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AUTOBIOGRAPHY ABSTRACT LIST OF REFERENCES REPORT ABOUT SEARCH INDIVIDUAL TASK
Currently, there is a great deal of interest in the use
of ceramic components in the automotive industry. The machining of these is
often a slow, costly process, so new and improved processes are being sought.
This article deals with the grinding of flat surfaces on the ceramic components.
The grinding of single or opposed flat surfaces can be carried out at
relatively high production rates using disc-grinding methods. With any disc
grinder, the large flat face of the disc wheel is used for grinding; there is no
line contact, as with reciprocating table surface grinders. A typical 18 in OD
wheel with a 305mm (12 in) center-hole gives you a 910cm^2 (141 in^2) cutting
surface; the disc method of grinding can increase production rates to 100 per
cent compared to other processes. This includes reciprocating or rotary table
methods, which, in many cases, may also require the use of vacuum chucks or
complicated clamping or blocking of the parts being ground.
In the process used to manufacture ceramics, the casting may necessitate
leaving excess material on the component, which must be removed in order to get
into the non-porous base material. This can be done by several methods of disc
grinding, as will be explained later.
Other ceramics parts may not have excess material to be removed, and are
formed to near net shape. Such parts require minimum stock removal, and are
ground at high production rates to very close tolerances. Depending on the
specified tolerances, disc grinding can be used to finish the part, thereby
either eliminating the lapping process or preparing the part for a final lap. In
the latter case, disc grinding will reduce lapping process time.
Various methods for disc grinding ceramic components are reviewed later. The
particular method appropriate to a given application should be selected, based
on a consideration of the following specifications: part shape; following
process; stock removal; automation; tolerances; quality; costs; variety of
parts; finish; lot sizes; preceding process; yearly production; and hardness of
ceramics.
Horizontal spindle feed-through fixture method. With this method of disc
grinding, work-parts are fed between the opposed abrasive discs by various
methods, such as power-driven belts, pick-off chains, rollers, sprockets and
magnetic discs. Upper and lower bars support and guide the parts through the
grinding area. Feed-through fixturing provides the highest rate of production of
all fixturing methods.
Horizontal and vertical spindle rotary method. Here, work-parts contained or
clamped in the rotary carrier are traversed in an arc between two abrasive
discs. Rotary-carrier fixturing provides the second highest rate of production
after the feed-through method. Abrasive discs are positioned to take off the
stock progressively.
Horizontal spindle reciprocating method. With this method of work support,
parts contained in the fixture are reciprocated between and across the disc
faces. Reciprocating fixtures are recommended for low lot-size runs, precise
tolerances, heavy-stock removal, holding relationships to other locating
surfaces, or for larger parts not suitable for other methods of disc grinding.
Special fixturing method. Special fixturing is designed to hold work-parts
that differ widely in size and shape from average parts and cannot be handled by
conventional tooling. Gardner engineers can design special work-handling
fixtures for a wide range of unusual parts.
Single-spindle vertical method. Here, grinding is done with a vertical
spindle and a single abrasive wheel. Depending upon the workpiece size, two
workstations are in the grinding zone simultaneously, so each index of the table
produces two ground parts. During grinding, the individual work- holding
stations rotate their parts under the rotating abrasive disc as the abrasive is
fed downward. Workpieces rotate only while in the grinding zone. Parts usually
nest in a free-state condition within their holding stations, so there is no
distortion from clamping or magnetic chucking.
Here are four typical examples, which illustrate disc grinding of ceramic
components.
Tooling, in this case, is inexpensive and can be made from various materials.
The component to be ground is placed in the keeper, with the diamond abrasive
wheels retracted 0.254mm (0.010 in) beyond the finish thickness as well as the
stock to be removed. The keeper is then advanced, placing the component between
the two abrasive wheels. The abrasive wheels are fed inward at the
pre-programmed rate as the workpiece is reciprocated across the abrasive faces,
breaking the OD and ID of the abrasive wheels. This continues until the finish
size is obtained.
