Hole cutter

A hole cutter having a generally cylindrical body provided with a plurality of first and second cutting teeth around its lower end. Each of the first cutting teeth has a bottom surface defining along its front edge at least two cutting edges which are separated from each other by a shoulder having a certain width and extending substantially circumferentially of the cutter body. Each of the second cutting teeth has a bottom surface provided with a ridgeline extending substantially circumferentially of the body and defining two cutting edges along the front edge of the bottom surface of each second cutting tooth. Each of the cutting edges on each of the second cutting teeth is partly located below one of the cutting edges on each first cutting tooth.

FIELD OF THE INVENTION 
This invention relates to a hole cutter having a generally cylindrical body 
provided with a plurality of cutting teeth around its lower end. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a hole cutter having a long 
life. 
A hole cutter has a generally cylindrical body provided around its lower 
end with a plurality of first cutting teeth and a plurality of second 
cutting teeth which are preferably arranged alternately. Each of the first 
cutting teeth has a relief surface defining at its front edge an inner 
cutting edge and an outer cutting edge which are separated from each other 
by a shoulder. Each of the second cutting teeth has two relief surfaces 
each defining at its front edge a cutting edge which is at least partly 
spaced below the outer cutting edge of the first cutting tooth. 
Each cutting tooth has, therefore, a portion not contributing to any 
cutting job. This means a reduction in the resistance to which the cutting 
teeth are subjected during a cutting job; therefore, they have a prolonged 
life.

DETAILED DESCRIPTION OF THE INVENTION 
A hole cutter embodying this invention is shown in FIGS. 1 to 12. It 
comprises a generally cylindrical body 1 provided at its upper end with a 
spindle 2 adapted for connection to the rotary shaft of, for example, a 
drilling machine not shown. The cutter body 1 is provided around its lower 
end with a plurality of first cutting teeth 3 and a plurality of second 
cutting teeth 4. The second cutting teeth 4 are shaped differently from 
the first cutting teeth 3 and arranged alternately therewith in 
circumferentially equally spaced apart relationship, as shown in FIGS. 2 
and 4. 
Each of the first cutting teeth 3 has an inclined bottom surface defining a 
relief surface 5. The relief surface 5 is downwardly inclined from its 
rear edge to its front edge and from its inner edge to its outer edge, as 
is obvious from FIGS. 2, 3, 5 and 6. The relief surface 5 has a relief 
angle .alpha.1 (FIG. 5) of 6.degree. and a rake angle .alpha.1 (FIG. 6) of 
5.degree. relative to the horizontal plane in which the lower end of the 
hole cutter is rotatable. The term "front edge" as herein used means the 
leading edge of the relief surface of a particular cutting tooth moving 
when the hole cutter is rotated, and the term "rear edge" means the 
opposite edge. The "inner and outer edges" are believed to be 
self-explanatory. 
The front edge of the relief surface 5 defines an inner cutting edge 7 and 
an outer cutting edge 8 which are separated from each other by a 
substantially circumferentially extending shoulder 6 which is located 
approximately midway between the inner and outer edges of the relief 
surface 5. The outer cutting edge 8 is, therefore, located behind the 
inner cutting edge 7. The outer cutting edge 8 projects downwardly 
slightly beyond the inner cutting edge 7 due to the presence of the rake 
angle .beta.1, as shown in FIG. 5. 
Each of the second cutting teeth 4 has a bottom surface having a generally 
circumferentially extending ridgeline 9 which defines an inner relief 
surface 10 on one side thereof and an outer relief surface 11 on the other 
side thereof, as shown in FIGS. 2, 3, 7 and 8. The inner and outer relief 
surfaces 10 and 11 are upwardly inclined from the ridgeline 9 to the inner 
and outer edges, respectively, of the tooth 4. The inner and outer relief 
surfaces 10 and 11 have a relief angle .alpha.2 (FIG. 7) of 6.degree., 
which is equal to the relief angle .alpha.1 on the first cutting tooth 3. 
The inner relief surface 10 has a rake angle .beta.2 (FIG. 8) of 
5.degree., which is equal to the rake angle .beta.1 on the first cutting 
tooth 3, relative to the horizontal plane in which the lower end of the 
hole cutter is rotatable, while the outer relief surface 11 has a rake 
angle .beta.2' (FIG. 8) of 20.degree.. 
