Hello, I’m Jim Salsbury with Mitutoyo America Corporation and welcome to
the metrology training lab in this episode we're going to talk about the
calibration of an outside micrometer in past episodes we've defined the concept
of calibration discuss the purpose of calibration and introduced important
aspects of what makes a good calibration method in this episode we'll apply all
of those ideas to the calibration of an outside micrometer calibration is a
search for airs and we want to develop a calibration method that efficiently
hunts down and finds any potential errors in the measuring instruments so how do
we do that best for a micrometer? well there's a nice American standard called
ASME B89.1.13, and the calibration method that
we'll be discussing is based on that standard calibration usually starts with
checking out the proper operation of the micrometer is there any obvious damage
to it does it move smoothly across its measuring range do you feel any play in
it do the buttons work correctly does the clamp work once you are satisfied
with the operation then we begin the accuracy tests here at minatoya we
recommend using a micrometer stands
micrometer is a precision measuring instruments and the stand helps
eliminate the transfer of your body heat to the tool the stand also makes the
measurement a bit easier which may help improve your accuracy the first step is
to clean the measuring faces we'll put a little bit of solvent on a
cloth and we'll pinch the cloth between the measuring faces like this right then
we'll bring the measuring faces together to set our zero for all of our
measurements will want to properly use the micrometer this one includes a
ratchet mechanism to control the force we want to apply a nice smooth
consistent force and listen for several clicks of the ratchets the zero is so
important we'll want to check that a few times
the most important skill and calibrating micrometer x' is simply using a
micrometer correctly if you are having some struggles getting good results find
yourself a mic that is known to be good grab some gauge blocks and practice it
doesn't take too long to get the right feel once we have our zero sets
we'll check the micrometer at the first test points we recommend the use of
gauge blocks there really isn't anything better to use and you need something
pretty accurate so gauge blocks make a good reference standard for calibrating
micrometers the ASME standard recommends five specific size gauge blocks for
calibration in inches these sizes are the 210 block for twenty six oh five
eight 15 and one inch the reason for these specific sizes is that they will
force the micrometer thimble to be in different orientations for each test
points remember that the goal of a good calibration method is to have an
efficient test and the use of these five test points will look for airs across
the measuring range simultaneously with the rotation of the
thimble these five blocks are the most efficient tests but if you don't have
these special sizes you can choose to use others that achieve the same goal
the principle is what is important not these specific sizes if you do use
different sizes such as from a standard set of gauge blocks you will likely find
yourself needing a few extra test points you can use any style or material gauge
blocks for this calibration I'm using rectangular ceramic blocks we like
ceramic blocks for calibrating mics due to their thermal properties ceramic
blocks thermally expand about the same as steel but they have a much lower
thermal conductivity which means your body heat won't affect them as much as
steel blocks and since you have to handle the blocks during calibration
this is a useful advantage we also recommend the use of grade zero
gauge blocks grade zero blocks are sufficiently accurate for micrometer
calibration so you can just use the value that's on the block which makes
the calibration more efficient so let's clean our gaze blocks and take our first
test point using a two ten block glaze box should be properly calibrate across
the measuring faces so it doesn't matter where you measure on the blocks some
people like to measure near the ends
others in the middle it shouldn't matter
again we want to use them a crumber properly and control the forest as best
as possible make sure the measuring faces are contacting the grades blocks
without trapping any air or dust between them a proper measurement should not be
too tight or too loose but the gauge block should feel like it is in full
content it should allow for some slight smooth movements like this it's common
to take a few readings with the goal to ensure that you have a consistent value
this is typical practice when using a micrometer it is not correct however to
take a bunch of readings and report the average during the calibration
micrometer should repeat within the specification limits for the length
accuracy and if not then you should reject the micrometer the reported error
is the measured value minus the value of the gauge block to be more efficient it
is often useful to not record the entire measured value but just the observed air
this digital micrometer in inches as a 50 millionth resolution and so the
readings will be something like zero plus 50 as in this case or minus 50
millions in metric the errors will be something like zero one or minus one
microns now if you don't understand millions for microns go watch the
episode of the metrology training lab that covers units of measure I'm using
this datasheet that we use in some of our calibration classes and it's just
for training purposes you can use any sort of form that you are I'll write
down the entire value in inches and the air and millions for this digital
micrometer the tolerance is plus or minus 50 millions or plus or minus one
micron in metric which is typical for a digital micrometer
I'll now continue along with the other readings until I complete all four
positions
the length tests are now complete but that is not the only important test for
micrometers the flatness and parallelism of the measuring faces also needs to be
checked historically the flatness and parallelism was checked using an optical
parallel like this now this method is a bit cumbersome and requires some special
skills in reading optical fringe patterns so instead of checking the
micrometer in this manner the ASME be 89 113 standard presents a much simpler
method the reference standard that we'll use instead of the optical flat in
parallel is a small sphere like this ball gauge here you can also use a
stylus from a coordinate measuring machine if you'd like so we're going to
start by measuring in the middle of the measuring faces and then set our zero
there
all right now we're going to move the ball across the measuring faces up and
down backwards forwards across the measuring faces any variation in the
size is due to the flatness or parallelism of those two measuring faces
then we'll report the range of those results the maximum minus the minimum is
reported as what we'll call the parallelism in this case I saw no range
and so the reading would be zero now this method is generally not as accurate
as using the optical parallel but it's easy and efficient to do when a
calibration method is not easy to do and it is not the primary test we often see
labs skip units so we recommend using the ball because anybody can do it it's
a much better than not doing anything at all we've also heard that some labs will
use the optical parallel or optical flats if the test with the ball fails
because you can get a better idea of why it's failing that seems like a pretty
good idea but we'll save using the optical parallel or flat for future
episodes of the metrology training lab so that completes this micrometer
calibration it looks like this micrometer is in tolerance so that was
the calibration of a 0 to 1 inch micrometer now in a 0 to 1 inch
micrometer we can bring the measuring faces together to set the zero
what about calibrating larger micrometers a one to two inch micrometer
2 or 3 inch micrometer or up nineteen to twenty inch micrometer
well they're calibrated in a similar manner but you need to use a reference
standard so let's cover that larger micrometers generally come with
micrometer standards such as this these standards are useful for the shop floor
setting of the zero by the user but they are not recommended for the calibration
of the micrometer instead you should use a gauge block in this case this one inch
gauge block I'll use for this one to two inch micrometer now I'll set my zero
using this gauge block
and then I'll proceed using the same five gauge blocks that I used before for
the one to two inch mic this time however I have the added complication of
needing to ring each of these five blocks to the one inch block it's very
important to ring each of these blocks one at a time to the one inch reference
thereby eliminating any air in this one inch block from affecting the readings
this is particularly important as the micrometers get much larger let me wring
these blocks together and then we'll take a reading to demonstrate the method
you would then continue with the other four blocks to complete the test
for the parallelism of a larger micrometer the benign one thirteen
standard recommends a test similar to the small ball but instead uses the edge
of a gauge block my zero is already set with this gauge block now what I'm going
to do is move the gaze block up down in and out to look for variation and I'm
going to measure right at the edges like that and like that
and then down and then up this time I see a minus fifty millions reading so
the range of all my numbers would be fifty millions and that would be in
tolerance and that's the calibration of outside micrometers remember the method
we just discussed comes from the American national standard on
micrometers asme dat nine point one point thirteen if you don't like the
method you're always welcome to come attend the next ASME B89 standards
meeting to discuss it further thank you I'm Jim Salsbury and we'll see you next
time from the metrology training lab