Cutting by use of rotating axially moving tool – Tool or tool with support – Having peripherally spaced cutting edges
Reexamination Certificate
1999-08-12
2001-03-13
Bishop, Steven C. (Department: 3722)
Cutting by use of rotating axially moving tool
Tool or tool with support
Having peripherally spaced cutting edges
C407S113000, C408S227000, C408S233000, C408S713000
Reexamination Certificate
active
06200077
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a twist drill of the type that comprises a shank having one or more helical, chip-transporting grooves, each one of which cooperates with at least one cutting insert having a cutting edge located between a front surface and a flank surface, the flank surface extending at an angle to a reference plane intersecting the center axis of the drill shank.
BACKGROUND OF THE INVENTION
Generally, twist drills tend to oscillate in the direction of torsion during machining. When the drill is in operation, the shank oscillates to and fro in the direction of torsion, whereby the axial position of the cutting edges is alternately moved forward and backward relative to the workpiece. Thus, the effective drill length is alternately extended and shortened during the oscillation. This cycle creates a regenerative effect in the form of a feed-back vibration phenomenon, which briefly may be described in the following way.
When a drill shank oscillates in the above-indicated way, the individual cutting edge cuts a wavy surface in the bottom of the bore hole, as is generally illustrated in
FIGS. 3 and 4
. Thereafter, when either one and the same cutting edge or a subsequent cutting edge (depending on whether the tool includes one or more cutting edges which cut in the same path) runs across this wavy surface, a secondary wavy surface is created (compare the continuous and dashed wave lines, respectively, in FIG.
4
). In practice, this means that the nominal chip thickness will vary depending on the phase position or phase displacement between the wavy surfaces in the bottom of the bore hole. When this nominal chip thickness varies, the cutting force will also vary. This constitutes the driving force behind the above-mentioned vibration mechanism. If the chip width is increased, due to the fact that the diameter of the drill is increased at the same time as the other properties of the drill remain intact beyond a certain value, the mechanical system is no longer capable of resisting, and therefore the oscillating amplitude will grow and become unstable. By “mechanical system” is here meant primarily the helically cut drill as such. This mechanical system is primarily effected by the length of the drill shank. More precisely, a longer drill becomes more unstable in the torsion direction than a shorter one.
If the regenerative effect is triggered off, i.e. the amplitude of the vibrations grows, after a short while the vibration level will be very high. However, the growth of the amplitude decreases after a time as a consequence of other mechanisms, for instance the fact that the cutting edge cannot cut itself free.
OBJECTS AND FEATURES OF THE INVENTION
The present invention aims at solving the above mentioned problems and at providing an improved twist drill. Thus, a primary object is to provide a twist drill which directly at the beginning of use counteracts the development of an unstable regenerative effect, i.e. counteracts a tendency for increasing oscillating amplitude to occur in the drill shank when the drill operates.
According to the invention, the above-mentioned object is attained by a twist drill which comprises a shank that defines an axis of rotation and has a helical, chip-transporting flute. The drill further includes a cutting part arranged at a front end of the flute. The cutting part includes a body which forms a cutting edge, and a clearance surface arrangement which is situated behind the cutting edge with reference to a direction of drill rotation. The clearance surface arrangement includes first and second clearance surfaces each extending at respective first and second acute angles relative to a reference plane containing the axis of rotation. The sizes of the first and second acute angles are different from one another.
In one embodiment of the invention, both of the first and second clearance surfaces intersect the cutting edge, and one of the first and second clearance surfaces constitutes a recessed clearance surface that is recessed into the body relative to the other of the first and second clearance surfaces.
In another embodiment of the invention, only the second clearance surface intersects the cutting edge, and the first clearance surface extends from an end of the second clearance surface that is remote from the cutting edge.
REFERENCES:
patent: 3460409 (1969-08-01), Stokey
patent: 5049011 (1991-09-01), Bohnet et al.
patent: 5478179 (1995-12-01), Kress et al.
patent: 5688083 (1997-11-01), Boianjiu
Weck, M. and Teipel, K., “Dymanisches Verhalten spanander Werkzeugmaschinen”, Springer-Verlag 1977, ISBN 3-540-08468-1.
Sjolander Åke
Svenningsson Inge
Bishop Steven C.
Burns Doane Swecker & Mathis L.L.P.
Sandvik AB
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