What is the proper technique? - Inside dial caliper

QC Dude · Oct 17, 2012

Hey guys...FNG metrologist here.

Got a new instrument and I am trying to verify calibration of a SPI 0-1" inside dial caliper. I haven't used these before and I want to make sure that I am using a proper technique for measurement. I don't see any tutorials online. Can anyone suggest best way to use this variable tool? Would be much appreciated!

-Rich

The real Leigh · Oct 17, 2012

The trick to any inside gage is ensuring that your measurement is of the minimum dimension of the feature.
It's very easy to get the measurement axis skewed such that it's reading a distance greater than the dimension you wish to measure.

The proper way to check the instrument calibration is with a ring gage, which would cost more than the instrument.

- Leigh

QC Dude · Oct 17, 2012

Thanks Leigh, we have the ring gages. My concern was finding a consistent manner to position the gage itself. (Didn't know if there was a certain way to hold the gage...as in keeping the longer arm parallel to the surrounding surface...like someone told me.)

Knowing that I should look for the minimum dimension is a help. Thanks!

-Rich

The real Leigh · Oct 17, 2012

Hi Rich,

There are no hard and fast guidelines since it depends on the workpiece being measured.

The basic rule is to move the gage around to find the minimum dimension, and use that value.

As an example, if you envision measuring the distance between two parallel flat surfaces, it's obvious
that aligning the axis of measurement perpendicular to the faces will yield a minimum.

The two faces of a micrometer, spread to an appropriate size and locked in place, will give a good practice jig.
You know in advance what the proper value is.

- Leigh

The real Leigh · Oct 17, 2012

Gordon B. Clarke said:
I've often wondered why these types of internal gauges don't have ground (short) cylinders instead of balls as this would make alignment easier.

You would have the flat end of the cylinder bearing against a concave radius, so you'd be measuring from the extreme points of contact rather than the center.

- Leigh

LKeithR · Oct 17, 2012

The real Leigh said:
You would have the flat end of the cylinder bearing against a concave radius, so you'd be measuring from the extreme points of contact rather than the center.

- Leigh

I think Gordon's idea is to place the centreline of the "cylinder" so it's parallel to the centreline of the bore being measured...

The real Leigh · Oct 17, 2012

LKeithR said:
I think Gordon's idea is to place the centreline of the "cylinder" so it's parallel to the centreline of the bore being measured...

That would work for measuring the minor diameter of the thread, but for nothing else.

- Leigh

The real Leigh · Oct 17, 2012

But this thread is about threads.

- Leigh

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