacement. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a trepanning tool which 
does not require a plurality of cutting tips to produce a hole in sheet 
metal. 
It is a further object of the present invention to provide a trepanning 
tool which is capable of producing smooth holes in sheet metal in field 
locations using a common hand held drill. 
It is another object of the present invention to provide a tool which is 
easily manufactured, producing a tool with substantially enhanced life at 
low cost. 
It is another object of the present invention to provide a trepanning tool 
which cuts holes substantially faster than conventional tools, with a 
minimum of resistance, thereby reducing operator fatigue. 
Accordingly, a metal cutting tool is disclosed which comprises a 
cylindrical cutter body which is substantially the diameter of the desired 
hole The cutter body includes an outer wall and an inner wall which tapers 
inwardly from a cutting end to a base in the cutter body, with the inner 
and outer walls joined at a forward edge thereof The cutter body includes 
a notch extending about the length thereof for removing cut material 
therefrom. A socket is disposed on one wall of the notch, with the socket 
sized to accept a cutting tip therein The body further includes an axially 
disposed bore extending therethrough for inclusion of pilot means which 
align the cutting tool with the desired hole location. The cutting tip is 
substantially pyramidically shaped and includes a blunt cutting edge which 
extends above the cutter body, further including a bottom and rear wall 
for mating with the socket in the cutter body.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 2, the metal cutting tool of the present invention 
is shown. The tool 10 includes a drive engaging section 11, and a cutter 
body 12. The drive engaging section may be of any suitable shape for 
engaging a conventional drill chuck common, for example, to 3/8 or 1/2 
inch drive drills. 
The cutter body 12 includes an outer cylindrical surface 13, essentially 
matching the size of the hole desired, with the cutter body further 
including an inner, inwardly tapered surface 14 which joins at a base 15. 
The inner and outer surfaces join at a forward edge 16 of cutting body 12. 
Having an inwardly tapered surface allows plug discharge without requiring 
auxiliary plug removers such as spring loaded arms. Essentially, as a plug 
is cut, it tends to bow due to the cutting pressure, which forces the plug 
into the cutting body. After the plug is released from the sheet metal, it 
is driven by its own resiliency, acting on the tapered surface to self 
discharge, an important feature for field operation. Generally a taper of 
from 10-40 degrees may be used with about 27 degrees preferred in a 7/8 
inch diameter tool. 
The cutter body further includes a notch 17 extending from the forward edge 
16 to the drive section 11 of the cutter body. The notch 17 is essentially 
rectangular, including sides 18 and 19, and a bottom 20. While a 
rectangular notch is shown, it will be understood that other shaped 
notches may also be used. The notch is preferably narrow, for example, on 
the order of about 0.25 inches wide in a 7/8 inch tool, to prevent hooking 
during hole cutting. A leading edge 21 of the side 18, in reference to the 
rotation of the tool, includes a socket 22 having a bottom wall 23 and a 
side wall 24, with the socket sized to accept a cutting tip therein The 
cutter body further includes a concentric bore 25 extending from the drive 
end through the base 15 of the cutter body. 
Generally, the cutter body may be composed of steel, alloy or a high carbon 
steel known as "tool steel", and is preferably composed of A.I.S.I. 6150 
steel, heat treated for maximum hardness and toughness. 
The metal cutting tool includes a cutting tip 26 which is secured in the 
socket 22. Referring to FIGS. 3A-3C, the cutting tip 26 includes an outer 
wall 27 which is straight sided to essentially match the outer wall 13 of 
the cutter body 12. The tip outer wall 27 may optimally include a beveled 
outer leading edge for assuring that a smooth sided hole is provided 
during through cutting. The cutting tip further includes a bottom wall 29 
and a rear wall 30 which mate with the bottom wall 23 and side wall 24 of 
the socket 22. In a preferred embodiment, best seen in FIG. 3A, the rear 
wall to bottom wall junction is curved rather than square edged in order 
to assure proper alignment and maximize distribution of the cutting forces 
over the surface of the cutter body. The bottom wall and rear wall should 
also be sized to provide a maximum surface area for bonding. For example, 
a bottom width of about 0.165 inches and a rear wall height of about 0.265 
inches, joined with a radius of about 0.0625 inches, provides a large 
surface area for assuring firm bonding in a 7/8 inch diameter tool. 
The cutting tip further includes a flat front face 31 which is generally in 
alignment with the side wall 18 of the notch 17. The carbide tip also 
includes an inwardly sloped wall 32, tapering towards the base 15 of the 
cutter body, which joins with a vertical section 33 forming an essentially 
pyramidically shaped tip. The wall 32 tapers essentially in about the same 
amount as the cutter body inner wall, i.e. about 10-40 degrees, with 30 
degrees preferred (angle D). The carbide tip further includes a top flat 
surface 34 which slopes and tapers rearwardly in the direction of 
rotation. The top surface slope may be fairly steep, on the order of about 
5-20 degrees, sloping rearwardly until the back of the top surface 34 is 
essentially flush with the forward edge 16 of the cutter body 12 (see FIG. 
