Thread Gauge Calibration

Thread gauge calibration

Thread gauge is a fairly common measuring tool. The purpose is to check the details of the diameter of the threaded hole and the threaded screw head when processing has reached the standard within the allowable limit or not. The technician will use a thread gauge device to check. It is very simple because the principle of the standard thread gauge device includes 2 heads Go – No Go for thread plug gauge or Go Ring – No Go Ring for thread ring gauge.

Before learning about thread gauge calibration, we need to know what thread plug gauge and thread ring gauge are.

I. Classification

1. Thread plug gauge

To check whether the threaded hole processing product meets the standards or not? The technician only needs to insert the Go and No Go heads of the device into the parts with threaded holes. First, the Go head of the device gets inside to the last edge of the threaded hole of the part. Next, the technician will take the No-Go head of the device and screw it back into the details with threaded holes. If the No-Go device can only enter a little, the threaded hole work piece is qualified. The remaining cases are rejected because they do not meet the standards of the thread.

Thread plug gauge device.

2. Thread ring gauge

To check whether the threaded screw head processing product meets the standards or not? The technique only needs to use the thread ring gauge device to check through 2 holes Go and No Go Ring. First, the Go threaded hole of the device will be screwed through the threaded screw head of the components. Next, the technique will use No Go Ring to screw on the threaded screw head. If the No Go Ring thread gauge can only enter a little, the work piece is qualified. The remaining cases are rejected because they do not meet the standards of the thread.

Thread ring gauge device.

These are some of the applications and uses of thread gauges. In terms of specifications, equipment is usually processed according to certain standards such as: ANSI/ASME standards (UNC, UNF, UNEF, UNS…); JIS standards (GRIRI, GPWP and STUD GRIR,…); ISO standards (GPNP, GRNR,…)

3. Is thread gauge calibration necessary?

The answer is yes. If you don’t want your products’ threaded holes or threaded screw heads to be invalid size and unusable.

Calibration is used to check the regularity of the thread ring gauge or the thread plug gauge. The devices used for a long time will be worn out, resulting in size deviations, inaccuracies, difficult to see with the naked eye. So you need to take them periodically to calibrate to check the accuracy, ensure the production quality for your products.

4. Thread plug gauge calibration process

4.1. Standard and supporting device

Gauge block set grade 0, 1, 2 with size similar or higher than Unit Under Test (UUT), accuracy better than 4 times accuracy of UUT.
Dimensional Length Measuring System (DMS), super micrometer or equivalent.
3 wires set supplied with DMS, or equivalent.
Alcohol, lint-free cloth, gloves.

4.2. Environment conditions

Temperature: (20 ± 1) ºC.
Relative Humidity: (50 ± 20) %RH.

Note: environment conditions above apply for Techmaster in-house laboratories. Outside this bound conditions will be considered with customer for uncertainty and measurement results.

4.3. Preparation

Ensure that each UUT threaded segment is clean, free from burrs, indentations, scratches, nicks, and free from any obvious defects that would impair its use.
Clean all measuring anvils of DMS, 3 wires set, gauge block set, UUT.
Place UUT, gauge block set, 3 wires set near the DMS at least 2hours before calibration.

4.4. Perform

4.4.1 Major diameter

When 2 anvils of DMS are fully contact, set zero.
Base on UUT unit inch or mm, change the same measuring unit for DMS.
If UUT has 2 side GO & NOGO, perform for GO side first, then repeat for NOGO.
Reference reading for DMS by gauge block with size within ±1 inch (or 25mm) of UUT size.
Using DMS measure major diameter of UUT with set up as below:
For UUT with threaded segments that are up to and including 0.50 inch length: Measure and record the UUT Major Diameter near the center (2) of the thread segment length at 0 and 90° for a total of two measurements.
For UUT with threaded segments that are larger than 0.50 inch and up to and including 1.0 inch in length: Measure and record the UUT Major Diameter approximately halfway between the center (2) and each end of the threaded segment length at 0 and 90° for a total of four measurements.
For UUT with threaded segments that are larger than 1.0 inch in length: Measure and record the UUT Major Diameter near the center (2) of the thread segment length and approximately halfway between the center and each end (1&3) of the threaded segment length at 0 and 90° for a total of six measurements.

