This invention relates to rotary dressing tools designed
for truing and dressing the profiled faces of abrasive grinding wheels, (see for
example DE 3811784).
Rotary diamond dressing tools impart the required form
onto a grinding wheel and must be designed and made to specifications driven by
the design of the grinding wheel. These tools have narrow quality specifications
with low tolerances for deviations in geometry and mechanical attributes. Although
dressing tools have been constructed in a variety of ways utilizing various materials
and processes, most processes known in the art are demanding and inefficient.
For example, in one commercial process, diamond grains
are hand set into a pattern in the cavity of a mold with an adhesive, then a powdered
metal bond material is added and pressed into place around the diamonds. The pressed
materials are densified by processes such as infiltration, hot pressing, sintering,
or a combination thereof, to fix the diamonds in place and form the tool. In another
typical process, a diamond layer may be set onto a custom designed mold and fixed
in place by reverse electroplating. See, e.g., US-A-4,826,509. The sintering or
plating step is followed by an extensive grinding step to remove grain high spots
and to flatten the surface.
In another process described in U.S. Pat. No.-A-4,805,586,
the diamond grains are pretreated to roughen and enlarge their surface area and
to permit the grains to be arranged within the bond so that the majority of the
grains are in direct contact with adjacent grains. These pretreated diamond grains
are then electroplated to the surface of a base body with nickel or cobalt or alloys
of nickel or cobalt.
In US-A-5,505,750, the diamond grains and metal powder
bond are infiltrated with a near-eutectic copper-phosphorus composition during sintering.
Many powder metal matrix abrasive components for dressing
tools utilize relatively small diamond grains (e.g., less than 0.5 mm in diameter)
embedded within the powder matrix and the resulting composite is ground to the required
geometry. Such abrasive components are not very sharp and grinding wheel dressing
with them is relatively inefficient due to rapid wear of the tool. When such a powder
matrix is used with large diamond grains, the finishing process loses considerable
amounts of diamond as the composite is ground to the required geometry. It is not
possible to achieve a durable, fine (e.g., about 0.127 mm (0.005 inch)) dressing
tip radius in tools made from diamond grains in a powder metal bond.
Polycrystalline diamond (PCD) inserts have been used to
construct rotary dressing tools. PCD inserts are embedded in a powder metal matrix,
sintered onto the tool, and then ground to the required geometry and surface finishing.
See, e.g., US-A-4,685,440. PCD inserts offer a relatively flat surface and can be
easily ground to the required geometry during finishing operations, or, for some
shapes, can be provided as a near net shape piece. However, PCD is not 100% diamond.
PCD material initially contains significant quantities (10-12 wt%) of metal catalyst
and the metal catalyst is typically leached from the PCD material, leaving voids,
to yield essentially pure diamond with a density of about 90 to 95 % of the theoretical
density. Therefore, dressing tools made with PCD inserts lack the durability of
dressing tools made with diamond abrasive grains which are fully dense, 100% diamond
materials.
The rotary diamond tool for dressing abrasive wheels described
in US-A-5,058,562 is made by using a chemical vapor deposition (CVD) process to
deposit a layer of diamond film directly onto a base plate of the tool and assembling
the base plate with a pair of backup plates to provide stiffness. With this approach,
there are no diamond cutting points created, merely a hard, flat diamond surface.
In a dressing tool, a flat diamond surface merely acts to crush the wheel face,
rather than to cut bond and spent abrasive grains from the face and, thereby, open
the face of the wheel for further grinding.
The rotary diamond tool for dressing abrasive wheels described
in US-A-4,915,089 is made by forming a single layer of diamond grains in a plane
orthogonal to the rotational axis of the tool. The layer of diamond grains is sandwiched
between two layers of metal backup plates. The diamond layer is bonded to the plates
by hot pressing the diamond grains and metal powder between the metal backup plates
in a suitable mold to sinter the metal powder. The 4,915,089 patent mentions an
alternative design wherein diamond grains are attached to one or both sides of the
tool by plating or metal bonding, but teaches that the alternative design suffers
the disadvantage of poor diamond retention. In the preferred design, arcurate segments
of the laminated assembly of diamond grains and plates are brazed to the circumference
of a disc-shaped metal wheel to form a dressing tool, optionally with a continuous
abrasive rim. However, consistent with the geometry of this tool design, the patent
teaches that the tool is used to dress a straight face wheel and the tool would
not be useful for dressing a profile into the face of a grinding wheel.
EP-B-116668 discloses a dressing tool having a single layer
of electroplated diamond grains arranged in a geometric design similar to that of
the tool of U.S.-A-4,915,089. In contrast to the active braze bond used in the tools
of the invention, with the electroplated bond of the EP-B-116668 tool, poorer diamond
grains retention, shorter tool life and higher manufacturing costs are predicted.
