WO1989001086A1

WO1989001086A1 - Masonry two-prong rotary drill bit

Info

Publication number: WO1989001086A1
Application number: PCT/US1988/002268
Authority: WO
Inventors: Edmund Isakov
Original assignee: Kennametal Inc.
Priority date: 1987-07-23
Filing date: 1988-07-06
Publication date: 1989-02-09
Also published as: AU2084488A

Abstract

A drill (11) for forming holes in masonry includes a supporting body (13) having an axis of rotation (15) therethrough and a pair of inserts (17, 19) mounted at one end (2) thereof by mounting portions (23, 25). Each insert has a main cutting edge (27), and external cutting edge (33), an upper internal cutting edge (29) and a lower internal cutting edge (31). The upper and lower internal cutting edges define a center relief (35) having an upper conical shaped portion (37) with an angle of relief between about 60 to 90 degrees and a lower cylindrical shaped portion (39). There is a ratio of between about 5 to 8 between the diameter of the drill and the diameter of the lower cylindrical shaped portion. There is a ratio of between about 2 to 2.5 between the diameter of the drill and the largest diameter of the conical shaped portion.

Description

MASONRY TWO-PRONG ROTARY DRILL BIT

BACKGROUND OF THE INVENTION The invention relates to a drill bit for forming holes in masonry. More particularly, the invention provides a two-prong rotary bit which incorporates two inserts with improved bit geometry, as well as, a relieved center portion of the bit, to increase penetration rate, reduce thrust and to provide enhanced tool life. Masonry drill bits have been long known and are commonly employed for drilling holes in especially hard friable material such as masonry or stone. Such drill bits usually comprise an elongated body or shank having a spiral groove or grooves formed along its length and a diametrically extending straight groove on its leading edge. A hard insert is provided in the straight groove and is held in place by soldering or brazing. The insert typically employs sharp cutting edges on its leading end so that the drill bit might effectively be used in hard masonry or stone material. A spiral provides a channel for the discharge of chips, particles, dust and other drilling debris loosened by the leading end of the drill bit during the drilling operation. The insert must be capable of resisting wear, fracture, and the abrasive action of the chips from the material being drilled, while the body of the drill bit must maintain sufficient strength in the presence of heat generated in use. It is the conventional practice in the manufacture of prior art masonry drill bits to make the drill bit body of a material such as steel and to mount the abrasion-resistant insert or cutting elements at the cutting end. The body of the drill bit is normally formed by conventional rolling, machining or grinding a spiral thread in a blank or rod of suitable length. The straight groove for the mounting of the insert or cutting element is then machined in the leading end of the drill bit body, and the cutting is secured therein. The cutting element is formed of hard material, such as tungsten carbide, and is usually anchored in place in the body of the drill bit by soldering or brazing it. U. S. Patent No. 4,314,616, to Rauckhorste et al, illustrates a typical prior art masonry drill bit with a carbide insert having a continuous cutting edge. A distinct disadvantage to this commercial design is a solid center or chisel edge. The chisel- point geometry requires considerable thrust since the chisel must push through the work material .at almost zero cutting speed. The higher thrust reduces tool life and may cause the bit to fail, especially when drilling in concrete blocks, bricks or other types of hard masonry.

It has been suggested in British Patent Specification No. 832,238, to Taylor, that a rotary masonry drill bit can be designed to incorporate a center relief. The center relief of the drill bit disclosed in this British patent specification has an obtuse-angled shape. That is to say, the angle of relief is greater than 90 degrees. Accordingly, the center relief is very shallow and it does not provide significant cutting action. A zero cutting speed zone exists in this drill bit configuration.

It has also been suggested in Frenc Patent No. 1,009,121, that a center relief can be used in a drill bit for drilling coal and rock. In this French patent, the bit includes a center relief of a cylindrical shape. This design indicates that the inner edges are parallel to the axis of the bit and the angle of relief is zero degrees. The cylindrical relief provides shearing of the rock, rather than the cutting of it. This tool geometry increases the dynamics of drilling and can precipitate bit failure, because the cutting edges have an eccentric sharpening which causes wobble of the rotary bit and, in turn, can lead to a rapid fatigue of the drill bit, especially when hand-held drilling machines are employed.

In my U. S. Patent No. 4,550,791, which is assigned to the assignee of the present invention, a two prong rotary bit has a pair of inserts each of which has a cutting portion facing in the direction of rotation. When viewed in a direction parallel to the axis of rotation, each of the inserts displays a cross sectional configuration which is generally wedge shaped. The invention as claimed-is intended to remedy the drawbacks as identified above which are associated with certain prior art configurations. It solves the aforedescribed problems by providing a masonry drill bit which incorporates two inserts with improved bit geometry as well as a relieved center portion to increase penetration rate, to reduce thrust and to enhance the tool life.