4.4.2 Pitch diameter

Select suitable 3 wire set for UUT thread pitch as table below:

_ Best wire size = 0.57735 / TPI

_ Minimum wire size = 0.505182 / TPI

_ Maximum wire size = 1.010363 / TPI

_ Number of threads per one inch

Insert 1 wire at one side and 2 wire at opposite side of UUT threads.
Using DMS measure the MOW of UUT with set up as below.
Measuring positions are the same with major diameter measurement.
Then, calculate pitch diameter from MOW by equation:

PD = MOW + TPI – 3W (inch)

_ PD: Pitch Diameter, inch

_ MOW: Measuring Over Wires, inch

_ TPI: Number of threads per one inch

_ W: Diameter of wire, inch

Or PD = MOW +0.86603*P – 3W (mm)

_ PD: Pitch Diameter, mm

_ MOW: Measuring Over Wires, mm

_ P: Distance between 2 continuous pitch, mm

_ W: Diameter of wire, mm

4.5. Tolerance

Customer required tolerances are to be used for all calibrations unless otherwise noted.
If the customer does not supply instrument tolerances then manufacturing specifications or other approved sources are to be used as the default tolerance.
If not, tolerance shall be follow to JIS B thread gauge, ISO 965 or ANSI/ASME B1.1, B1.2…
If all Specification Standard above is not complied with UUT, then applied non-judgement results.

5. Thread ring gauge calibration process

5.1. Standard and supporting device

Reference ring gauges size up to 150mm, accuracy 1.5μm or better.
Dimensional Length Measuring System (DMS) or equivalent.
Coordinate Measuring Machine (CMM) or Inside micrometer sets, range up to 150mm or equivalent.  T-sphere all sizes, supplied as DMS accessories.
Alcohol, lint-free cloth, gloves.

5.2. Environment conditions

Temperature: (20 ± 1) ºC.
Relative Humidity: (50 ± 20) %RH.

Note: environment conditions above apply for Techmaster in-house laboratories. Outside this bound conditions will be considered with customer for uncertainty and measurement results.

5.3. Preparation

Ensure that each UUT threaded segment is clean, free from burrs, indentations, scratches, nicks, and free from any obvious defects that would impair its use.
Clean all reference ring gauges, UUTs.
Place UUT, ref. ring gauge near the DMS at least 2hours before calibration.
Technician must be familiar with using of DMS, T-sphere and other accessories.
Extremely careful while handle with T-sphere.
Wear gloves during calibration process.

5.4. Perform

5.4.1 Minor diameter

Minor (inside) diameter of UUT can be measured by DMS, CMM or inside micrometer.
Base on UUT unit inch or mm, change/convert the same measuring unit for DMS, CMM or micrometer.
Reference reading for DMS/CMM/micrometer by ring gauge size within ±1inch (or 25mm) of UUT size.
Using DMS/micrometer measure minor diameter of UUT at 3 difference locations.
Rotate UUT 90° and repeat measure for a total of 6 measurements.

5.4.2 Pitch diameter

Select suitable T-sphere size and attached on measuring arm of DMS.
Select suitable ring gauge size and reference the DMS reading, ring size ±1 inch (or 25mm) of UUT size.
Take out ref. ring gauge and replaced by UUT, clamp it on measuring table.
Using DMS measure at 3 positions on segment threaded of UUT as below:
Position 1 at right side, then move to position 2 at left side with lower ½ thread pitch height, then move again to position 3 at right side with lower 1 thread pitch.
At each position, measurement can be done with suitable measuring force for vertical and horizontal axis in the green indication:
Click “Next” button and record final pitch diameter on DMS.
Rotate UUT for 90o and repeat measurement.
Calibration now completed, un-clamped, take UUT out, turn of DMS and secure all equipment.

5.5. Tolerance

Customer required tolerances are to be used for all calibrations unless otherwise noted.
If the customer does not supply instrument tolerances then manufacturing specifications or other approved sources are to be used as the default tolerance.
If not, tolerance shall be follow to JIS B thread gauge, ISO 965 or ANSI/ASME B1.1, B1.2…

Nguyen Anh