Annular cutter

A hole cutter has a fluted annular wall formed with a plurality of circumferentially spaced teeth around the lower end thereof, the successive teeth being connected by webs of reduced thickness. Between successive teeth each web is formed with a gullet, the upper end of which is defined by a wall which inclines upwardly in a radially outward direction. The upper end wall of the gullet comprises a radially inner section and a radially outer section. The radially inner section is inclined to the horizontal at a substantially less angle than the radially outer section. The two wall sections intersect in a shoulder which is spaced substantially closer to the radially inner face of the web that the radially outer face of the web.

This invention relates to an annular cutter of the type having a 
cylindrical side wall formed with a plurality of circumferentially spaced 
teeth around the lower end thereof and with a plurality of flutes 
extending lengthwise of the cutter around its outer periphery. 
The efficiency of an annular cutter, and particularly the ease with which 
it can be rotated and advanced into a workpiece, depends to a large extent 
on the ease with which the metal chips generated by the teeth of the 
cutter can be directed upwardly through the flutes of the cutter. Unless 
the cutter is designed to promote a smooth chip flow upwardly through the 
flutes, the flutes will become clogged with chips which, if the cutter 
continues to advance into the workpiece, can be dislodged upwardly through 
the flutes only by the chips being cut therebelow. This results in surging 
which produces an erratic cutting action and also results in excessive 
wear and breakage. One way of improving the chip flow is to form a 
plurality of cutting edges on each tooth designed to cut separate chips, 
each of which has a width not greater than the depth of the adjacent 
flute. This type of cutter is shown in U.S. Pat. No. Re. 28,416. However, 
even with cutters of the general type shown in this patent, unless the 
chips cut by the radially innermost cutting edges of the teeth can be 
directed freely, not only radially outwardly, but also upwardly through 
the radially adjacent flutes, the problems referred to above are not 
overcome. 
Cutters of the general type shown in the aforesaid patent are formed with a 
gullet between successive teeth in the web section of the cutter side 
wall, a gullet extending upwardly from each innermost cutting edge. In the 
prior art cutter the upper end wall of the gullet is inclined upwardly and 
radially outwardly and/or the radially inner cutting edge is inclined 
radially for the purpose of directing the chips cut by the inner cutting 
edges outwardly into the adjacent flute. Experience has shown that when 
the upper end wall of the gullet is inclined to the horizontal at an angle 
greater than about 40.degree., the web section of the cutter side wall is 
excessively weakened and breakage becomes a serious problem. Likewise, 
when the inner cutting edge is inclined radially in excess of about 
20.degree., the chips generated by the inner cutting edges are directed 
too sharply in a radially outward direction and, as a consequence, impinge 
immediately against the wall of the hole being cut. This also results in 
clogging of the flutes. Consequently, it has been the practice to incline 
this upper end wall of the gullet a maximum of about 38.degree. to the 
horizontal. However, I have found that when the upper end of the gullet is 
inclined to the horizontal at an angle of not more than about 40.degree. 
and even when the inner cutting edges are only slightly inclined radially, 
the chip cut by the inner cutting edge is directed radially outwardly too 
abruptly by the upper end wall of the gullet. The chip is actually 
directed radially outwardly against the wall of the hole being cut rather 
than upwardly through the adjacent flute. 
The present invention has for its object the provision of a cutter of the 
type described wherein each gullet is formed with a wall at its upper end 
which assists in directing the chip cut by the inner cutting edge upwardly 
through the flute without materially affecting the strength of the web 
section of the cutter. 
More specifically, it is an object of the present invention to form the 
upper end wall of each gullet with a radially inner section inclined to 
the horizontal at an angle of not more than 40.degree. and with a radially 
outer section inclined to the horizontal at a substantially greater angle.

