Disc Grinder Use
The prototype Disc grinder is used to finish-grind the annular coated abrading
surface of island type abrasive discs to ensure that it has a thickness variation
of less than .0001”. This thickness accuracy is required for high speed
abrading. Also, the Disc grinder is used to “condition” the surfaces of wafers or
workpieces before they are lapped or polished. All other individual wafers in a
group must have the same thickness so that their abraded surfaces lie in a
common plane. During lapping or polishing, material is uniformly removed from
all portions of the wafers. Large wafers or groups of wafers attached to a
backing disc, are quickly attached to the grinder platen with vacuum. Ground
wafers can then be quickly attached by vacuum to a lapper wafer head to obtain
immediate precision flat-surfaced contact with the island disc abrasive surface.
Disc Grinder Machine
The prototype Disc Grinder is a large, heavy and stiff precision machine that
has an internal-motor driven sealed air bearing spindle with a 19” diameter platen. The platen is flat to within .00003” and rotates from 0 to 3000 rpm. Vacuum port holes in the platen surface allow wafers or abrasive island discs to be quickly and strongly attached to the platen for abrading. The robust air bearing platen spindle is sealed from abrading debris and is maintenance free.
Disc Grinder Carriages
The Disc grinder has large standard and preloaded THK precision ball bearing slides that are used for both a horizontal and a vertical carriage. The carriages support a precision machine tool spindle having a diamond abrasive wheel that rotates from 0 to 2500 rpm. The mass of the heavy 700 lb horizontal carriage allows the abrasive wheel to be vibration free while it is translated 10” laterally across the radius of a 18” abrasive disc with a vertical accuracy variation of less than .0001”. To provide uniform grinding, the horizontal carriage is motor driven horizontally. In addition, the carriage is adjustable vertically in increments of .0001” to accurately control the amount of material removed from the discs or wafers. A precision digital readout on the machine indicates the amount of material removed during the grinding procedure.
Diamond Grinding Wheels
The disc grinder uses a variety of diamond abrasive wheels with a range of different particle sizes. Wheels having large-sized diamonds (that are electroplated to a balanced aluminum wheel) can be used for high removal
rates for hard materials such as sapphire. Wheels having smaller diamonds remove less material, but less time is required for polishing these wafers. It is very efficient to use the diamond abrasive wheel on the Disc grinder to quickly rough grind the surface of a wafer or a group of wafers attached to a backing disc. Lapping or polishing uses very small abrasive particles that provide smooth surfaces but are very slow at removing the large amounts of material required to flatten uneven wafer surfaces which are common with less than perfect workpieces.
Controls, Instruments, Gauges
The Disc grinder has a number of electrical, fluid, and mechanical control and
monitoring devices. The air bearing platen internal torque motor has a control
system that allows different speed profiles to be used for abrading procedures.
Many of the motor operating parameters including speed, torque (abrading friction),
power, acceleration, and deceleration are displayed during the lapping and
polishing operation. Also, a precision position read-out device provides information
for setting up the disc grinding process for different thickness wafers. It is used to
monitor and accurately control the thickness of the abrasive discs and wafers.
Operator access to remove or install abrasive discs or wafers on the Disc grinder
platen is provided by motor driving the heavy carriage vertically.