Annular hole cutter

An annular hole cutter having an inverted cup-shaped body provided with a plurality of circumferentially spaced teeth around its lower edge. Helical flutes extend upwardly between successive teeth and form around the outer periphery of the cutter a plurality of radially relieved lands, each having a circle ground narrow margin at the leading edge thereof. Each tooth has a plurality of radially and axially staggered cutting edges thereon.

This invention relates to an annular hole cutter of the fluted type. 
The object of this invention is to extend the life of the fluted annular 
hole cutter and to improve the finish of the holes formed by such cutters. 
Fluted type hole cutters, such as conventional twist drills, and annular 
cutters, such as shown in U.S. Pat. No. 2,444,099 and my prior Reissue 
Patent No. 28,416, have radially extending cutting edges at the leading 
end of the tool and generally helical flutes extending upwardly from the 
cutting edges for discharging chips from the hole being formed. The outer 
periphery of such hole cutters is defined by lands between the successive 
flutes. With such cutters it is universal practice to grind or otherwise 
form the lands with radial relief rearwardly of a circle ground margin at 
the leading edge of each land. These margins extend axially to the 
radially outer end of the cutting edges at the leading end of the tool and 
are provided to impart radial or lateral stability to the cutting tool. 
Lateral stability is absolutely necessary with end cutting fluted tools of 
this type in order to maintain hole size and finish. 
In annular cutters of the type previously referred to the width of these 
margins have consistently been in the range of about 0.060 to 0.100 inches 
regardless of the diameter of the cutter. The width of the margin on such 
cutters does not vary in accordance with the diameter of the cutter 
because the tooth load does not vary significantly with diameter since 
larger cutters usually have more teeth. On the other hand, in the case of 
twist drills the margin width increases with an increase in diameter since 
the tooth load on a drill varies in accordance with the diameter of the 
drill. For example, the margin is about 0.040 inches on a one-half inch 
drill, about 0.050 inches on a three-quarter inch drill, and about 0.080 
inches on a one and one-quarter inch drill. The widths on margins of 
fluted end cutting tools in the ranges stated above have always been 
considered necessary not only from the standpoint of imparting lateral 
stability to the cutter, but also to prevent rapid and excessive wear at 
the junction of the margins and the cutting edges at the leading end of 
the tool. 
Heretofore it has always been believed that the susceptability to rapid 
wear of the outer ends of the cutting edges of such tools was related to 
the strength of the cutting edges at this location. Thus, a relatively 
large margin was deemed necessary in order to provide sufficient mass at 
the outer end of the cutting edges to withstand the cutting load and to 
dissipate the heat generated at this location. In the case of twist drills 
having two cutting edges, this reasoning is apparently sound. However, in 
the case of annular cutters of the type disclosed in my prior Reissue 
Patent No. 28,416, contrary to the assumptions which have governed the 
determination of the minimum margin width required, I have found that if 
the leading edge of each land is provided with a relatively narrow margin 
the tool life is actually extended substantially and the hole finish is 
improved as compared with the provision of a relatively wide margin.

Referring first to FIG. 1, the annular hole cutter of this invention is 
generally designated 10 and includes a cutter body 12 and an arbor 14. 
Cutter body 12 is of inverted cup shape having a side wall 16 and a top 
wall 18. The lower end of side wall 16 is formed with a plurality of 
regularly spaced cutting teeth 20. Each cutting tooth 20 is formed with a 
radially inner cutting edge 22 and a radially outer cutting edge 24. These 
cutting edges are best illustrated in FIGS. 2 and 3. As shown in FIG. 2, 
cutting edge 22 is spaced forwardly (in the direction of rotation as 
designated by the arrow D) from the cutting edge 24. These two cutting 
edges are separated circumferentially by a shoulder 26 (FIG. 4) and are 
staggered vertically or axially as shown in FIG. 3. The two cutting edges 
22,24 are staggered both radially and axially so that when the cutter is 
fed into a workpiece the cutting edges 22,24 cooperate to cut an annular 
groove in the workpiece and each cutting edge cuts its own individual 
chip. The bottom of the groove cut in the workpiece has a contour in cross 
section which is complementary to the contour of the cutting edges 22,24 
as shown in FIG. 3. In other words, the groove cut in the workpiece has a 
concentric radial shoulder defined by the portion of the circumferential 
shoulder 26 on each tooth designated 28 in FIG. 3. 
