Cutting by use of rotating axially moving tool – Tool or tool with support – Having peripherally spaced cutting edges
Patent
1993-09-10
1994-08-23
Bishop, Steven C.
Cutting by use of rotating axially moving tool
Tool or tool with support
Having peripherally spaced cutting edges
408231, 408713, 408224, B23B 5102
Patent
active
053402464
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
It is previously known to make drills for metal drilling with two indexable inserts of tungsten carbide, where the first insert is a center insert which will cut the central parts of the hole, and the second insert is a peripheral insert which will cut the peripheral parts of the hole. The inserts are substantially located each on one side of the axis of the drill, but a portion of the center insert will often extend past the axis. Some such drills are described in the patents EP 088 505, EP 181 844, EP 292 455, U.S. Pat. No. 4,124,328 and U.S. Pat. No. 4,268,198. Compared to drills with a continuous insert over the whole diameter of the drill, such as U.S. Pat. No. 4,527,931, drills with center and peripheral inserts have the advantage of cutting narrower chips which are more easily conveyed out of the hole, but the disadvantage of large unbalanced lateral cutting forces, tending to bend the drill or displace the work piece resulting in impaired precision. For this reason, drills with center and peripheral inserts have up to now been used for short holes only, and require rigid clamping of the work piece. Since the forward extremity of the center insert is not on the axis of the drill, it has no tendency to center itself on an existing hole of smaller diameter and is not suitable for chamfering.
This is especially disturbing in starting a new hole when the drill is not yet stabilized by the outer edge of the peripheral insert. Unbalanced forces in the plane of the edges will bend the drill to give the hole an incorrect diameter, and forces at right angles to the inserts will make it difficult to start a hole at the point intended, especially on work pieces with small thickness. Unbalanced lateral forces will occur either because the length of actively cutting edge on one side of the drill axis is not equal to that on the other side, or because the axis does not lie in the planes of the edges. The unbalanced forces will be particularly severe if the surface of the work piece is uneven or is not at right angles to the drill axis.
SUMMARY
The invention concerns a drill with center and peripheral inserts, the edges of which have been shaped to balance the forces completely at the start of a hole, and later at least to keep lateral forces in the plane of the edges as small as possible. This makes it possible to use this insert arrangement for long holes and deep holes. Drills according to the invention are as easy to use as symmetrical twist drills, and they do not require as much rigidity in the machine and in the clamping as do other drills with two inserts.
According to the invention, the center insert is made with a centering drill point, preferrably with a cutting cross edge, such as split-point, relief ground or according to U.S. Pat. No. 4,222,690. These drill point designs need small feed forces and will center the drill efficiently. On one side of the drill point the cross edge is continuous with a long edge and on the other side with a short edge The long edge should be longer than 25% of the hole diameter, the short edge should be shorter than 25% of the hole diameter. At the beginning of a hole there are no lateral forces until the actively cutting part of the long edge is longer than the short edge.
The long edge lies in the conical surface cut by the center insert, but is made concave by a tangential recess in its middle part. In this way, the outer portions of the long edge get a small positive radial rake angle resulting in very small total radial forces. At its end the long edge joins the lateral edge with a short arcuate transition.
The peripheral insert shall have at least one long edge, longer than 25% of the hole diameter. It can be made with a straight cutting edge, and in such a case it is located with the drill axis in the plane of the edge. It can also be an insert of the same type as the center insert, and located in the same plane as the latter. In both cases the peripheral insert is preferrably retracted axially so far that the inner part of its edge
REFERENCES:
patent: 2576664 (1951-11-01), Berlien
patent: 4293252 (1981-10-01), Kress et al.
patent: 4563113 (1986-01-01), Ebenhoch
patent: 4844669 (1989-07-01), Tsujimura et al.
Bishop Steven C.
Sandvik AB
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