The invention is a rotary profile dressing tool having
a rigid, disc-shaped core and an abrasive rim around at least one surface of the
periphery of the core, the core and the abrasive rim being oriented in a direction
orthogonal to the axis of rotation of the tool, wherein the abrasive rim comprises
an abrasive component bonded to the core by means of an active braze, and the abrasive
component is selected from the group consisting of diamond grains arranged in a
single layer and diamond film inserts, and combinations thereof. In an alternative
design, the abrasive rim comprises a plurality of abrasive inserts mechanically
fastened to the core of the tool, and the abrasive inserts comprise an abrasive
component bonded to a backing element by means of an active braze, and the abrasive
component is selected from the group consisting of diamond grains arranged in a
single layer and diamond film inserts, and combinations thereof.
- Fig. 1 is an illustration of the operation of a rotary profiling dresser of
the invention showing a grinding wheel with a profiled grinding face.
- Fig. 2 is a planar view of a rotary profile dressing tool of the invention.
- Fig. 3 is a partial cross-section of a single layer of diamond abrasive grain
brazed onto a backing element in the rotary profile dressing tool of the invention.
- Fig. 4 is a partial cross-section of a single layer of diamond abrasive grain
brazed onto a rotary profile dressing tool of the invention without a backing element.
- Fig. 5 is a partial cross-section of a diamond film insert brazed onto a backing
element in the rotary profile dressing tool of the invention
As shown in Figure 1, the dressing tools of the invention
are effective in profile dressing and truing operations carried out on abrasive
grinding wheels. The dressing tool 3 is rotated about an axis (depicted in Fig.
1, with a dashed line numbered 5) and moved into contact with the profiled face
2 of the grinding wheel l in a direction along either an X axis (arrow 6) or a Y
axis (arrow 7) as needed to dress or true the profile of the wheel.
As used herein, "true" (or truing) refers to operations
used to make a grinding wheel round and profiled into the desired contours. Dress
or dressing refers to operations used to open the grinding surface (or face) of
the grinding wheel to improve grinding efficiency and avoid workpiece bum or other
damage caused as the wheel face dulls during grinding. The wheel face dulls, for
example, when the exposed sharp abrasive grains have been consumed, or the wheel
face becomes smooth due to failure of the bond to erode and expose new grain or
due to loading of the wheel face with debris from grinding operations.
Some operations permit a single dressing tool to be used
simultaneously for both purposes and others do not. Truing is generally required
when a grinding wheel is first mounted on a machine for use and whenever operations
cause the wheel to go out of round or lose its contour. Depending upon the particular
grinding operation, the dressing tools of the invention may be used to true or to
dress or to do both.
A typical rotary dressing tool of the invention is illustrated
in planar view in Fig. 2. A single layer of the diamond grain 8 is embedded in a
metal braze 9 and bonded to the metal core 11 of the tool. The metal core of the
tool contains a central hole for mounting the tool onto an drive spindle of a machine
equipped with a means for rotating the tool around an axis 5. Also depicted in Fig.
2 is an optional feature of the invention consisting of four holes 12 around the
central arbor hole for attaching the metal core of the tool to a support element
(not shown).
As shown in Figs. 3-5, the abrasive rim 4 of the dressing
tool 3 may be constructed in one of several preferred embodiments. In Fig. 3, the
abrasive grain 8 and braze 9 are supported by a backing element 13 which is part
of the unitary construction of the metal core 10. In Fig. 4, the abrasive grain
8 and the braze 9 are self-supporting and are brazed to the metal core 10 only along
the inner diameter of the abrasive rim 4. Such a construction has the advantage
that the dressing tool having exposed abrasive grain on each side of the tool may
be operated in either direction along the X axis (arrow 6) so as to approximately
double the efficiency of the dressing operation and, thus, to generate profiles
previously unobtainable with a single tool setup.
In either construction, after brazing, the diamond grains
8 are submerged within the braze 9 layer and are not necessarily visible in the
manner of metal bonded single layer abrasive cutting tools. Such a self-supporting
abrasive component cannot be constructed if utilizing an electroplating process
to bond the abrasive grain to the core of the dressing tool because the electroplated
metal diamond composite would lack sufficient strength to be used. It is only possible
when making a brazed single layer diamond abrasive tool utilizing an active braze
wherein the diamond grains function as a structural element of the tool, as described
herein.
As shown in Fig. 5, a diamond film insert 14 may be bonded
to the metal core 10 with an active braze 15 to construct a preferred embodiment.
