Bit for cutting sheetrock

Disclosed is a bit which can be cut both axially and laterally, and which is especially useful to cut an access opening in a sheetrock panel covering an electrical outlet box. The bit self-guides itself laterally around the periphery of the outlet box so that the opening will correspond to the shape of the box. The configuration of the bit permits it to be made in an unusually small diameter so as to minimize the width of the slot it cuts.

FIELD OF THE INVENTION 
This invention relates to router bits and more specifically to a bit for 
cutting sheetrock. 
BACKGROUND OF THE INVENTION 
In the construction industry sheetrock (or "drywall") is frequently used to 
cover large wall areas. In the interest of speed it is common first to put 
up a large sheetrock panel, covering apertures in the wall such as 
electrical outlet boxes and even window and door areas, and thereafter to 
cut openings in the sheetrock to expose or open the electrical outlet box, 
window, or door. This saves substantial time in comparison to precutting 
the panel to fit to the edge of the opening, before fastening the panel. 
Special bits are known for such cutting. They are generally used in a 
router operating at a speed in excess of about 20,000 rpm. 
For such use the bit must be able to cut both axially through a sheetrock 
panel, and then cut laterally in it, i.e., in a direction perpendicular to 
the length of the bit, thereby to make the opening in the panel. In other 
words, the bit must function both as a drill and as a saw. 
A conventional twist drill bit performs poorly for such use: it cuts well 
in the axial direction, but cuts laterally only poorly because of the 
flute configuration. It is convenient to use the box wall itself as a 
"template" for cutting an opening around a box, moving the bit sideways 
while guiding it along the side of the box. A conventional bit will not 
move smoothly around the side of an electrical outlet box or door; if 
guided against the box it will abrade or cut into the box wall, thereby 
dulling itself and marring the box or other opening. Moreover, the 
conventional drill configuration tends to convey powdered cut sheetrock 
material outwardly into the room, where it must ultimately be swept up. 
The so-called "WD-5" bit once marketed by the Roto-Zip Tool Corporation of 
Cross Plains, WI, was specially intended for cutting sheetrock. It cut 
both axially and laterally and had a single flute in the form of a 
so-called "down" or reverse spiral. (In a conventional twist drill bit, 
the spiral flutes advance toward the tip in the same direction as the bit 
is operated, all bits conventionally being rotated in the clockwise 
direction.) In the WD-5 bit the spiral advanced away from the direction of 
rotation, that is, oppositely to the usual flute direction. However, the 
WD-5 bit was subject to severe abrasion when it was guided around the 
periphery of an outlet box. Moreover, the bit was about 1/4" in diameter 
and so tended to cut a wider kerf or slot than was desirable. If made in 
smaller diameter, shear strength was lost and the bit tended to break off 
too easily. 
Kerf width is especially important when cutting around an electrical outlet 
box. The box opening is ultimately covered by a cover plate which usually 
extends outwardly about 1/2" beyond the periphery of the box itself. If a 
bit which is 1/4" wide is used to cut around the outside of the box, it 
can be seen that the resulting panel opening may extend almost to the edge 
of the cover, and in some cases the cover may have an insufficient margin 
of overlap to cover the opening in the panel. Moreover, if the bit is kept 
away from the box in order to minimize tip abrasion, the effective width 
of the opening it cuts is further increased. 
Thus there has been a need for a bit for cutting sheetrock (including 
drywall, gypsum board, plaster, "Gypsonite" and "Fiberbond" composite 
board materials, and other similar materials), which is sufficiently 
strong that it can be made in a diameter as small as 1/8" without an 
excessive breakage rate in use, yet which can be guided along the wall of 
an outlet box or other surface without undue abrasion. 
SUMMARY OF THE INVENTION 
In accordance with a preferred form of this invention a bit is provided 
which has a conically pointed, fluted tip for cutting axially through a 
sheetrock panel. The tip has two diametrically opposed straight flutes for 
cutting into and through the panel. In addition to the tip flutes, the bit 
has a plurality of body flutes for cutting laterally. The body flutes form 
a spiral on the bit that extends toward the tip in a direction which, when 
the bit is rotated, tends to resist advancement of the bit into the panel 
being cut. This spiral direction tends to convey the cut chips and powder 
into the wall, rather than outwardly into the room, so that cleanup is 
reduced. Preferably the bit has three body flutes. It has been found 
especially desirable that the body flutes be of a so-called "parabolic 
grind" configuration, having an S-curvature as viewed in transverse 
cross-section. 