As shown in FIGS. 2 and 4, the bottom face of each tooth is formed with two 
back-off or clearance faces 32,34. In the operative condition of the 
cutter the back-off face 32 inclines axially upward in a radially inward 
direction and back-off face 34 inclines axially upward in a radially 
outward direction. In addition, each of these back-off faces inclines 
upwardly from the cutting edges 22,24 in a circumferential direction to 
provide the necessary clearance. The two back-off faces intersect in a 
crest 36 which in turns intersects the outer cutting edge 24. 
The cutter is provided with a gullet 44 and a flute 46 between successive 
teeth. Each gullet 44 adjacent cutting edge 22 is defined by a front rake 
face 48 which slopes upwardly and rearwardly relative to the direction of 
rotation of the cutter. The upper end of each gullet 44 is defined by a 
curved surface 50 which slopes upwardly in a radially outward direction as 
shown in FIG. 3 for discharging chips cut by edge 22 into the adjacent 
flute 46. Each flute 46 extends spirally upwardly around the outer 
periphery of side wall 12. Each flute is defined by an inner 
circumferentially extending face 54 which is flush with shoulder 26, a 
leading face 56 and a trailing face 58. The leading face 56 of each flute 
46 comprises the rear face of a land 60 between each of the flutes 46. 
Face 56 inclines radially inwardly so as to provide radial relief directly 
adjacent each outer cutting edge 24. The cutter thus far described is 
generally the same as that shown in my Reissue Patent No. 28,416. 
Adjacent the leading edge of each face 56 each land 60 is provided with a 
narrow margin 62 which is circle ground to the desired diameter of the 
cutter. In the tool of this invention margin 62 has a circumferential 
width of between about 0.005 to about 0.030 inches and preferably not more 
than about 0.025 inches. For reasons not readily apparent, when the width 
of margin 62 lies within this range the life of the cutting edges 22,24 
before requiring resharpening is very markedly increased and the finish of 
the hole formed by the cutter is very substantially improved over the tool 
life and surface finish obtained when the width of the margin is at least 
0.060 inches. 
While the reasons for the improved performance of the cutter having the 
narrow margin referred to are not entirely understood, it is believed that 
the improved results are attributable to some extent to the fact that an 
annular cutter of the type disclosed is inherently much more stable in a 
lateral or a radial direction than a twist drill or other types of fluted 
annular cutters. As distinguished from a twist drill, the annular cutter 
of this invention has at least six teeth as contrasted to a conventional 
twist drill which has only two cutting edges. In addition, the cutter of 
this invention has at least two radially and circumferentially staggered 
cutting edges of each tooth, each cutting edge being designed to cut an 
individual chip. Therefore, the cutter inherently has substantial lateral 
stability because of the number of teeth on the cutter and also because of 
the concentric shoulder (designated 28 in FIG. 3) formed in the groove cut 
by the teeth of the cutter. It is believed that because of the inherent 
lateral stability of the cutter a very narrow margin at the leading end of 
each land can be tolerated without sacrificing lateral stability. 
A comparison of FIGS. 4 and 5 indicates one reason it is believed the 
cutter of this invention produces a much finer finish on the hold formed 
as compared with a prior art cutter having a relatively wide margin. For 
example, the prior art cutter illustrated in FIG. 5 is substantially 
identical to that illustrated in FIG. 4 except that the margin 64 has a 
width of at least 0.060 inches as compared with the margin 62 in FIG. 4 
having a width of about 0.010 inches. 