As used herein, diamond film refers to a thin layer of material made by a CVD or
jet plasma process, with or without diamond seed particles, consisting of approximately
100% diamond. Examples of diamond film preparations are provided in US- A-5,314,652;
US-A-5,679,404; and US-A-5,679,446 which are hereby incorporated by reference. The
diamond film is made into a thin layer (e.g., 100 to 1,000 microns) having the desired
size for a tool insert and then the diamond film insert is brazed to the backing
element 13 portion of the metal core 10 in substantially the same manner, and with
the same types of brazes, as the diamond abrasive grains are brazed to the metal
core.
These preferred embodiments differ from the prior art in
several significant ways. The abrasive components depicted in Figs. 3-5 require
less drastic finishing operations to achieve the precise surfaces desired for dressing
tools. Like PCD inserts, diamond film inserts (Fig. 5) are flat films. As for the
single layer diamond abrasive grain embodiments (Figs. 3 and 4), some initial grinding
of the surface may be needed, but the single layer of grain eliminates much of the
uneven character of a composite matrix of abrasive grain in a powdered metal bond.
The dressing tools of the invention are designed to present
the same tip radius to the wheel face throughout the life of the dressing tool because
the width of the single layer of diamond grain (or the diamond film insert) is not
affected by the dressing operation. As the outermost diamond grain is consumed,
a single grain below it is present at the radial tip of the dressing tool and the
radius of the dressing tip remains constant as the tool is used. Thus, the tools
of the invention are self-sharpening and maintain a precise geometry as they are
consumed.
In further contrast to the prior art tools, the dressing
tools of the invention have a long life and superior efficiency in dressing and
truing grinding wheels.
The angle of the backing element may range from 0 to 90°,
preferably from 10 to 45°, and most preferably ranges from 15 to 30° in
dressing tools designed for use on vitrified grinding wheels.
In constructing the tools of the invention, brazing is
typically carried out at 600-900° C, utilizing an active braze, and preferably
at 800-900° C utilizing an active bronze or nickel braze. An "active braze"
is a braze containing at least one material (e.g., titanium or chromium) that is
chemically reactive with the surface of the diamond grain. When heated, the braze
creates a chemical bond between the braze material, the diamond grain, and, optionally
the metal core of the tool. A preferred active bronze braze is made from a mixture
of copper, tin and titanium hydride powders, optionally with the addition of silver
powder, by the method described in commonly owned U.S. Ser. No. 08/920,242, filed
August 28, 1997, the contents of which are hereby incorporated by reference. A preferred
active braze comprises 55 to 79 wt% copper, 15 to 25 wt% tin and 6 to 20 wt % titanium.
Another preferred active braze suitable for use in the
invention is a nickel braze, comprising 60 to 92.5 wt% nickel, preferably 70 to
92.5 wt % nickel, and 5 to 10 wt% chromium, 1.0 to 4.5 wt% boron, 1.0 to 8.0 wt
% silicon and 0.5 to 5.0 wt % iron. The nickel braze optionally comprises other
materials, such as 0.1 to 10 wt % tin.
The rigid, disc-shaped core is constructed of a wear resistant
material having a use life complementary to the life of the diamond abrasive component.
Steel, particularly tool steel, tungsten carbide, iron, cobalt, and composites thereof
and combinations thereof, are suitable for use in the core. Steel is preferred.
Suitable composites include ceramic particles or fibers contained in a metal matrix
continuous phase. The core may be molded or machined into the desired tool dimensions
by methods well known in the art.
Figures 2-5 show a continuous abrasive rim construction.
In an alternative embodiment, the abrasive component is inserted as strips along
the metal core. The strips may rest within indentations upon a backing element,
or they may be filled into slots machined into and through the perimeter of the
metal core.
In another embodiment of the invention (not shown in the
drawings) the layer of brazed diamonds is present as a plurality of offset strips
located alternately on the periphery of either of the two sides of the rigid core.
In this zig-zag configuration, the periphery of the rigid core appears fluted and
the diamond is brazed in strips within the indentations of the fluted periphery.
In another embodiment of the invention (not shown the drawings)
the diamond is brazed to a backing element to form an abrasive insert and a plurality
of the abrasive inserts are mechanically fastened (e.g., bolted) to the periphery
of the rigid core.
Other embodiments are suited for use in the rotary profile
dressing tools of the invention, provided the diamonds are oriented such that a
set of diamond grains at any given point around the periphery of the tool is presented
to the face of the wheel as a single cutting point and, as this single diamond point
is worn, the set of remaining diamond grains consecutively presents another diamond
grain to replace the worn one and become the single cutting point until the set
has been exhausted.
