Cutter bit having hard tip with middle section defined by concave surface of revolution

Abstract

 A cutter bit hard tip (62) has an outer tip section (76), an inner flange section (78) , and a middle section (80) disposed between and merging at its opposite ends with the tip and flange sections and being defined by a continuous concave surface of revolution (82). The tip, flange and middle sections lie along a common longitudinal central axis (84). The tip section has a rounded-off end, and a frusto-conical surface of revolution (88) merging from the end. The flange section has a right cylindrical surface whose diameter is greater than the maximum diameter of the tip section. The middle section has a maximum diameter less than or equal to the diameter of the flange section and a minimum diameter substantially equal to the maximum diameter of the tip section. The concave surface of revolution of the middle section in profile has an outwardly and downwardly sloping configuration generally similar to that of the sides of a bell-shaped curve.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to excavation and construction tools and, more partic­ularly, is concerned with a cutter bit having a hard tip with a configuration which decreases the areas of high stress concentration created during fabrication and thereby increases wear life during use.

[0002] Road maintenance techniques have involved the process of road planing which involves the mounting of cutter bits on a power driven rotary drum. Asphalt is planed off the old road surface as the drum rotates and the bits strike or dig into the roadway. A typical rotating drum has 150 to 160 cutter bits. In the case of asphalt cutting, the cutter bits will wear out and have to be replaced frequently. Under some circum­stances, depending on the asphalt material in use, the wear life of an average cutter bit is as short as 2 to 3 hours, whereas in other circumstances, the same bit can last as long as 8 hours.

[0003] The most expensive portion of the cutter bit is its hard tip. Typically, over two-thirds of the cost of the bit resides in the tip. Consequently, it is highly desirable to be able to use the tip as long as possible, i.e., to maximize its useful life. Additionally, since the tip is so costly, it is equally desirable to fabricate it in a way which reduces the rate of rejections.

[0004] Hard tips on cutter bits can take various shapes for use in different applications. Further, there are a multitude of mixtures of different ingredients from which to form the tips. Hard tips for cutter bits used in surface planing are typically formed of cemented tungsten carbide, e.g., a mixture of tungsten carbide and cobalt, hereinafter referred to as "carbide." (However, there are many different grades of carbide, which are based on percentages of cobalt included, the grain size of the tungsten carbide, porosity type, and the presence of other metal carbides and metal binders, etc.).

[0005] Representative of the prior art are the cutter bits and tips thereon disclosed in U. S. Patents to Kniff (3,499,685), Engle et al (3,519,309), McKenry et al (3,720,273), Stephenson (4,216,832), Taylor et al (4,316,636), Ojanen (4,497,520) and Ewing et al (4,627,665).

[0006] Basically, as shown in the Ewing et al Patent No. 4,627,665, the conventional hard tip has a tip section, a flange section and a middle section which extends between them. The tip section of the conventional tip is defined by a rounded forward end which merges rearwardly into a shallow frusto-conical surface of revolution generated by a line at forty-five degrees from the longitudinal central axis of the tip. The flange section has a flange portion defined by a right cylindrical surface which has a diameter substantially greater than the maximum diameter of the tip section and a lower valve seat portion extending below the flange portion.

[0007] The middle section of the conventional tip is partially defined by a steep frusto-conical surface of revolution generated by a line at twelve degrees from the tip axis. This steep frusto-conical surface of the middle section at its upper end merges at a rounded-off transition with the lower end of the shallow frusto­conical surface of the tip section. The middle section also has an extremely sharply-curved surface of revolution, being substantially shorter in axial length than its frusto-conical surface, which is generated by an arc having a very short radius, for example, from about 0.178 cm to about 0.229 cm. The sharply-curved surface of the middle section at its upper end merges with the lower end of the steep frusto-conical surface of the middle section and at its lower end merges with the upper end of the cylindrical surface of the flange section.

[0008] The carbide tip is formed by a powder metallurgy method. Basically, the powder is first compacted under very high pressure to a "green" state wherein it forms a mass of chalk-like consistency. The tip is formed upside down within a cylindrical bore in a die wall between a lower stationary die having a forming cavity in the shape of an upper portion of the tip and an upper movable die having a forming cavity in the shape of a lower valve seat portion of the tip. In forming the conventional hard tip, initially the space of the bore and the forming cavity in the lower die are filled with powder. The upper die is then moved toward the lower die. to form the tip in the green state. Normally, pressures in the range of 2,000 - 30,000 psi are applied to form the tip, with the actual forming pressure being used depending on the grade of carbide, grain size, etc.