SUMMARY OF THE INVENTION The advantages offered by the present inven- tion include a center relief portion which consists of two parts, a conically shaped upper portion and a cylindrically shaped lower portion. The cutting edges of the bit have a concentric sharpening which permits drilling without wobble. More particularly, the drill bit for forming holes in masonry according to this invention comprises a supporting body having an axis of rotation therethrough and a pair of inserts mounted at one end of the supporting body by means of a mounting portion. Each of the inserts has a main cutting edge facing in the direction of rotation. An external cutting edge of each insert is generally parallel to the axis of rotation. An upper internal cutting edge of each insert and a lower internal cutting edge of each insert define in combination a center relief having an upper conical shaped portion and a lower cylindrical shaped portion. The center relief as defined by the upper internal cutting edges has an angle of relief of between about 60 to 90 degrees. The external cutting edges define the outside diameter of the drill bit and the lower internal cutting edges define the diameter of the lower cylindrical shaped portion of the center relief. There is a ratio of between about 5 to 8 between the diameter of the drill •bit and the diameter of the lower cylindrical shaped portion of the center relief. The upper internal cutting edge and the main cutting edge^" of each insert define an apex and the opposed apexes of the inserts define the largest diameter of the conical shaped portion of the center relief. There is a ratio of between about 2 to 2.5 between the diameter of the drill bit and the largest diameter of the conical shaped portion of the center relief.

BRIEF DESCRIPTION OF THE DRAWINGS The above as well as other features and advantages of the present invention can be appreciated through consideration of the detailed description of the drawings, in which:

Fig. 1 is an isometric view of the masonry drill bit of this invention;

Fig. 2 is a plan view of the drill bit according to this invention; Fig. 3 is a side elevational view with portions thereof cut away of the drill bit of this invention; Fig. 4 is an isometric, detail view of the drill bit of the bit inserts of this invention illustrating the external side relief angle between the bit insert and a tangent to the external side cutting edge and the internal side relief angle;

Fig. 5 is a detail illustrating the clearance angle between the main cutting edge of the insert and the bit body;

Fig. 6 is a detail view of the drill bit of this invention illustrating the clearance angle between the internal cutting edge of the bit insert and the bit body;

Fig. 7 is an exploded, side elevational view of a two-prong bit for drilling in masonry with a screw-on head design; and

Fig. ^"8 is a graph illustrating comparative test results utilizing the drill bit of this invention. DETAILED DESCRIPTION OF THE INVENTION The masonry two-prong rotary bit of this invention is generally indicated by reference character 11 in Fig. 1. The bit 11 includes a supporting body 13 having an axis of rotation indicated at 15. A pair of cutting inserts 17 and 19 are mounted at one end 21 of the supporting body 13 by means of a pair of mounting portion means 23 and 25. Preferably, the mounting portion means 23 and 25 are integral with the supporting body 13. Typically, each of the cutting inserts 17 and 19 are mounted onto their respective mounting portion means 23 and 25 by brazing. Considering now Figs. 1, 2 and 3, it can be seen that each of the cutting inserts 17 and 19 include an external or main cutting edge 27, an internal cutting edge 29, an internal side cutting edge 31, and an external side cutting edge 33. The opposed inserts 17 and 19 define in com¬ bination a center relief 35 having an upper conically shaped portion 37 and a lower cylindrically shaped portion 39. The upper conical portion 37 of the relief 35 is defined by the internal cutting edge 29 of each insert 17 and 19. The lower cylindrical portion 39 of the center relief 35 is defined by the internal side cutting edges 31 of the inserts 17 and 19. As can be clearly seen in Fig. 3, the internal cutting edge 29 of each insert 17 and 19 define in combination an angle of relief of between about 60 to 90 degrees. This angle is indicated by the reference character a. The external side cutting edge 33 of each of the inserts defines the outside diameter D of the drill. The lower internal side cutting edges 31 of the inserts defines the diameter of the lower cylindrically shaped portion 39 of the center relief 35. There is a ratio of between about 5 to 8 between the drill bit diameter D and the diameter B of the lower cylindrically shaped portion 39 of the center relief 35.

The upper internal cutting edge 29 and the main cutting edge 27 of each insert define an apex 41. The opposed apexes 41 of the inserts 17 and 19 define the largest diameter of the conical shaped portion 37 of the center relief 35. There is a ^"ratio of between about 2 to 2.5 between the diameter D of the drill bit and the largest diameter A of the conically shaped portion 37 of the center relief 35.