Example 1
A test tool was constructed from a 10 cm (4 inch) outer
diameter stainless steel (304L) core by vacuum brazing approximately 100% concentration
of SDA 100+ diamond grit (425 to 500 microns, obtained from DeBeers) onto a 20°
included angle backing element on the rim of the core. The tool was designed to
yield a dressing tip radius of about 0.25 mm (0.01 inch), a radius approximately
equal to the radius of the diamond grit selected for the tool after a minor amount
of grinding to finish the abrasive component to the desired initial dressing tip
radius.
Brazing was carried out at 880° C utilizing an active
bronze braze. The active bronze braze was made from a mixture of 100 parts by weight
of 77/23 copper/tin alloy powder and 10 parts by weight of titanium hydride powder.
The powder mixture was blended at 13 wt % with Braz™ organic binder to make
a paste composition, and the paste was spread onto designated portions of the rim
of the metal core of the tool. Diamond grain was dusted onto the paste in a single
layer and excess diamond grain was shaken off of the tool. The tool was oven dried
to evaporate the water from the binder and the dried tool was heated to 880°
C for 30 minutes under a low oxygen atmosphere at less than 0.133 Pa (<10-3
Torr) pressure, and then permitted to cool. In the finished tool, the braze contained
70.2 wt% copper, 21.0 wt% tin and 8.8 wt% titanium.
A second tool was made in the same fashion, except that
the dressing tip radius was 0.12 mm (0.005 inch) and the diamond grit size was 0.212
to 0.25 mm.
The 0.25 mm (0.01 inch) tip radius tool was tested in a
commercial setting on thread grinders. The grinding wheels were 46 x 1.3 x 25 cm
(18 x 0.50 x 10 inch), 3SG100-VBX467 (sol gel alumina abrasive grain) wheels (obtained
from Norton Company, Worcester, MA) operating at 30 surface meters/second (6000
surface feet/minute) during dressing, at an infeed of 0.013 mm (0.0005 inch) per
pass after the initial form dressing (0.025 mm (0.001 inch) per pass). No wear of
the abrasive component of the dresser was observed after 12 weeks of continuous
operation. This compares favorably to a typical commercial rotary dressing tool
used in this commercial setting which has measurable wear after 6 weeks of continuous
operation. In addition, about 50% improvement in grinding wheel productivity was
observed due to the sharpness of the rotary dressing tool.
The 0.12 mm (0.005 inch) tip radius tool was tested in
the same commercial setting and has shown very little measurable wear after 5 weeks
of continuous operation (i.e., about 2 microns per day).
Example 2
A dressing tool was constructed utilizing a 15 cm (6 inch)
stainless steel core having slots preformed along the rim into which 0.60-0.71 mm
(about 0.025 inch) diameter diamond grains were brazed to yield a tool with a dressing
tip radius of 0.3 mm (0.012 inch). The diamond was brazed into the slots using the
braze and the method of Example 1. This striped construction had straight sides
(0° included angle). The tool was effective in dressing profiles into vitrified
bonded CBN wheels.
In a further aspect, the present invention is directed
at a rotary profile dressing tool having a rigid, disc-shaped core and an abrasive
rim around at least one surface of the periphery of the core, the core and the abrasive
rim being oriented in a direction orthogonal to the axis of rotation of the tool,
wherein the abrasive rim comprises an abrasive component bonded to the core by means
of an active braze, and the abrasive component is selected from the group consisting
of diamond grains arranged in a single layer and diamond film inserts, and combinations
thereof.
In a preferred embodiment of this aspect, the abrasive
rim of the dressing tool further comprises a backing element upon which the abrasive
component is brazed.
In a further preferred embodiment of this aspect of the
invention, the rigid core consists of material selected from the group consisting
of steel, tool steel, tungsten carbide, iron and cobalt, and reinforced composites
thereof, and combinations thereof.
In yet a further preferred embodiment of this aspect of
the present invention, the active braze is a bronze braze containing an effective
amount of titanium to react with the abrasive component. It is especially preferred
that the active braze comprises 55 to 79 wt% copper, 15 to 25 wt% tin and 6 to 20
wt% titanium.
In yet a further preferred embodiment of this aspect of
the present invention, the abrasive component is diamond grains and the diamond
grains have an average diameter of 0.15 to 2.0 mm. Preferably, the abrasive rim
has a tip radius equal to about one-half of the average diameter of the diamond
grains.
In yet a further preferred embodiment of this aspect of
the present invention, the core and the backing element are of a unitary construction.
In yet a further preferred embodiment of this aspect of
the present invention, the active braze comprises 60 to 92.5 wt% nickel, 5 to 10
wt% chromium, 1.0 to 4.5 wt% boron, 1.0 to 8.0 wt% silicon and 0.5 to 5.0 wt% iron.
In yet a further preferred embodiment of this aspect of
the present invention, the active braze further comprises 0.1 to 10 wt% tin.