[0009] Due to the inherent nature of the powder and the die forming method, compaction of the powder is not achieved evenly throughout. Instead, a compaction density gradient is produced axially through the tip, such that the lower valve seat portion of the tip being formed in the upper die cavity is compacted more than the upper portion of the tip being formed in the lower die cavity. Thus, the flange section of the tip is more dense than the middle section which, in turn, is more dense than the tip section. The usual difficulty in compacting the middle and tip sections to the required level is exaggerated in the case of the conventional tip due to the formation of the sharply-­curved surface of its middle section which provides the transition of the flange section to the steep frusto-­conical surface of the middle section. This is equivalent to an annular surface portion of the lower die located at the mouth thereof and adjacent to the cylindrical portion of the die wall. The sharp curvature of this surface portion, in being between the cylindrical portion of the die wall and a steeply-­inclined portion of the lower die which is equivalent to the steep frusto-conical surface of the tip middle section, in effect, forms the sharply curved surface which constitutes a constriction at the mouth of the lower die around which powder must be forced by the upper die to push it further down into the lower die cavity. Consequently, the already high pressures required to form the conventional tip must be increased even further to overcome the presence of this constriction. However, this creates a high concentration of stress at the circumferential region of the sharply-curved surface of the tip middle section which produces stress cracks in many conventional tips causing them to be rejected upon inspection at the factory or to prematurely wear out later in the field.

[0010] After the conventional tip has been formed in its green state, it must then be removed from the die wall bore and lower die cavity. The upper die is withdrawn and the lower die is then raised to push out the tip. However, due to the extra extremely high pressure used in forming the tip, as explained above, to overcome the constriction caused by the annular portion at the mouth of the lower die, the flange portion of the flange section tends to be compacted to such a severe degree that it expands radially and begins to bulge the die wall radially outward at the short cylindrical portion thereof. Then, when the lower die is raised to push the tip out of the die wall, frequently either or both the tip is destroyed or the portion of the die wall defining its bore is damaged.

[0011] The mass is then sintered in a furnace at high temperature to make the end product extremely hard. If a. tip is found to be unacceptable and thus rejected while in the green state, it can be saved and reprocessed. However, after the "green" tip has been sintered, if then rejected, it cannot be reprocessed and must be discarded.

[0012] It has been found that the configuration of the conventional bit tip deleteriously promotes the creation of high concentrations of stress during its formation which produces stress cracks therein that result either in rejection of a portion of the tips during quality inspection at the factory or early failure during field use.

[0013] Consequently, a need exists for a different approach to the design of hard tips for cutter bits which will overcome the problems identified above.

[0014] With this object in view, the present invention resides in a cutter bit hard tip, comprising: (a) an outer tip section; (b) an inner flange section; and (c) a middle section extending between and merging at its opposite ends with the tip and flange sections. The middle section is defined by a continuous concave surface of revolution.

[0015] More particularly, the tip, flange and middle sections lie along a common longitudinal central axis. The flange section has a diameter which is greater than that of the tip section. The middle section has a maximum diameter less than or equal to the diameter of the flange section and a minimum diameter substantially equal to the maximum diameter of the tip section. Such diametrical relationships provide the concave surface of revolution of the middle section in profile with an outwardly and downwardly sloping configuration similar to that of the sides of a bell-shaped surface.

[0016] Still further, the tip section has a rounded­off end, and a frusto-conical surface of revolution merging from the rounded-off end and being generated by a line disposed at a predetermined angle from the longitudinal axis of the tip. On the other hand, the flange section has a flange portion defined by a right cylindrical surface and a lower valve seat portion extending below the flange portion.

[0017] The present invention provides a cutter bit with a hard tip designed to satisfy the aforementioned needs. Particularly the cutter bit has a hard tip with a configuration which decreases the areas of high stress concentration created during fabrication and thereby increases wear life during use. These improved and highly beneficial results are brought about by the provision of a concave surface of revolution on a middle section of the hard tip by an identically configured surface portion at the mouth of the lower one of the set of forming dies. The continuous concave surface replaces both the steep frusto-conical surface and sharply-curved surface on the middle section of the conventional tip, and, by doing so, eliminates the constriction to powder flow in the lower forming die.

[0018] The tip of the present invention, although made of the same material as the prior art tip, eliminates the high stress concentrations and decreases the rejection rates experienced heretofore. Further, the concave configuration results in less compaction of the flange section of the tip which allows the tip to be pushed easier out of the die without risk of its own destruction or damage to the die.