Because the center relief consists of two parts, the conical shape of the upper portion and the cylindrical shape of the lower portion, significant improvement in the penetration rate and thrust reduction has been achieved. This conical relief is characterized by the angle and the ratio D/A. ^" The cylindrical relief is characterized by the ratio of D/B. The dimension A is a distance between the two points defining the opposed apexes 41 of the inserts and the dimension B is a distance between the two inner cutting edges. Both dimensions A and B are related to the bit diameter D. Considering Figs. 3 through 6, a bevel angle β of the main cutting edge 27 is measured from a cut¬ ting surface face schematically indicated by the line 43. Cutting surface face 43 is generally perpendicular to the axis of rotation 15. Preferably, the bevel angle of the main external cutting edge 27 is between about 20 to 30 degrees. A clearance angle σ ' between the main cutting edge 27 and the bit body behind the cutting edge is preferably between 12 and 18 degrees. The clearance angle σ2 between the cutting edge of the internal cutting edge 29 and the bit body 1.1 is preferably between about 10 to 15 degrees. Because smaller angles reduce the penetration rate and increase friction between the material of the workpiece and the bit body, the maximum clearance angle is selected to be consistent with the strength of the bit body.

Δl is a side relief angle between the bit body 11 and a tangent 45 to the external side cutting edge 33. Preferably, the angle Δl is between about 8 and 12 degrees. The internal side relief angle Δ2 between the internal side cutting edge 31 and the tangent 45 is between approximately 3 to 8 degrees.

In Fig. 7, there can be seen an exploded side elevational view of the two-prong bit for drilling in masonry with a screw-on head design. The bit 11 includes an interior portion 47 adapted to receive therein a threaded member 49 of the drill bit body 51. The drill bit body 51 includes flutes 53 and lands 55. A shank portion 57 is disposed opposite the threaded portion 49.

Turning now to Fig. 8, a graph representing the results of a comparative test between a two-prong masonry drill bit of this invention and a conventional masonry drill bit of the type generally illustrated in u. S. Patent No. 4,314,616 is presented;

The graph indicates the penetration rate in inches per minute versus the bit diameter and indicates the amount of thrust required to achieve the penetration. As can be seen in this graph, line 61 indicates the plot of the two-prong bit of the current invention and line 63 illustrates the plot when the prior art masonry bit is employed. This information is also presented in the table below in which the results are summarized.

TABLE TEST RESTJLTS

THRUST, LBS,

AVERAGE MAXIMUM

39 48

37 45

I I

46 53

44 53

59 65

52 58

In conclusion, it can be seen that the two- prong bits of the present invention have a significantly higher penetration rate when compared against prior art bits. For example, the penetration rate is approximately 3.8 times greater for an 11/16 inch diameter drill bit of the present invention compared to a prior art bit, penetration is approximately 6.2 times greater for a 7/8 inch diameter drill bit and penetration is approximately 8.8 times greater for a one inch diameter bit. The higher penetration rates for the two-prong bits is achieved at the same average value of the applied thrust. In conducting the tests to obtain the results shown in this graph and table, the commercial, i.e., prior art bits were rtin for 30 seconds each and the depths of the holes were measured. As can be appreciated through consideration of the Table, the bits of this invention required only between eight to seventeen seconds to completely penetrate the masonry. Thrust was also recorded during each test and is presented in the Table.

It will be obvious to those skilled in the art that various changes may be made in the invention without departing from the spirit and scope thereof, and thereof the invention is not limited by that which is shown in the drawings and described in the specification, but only as indicated in the appended claims.

Claims

WHAT IS CLAIMED IS; 1. A drill for forming holes in masonry, said drill having a supporting body with an axis of rotation therethrough; a pair of inserts mounted at one end of said supporting body by means of mounting portion means, each of said inserts having a main cutting edge facing in the direction of rotation, ..an external cutting edge which is generally parallel to the axis of rotation, an upper internal cutting edge and a lower internal cutting edge; characterized by said upper internal cutting edges and said lower internal cutting edges defining in combination a center relief having an upper conical shaped portion and a lower cylindrical shaped portion wherein said upper conical shaped portion of the center relief as defined by said upper internal cutting edges has an angle of relief of between about 60 to 90 degrees.

2. The drill for forming holes in masonry according to. claim 1 wherein the main cutting edge has a bevel angle of between about 20 to 30 degrees to a cutting surface face.

3. The drill for forming holes in masonry according to claim 1 wherein the mounting portion means of the supporting body includes a clearance angle of between about 12 to 18 degrees relative to the main cutting edge.

4. The drill for forming holes in masonry according to claim 1 wherein the mounting portion means are integral with the supporting body.

5. The drill for forming holes in masonry according to claim 1 wherein the mounting portion means of the supporting body includes a clearance angle of between about 10 to 15 degrees relative to the upper internal cutting edge.

6. The drill for forming holes in masonry according to claim 1 wherein at least a portion of the supporting body proximate the mounting portion itfeans is disposed so as to define a relief angle relative to the external cutting edge of between about 8 to 12 degrees.

7. The drill for forming holes in masonry according to claim 1 wherein the lower internal cutting edge defines an internal side relief angle of between about 3 to 8 degrees.