Hello, I’m Jim Salsbury with Mitutoyo America Corporation
and welcome to the Metrology Training Lab.
In this episode, we want to recognize and appreciate
a piece of measuring equipment that rarely get much glory
but is the foundation for much of the work done in this business,
the granite surface plate.
The surface plate is simply a big flat rock.
Designed stiff and massive to not distort,
Designed stiff and massive to not distort, nice and hard to limit wear,
and flat, so we can use it as the reference in measurement.
A typical measurement on a surface plate involves something like this height gage,
where either the part or the gage moves around on the plate.
An example calibration done on a surface plate is the calibration of micrometer standards,
shown here, where the standard stands upright on the plate,
and we do a comparison method using a gage block
and lever head electronic detector, like this Mitutoyo Mu-checker.
There is an American National Standard on granite surface plates,
called ASME B89.3.7,
which replaced an old Federal Specification
called GGG-P-463c, which was used for decades.
Both standards define the same accuracy grades
for surface plates, the highest accuracy is Grade AA,
which is often found in laboratories, Grade A, which is common in
inspection areas, and Grade B, which is more a shop floor grade.
We have developed a technical bulletin on surface plates that includes
some information on tolerances and calibration, so you can find that
in our on-demand educational resources section of our website.
There are two important FLATLINES specifications for surface plates,
and the tolerances are defined by the grade and the size of the plate.
This plate here is 30 by 48 inches, length and width,
and it's Grade AA.
So based on the size and the grade, the overall flatness tolerance
for this plate in accordance to the standard is 180 millionths of an inch.
That tolerance applies across the entire surface of the plate
and is a maximum separation form tolerance, from the highest peak
to the lowest valley, the entire surface must lie between those limits
and must be less than that flatness tolerance.
Surface plates also have a smaller localized flatness tolerance,
which applies over a much smaller area, about the footprint of a height gage.
That location variation in flatness,
is usually called the repeat reading in practice,
which is an old historic term coming from calibration practice.
For this plate, with the overall flatness of 180 millionths,
the tolerance for the repeat reading is 45 millionths.
Now it’s critical to get a surface plate calibrated for both
the overall flatness and the repeat reading, over time.
We see in practice that some commercial labs
will suggest skipping the check of overall flatness.
Be careful with this, your surface plate calibration program
needs to address both the local and overall flatness of the plate.
With regular calibration and occasional relapping,
your surface plates should last for decades.
Your primary maintenance is mostly to clean it regularly.
With such a large horizontal surface,
surface plates can collect dust and dirt quite easily.
There are a number of different cleaners on the market,
just for surface plates, but it’s a rock, there are lots of options for cleaning.
but it’s a rock, there are lots of options for cleaning.
In our calibration lab we have a clean room, so our plates don’t get too dirty.
But prior to any precision work,
we will clean the area with denatured alcohol,
which is the cleaner of choice in our lab.
Our experience has also shown that a light stoning of the plate will improve repeatability.
We like the larger ceraston that we sell at Mitutoyo, it works well for this job.
We are not trying to change the flatness, just a few passes
with the stone to remove any little microinch contaminates.
We like our gages to almost float across our plate
like the puck in air hockey.
In this episode we just wanted to give you an introduction to surface plates and
some things to consider in the calibration and maintenance of your plates.
Most people do not calibrate their own plates
so we are skipping those details, but please let us know if you would like
to see more on the calibration of surface plates
and maybe we can cover that in a future episode.
Thank you, I’m Jim Salsbury, and I’ll see you next time
from the Metrology Training Lab.