[0019] The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings in which:

Fig. 1 is a side elevational view of a cutter bit mounted on a block, being shown in fragmentary sectional form, and incorporating a hard tip constructed in accordance with the principles of the present invention.

Fig. 2 is a side elevation view of the hard tip of the present invention.

Fig. 3 is a top plan view of the hard tip taken along line 3--3 of Fig. 2.

Fig. 4 is a bottom plan view of the hard tip taken along line 4--4 of Fig. 2.

DETAILED DESCRIPTION OF THE INVENTION

[0020] In the following description, like reference characters designate like or corresponding parts throughout the several views. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms.

[0021] Referring now to the drawings, there is shown the preferred embodiment of a cutter bit, generally designated 60, having a hard tip 62 constructed in accordance with the present invention. The cutter bit 60 has a forward body portion 64 and a rearward shank portion 66 which are constructed as a single piece of steel. A cylindrical retention spring 68, which is longitudinally slotted and made of resilient material, encompasses the shank portion 66 of the bit 60 and adapts the bit for mounting in a socket 70 of a block 72 which is, in turn, mounted on an excavating drum (not shown). The retention spring 68 tightly engages the socket 70 and loosely engages the bit shank portion 66, allowing the bit to rotate during use.

[0022] The hard tip 62 of the present invention is attached on the front end of the forward body portion 64 of the bit 60. Whereas the body and shank portions 64,66 of the bit 60 are constructed as a single piece, the tip 62 is constructed separately and then inserted and either cemented or brazed into a generally concave tapered cavity 74 formed in the front end of the bit body portion 64.

[0023] The hard tip 62 is preferably made of a wear resistant material such as cemented tungsten carbide which includes a cobalt content in the range of 5.3 - ­8.0 weight percent, with the preferred cobalt weight percentage being 5.4 - 6.0. The tip has a hardness in the range of 87.5 to 89.0 Rockwell A, and preferably 87.8 to 88.6 Rockwell A. Basically, the hard tip.62 includes an outer tip section 76, an inner flange section 78, and a middle section 80 extending between and merging at its opposite ends with the tip and base sections. The flange section 78 has a flange portion defined by a right cylindrical surface 79 and a lower valve seat portion 81 depending below the flange portion 79. The lower valve seat portion 81, being similar in shape and configuration to the valve seat portion of the conventional tip, is adapted to be inserted into the tapered cavity 74 of the body portion 64 and then brazed or cemented therein for mounting of the tip 62.

[0024] The middle section 80 of the hard tip 62 is uniquely defined by a continuous concave surface of revolution 82. The flange portion 79 of the flange section 78 has a diameter which is greater than the maximum diameter of the tip section 76, whereas the middle section 80 has a maximum diameter which is slightly less, but for all practical purposes is substantially equal to the diameter of the cylindrical flange portion 79 and a minimum diameter substantially equal to the maximum diameter of the tip section 76.

[0025] The tip section 76 of the hard tip 62 is defined by a rounded-off end 86 and a frusto-conical surface of revolution 88 merging from the rounded-off end and being generated by a line disposed at predetermined angle from the longitudinal axis 84. In view of the diametrical relationship of the middle section 80 with both the tip and flange sections 76,78, the concave surface of revolution 82 of the middle section 80 in profile has an outwardly and downwardly sloping configuration generally similar to that of the sides of a bell-shaped surface.

[0026] The tip 62 is fabricated by the same method as described above for the conventional tip. However, now the concave surface of revolution 82 on the middle section 80 of the hard tip 62 is formed by an identically configured surface portion at the mouth of the lower one of the set of forming dies. Thus, the concave surface 82 replaces both the steep frusto-­conical surface and sharply-curved surface on the middle section of the conventional tip and by doing so, eliminates the constriction to powder flow in the lower forming die.

[0027] As well appreciated and understood by those skilled in the art in the forming process of a carbide tip, as described above, that due to manufacturing tolerances it is difficult to have the terminal edge of the sidewall of the lower die, which defines the cavity for forming the upper portion of the carbide tip, to terminate in a sharp acute point, but rather the edge is normally flattened off to a narrow width thickness of approximately 3-5 mils. Thus, as a result of the flattened edge of the lower die sidewall, during the forming process a corresponding narrow flat, being indicated by the numeral 83 in Figs. 2 and 3, is formed on the tip 62. The edge or flat 83 is narrow, being approximately 3-5 mils in width thickness when the tip 62 is formed in its green state, and, after sintering of the tip which causes shrinkage, the flat 83 is narrower. While it is not necessary or preferred in that it adds additional manufacturing expense, the narrow flat 83 may be ground off if desired. Therefore the maximum diameter of the middle section 80 may be less than or equal to the diameter of the flange portion 79.