The tip flutes are spaced axially from and do not join or open into the 
body flutes. Between the tip flutes and the outer end of the body flutes 
there is an unfluted annular cylindrical band which extends around the 
body of the bit. This unfluted band has been found to provide a guide or 
bearing surface on which the bit can be guided around an electrical outlet 
box. Provision of this guide surface minimizes abrasion of the cutting 
edge of the flutes as well as minimizes damage to the electrical guide box 
or other structure along which the bit is guided. Moreover, the absence of 
body flutes at the tip has been found to reduce nicking and cutting of 
electric leads to the box. 
Surprisingly, the bit has sufficient shear strength, even in a diameter as 
small as 1/8", to display much better resistance to breakage in use in 
comparison to prior bits.

DETAILED DESCRIPTION 
FIG. 1 illustrates one use of the bit of the invention. A conventional 
metal electrical outlet or junction box 10, having a generally rectangular 
interior opening 11, is mounted to studding or other wall structure, not 
shown. A sheetrock panel 12 has been nailed to the wall structure to cover 
the outlet box. A bit 14 in accordance with the invention, mounted in the 
chuck 16 of a router tool not illustrated in detail, is used to cut an 
opening 20 through the panel corresponding to the shape of box 10, so that 
the interior of the box will be accessible for wiring and mounting a 
switch or plug socket. In cutting opening 20, the bit first is moved 
axially to cut into panel 12, and then is moved laterally, i.e., 
perpendicular to the bit axis and parallel to the surface of panel 12, to 
cut around the outside perimeter of the box. It is desirable to use a 
router having a depth gauge, to insure proper depth of bit insertion. It 
can be seen that if the bit 14 tracks on or is guided along the outside 
wall 18 of box 10, a gap or slot 22 will be formed between outside wall 18 
and the cut edge 24 of panel 12. The width of slot 22 will at least be 
equal to the cutting diameter of bit 14. (It should be noted that the bit 
may also be used to cut around the inside of a door, window or box, as 
well as around the outside of a box.) 
Once a switch or outlet has been mounted in box 10, the opening 20 will be 
covered by a cover plate (not shown). A conventional cover plate is 
usually only slightly wider and longer than the OD dimensions of the box 
itself. Thus if gap 22 is too wide the cover plate will not adequately 
cover it. It is desirable that the cutting width of bit 14 be no more than 
about 1/8 or 3/16" diameter, to cut a slot so narrow that the opening will 
be completely covered. 
At one end (or in the middle, if double ended) the bit 14 has an unfluted 
shank 26 by which it can be gripped in chuck 16. The bit has a series of 
spirally curving body flutes 28, three in the embodiment shown (FIGS. 2 
and 3), which extend from the shank toward but not to the tip 30 of the 
bit. Tip 30 is conical, preferably having a 120.degree. included angle at 
the tip, and is provided with two tip flutes which, as best shown in FIG. 
4, are straight V-shaped flutes 32, 32, on opposite sides of the bit. It 
is the function of the tip flutes to cut axially through the panel, so 
that the body flutes can then cut sidewise through it. 
Between tip 30 and the flutes 32 is an unfluted annular, circular 
cylindrical guide band 36 which extends around the bit. Neither the spiral 
body flutes 28 nor the tip flutes 32 extend across this guide band, that 
is, the guide band is continuous and unfluted around the periphery of the 
bit. The diameter of the guide band may be the same as that of shank 26 
and substantially the same as the cutting diameter of the body flutes (see 
FIG. 3). 
As shown in FIG. 2 the body flutes 28 extend in "left hand" or "down" 
spiral toward the guide band and tip. In use, the bit is rotated in 
clockwise rotation, as virtually all router motors conventionally do. The 
"down" orientation of the spiral tends to resist advancement of the bit 
into the workpiece, that is, the engagement of the cutting edges of the 
body flutes with the sheetrock panel tends to urge the bit outwardly, 
rather than pulling the bit inwardly. This direction of the spiral is the 
reverse of the usual direction of spiral flutes. It has the effect of 
carrying the cuttings 38 into the wall, rather than outwardly into the 
room. 