On a part with a surface of approximately 129cm^2 (20 in^2), 3.81mm (0.150
in) of ceramic material can be removed at a depth rate of approximately
0.030mm/s (0.0012 in/s), holding tolerances of 0.0051mm (0.0002in) parallel and
0.010mm (0.0004 in) size. For this operation, 100 grit, 584mm (23in) diameter
diamond wheels would typically be used. Programmed dressing of the wheels on a
regular basis will ensure faster cutting and less power consumption by the
spindle drives. Abrasives are run at a speed of 1889m/min (6200 sfm).
Coolant (Rust Veto, 2350 with 1:12 mix) is used to keep the workpiece cool
and to assist in flushing the ground material out of the grinding zone. Coolant
temperature is precisely controlled to provide process stability. Temperature
control is recommended on most grinding applications, and it can increase
abrasive wheel life and reduce dress frequency, thus giving more uptime and
lower parts costs.
The opposed rotation prevents the rings from being rotated by the abrasives
at the same speed at which the wheels are running (in this case, 3050m/min
(10000 sfm). The abrasives are set at slight angles to each other, so that the
ceramic being ground off is removed progressively across the abrasive faces.
Approximately 0.254mm (0.010 in) of stock is removed in a single pass. This
method of grinding is the fastest available by double disc grinding. Abrasives
are only adjusted inward to compensate for diamond wear.
Similar parts are ground holding flatness tolerances of 0.0001 in or less, to
size tolerances of 0.0002 in. In this case, the abrasive speed is 1706m/min
(5600 sfm), and 220 grit diamond wheels are used. The upper abrasive is slightly
tilted to enable the stock to be taken off progressively as the workpiece
travels in an arc across the abrasive face. Abrasives are fed only when diamond
wear occurs.
For more information on disc grinding contact Ron Hanson at the following
address:
DISC-GRINDING EXAMPLE NUMBER ONE
Ceramic components with round, square
or irregular shapes are ground on a double disc grinder with a reciprocating
fixture. In this method of grinding, the workpiece is contained in a keeper
(normally unclamped), which has a cut-out the shape of the component.
EXAMPLE NUMBER TWO
A second example of disc grinding is the grinding of
ceramic rings approximately 51mm (2.0 in) diameter x 38mm (1.5 in) ID x 9.5mm
(0.375 in) thick. These are typically ground by the feed-through method, both
sides of the rings being ground at a rate of 10000 to 16000 an hour. The machine
would typically use 406mm (16 in) diameter diamond abrasives. The ceramic rings
would be supported between the wheels by two simple steel bars. The rings are
driven through the machine by a pair of feed-belts at the front of the machine.
The diamond wheels should be run opposed to each other (one clockwise, one
counter-clockwise), to reduce tooling wear.
EXAMPLE NUMBER THREE
In a third example, a vertical double
spindle/double disc grinder is used to grind ceramic rings and other components
measuring approximately 28.57mm (1-125 in) OD x 3.56mm (0.140 in) thick. The
production rate every hour is 1000, with between 0.127mm and 0.254mm (0.005 in
and 0.010 in) of stock removed from each face to a tolerance of 0.0254mm (0.0005
in) and a size of +/-0.0254mm (0.0005 in).
EXAMPLE NUMBER FOUR
A fourth method of disc grinding to be considered
for extremely thin workpieces (for example, 0.75mm or 0.030 in) is the
vertical-spindle method. This method uses a single narrow- face diamond-wheel to
plunge- feed, grinding two parts at the same time. During the plunge-feed, the
workpiece is rotated under the abrasive wheel. At the completion of the cycle,
the wheel is retracted and the table is indexed. This method can be used either
for parts that require grinding on one side only, or for parts that need to be
ground on both sizes.
CONCLUSION
Disc grinding should be considered in cases where both high
production and close-tolerance stock removal are required. Disc-grinding methods
are usually much less expensive than alternative processes that are less
precise.