With a relatively wide margin such as illustrated at 64 in FIG. 5 it 
follows that, if there is an obstruction between the margin 64 and the 
side wall of the hole, the unit pressure on the obstruction is 
substantially less and the friction is substantially greater than where 
the margin is very narrow as indicated at 62 in FIG. 4. Thus, if a chip 66 
cut by one of the cutting edges of the tool becomes wedged between the 
side wall of the hole being formed and the margin 64, the portion of the 
chip wedged between margin 64 and the side wall of the hole becomes 
trapped and may become heated to a relatively high temperature because of 
the friction generated therebetween. Under such conditions the chip 66 can 
produce galling on the tool and on the side wall of the tool and, if 
heated to a sufficiently high temperature, can actually weld to the margin 
64. On the other hand, if the margin is relatively narrow as illustrated 
at 62 in FIG. 4, the portion of the chip 66 which may become trapped 
between the narrow margin and the side wall of the hole is subjected to an 
extremely high unit pressure which will tend to shear or break the chip 
before it becomes sufficiently heated to cause galling or welding. 
It is believed that the high unit pressure resulting from a very narrow 
land and the ability to more effectively cool the outer end portion of the 
outer cutting edges 24 contribute to the substantially longer life of the 
tool of the present invention. As shown in FIG. 1, a conventional coolant 
passageway 68 is formed in the shank of the tool for conducting coolant 
within the cup-shaped cutter down to the cutting edges 22,24. The coolant 
flows down to the cutting edges and then radially outwardly to the outer 
periphery of the cutter. This coolant is designated 68 in FIGS. 4 and 5. 
It will be observed from the showing in FIG. 4 that the coolant 68 flows 
to an area much closer to the outer end of cutting edge 24 than does the 
coolant where the cutter has a wide land such as shown at 64 in FIG. 5. 
The flow of coolant closer to the outer end of cutting edge 24 coupled 
with the fact that the mass of the tooth directly behind the outer end of 
cutting edge 24 is substantially smaller with a narrow margin as compared 
with a wide margin maintains the temperature of the outer end portion of 
each cutting edge 24 (the most vulnerable portion of the cutting edge) 
substantially lower. It follows that the cutting edge will stay sharp a 
substantially longer period of time if it is prevented from becoming 
overheated. 
It is also believed that a very narrow land prevents rapid and excessive 
wearing of the outer end portion of the outer cutting edge because, as 
pointed out previously, a very narrow land results in a relatively high 
unit pressure. The high unit pressure, as distinguished from a relatively 
low unit pressure, will enable the outer end portion of the cutting edge 
to penetrate sufficiently into the material being cut so that it will 
shear the material rather than producing frictional drag thereover. It 
must be appreciated that, because of the run-out of tool spindles and 
because of the impracticality of forming a tool where the margins define a 
circle truly concentric to the axis of the cutter, a radial load between 
the margins at the leading end of the cutter and the side wall of the hole 
being formed is inevitable. However, if this radial load is reflected by a 
relatively high unit pressure, the margin will actually penetrate into and 
shear the metal in the manner of a cutting edge. However, when the margin 
is relatively wide, the radial load on the leading end of the cutter 
results in a relatively low unit pressure which is not sufficient to cause 
penetration. As a result, heat is generated and wear is encountered. This 
becomes progressively worse and results in tooth chipping and excessive 
wear of both cutting edges and especially at the outer end portions of the 
outer cutting edges 24. 
It has also been found that, as distinguished from conventional annular 
cutters of the fluted type which have a back taper of about 0.002 to 0.003 
inches per axial inch, when the cutter is provided with a margin of not 
more than about 0.030 inches, and preferably less, the performance of the 
cutter is enhanced if the back taper is entirely eliminated or reduced to 
a value of not more than about 0.0005 inches per axial inch of length. By 
reducing the back taper to a minimum is is believed that the lateral 
stability is improved since even a narrow land of sufficient length will 
not result in an unduly high unit pressure as to cause the margin to 
penetrate into the side wall of the hole being formed along its entire 
length. In addition, when the back taper is eliminated or reduced to about 
0.0005 inches, the cutter can be resharpened repeatedly without 
substantially reducing its outer diameter. 
TABLE I 
______________________________________ 
Number 
Margin 
of Peripheral 
Tooth Surface 
Width Holes Wear (in.) 
Wear (in.) 
