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Calibrating Thread Plug Gages
By: Carlis Stuber
For our purposes here we are going to examine the measurements of
Outside Diameter (OD), often referred to as the Major Diameter,
and Pitch Diameter (PD). These are the two diameters most
laboratories measure when calibrating your thread plug gages.
The OD measurement is taken directly over the threads and is
therefore not a great problem in calibration. Its function is
to check the cleared diameter in the thread to be checked.
The Pitch Diameter is most accurately measured using the three wire method.
It should be noted that certain difficulties or
process procedures may result in higher uncertainties. Some
examples of practices that may result in higher uncertainties or
even inaccurate readings are: Using elastic bands to hold the three
wires, not using proper force on the wires, or using the wrong
size wires.
PROCESS:
1. Thoroughly clean the thread plug gage. If possible use an
ultrasonic cleaner to remove the tough dirt that hides in the
threads.
2. Exam the threads for nicks or material stuck in the threads.
This is especially a problem if gaging aluminum or cast iron
parts.
3. Allow the gages to "soak" or acclimate to a temperature of
68°F to 72°F. This soaking process takes about 24 hours.
OD or Major Diameter
1. Set your micrometer to the proper force. One Pound force for
threads less than 20 TPI. Two and one-half pounds for 20 or
greater TPI. This force is not required for Major Diameter
measurement, but if you set it here then you are less likely to
forget to set it when measuring PD.
2. Measure over the threads with the micrometer at a point just
past the gage notch.
3. Record reading.
4. Turn gage 90° and take another reading.
5. Repeat steps 2-4 at a point midway between gage end and gage
handle.
6. Repeat steps 2-4 at a point near the gage handle. Depending
on the length of the gage you may only be able to take two
readings vs. three readings.
PD
1. Determine the "best size" wires for the TPI gage you are
calibrating. If you have purchased thread measuring wires from a
manufacturer the size should be listed on the container. If
measuring 60° threads and the size is not listed determine best
size wires using the formula .57735 x P.
2. Using the "best size" three wires, place two wires in the
threads on one side of the gage just above the gage notch and
the third wire will be placed in the thread opposite the two
wires.
3. Using your micrometer, measure the gage over the wires. The
micrometer anvils must be over the wires.
4. This gives you the measurement over wires or "MOW".
5. Find the constant listed for the "best size wires" you are
using.
6. Subtract the Constant from the MOW.
7. Record this result as your PD.
8. Turn gage 90° and repeat steps 3-7.
9. Repeat steps 2-8 at a point midway between gage end and gage
handle.
10. Repeat steps 2-8 at a point near the gage handle. Depending
on the length of the gage you may only be able to take two
readings vs. three readings.
About the Author
Carlis Stuber is the president of IICT. He has nearly 30 years of Quality experience dealing with
automotive and agricultural manufacturing. His experience
includes Quality Management, Quality Engineering, Inspection,
Sales, Calibration and Calibration Training, Teaching and
Consulting.
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