Cutting by use of rotating axially moving tool – Tool or tool with support – Having peripherally spaced cutting edges
Reexamination Certificate
1999-12-23
2001-10-30
Bishop, Steven C. (Department: 3722)
Cutting by use of rotating axially moving tool
Tool or tool with support
Having peripherally spaced cutting edges
C408S0010BD, C408S227000
Reexamination Certificate
active
06309149
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a twist drill preferably for dry drilling, which drill has a central center web with a drill tip lying on the drill axis, and with the drill axis as the axis of symmetry both of the chip flutes and also of the primary cutting edges, namely those parts of the cutting edges, the chip faces of which lie in the vicinity of the chip flutes.
2. Background Information
In at least one embodiment according to the present invention, the chip flutes and the drill center web are therefore substantially no different from those of an altogether standard drill, the chip flutes and drill center web of which are realized so that they are substantially symmetrical to the drill axis. The axial symmetry also relates to those parts of the cutting edges, the chip faces of which lie in the vicinity of the chip flutes.
The elimination of the need to add a drilling fluid or coolant-lubricant during drilling significantly reduces costs for disposal as well as the other well-known expenses that are incurred as a result of the use of the drilling fluid. In general, therefore, attempts are made to manufacture drills using such materials and having such cutting geometries that dry drilling is possible. On the other hand, dry drilling causes problems, such as difficulties in chip removal, increased wear on the cutting edges and the circular lands, severe heating of the drill, and when the drill is extracted at the end of the drilling process, loud noises and flying sparks, or even the breaking of the drill. Cutting force measurements show that during the retraction of the drill from the hole, torques can occur that even exceed the torque required for drilling. An additional force component that occurs during the retraction of the drill is a negative thrust force which can even lead to the tearing of the drill.
The problems observed during the retraction of the drill after the drilling process has been completed are caused by the increased heating of the drill in the vicinity of the drill cutting edges that occurs during dry drilling. This increased heating of the drill during the drilling process leads to a continuous increase in the diameter of the drill and thus also of the hole, which increases as the drilling depth increases. This phenomenon can easily be determined mathematically as a function of the temperature of the drill in its cutting area. In addition to the simple heat of chip removal caused by the chip formation, a further heating results from the friction of the circular lands or secondary cutting edges on the wall of the hole. At this point, no further cutting occurs, but rather a scraping with negative chip angles. However, the diameter enlargement related to the heating also has a disadvantageous effect at this point. In short, the consequence is that at the end of the drilling process, the diameter of the drill and accordingly the diameter of the hole are greater than at the beginning of the drilling. When the drill is retracted, this results in a severe jamming in the hole. One consequence of this jamming is an even more severe heating of the tool, the resulting further expansion, the above-mentioned loud drill noises and the significant increase in torque when the drill is retracted from the hole.
In the known technical literature, there are repeated references to the fact that the problems described above during dry drilling can be avoided by cooling the drill by means of compressed air. Other remedies include a minimal lubrication or the use of special lubricant coatings.
OBJECT OF THE INVENTION
The object of the invention is to create a drill for dry drilling which substantially eliminates or minimizes the disadvantages described above merely on the basis of its cutting edge geometry. The invention teaches that only those parts of its cutting edges, the chip faces of which lie in the vicinity of the center web diameter, in particular the drill chisel edges, have a deliberate asymmetry.
SUMMARY OF THE INVENTION
The known art (German Patent No. 26 55 452 A) already teaches, for drills used to drill printed circuit boards made of synthetic resin, to configure the entire drill cutting edges eccentrically, i.e. asymmetrically. In that case, however, the drill in question is for a special type of application, where the material to be drilled, namely the printed circuit boards made of synthetic resin, causes only low wear to the cutting edges. In that case, however, the special material causes the problem of the removal of the drilling dust, and, to solve that problem, the known art recommends a basically asymmetrical configuration of the cutting edges. A transfer of the solution applied there to universal drills is problematic, simply because the cut is no longer central. Asymmetrical cutting edges, e.g. primary or major cutting edges of different lengths, also lead to an undesirable drilling result. The applicable theory in the field of drill design is aimed at avoiding such a situation at all costs, i.e. differences in the tip angle or an eccentric location of the drill tip and differences in the height of the cutting edges of the primary cutting edges. Even differences in the height of the cutting edges specifically result in different chip cross sections, and significantly increase the wear of the drill.
A drill of the known art described in U.S. Pat. No. 5,078,554 and corresponding European Patent No. 0 427 857 B1 has an asymmetrical center web diameter and asymmetrical chip flutes—each in relation to the axis of the drill. Accordingly, the curve of the chisel edges is also asymmetrical. The total asymmetry of this drill is intended to avoid an excessive enlargement of the hole being drilled as the depth of drilling increases, a phenomenon that the object of the current application not only accepts but even strives for. One primary application in particular is the machining of crankshafts for purposes of balancing said crankshafts. The holes are thereby made to alter the balance of the crankshaft. The drilling process is performed on the balancing machine itself, namely to balance the chucked crankshaft, the chucking of which does not need to be changed to change the balance. Sensors associated with the balancing machine are used for the balancing. The accuracy and precision of the work, however, are negatively influenced by cooling water or sprayed water. Therefore, under the conditions described above, dry drilling must be performed. In that case, it is less a question of the accuracy and precision of the hole drilled than the rapid and controlled removal of material.
Compared to the solutions of the known art explained above, the asymmetry taught by the present invention is essentially restricted to the design of the drill tip, namely to the configuration of the chisel edges, without practically any influence on the primary cutting edges. Therefore the area of the cutting edges that lies in front of the drill center web in the axial direction can therefore be affected by the desired asymmetry. As a result of the solution taught by the present invention, the drill cuts substantially free eccentrically during drilling. The attack can be so effective that the drill initially drills in or taps centrally. The drill tip can lie on the drill axis. As the drilling proceeds, the asymmetrical portions of practically the chisel edges are engaged. Pressure components can be generated by their asymmetrical portions that, during the drilling process, cause the drill to deviate from the ideal line specified by the drilling axis radially toward the center line between the cutting edges. Thus, as the drill rotates, a hole can be produced, the diameter of which can be approximately 0.01% to 1% larger than the actual diameter of the drill. During the retraction of the drill, the jamming of the drill in the hole is thereby substantially prevented, as can be determined by a measurement of the cutting force. Disadvantages of the type that would be expected with the asymmetrical configuration of the primary cutting edges, as
Borschert Bernhard
Seidel Jurgen
Bishop Steven C.
Kennametal PC Inc.
Nils H. Ljungman & Associates
Prizzi John J.
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