The annular hole cutter of the present invention is designed for cutting 
holes in metal and is generally designated 10 in FIG. 1. The cutter 
includes a body 12 and a shank 14. Cutter body 12 is of inverted cup 
shape, having a side wall 16, the length of which is greater than the 
thickness of the workpiece in which the hole is to be cut. The lower end 
of side wall 16 is formed around its periphery with a plurality of 
circumferentially spaced cutting teeth 18. Around its outer periphery side 
wall 16 is formed with a spiral flute 20 adjacent each cutter tooth. The 
successive flutes 20 are separated by a land 22 at the periphery of the 
cutter. The leading edge of each land 22 is formed with a narrow margin 
24. The portions of the annular side wall 16 of the cutter between 
successive teeth comprise webs 26. Webs 26 have a thickness approximately 
one-half the thickness of side wall 16. The radially outer face 28 of each 
web 26 defines the radially inner wall of each flute 20. Each flute 20 
includes a circumferentially leading side wall 30 and a circumferentially 
trailing side wall 32. 
In the cutter illustrated in the drawings each tooth is formed with a 
radially inner cutting edge 34 and a radially outer cutting edge 36. 
Cutting edge 34 is spaced forwardly in the direction of rotation of the 
cutter from cutting edge 36. Cutting edge 34 is located at the lower end 
of the trailing face 38 of a gullet 40. Cutting edge 36 is located at the 
lower end of the trailing face 32 of flute 20. 
The bottom face of each tooth 18 is formed with two back-off or clearance 
faces 42,44. In the operative condition of the cutter (FIG. 1) the 
radially inner back-off face 42 inclines axially upwardly and radially 
inwardly and the radially outer back-off face 44 inclines axially upwardly 
and radially outwardly. In addition, each of these back-off faces inclines 
upwardly from its respective cutting edge in a circumferential direction 
to a slight extent (for example, 8 to 10.degree.) to provide the necessary 
clearance for the cutting edges as the tool is rotated and fed axially 
into a workpiece. The two back-off faces 42,44 intersect in a downwardly 
projecting crest 46 which in turn intersects the radially outermost 
cutting edge 36. The radial inclination of the outer back-off face 44 is 
in the range of about 5 to 35.degree. to the horizontal, and preferably 
about 10.degree.. The radially inner back-off face 42 inclines radially to 
the horizontal at an angle of between -3 to +20.degree., and preferably 
not more than about 15.degree.. As a result of the inclination of these 
back-off faces 42,44 in both a radial and a circumferential direction, 
cutting edges 34,36 are not only staggered circumferentially as shown in 
FIG. 3, but are also staggered vertically as shown in FIGS. 2, 4 and 5. 
The present invention is concerned primarily with the gullet 40 and, more 
specifically, with the configuration of the upper end wall 50 of the 
gullet. As shown in the drawings, the upper end wall of the gullet is 
formed as two sections, a radially inner section 50a and a radially outer 
section 50b. It will be observed that the angle of inclination a of wall 
section 50a to the horizontal is substantially less than the angle of 
inclination b of the wall section 50b. The wall sections 50a,50b intersect 
at 52. It will be observed that the shoulder formed by the line of 
intersection 52 is located radially much closer to the radially inner face 
54 of web 26 than the radially outer face 28 thereof. The angle a may vary 
from zero to about 40.degree. and angle b may vary from about 45.degree. 
to 70.degree.. Although it is preferred that both angles a and b are at or 
near the maximum of their respective ranges, on cutters where side wall 16 
is relatively thin (0.150" or less) these angles may be progressively 
reduced as the wall thickness is reduced in order to maximize wall 
strength. These angles can be reduced on thinner wall cutters since the 
narrower inner chip generated by a thinner web can be directed into the 
flute with less difficulty than a wide chip. The wall section 50a should 
be spaced vertically above the inner end 56 of the inner cutting edge 34 a 
distance of not less than about 0.020". The line of intersection 52 
between the wall sections 50a,50b should be spaced radially from the inner 
periphery 54 of web 26 a distance of between about 0.020" and 0.035". As 
explained hereinafter, the radial location of the line of intersection 52 
is related to a large extent to the angle of inclination of the inner 
cutting edge 34 (angle c in FIG. 5) and the daylight height h of the 
gullet (the vertical distance between the point 56 and the portion of wall 
section 50a which is circumferentially adjacent the trailing face of 
gullet 40). 
In the embodiment illustrated, gullet 40 is formed by means of a 
cylindrical cutting tool so that the leading face 58 of gullet 40 is 
generally parallel to the trailing face 38 of the gullet and the wall 
sections 50a, 50b are generally perpendicular to these faces. If desired, 
the gullet can be formed so that the wall sections 50a,50b are of arcuate 
configuration similar to that shown in the aforesaid patent rather than of 
generally rectangular configuration. 
As mentioned previously, the radial location of the intersection line 52 is 
related to a large extent to the angle c and the distance h. This is 
predicated on the fact that the chip generated by cutting edge 34 is 
directed upwardly away from the cutting edge in a direction generally 
perpendicular to the radially inclined cutting edge. Thus, if the angle c 
is about 15.degree., then the chip 60 generated by this cutting edge will 
also be directed upwardly and radially outwardly at an angle of about 
15.degree. to the horizontal as shown in FIG. 2. To the extent possible, 
the line of intersection 52 should be located so that the chip 60 directed 
upwardly by cutting edge 34 will clear line 52 or at least will pass the 
line of intersection 52 with minimum obstruction from the shoulder it 
forms. In this way the chip will be guided smoothly upwardly into the 
adjacent flute 20 by the wall section 50b. If the line of intersection 52 
is located radially too far from the inner periphery 54 of web 26 or is 
located too close to the end 56 of cutting edge 34, the chip 60 would 
impinge excessively against wall section 50a and would consequently be 
directed radially outwardly in an abrupt manner against the wall of the 
hole being bored in the workpiece. When this occurs, the chips will not 
flow freely upwardly through flute 20 and the flutes tend to be clogged 
with chips. As a consequence, excessive surges occur in the thrust loads 
on the cutter and an erratic cutting action results. However, when the 
chips 60 generated by cutting edges are not directed abruptly radially 
outwardly, they are guided smoothly upwardly into flutes 20 by the steeply 
inclined wall section 50b. 
With the above-described arrangement it will be observed that the portion 
of web section 26 directly above gullet 40 does not terminate in a thin 
knife edge, but rather in a blunt thickened edge defined by wall section 
50a. Wall section 50a thus imparts substantial strength to the thinned 
portion of the web and thus reduces very substantially the tendency for 
the cutter to break because of an excessively weak web section. In 
addition, tests have shown that with a cutter constructed in accordance 
with the present invention the cutting action is smooth as compared with 
the erratic cutting action produced by cutters in accordance with the 
prior art and the thrust required to advance the cutter into the workpiece 
is likewise very substantially lessened and the power required to rotate 
the cutter is somewhat lessened.