The inner relief surface 10 has a front edge defining an inner cutting edge 
12, and the outer relief surface 11 has a front edge defining an outer 
cutting edge 13. The ridgelines 9 on the second cutting teeth 4 are 
located radially outwardly of the shoulders 6 on the first cutting teeth 3 
and in a plane below the plane of the outer cutting edge 8 on the first 
cutting tooth 3. The inner cutting edge 12 lies in the same plane with the 
inner cutting edge 7 on the first cutting tooth 3 and extends downwardly 
to the ridgeline 9, as shown in FIG. 9. The outer cutting edge 13 has a 
portion lying below the outer cutting edge 8 on the first cutting tooth 3, 
but the plane in which the edge 13 lies intersects the plane of the edge 8 
so that the remaining or outer portion of the edge 13 may lie above the 
edge 8, as shown in FIG. 9. Therefore, a portion of the inner cutting edge 
12 and a portion of the outer cutting edge 13 are located below the outer 
cutting edge 8 on the first cutting tooth 3 as shown at A, while the 
remaining or outer portion of the outer cutting edge 13 is located above 
the outer cutting edge 8 as shown at B. 
The cutter body 1 has an outer peripheral surface provided with a plurality 
of spiral projections 14 each extending upwardly from one of the cutting 
teeth 3 and 4, and every two adjoining projections 14 define a spiral 
groove 15 for the ejection of chips therebetween, as shown, for example, 
in FIG. 1. A gullet 16 for chip ejection is formed at the lower edge of 
the cutter body 1 between every two adjoining cutting teeth 3 and 4. 
The operation of the hole cutter hereinabove described will now be 
described with reference to FIGS. 10 to 12. If the first cutting teeth 3 
are brought into contact with the work W, the inner and outer cutting 
edges 7 and 8 of each tooth 3 form an inner cut surface 17 and an outer 
cut surface 18, respectively, on the work W, as shown in FIG. 10, while 
producing two separate chips, which are ejected through the ejection 
groove 15 or gullet 16. If the second cutting teeth 4 are, then, brought 
into contact with the cut surfaces 17 and 18, the entire inner cutting 
edge 12 and an inner portion of the outer cutting edge 13 form new cut 
surfaces 19 and 20, respectively, on the work W, as shown in FIG. 11. The 
outer portion of the outer cutting edge 13 does not, however, contact and 
cut the cut surface 18. The second cutting teeth 4 are, therefore, 
subjected to a smaller amount of resistance than if the entire cutting 
edges thereof were used for the cutting operation. The teeth 4 have a 
prolonged life without getting broken quickly even if the cutter is used 
at a high speed. 
The first cutting teeth 3 are, then, brought into contact with the cut 
surfaces 19 and 20. The inner cutting edge 7 of each tooth 3 forms a new 
inner cut surface 21 in a cut surface 19, and the outer portion of its 
outer cutting edge 8 forms a new outer cut surface 22 in a cut surface 20, 
as shown in FIG. 12. The inner portion of the outer cutting edge 8 does 
not contact and cut the cut surface 19. This means a reduction in the 
resistance to which the first cutting teeth 3 are subjected, and 
therefore, a drastic prolongation of their life. The chips produced by the 
inner and outer cutting edges 7 and 8 of each tooth 3 are completely 
separate from each other. This means a reduction in the resistance exerted 
on the cutter by the chips when they are ejected, and therefore, an 
improvement in the working efficiency of the cutter. 
Each of the first and second cutting teeth 3 and 4 has a portion which does 
not contribute to the cutting operation, as hereinabove stated. This 
feature enables a reduction in the possibility of the teeth 3 and 4 being 
damaged if the hole cutter is operated at a high speed for a long period 
of time. The relief surfaces 5 of the first cutting teeth 3 and the relief 
surfaces 10 and 11 of the second cutting teeth 4 have an equal relief 
angle, and the relief surfaces 5 of the first cutting teeth 3 and the 
inner relief surfaces 10 of the second cutting teeth 4 have an equal rake 
angle, as hereinbefore stated. This feature greatly facilitates the 
sharpening of the relief surfaces 5, 10 and 11. 
Another hole cutter embodying this invention is shown in FIGS. 13 to 21. 
The principal difference between the cutter of FIGS. 1 to 12 and that of 
FIGS. 13 to 21 resides in the shape of the first cutting teeth 3. The 
cutter which is going to be described likewise has a plurality of first 
cutting teeth 3 and a plurality of second cutting teeth 4 which are 
arranged alternately with each other. Each of the first cutting teeth 3, 
however, has at its bottom a ridgeline 23 lying substantially 
circumferentially of a cutter body 1, and defining an inner relief surface 
24 on one side thereof and an outer relief surface 25 on the other side 
thereof, as shown in FIGS. 13 to 15 and 17. The relief surfaces 24 and 25 
have an equal relief angle. The inner relief surface 24 has a rake angle 
.gamma. (FIG. 17) of 20.degree. to 25.degree. relative to the horizontal 
plane in which the lower end of the hole cutter is rotatable, while the 
outer relief surfaces 25 has a rake angle .gamma.' of 15.degree. to 
25.degree.. Each of the second cutting teeth 4 has an inner relief surface 
10 and an outer relief surface 11 which are defined by a ridgeline 9, as 
is the case with the hole cutter of FIGS. 1 to 12. The inner relief 
surface 10 has a rake angle .beta.2 (FIG. 16) of 20.degree. to 25.degree. 
which is equal to the rake angle .gamma. of the inner relief surface 24 on 
the first cutting tooth 3, and the outer relief surface 11 has a rake 
angle .beta.2' (FIG. 16) of 15.degree. to 25.degree. which is equal to the 
rake angle .gamma.' of the outer relief surface 25 on the first cutting 
tooth 3. 