It is preferred that the bit have at least three evenly spaced flutes, each 
made by the so-called "parabolic" grinding process, as shown in FIG. 5. 
This configuration provides an inwardly curving web 44 of substantial mass 
behind each cutting edge 42, thereby strengthening the bit and reducing 
breakage, especially in smaller diameters, in comparison to bits having a 
different flute section. In contrast to a conventional grind, the web 44 
does not have a circumferential land of the same diameter as the cutting 
edge. So-called parabolic grinding (which does not necessarily form a true 
parabolic curve in the flutes) is known in flute grinding art; however, so 
far as I know, it has not previously been used in a left hand flute 
configuration, or in a sheetrock bit. 
To grind a "parabolic" flute, a grinding wheel 50 is used, having opposite 
side faces 52, 53 and a peripheral rim 54. Wheel rim 54 meets side face 52 
at an included angle 55 of about 70.degree. between them. For a bit of 
about 0.125" diameter, wheel 50 may be about 0.200" thick. To form the 
spiral the wheel, skewed to the axis of the bit, is advanced into a bit 
blank as the blank is rotated (counterclockwise in FIG. 5), in accordance 
with known flute grinding procedure. Wheel corner 56 cuts a concave curve 
at the bottom of the leading face 40 of a web 44b, and rotation of the bit 
forms a convex curve 57 leading to the adjacent web 44a. Thus, as seen in 
section (FIG. 5), the flute has an S-curve: behind the cutting edge 42 of 
a first web 44a, the surface curves inwardly along a convex curve 57 to a 
flute bottom 58 which is substantially flat, then curves outwardly in a 
tight concave curve 60, to the cutting edge 42 of a following web 44 b. 
Each web 44 thus has a relatively wide and rounded section, but without a 
land of equal diameter behind it. The maximum depth of the flute (at 
surface 58) is preferably less than about 1/4 the diameter of the bit. 
Surprisingly, it has been found that the use of a parabolic grind flute 
configuration tends to strengthen the bit by providing a larger and 
stronger body of metal in the land behind each cutting face so that even 
in diameters as small as 1/8" diameter, the bit will not break or wear 
unduly. Moreover, this configuration facilitates the use of three flutes 
rather than two, which of itself strengthens the bit. With other flute 
configurations the bits tend to break more easily. 
Bit 14 is preferably formed from a cylindrical rod of tool steel, for 
example molybdenum type high speed tool steel such as grade Ml. After 
grinding the bit is hardened, preferably to 60-65 Rockwell C hardness. 
Hardening is preferably carried out by heating an Ml steel bit to about 
2205.degree. F. in a diamond block controlled (reducing) atmosphere 
furnace; oil quenching it; and twice annealing it by heating to 
1020.degree. F. for 2 hours followed by slow cooling to room temperature. 
FIG. 2 shows the bit in use. The guide band 36 tracks along a box wall to 
guide the bit closely along the box periphery. The continuous 
circumferential band 36 rides against the outside wall 18 of the box, 
preventing the cutting edges 42 of the flutes from being abraded by the 
box wall while they are cutting through the sheetrock. The bit is 
preferably rotated at a speed of about 25,000 rpm, but this is not 
critical. 
The axial length of guide band 36 need not be great. For a bit of about 
1/8" diameter, a band 36 with an axial dimension of only 1/16-1/8" is 
sufficient. It is the fact that the guide band is continuous around its 
periphery, rather than a particular axial length of the guide band that is 
critical to the guide function. The overall flute length may be about 1" 
up to the shank. (Double-ended bits may have flutes on both ends, with a 
shank in the middle.) 
In comparison to prior sheet rock bits, the present bit cuts faster and 
with a lower breakage rate. It conveys chips inwardly rather than 
outwardly, and cuts a narrower slot. It is effective even on 
fiber-containing composite sheet materials such as "Fiberbond" which it 
cuts with a smoother surface finish because the fibers are curled inwardly 
rather the outwardly.