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"No-point" drill reduces thrust.

"No-point" drill reduces thrust

The twist drill is one of the most widely used, yet most complex, of all cutting tools. There are many factors that influence drill performance, including both the drill and workpiece materials as well as the drill point shape. The drill point is the most critical part of the drill and small variations in its geometry or symmetry can have a strong influence on its performance.

Many variations of drill geometry have been proposed in order to reduce cutting forces induced and to improve tool life of the drill. For example, point thinning methods, that provide a greater clearance near the corners of the chisel edge, were found to both reduce drill thrust and improve drill life. Another factor influencing performance is web thickness. With a thicker web, the chisel edge length increases and a greater amount of metal removal is done by extrusion rather than by the cutting lips of the drill, resulting in higher thrust forces.

A major step in the evolution of the twist drill occurred with the introduction of carbide tipped drills. These drills combine the advantages of lower material cost and higher toughness of HSS drills with the enhanced cutting performance of carbide. Carbide tipped drills resist wear better than HSS drills and are capable of drilling at higher speeds and penetration rates. However, carbide tips are more suspectible to chipping, especially in the chisel edge area.

Considerable R&D efforts have been expended in obtaining suitable point geometries for this category of twist drills. Many solutions analogous to web thinning techniques for HSS drill points have been proposed for carbide tipped drills. One such drill is the New Point Drill (NPD) developed by H Negishi, of Mitsubishi Metal Corp, Gifu, Japan. The NPD is a cemented carbide tipped drill with a noncutting center, Figure 1. The cemented carbide tips are preformed so that they produce an insert with a convex rake face and a planar relief surface.

The carbide inserts are brazed into the drill body in such a way that the inside edges of the inserts do not meet and a noncutting center area is formed. The space inside the recess of the NPD exists before the inserts are brazed and is not produced by any kind of grinding or machining after brazing. With the chisel edge eliminated, the work material in this area of the drill is removed by a twisting action.

The procedure required for drill regrinding affects the economics of the drilling operation. The high initial cost of carbide tipped drills dictate that they be resharpened periodically. Regrinding of the NPD becomes easier with the elimination of the central chisel edge. Only the relief surfaces need be ground and this task is greatly simplified because the relief surfaces are planes and not some complex curved surface. Rake surface regrinding is not necessary as these surfaces are molded into the insert.

Drilling test procedure

An experiment was conducted involving a comparison of the NPD with different geometries of carbide tipped drills and HSS drills based on cutting performance characteristics and chip structures. Table 1 lists the drills used in this experiment and presents some of their characteristics.

The machine tool used was a Toyoda Model DN-IV NC machining center, with an 11-kW motor and mechanically driven axes of motion. A Kistler Model 9271A two-channel piezoelectric dynamometer was used to measure the drill thrust and torque developed during drilling. A suitable cutting fluid was supplied during drilling.

The workpiece material consisted of two grades of steel, S15C (AISI 1015) and S45C (AISI 1045), and the samples were 50 mm (1.97 inches) dia and 60 mm (2.36 inches) long. The S45C material had samples prepared at two different hardness levels. The S15C material had a hardness of 65 HRB and a microstructure of ferrite matrix and approximately 20 percent pearlite. The S45C materials had a hardness of 96 HRB and 104 HRB. The S45C material at 96 HRB consisted of pearlite and approximately 25 percent ferrite. The S45C material at 104 HRB consisted of tempered martensite.

The carbide tipped drills were run at a cutting speed of 60 m/min (197 ft/min) and a drill feed of 0.3 mm/rev (0.012 inches/rev). The HSS drills were run at a cutting speed of 20 m/min (65.6 ft/min) and a drill feed of 0.4 mm/rev (0.016 inches/rev).

Chips were collected at the conclusion of each drill test. In all of the tests for drill C-5 (the NPD) core chips were collected separately from the large chips. Spindle horsepower readings were recorded by hand at timed intervals during each drilling test.

Cutting-edge geometry

The New Point Drill has no cutting edge in the central portion of the drill tip, thus it has neither a rake face nor a relief surface in this area. This is the key reason why this drill operates at a decreased thrust force level. The relief surfaces along the cutting edges in the central portion of an ordinary twist drill create a rubbing phenomenon between the relief surface and work material. The vertical feed speed at all points along the relief surface is the same. However, the horizontal and tangential cutting speed varies depending on the drill radius.

Along the main cutting edge, where the radius is large, normal cutting occurs because there remains enough clearance between the relief surface and the bottom surface of the workpiece. As the radius becomes smaller, the tangential cutting speed decreases and rubbing under the relief surface occurs because there is a lack of clearance. This problem becomes more severe at the center of the drill. Squeezing or piercing occurs at the center due to the large negative clearance. The NPD solves this problem by eliminating the cutting edge and the relief surface in the central region of the drill.

Twisting-off mechanism

The NPD removes material in the vicinity of the chisel edge area by a mechanism that involves the twisting off of small cores produced inside the recess. As the drill penetrates into the workpiece, the core length increases. As the drilling continues, the solid core grows to a length of 1.0 mm (0.039 inches) or so then fractures from the base material. Fracture occurs because the stress introduced by the couple of the friction forces, which is produced by the opposite side walls of the recess, becomes greater than the shearing strength of the central core. The twisted off cores are discharged from the open top of the recess into the flutes as minute drum-shaped objects. As drilling continues, the next core is produced, grows to a certain length, and fractures by the twisting-off mechanism. This phenomenon continues in a stable manner.

Performance characteristics

The analysis of performance characteristics were based on the average drill thrust, torque, and spindle horsepower. The results are shown in Graphs 1,2,and 3. Results for Drill C-1 exist only at one hardness level because there was only one drill available and that had severe chipping of the carbide tip take place during drilling of the S15C material.

The graphs show a general tendency for all three performance characteristics to increase as the Rockwell B hardness increases. The one exception is the conventional HSS drill without web thinning (Drill H-4). Another general tendency is that the average drill thrust increases by a larger percentage than either the torque or spindle horsepower over the range of hardnesses except for Drill H-4.

In comparing the NDP (Drill C-5) with the other carbide tipped drills, it can be observed that there is a smaller increase in the average thrust and spindle horsepower for the NPD as the hardness increases than for the other carbide tipped drills. The average thrust for the NPD increased by 25 percent from the hardness of 65 HRB to 104 HRB while the other carbide tipped drills increased between 63 percent and 97 percent for the same increase in hardness.

There was no significant difference in the increase in average torque between the NPD and other carbide tipped drills. The average spindle horsepower for the NPD increased by 12 percent, while the other carbide tipped drills increased between 17 percent and 27 percent. Thus, the primary difference between the NPD and other carbide tipped drills with respect to their performance characteristics is that the drill thrust of the NPD is significantly lower.

TABLE : Table 1. Drills used in the experiment
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:DeVries, M.F.; Crosheck, M.K.
Publication:Tooling & Production
Date:Apr 1, 1989
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