H.P. Finish 
______________________________________ 
.090" 50 .005 .004-.0075 
1.5-2.2 
250 
100 .005 .010-.012 
1.5-2.6 
350 
.040" 50 .006 .045-.010 
1.6-1.8 
100 
100 .006 .009-.013 
1.7-2.3 
250 
.030" 50 .002 .005-.0075 
1.5-1.7 
100 
100 .003 .007-.008 
1.6-2.0 
250 
150 .005 .0105-.0125 
1.4-2.0 
250 
.0275" 
50 .001 .0025-.004 
1.5-1.7 
100 
100 .003 .006-.008 
1.4-1.9 
100 
150 .0045 .008-.013 
1.5-1.9 
125 
.020" 50 .0005 .003-.005 
1.5-1.7 
125 
100 .0015 .0045-.006 
1.5-1.8 
250 
150 .003 .005-.008 
1.5-1.9 
350 
200 .005 .008-.010 
1.6-1.9 
350 
.015" 50 .002 .003-.0045 
1.5-1.8 
63 
100 .003 .004-.0055 
1.6-1.9 
75 
150 .004 .006-.008 
1.5-1.8 
100 
200 .005 .009-.011 
1.6-1.8 
100 
250 .005 .010-.013 
1.6-1.9 
100 
.005 to 
.010" 50 .001 .0015-.0025 
1.4-1.6 
75 
100 .002 .004-.005 
1.5-1.7 
75 
150 .003 .006-.008 
1.5-1.7 
75 
200 .0035 .007-.009 
1.4-1.7 
75 
250 .004 .008-.010 
1.4-1.6 
75 
300 .005 .011-.0135 
1.4-1.7 
75 
______________________________________ 
Table I sets forth the results of tests conducted with cutters of the type 
shown in FIG. 1 having margins of different widths. The holes were cut in 
one inch thick steel commercially known as "Jalloy" which has a carbon 
content of about 0.30%, a manganese content of about 1.65%, chromium about 
1.2%, silicon in the range of about 0.15 to 0.30% and other lesser 
ingredients. The steel had a Rockwell C hardness of between 28 and 32. 
This steel was selected because it is normally considered difficult to cut 
holes therein and obtain a good finish. In all of the tests conducted the 
machine tool spindle on which the cutters were mounted was rotated at 250 
r.p.m. with a feed rate of 3.5 inches per minute. The horsepower required 
to rotate the spindle was continually observed and recorded. After every 
fifty holes the peripheral wear, the tooth wear, and the surface finish of 
the hole were measured. The peripheral wear reflects the decrease in 
diameter at the leading end of the cutter. The column designated "tooth 
wear" is a measurement of the widths in a circumferential direction of the 
flat areas worn on the originally sharp pointed teeth. The surface finish 
indicated is in terms of the shape turn microfinish. Each cutter was 
formed with six teeth, had a diameter of 13/16 inches and a wall thickness 
of about 0.155 inches. 
The first hole cut with each cutter had a surface finish of 75 shape turns 
with the exception of the cutter having a margin width of 0.015 inches 
which had a surface finish of 63 shape turns. A visual examination of the 
cutters after every fifty holes clearly showed that as the outer corner of 
each outer cutting edge, the corner designated 70 in FIG. 3, became 
increasingly rounded through wear the horsepower required to rotate the 
cutter increased. Through experience it has been learned that with the 
particular machine used in the test, when the horsepower begins to exceed 
2, there is a strong likelihood that, if use of the cutter is continued 
substantially without resharpening, it will sieze in the work and break. 
Accordingly, in view of the results of the tests conducted, it has been 
determined that the width of the margin should not exceed about 0.030 
inches. The minimum margin width should be about 0.005 inches to allow use 
of the cutter in a drill bushing, but a margin width of this magnitude is 
impractical from the commercial standpoint because it would be difficult 
to maintain such close tolerances economically on a production basis. 
Therefore, if grinding tolerances can be held to about .+-.0.0125", the 
nominal width of the margin should be about 0.0175". As a measure of 
safety it is preferred to maintain the margin width at not more than about 
0.025".