[0028] Therefore, the middle concave surface 82 provides the tip 62 of the present invention with a configuration that eliminates the high stress concentrations and decreases the rejection rates experienced heretofore. Further, the concave configuration results in less compaction of the flange section 78 of the tip 62 which allows the tip to be pushed easier out of the die without risk of its own destruction or damage to the die.

[0029] By way of example, in one embodiment of the tip 60, the rounded-off end 86 of the outer tip section 76 has an internal radius from about 0.3048 to about 0.3302 cm, and its frusto-conical surface 88 is generated by a line extending from about 49 to 51 degrees from the longitudinal axis 84. The tip section 76 also has a maximum diameter from about 0.9677 to about 0.9931 cm, and a length from about 0.3962 to about 0.4216 cm. The cylindrical flange portion 79 has a diameter from about 1.5748 to about 1.6002 cm, and a length from about 0.1651 to about 0.1905 cm. The concave surface 82 of the middle section 80 has an external radius 90 from about 1.3843 to about 1.4097 cm. The middle section 80 has a length from about 0.9829 to about 1.0083 cm. And the lower valve seat portion 81 of the tip 62 at which the tip is brazed to the bit body 64 is not appreciably different than that same portion of the conventional tip, so it need not be described.

[0030] It is thought that the cutter bit hard tip of the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.

Claims

1. A cutter bit (60) having a hard tip (62), an outer tip section (76); an inner flange section (78); and a middle section (80) extending between and merging at its opposite ends with said tip and flange sections, characterized in that said middle section (80) is defined by a continuous concave surface of revolution (82).

2. The hard tip (62) as recited in Claim 1, wherein said flange section (78) has a diameter which is greater than that of said tip section (76).

3. The hard tip (62) as recited in Claim 1, wherein said tip (76), flange (78) and middle section (80) lie along a common longitudinal central axis.

4. The hard tip (62) as recited in Claim 3, wherein said tip (76) section has a rounded-off end (86), and a frustoconical surface of revolution (88) merging from said rounded-off end and being generated by a line disposed at a predetermined angle from said longitudinal axis (84).

5. The hard tip (62) as recited in Claim 4, wherein said rounded-off end (86) of said tip section (76) has an internal radius from about 0.3048 to about 0.3302 cm.

6. The hard tip (62) as recited in Claim 4, wherein said tip section (76) has a maximum diameter from about 0.9677 to about 0.9931 cm.

7. The hard tip (62) as recited in Claim 4, wherein said tip section (76) has a length from about 0.3962 to about 0.4216 cm.

8. The hard tip (62) as recited in Claim 4, wherein said predetermined angle from said axis to said line generating said frusto-conical surface of revolution of said tip section is from about 49 to 51 degrees.

9. The hard tip (62) as recited in Claim 1, wherein said flange section (78) includes a flange portion defined by a right cylindrical surface.

10. The hard tip (62) as recited in Claim 9, wherein said cylindrical flange portion has a diameter from about 1.5748 to about 1.6002 cm.

11. The hard tip (62) as recited in Claim 9, wherein said cylindrical flange portion has a length from about 0.1651 to about 0.1905 cm.

12. The hard tip (62) as recited in Claim 9, wherein said flange section further includes a valve seat portion (81) depending below said cylindrical flange portion.

13. The hard tip (62) as recited in Claim 1, wherein said concave surface of said middle section (80) has an external radius from about 1. 3843 to about 1.4097 cm.

14. The hard tip (62) as recited in Claim 1, wherein said middle section (80) has a length from about 0.9829 to about 1.0083 cm.

15. The hard tip (62) as recited in Claim 1, wherein said concave surface of revolution (82) of said middle section (80) in profile has in outwardly and downwardly sloping configuration similar to that of the sides of a bell-shaped curve.

16. In a rotatable cutting bit (60) including a forward body portion having a front cavity (74), a shank portion (66) depending rearwardly from the forward body portion for mounting the bit in a socket (70), an improved hard tip (62) adapted to be mounted on said forward body portion, said hard tip (60) having an outer tip section (76); an inner flange section (78) including a flange portion defined by a right cylindrical surface (79) having a diameter greater than the maximum diameter of the tip section and a valve seat portion (81) depending below the cylindrical flange portion and correspondingly shaped to be inserted into the front cavity defined (74) in said forward body portion for mounting of said tip; and a middle section (80); characterized in that said middle section (80) is defined by a continuous concave surface of revolution (82) and is disposed between said tip and flange sections.

Drawing