Metal tools and implements – making – Blank or process – Drill
Reexamination Certificate
1998-12-07
2001-03-13
Watts, Douglas D. (Department: 3724)
Metal tools and implements, making
Blank or process
Drill
C175S426000, C175S430000
Reexamination Certificate
active
06199451
ABSTRACT:
BACKGROUND OF THE INVENTION
Machines are available for breaking up and excavating hard surfaces such as concrete, stone and asphalt. These machines have a rotating member, such as a wheel or a drum, with a plurality of tools located on the outer surface of the rotating member. When the rotating member is forced against the surface to be excavated, the cutting ends of the tools successively impact against the surface to be broken up, resulting in small amounts of material being removed by the impact of each tool.
The tools mounted on the rotating member have a generally concave seat at the forward end in which a tungsten carbide insert is retained. A forward cutting tip of the insert cuts into the surface to be excavated, and the useful life of the tool is determined by a number of factors. Ideally, the tip of the insert will wear evenly around its circumference and not crack or dislodge during use and, therefore, replacement will be needed only after the tool and the cutting tip are so worn as to be unusable. To maximize the resistance of the tool and the cutting tip to wear, it is desirable that the tungsten carbide cutting tip be made as hard as possible and yet not be so brittle as to break. It is also desirable that the braze which retains the tip in the seat be sufficiently strong so that the insert is not dislodged during use. The tungsten carbide insert is the most expensive portion of the manufacturing cost of such tools, and a large portion of the cost of the insert is in the raw material of which the insert is made. To be a competitive manufacturer of such tools, a manufacturing company must provide a tool having inserts that are not subject to being dislodged or cracked, and yet be competitively priced.
Currently, inserts of this type are manufactured from raw tungsten carbide powder having average particle sizes in the range of 8 to 18 microns with an average particle size midway between the extremes such that the particle distribution is in the shape of a bell curve. The raw material further includes from 6 to 11 percent by weight powdered cobalt, and after sintering, such inserts have a mean hardness which does not exceed 89.0 on the R
a
scale, and the hardnesses of the inserts have tolerances which are no more than±0.5 R
a
.
When a small percentage of cobalt, such as about 6 percent, is used with smaller particles of tungsten carbide, such as less than 8 microns, the resulting product may be harder, but more brittle than presently available inserts, and would be subject to fracturing. As a result, commercially available inserts are not made from particles of raw material having average particle sizes of tungsten carbide of less than 8 microns, and present day inserts have hardnesses which do not exceed 89.0±0.5 R
a
.
It is well known that an insert having a greater hardness would have a significantly increased resistance to wear. Even a relatively modest increase in hardness, from 89.0 R
a
to 89.5 R
a
, for example, would result in a lengthening of the life of the tool by twenty or thirty percent. Therefore, it would be desirable to provide a tool having a cemented tungsten carbide insert which has a longer usable life, without being subject to breakage. It would also be desirable to have an insert for which the cost of manufacture is reduced below existing costs.
BRIEF DESCRIPTION OF THE INVENTION
Briefly, there is provided in accordance with the present invention an excavating tool having an insert made of a grade of tungsten carbide which is harder than that currently usable in such tools but is not subject to the fracturing which has prohibited prior efforts to manufacture inserts from such harder grades of material. Three factors much be optimized to make a fracture resistant insert which is harder than existing inserts, and those factors are: the percentage by weight of cobalt, the particle sizes of the powdered raw material, and the shape of the insert itself.
The harder grades of tungsten carbide are generally more brittle because they usually contain lower percentages of the more elastic binder. Inserts made of cemented tungsten carbide which is harder than 89.0 R
a
have been unreliable because they shatter when subjected to the impacts of the insert against hard surfaces during use of the machine.
Although the qualities of tungsten carbide have been the subject of extensive study, and empirical data is available relating to the compressive limits and tensile limits (transverse rupture strength) there is no information as to how or where a tungsten carbide object will shatter when subjected to a powerful shock. Product testing has been used to determine that 89.0 R
a
is the hardest grade of tungsten carbide which can be used in existing designs of cutting inserts to cut the hardest materials for which the machines are used. Harder grades of tungsten carbide could be used on machines which cut softer materials, but tools for cutting the softer materials do not incur the wear to the insert suffered from the harder materials, although the steel bodies to which the inserts are mounted are more subject to being washed away.
When an axially symmetrical insert of a cutting tool fails, the failure often includes a break along a plane which is transverse to the axis of the insert, and is positioned adjacent the base. The inserts used in excavating machines for cutting through the hardest of materials have bases which are 0.610 to0.750 inch in diameter depending on the manufacturer, and generally have a length of 0.600 or longer. Shorter inserts exist but they have been rarely used to cut hard surfaces because their usable life is too short. Existing configurations of inserts are made of tungsten carbide with a hardness greater than 89.0 R
a
fail when they are subjected to the impacts imparted by the machine. The failure often includes a break which is transverse to the axis of the insert, and frequently this transverse fracture is very near the base, often only 0.125 inch or less above the top of the base portion which is brazed into the tool body.
The compressive strength of tungsten carbide is dependent on a number of factors but is normally about 600,000 lb/in
2
whereas the shear strength of the material is only about 200,000 lb/in
2
. Tool failure, therefore, is much more likely to occur because of the shear strength of the material is exceeded and not because the compressive strength is exceeded.
The tools mounted on rotating members for cutting hard surfaces have an attack angle, that is, the angle between a line perpendicular to the surface being cut and the axis of a tool, which is usually between 40 to 60 degrees. In accordance with the present invention, the length the insert is reduced, and the forward end, or tip end, of the insert is enlarged such that it has a proportionally larger cross-sectional diameter than the configurations of most existing inserts. When a tool mounted on the rotating member of a cutting machine impacts a surface at an attack angle of 40 to 60 degrees, a line perpendicular to the surface being cut and passing through the point of impact, will pass through or very near the base of the insert. The result of this configuration is that the forces within the insert caused by the tool striking the surface to be cut are predominantly compressive forces and not shear forces, and the tool will be less subject to failure.
An insert in accordance with the present invention which is suitable for use on a machine for cutting hard materials is symmetric about a principal longitudinal axis has a forward cutting end and is axially disposed behind the forward cutting end is a base. The body of the insert diverges outwardly from the forward cutting end toward the base. The base has a maximum diameter of about 0.700 inch and the insert has a length from the bottom of the base retained in the seat to the forward end of the insert of no more than 0.600 inch. The forward cutting end has a tip section having a largest diameter of about 0.420 inch, and between the tip section and the base is a diverging central section The foreg
Marsh Robert L.
The Sollami Company
Watts Douglas D.
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