Cutting by use of rotating axially moving tool – Tool or tool with support – Having peripherally spaced cutting edges
Reexamination Certificate
2000-02-22
2001-07-17
Wellington, A. L. (Department: 3722)
Cutting by use of rotating axially moving tool
Tool or tool with support
Having peripherally spaced cutting edges
C408S057000, C408S229000, C408S705000
Reexamination Certificate
active
06261034
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a drill bit, especially a drill bit with multiple cutting edges, for example with point-symmetric arrangement of cutting edges with primary and secondary cutting edge. In this connection, the invention encompasses the most diverse types of drill bits, such as drill bits for drilling in solid material, boring bits and even deep-hole drills.
DISCUSSION OF THE BACKGROUND
Even today, drill bits are still being made predominantly as twist drills, or in other words as drill bits with helical flutes. The pitch angle of the drill helix and thus the side rake angle of the drill is then usually between 0° and 50°, and so is positive. It is explained in more detail with reference to FIG.
8
:
There is shown twist drill
10
with a shank
12
and a cutting part
14
, in which helical flutes
16
are machined. The side rake angle is denoted by &ggr;. This side rake angle &ggr; is positive, because it opens counter to the direction of rotation &PHgr;.
Positive rake angles are provided primarily because relatively favorable cutting conditions in the region of the secondary cutting edge can then be achieved in combination with high drill stability, and also because the chips can then be transported reliably out of the drillhole.
In the case of internally cooled drill bits, and especially drill bits for which the coolant is supplied under extremely high pressure, it is possible to use the coolant and lubricant to transport the chips out of the drillhole. In the case of short-cutting bits, therefore, a change to the use of straight-fluted bits has already been made, in order to take advantage of the higher torsional stiffness of straight-fluted bits as compared with the twist drill. However, the use of such bits has heretofore been limited to very short-cutting bits such as cast iron or AlSi alloys.
Intensive tests with conventionally shaped or fluted drill bits show that, especially in the case of slender bits, or in other words bits whose cutting part has a length-to-diameter ratio greater than
5
, difficulties are encountered in stabilizing the bit in radial direction. Certainly it is possible to reduce these vibrations by modern ground surfaces, which frequently are made with the help of CNC-controlled machine tools. However, it has been found that even slight asymmetric wear phenomena greatly impair the radial stability of the drill bit, leading to adverse effects on drillhole quality even after a short tool life. Not only the dimensional accuracy of the drillhole, but also the surface quality is affected in this way. This influence is felt particularly strongly in bits in which a drill-bit support part of steel or high-speed steel is tipped with an edge material having greater wear resistance, such as carbide or cermet.
SUMMARY OF THE INVENTION
The object of the invention is therefore to provide a drill bit of the type described above with which drilling can be achieved smoothly and with little vibration, even under unstable conditions, so that drillholes with good surface and dimensional accuracy can be produced.
According to the present invention, the disposition of the secondary cutting edge is used for stabilization of the bit. The effect then achieved according to the present invention can be best illustrated by comparison of FIGS.
8
and
1
:
In
FIG. 8
, a cutting force F
c
is shown, which points in a cutting direction. This cutting force F
c
represents the resultant of the normal force F
n
perpendicular to the cutting edge and the component F
s
of the cutting force. From FIG.
8
and
FIG. 8A
, which shows the force conditions at the corner of the cutting edge on a larger scale, it can therefore be seen that force component F
s
applies pressure on the helical part or the cutting part of drill bit
10
. In other words, force component F
s
subjects the relatively slender drill body to lateral bending stress, and so high-strength bits, such as high-speed steel or carbide, must be used for the drill bit or for the cutting part.
In comparison with conventional drill bits according to FIG.
8
and
FIG. 8A
, the arrangement of the drill bit according to the present invention is disposed, as shown in
FIGS. 1
,
1
A and
1
B, such that secondary cutting edge
118
is inclined in the direction of rotation, or in other words in the cutting direction, over a substantial axial effective length, and thus has a negative side rake angle &ggr;*. The resolution of cutting force F
c
into normal force component F
n
and a component F
z
perpendicular thereto means that force component F
z
—as is obvious from FIG.
1
A—applies tension to the drill bit or to cutting part
114
. This tensile force F
z
has a stabilizing effect on the body of the drill bit, with the result that, even when the drill bit has extremely slender geometry, lateral bending can occur only at much larger lateral forces than is the case for drill bits of conventional design. Tensile force F
z
therefore acts as a preload on the cutting part and in this way increases the dynamic stability of the drill bit. It has been proved in tests that the drill bit, according to the present invention, drills much more smoothly and with much less vibration, even under unstable conditions, than do conventional drill bits, and so it has become possible to make drillholes with better surface and dimensional accuracy.
It is therefore important for the subject matter of the present invention that the disposition of the secondary cutting edge is used for stabilization of the drill bit against lateral forces. In this connection, it has been found that it is not absolutely necessary to form the entire secondary cutting edge with a negative side rake angle. To the contrary, it is sufficient to incline the secondary cutting edge in the cutting direction over a substantial axial effective length, in order to achieve, in the sum of the dynamic forces being developed, the stabilization of the bit explained hereinabove. In this way it is possible to dispose the side rake angle in the positive range over a short axial distance and then to allow this angle to change gradually to a negative angle.
The side rake angle is advantageously matched to the lip angle of the secondary cutting edge. In this way it is possible to improve chip formation and, in fact, even for the case that a relatively large negative side rake angle is used.
The principle according to the present invention can be transferred to the most diverse drill bits, even for single-lip drill bits. In the case of deep-hole drill bits, the principle according to the present invention is particularly effective, since these bits have an extremely high slenderness ratio and thus react particularly sensitively to compulsory stresses.
A further particularly favorable area of application is found in drill bits in which the arrangement of cutting edges is formed on a cutting part and the cutting part is seated on a support part. Such a drill-bit design has advantages, if economies in expensive high-strength material for the support part are to be achieved. The drill-bit design, according to the present invention, makes it possible to rely on the stabilizing effect of the secondary cutting edges to assist the weaker material for the support part.
It has been shown that the design according to the present invention of the secondary cutting edge disposition is directly applicable for the common materials of the cutting part. The concept according to the present invention does not lead to an overstressing of materials, and on the other hand, the drillhole receives much better dimensional accuracy and surface quality.
A further embodiment provides an extremely simple design of the bit, because the secondary cutting edges are automatically defined by the disposition of the flutes. However, it is not absolutely necessary to provide the flutes with uniform helical pitch over the entire length of the cutting part. To the contrary, the side rake angle can vary over the axial length of the drill bit, or in other words, that the secondary cutting edge remai
Carter Monica S.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wellington A. L.
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