Hello, I'm Jim Salsbury with Mitutoyo America Corporation
and welcome to the Metrology Training Lab.
In this episode, we're going to revisit the calibration of calipers
and look at a specific issue with dial calipers.
In a past episode, we thoroughly covered the calibration of calipers,
following the guidelines from the American National Standard ASME B89.1.14.
In that episode, we focused on the more popular
digital caliper, and we missed discussing an issue
that is important to consider for dial calipers.
So first, let's do a very brief review
and summary of the general calibration method for calipers.
I'm going to first use a digital caliper, and I'm not going to try to be
particularly precise in this example, so please forgive me for that.
For the outside jaws, we take a series of measurements over the measuring range,
1 inch, 2 inch, 4 inch, 6 inches
and we also vary where we are across the jaws.
Then we do an inside measurements, 1 inch like this.
Then for the depth bar, we do a measurement like that.
This caliper has a step function, so we do a measurement like that.
And then to make sure that we don’t have any problems with the small inside
knife edges, we measure a two hundred thousandths ring gage
like that.
I'm going to now repeat some of those same measurements,
this time using a dial caliper.
I want you to look closely at the angular position
of the pointer in the dial during these measurements.
So I'm going to repeat the same
outside measurements as I did with the digital caliper,
so at 1 inch and notice that the dial is at the 12 o'clock position.
I go to the 2 inch,
once again the 12 o'clock position.
I go to 4 inches again, 12 o'clock position.
I go to 6 inches again, 12 o'clock position.
Inside measurements, once again, the same position.
I could show you that for the depth and step and for the ring gage,
all of those, the pointer would be at the 12 o'clock position on the caliper.
Now dial calipers use little gears inside, and the gears can wear.
It’s possible for there to be errors within a rotation of the dial
that you won't see if the calibration method results in the pointer
always being at the 12 o'clock position like you just saw.
For this reason, the ASME B89.1.14 standard requires some test points
that put the pointer in additional
angular positions, so they're not always at 12 o'clock position.
So for an example,
I have two gage blocks that are wrung together here.
So this is a 0.125 gage block and this is a 0.150 gage block.
These are standard blocks from any set of gage blocks, so most people have these.
For this style of caliper, this inch size caliper,
with the 0.125 block, if I was going to measure that,
I should be at the 3 o’clock position you see here.
If I measure the 0.150 block,
I should be at the 6 o’clock position like you see here.
And if I measure both blocks that are wrung together,
I should be at the 9 o’clock position.
So these two gage blocks
wrung together, give me three additional easy test points,
not too difficult to test the accuracy of the caliper
with the pointer in different angular positions.
the ASME B89.1.14 standard
does not prescribe specific test points,
so you have lots of flexibility here,
but we want to show you at least one option, one way
that does the job, and it's pretty easy to implement.
That's everything for this episode.
Thank you, I'm Jim Salsbury, and I'll see you next time
from the Metrology Training Lab.