| Granite Surface Plates Granite Surface Plates Ensure Precise Flatness Accuracy When Properly Maintained Manufacturers
worldwide still rely on flatness accuracy to varying degrees, and so
must properly select, install and maintain their granite surface
plates. Yet, even with the advances in calibration equipment technology
over the past 60 years, the basic plate has remained unchanged.
Of
interest, the granite surface plate was first fashioned because of a
desperate situation during the Second World War. An owner of a monument
and metal working shop in Dayton, Ohio, Mr. Wallace Herman, required a
flat table for checking parts, but the military was using all the metal
for artillery. Thankfully, Herman deduced that granite could be used,
just like metal, for a flat surface and he manufactured the first
granite plate from a piece of black stone. The original stone worked,
and the technologies have evolved and improved to the very precise
granite surface plates that are available today.
Granite Selection All
granite is not the same, and depending on the type, have different wear
characteristics. There is Black, Pink or Starrett Crystal Pink.
The
Black Granite or Diabase is harder than the other granites. The modulus
of elasticity is 9.5 x 10 to 6th power comparing to the Crystal
Pink which is 3.8 x 10 to the 6th power. However, due to the quartz
content of 32% in the Crystal Pink it will outlast the Black 5 to 1.
The quartz content is the determining factor in the wearability of
granite. Black granite is used primarily for its load bearing
capabilities. With any specific weight the Black will be thinner than
the “Crystal Pink” because it is literally just a piece of “rock” with
no particular mineral content. Whereas, the quartz content of the
Crystal Pink permits the instruments to slide much easier without any
risk of wearing the plate. In general, Crystal Pink does not carry the
load of the black, but its thickness can be compensated to carry the
same load.
Installation, Care and Maintenance The
granite surface plate is a precision piece of equipment and must be
properly installed and maintained. Before use, be sure the granite
surface plate is always used on the support system furnished with the
plate. This
will normally be a hard rubber pad attached to the bottom of the plate
forming a non-distortable 3-point support system. The pads are
installed during manufacturing and the plate rests on them throughout
lapping, inspection and shipping. They are a critical factor in surface
plate accuracy and must not be removed. Plates up to and including 6'
wide x 12' long are furnished with this non-distortable 3-point support
system. | | When
mounting the plate on a stand, be sure only the pads are resting the
stand. Never support the plate by the ledges or under its four corners
as this will completely void the guarantee of accuracy.
Plates
larger than 6' wide x 12' long are supported on multiple support points
(6 or more points) consisting of granite pedestals and leveling wedges.
The number of support points and positioning is determined by the size
of the plate. The height of the granite pedestals, or even if they are
used, is determined by the thickness of the plate and overall working
height desired.
Nylon slings are highly suggested when lifting
the granite. If you must use a forklift to move the granite surface
plate, always have protective padding between the metal forks and the
granite.
Once set-up, plates do not require extensive care and
maintenance. However, it is important to follow some basic rules.
Primarily, keeping the surface clean and free from buildup of dust,
dirt, grease, grime and other foreign particles will maintain accurate
tool readings and extend the life of the plate. Environment and usage
have much to do with the frequency of cleaning; however, as a rule, you
should clean a plate daily if it is used each day. Consult your
supplier for a good surface plate cleaner and if the plate is not used
for an extended period of time, it should be covered with a surface
plate cover.
Usage Because a surface plate is a
standard or “0” reference point, there are many uses for surface
plates. Whether alongside manufacturing or in final inspection, granite
surface plates are vital for quality control.When
using a height gage on the plate you must “0” that gage anywhere on the
plate with a gage block. Remove the gage block and you can check any of
your parts to see if they are flat. Large automotive manufacturers
plates to check for warpage of chassis' or crankshafts, as does the
aircraft industry etc. Usage for granite are virtually unlimited.
Do
not use granite surface plates as workbenches or lunch tables. Dropping
wrenches or hammers on plates can chip and nick the surface, and
spilling coffee or drinks on granite can cause permanent stains. Grease
from food particles will do the same. | |
Be
sure to set your work piece to be measured down gently on the plate, as
a sudden jolt or blow to the plate with a heavy metal object can chip
or nick the surface. Also, when inspecting work pieces, especially
small parts, try to used different areas of the surface plate because
using the same spot over and over, year after year, will wear a “hole”
in the plate.
When using threaded inserts in a granite surface
plate, use the minimum torque required on hold down bolts, but do not
exceed the limits noted on the caution label attached to the plate
which came with the threaded insert.
Inspection Surface
plates should be checked on a regular basis for wear using a repeat
reading gage with a manual indicator. Generally, long before a surface
plate has worn beyond specifications for overall flatness, it will show
worn or wavy spots, which will produce measurement errors. The reading
gage will readily detect these error-causing areas. Simply
set the gage on the plate and zero it at any point on the table. Move
the gage over the plate and if there are hand movements in the
indicator more than .000025” for a “AA” plate, .000050” for an “A”
plate or .000100” for a “B” grade plate then you have indication that
the plate may have some high and low spots and be out of tolerance.
Specifically, the tolerance must repeat from side to side within the
specified tolerance range in order to be an accurate plate.
| |
Also
note that actual calibration of overall flatness traceable to N.I.S.T.
can be performed periodically using an autocollimator.
When
inspection indicates that the overall accuracy of a surface plate is
out of tolerance, the plate should be relapped to restore it to its
original accuracy. Even with normal use a plate should be calibrated on
an established cycle, whether it be 1, 2 or 3 years, depending on
frequency of use.
The granite surface plate was invaluable for
ensuring flatness accuracy in the 1950's and provides the same benefit
today. Following the simple rules for maintaining your plates will
provide you with precise measurement readings in your manufacturing
process. | | | | “When
a life is on the line, every second counts. The last thing you need is
a saw blade that won’t perform, or worse yet breaks. That’s why I urge
rescue groups to use only the best, like yellow recips from Starrett.” Dwight Clark, Former President First Care | | |