3A, angle A). For example, in a 7/8 inch tool, an 8 degree slope may be 
used. Similarly the taper may vary with tool size, being dependent on the 
desired clearance, with inner and outer tapers of 5-15 degrees possible 
(see FIG. 3B, angles B and C). Again, for a 7/8 inch tool, an inner taper 
of 10 degrees (B), and an outer taper of 12 degrees (C) are exemplary, 
with the difference in taper angle resulting from the difference in radius 
between the inner and outer walls. 
A blunt cutting edge 35 is formed at the junction of the front face 31 with 
the top surface 34. The tip, at the cutting edge 35, extends above the 
forward edge of the cutter body, generally from 0.010 to 0.020 inches. For 
example, in a 7/8 inch tool for cutting sheet metal up to 0.14 inches 
thick, the tip extends about 0.015 inches above the forward edge of the 
cutter body. The cutting edge may be from 0.05 to 0.10 inches wide, with 
the top surface narrowing in the direction of rotation until it 
approximates the width of the cutter body forward edge, i.e. about 0.04 
inches. The blunt cutting edge provided by this geometry is substantially 
more durable than the pointed cutting edges common to other tools which 
are prone to rapid wear or breakage. For the illustrative 7/8 inch tool, 
the cutting edge is preferably about 0.07 inches wide. 
The cutting tip may be composed of tungsten carbide such as industry code 
number C-10, and is preferably composed of micro fine grain tungsten 
cobalt carbide. The cutting tip may be brazed, welded or otherwise bonded 
to the cutter body to assure that the tip does not move during cutting. 
Generally, brazing is preferred for bonding the tip to the cutter body, 
providing an economic yet reliable means of producing the metal cutting 
tool. 
Referring to FIG. 2, the metal cutting tool of the present invention 
further includes a pilot drill bit 36 disposed within the bore 25 
extending through the cutter body 12. Such a pilot bit includes a fluted 
section 37 which preferably has a double sided flute and a pointed end 38. 
A 135 degree split point is preferred to assure proper hole placement 
without requiring a prick punch mark. The pilot bit is a modified version 
of a common body drill used in the sheet metal trade, having no margin or 
clearance which is common to twist drills. Generally, if a common twist 
drill with standard fluted end were used, the side thrust generated by the 
trepanning tool cutting edge would cause the drill to cut on the side as 
an end mill, quickly producing an oversized egg shaped pilot hole which 
could allow the carbide tip to bind, producing a ragged hole. The pilot 
bit therefore includes a limited flute section to allow for first cutting 
a pilot hole using the fluted end and then insertion of an unfluted shaft 
section 39 within the pilot hole for guiding the cutting tip in the proper 
orbit without the side thrust causing hole enlargement. 
The fluted section extends for at least the depth of the maximum gauge 
sheet metal for which the tool is designed, with about 0.25 inches 
generally adequate for most applications. The fluted section extends above 
the maximum height of the tip to allow the fluted end to disengage from 
the sheet metal prior to the tip contacting the sheet metal. In a 
preferred embodiment, shown in FIG. 2, the pilot is double ended, having a 
second fluted section 37a. The pilot may be removed by loosening a locking 
screw 40 which is engageable with a recess 41, allowing removal of the 
pilot and exchange of the dull end with a sharp end. 
The metal cutting tool of the present invention is particularly suited for 
field use using a hand held driver such as a standard drill, being 
surprisingly durable and therefore capable of long term use. Utilizing a 
blunt cutting edge provides a smooth hole with a minimum of resistance, 
while substantially increasing wear resistance. In addition, limiting the 
metal cutting tool to a single tooth design prevents uneven wear, further 
reducing the potential for binding. Placing such a tip with a large 
bonding area in the cutting body previously described assures firm holding 
of the tip with a minimum of distortion, thereby reducing the chances for 
breakage. The shape of the cutter body also presents an outside body 
surface which rubs in the cut such that any misalignment caused by the 
lack of rigidity in the hand held operation causes a reverse thrust to 
return the tool to the proper orbit. Utilizing a narrow notch prevents the 
notch from hooking when the hand held drill is tipped at an angle to the 
hole axis, allowing intersecting holes to be drilled without binding, so 
long as a pilot hole can be provided. The tapered inner wall of the cutter 
body also provides near automatic discharge of the cut plug without 
requiring auxiliary devices. 
While the preferred embodiment of the present invention has been described 
in relation to a metal cutting tool including a rectangular notch and a 
particularly shaped cutting tip extending from a cutter body, it will be 
understood by those skilled in the art that other similarly shaped cutting 
tips, notches or pilot bits can be used without varying from the scope of 
the present invention. In addition, it will be understood by those skilled 
in the art that the means for attaching to a drive member such as a hand 
held drill may vary in accordance with the various drill chucks common to 
the art.