The inner relief surface 24 of each first cutting tooth 3 has a front edge 
defining an inner cutting edge 26 and a middle cutting edge 27 located 
behind the inner cutting edge 26. The middle cutting edge 27 is completely 
separated from the inner cutting edge 26 by a shoulder 6 provided 
substantially midway between the inner and outer peripheral edges of the 
tooth 3 and extending generally circumferentially of the cutter body 1, as 
shown in FIGS. 14, 15 and 17. The rake angle .gamma. of the inner relief 
surface 24 makes a difference in height between the inner and middle 
cutting edges 26 and 27, as shown in FIG. 17. 
The outer relief surface 25 has a front edge defining an outer cutting edge 
28 which is smaller in width than the inner and middle cutting edges 26 
and 27. The ridgeline 9 on each second cutting tooth 4 has a front end 
located in a plane below the plane in which approximately the middle point 
of the middle cutting edge 27 on each first cutting tooth 3 is located, as 
shown in FIG. 18. The second cutting tooth 4 has an inner cutting edge 12 
extending radially inwardly from the front end of the ridgeline 9, and an 
outer cutting edge 13 extending radially outwardly therefrom. Therefore, a 
portion of the inner cutting edge 12 and a portion of the outer cutting 
edge 13 are located below the middle cutting edge 27 on each first cutting 
tooth 3 as shown at A in FIG. 18, while the remaining or outer portion of 
the outer cutting edge 13 is located above the middle and outer cutting 
edges 27 and 28 on the first cutting tooth 3 as shown at B. 
If the first cutting teeth 3 are brought into contact with the work W, the 
inner, middle and outer cutting edges 26, 27 and 28 of each tooth 3 form 
three cut surfaces 29, 30 and 31, respectively, as shown in FIG. 19. If 
the second cutting teeth 4 are, then, brought into contact with the cut 
surfaces 29 to 31, the inner cutting edge 12 of each tooth 4 forms a new 
inner cut surface 32 in the cut surfaces 29 and 30, and the inner portion 
of the outer cutting edge 13 forms a new cut surface 33 in the cut surface 
30, as shown in FIG. 20. The remaining or outer portion of the outer 
cutting edge 13, however, does not contact and cut the cut surfaces 30 and 
31. This means a reduction in the resistance to which the second cutting 
teeth 4 are subjected, and the teeth 4 have a prolonged life accordingly. 
The first cutting teeth 3 are, then, brought into contact with the work W 
again. The inner cutting edge 26 of each tooth 3, an outer portion of its 
middle cutting edge 27 and its outer cutting edge 28 form new cut surfaces 
34, 35 and 36, respectively, as shown in FIG. 21. As is obvious, the 
remaining or inner portion of the middle cutting edge 27 does not contact 
any part of the work W, and thereby contributes to a reduction in the 
resistance to which the first cutting teeth 3 are subjected. 
The presence of the cutting edge portions not contacting any part of the 
work, while the remaining portions contribute satisfactorily to the 
cutting operation, ensures a prolonged life for both of the first and 
second cutting teeth 3 and 4. 
The invention has been described with reference to two embodiments thereof. 
They are not intended to limit the scope of this invention, but a variety 
of modifications may be possible within the scope of this invention, as 
will hereinafter be set forth briefly by way of example: 
(1) The cutting teeth do not necessarily need to be formed on the cutter 
body itself, but may be formed on tips which are separate from the cutter 
body, and which are joined to its lower end by brazing or otherwise. 
(2) The first and second cutting teeth do not necessarily need to be 
disposed exactly alternately with each other, but other arrangements may 
be possible. For example, it is possible to dispose two first cutting 
teeth and one second cutting tooth in alternate relationship. It is, 
however, desirable to ensure that every two diametrically opposite teeth 
be of the same type. 
(3) A plurality of shoulders may be provided along the front edge of each 
first cutting tooth to define three or more cutting edges which are 
completely separated from one another. 
(4) A variety of modifications may be possible to the relief and rake 
angles of the relief surfaces on the first and second cutting teeth.