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Screw Thread Major/Minor Diameters

Screw Thread Major and Minor Diameters - parts vs. gages

The minor diameter of a screw thread ring gage is easily measurable as is the major diameter of a screw thread work plug gage. People frequently check these diameters and then they have questions as to apparent discrepancies in the gages compared to the part sizes. The three questions raised are:

1. Given that on a screw thread ring gage the NOGO pitch diameter is smaller than the GO pitch diameter, why is the NOGO minor diameter larger than the GO minor diameter?

2. Given that on a screw thread plug gage the NOGO pitch diameter is larger than the GO pitch diameter, why is the NOGO major diameter of a screw thread work plug gage smaller than the GO minor diameter?

3. Given that the screw thread pitch diameters are the same as the screw thread gage pitch diameters, why are the screw thread gage minor and major diameters so much different than the product screw thread minor and major diameters?

To totally discuss the major and minor diameters of thread gages, and the related inspection issues, let me state the following:

1. The screw thread gage major and minor diameters are different than the part major and minor diameters for specific reasons based on one of Taylor’s Principles: The GO should gauge all features simultaneously and the NOGO should gage each feature independently.

2. The GO ring gage major diameter is made as large as possible and is tested to assure that it clears a specific diameter. By design, the GO ring gage major diameter has no measurement function. The part thread major diameter is to be measured via alternate measurement methods.

3. The GO ring gage minor diameter is set at the maximum material condition. This tests the part to assure that the minor diameter has not grown to a point that it would interfere with the mating part.

4. The GO plug gage major diameter is set at the maximum material condition. This tests the part to assure that the major diameter has not shrunk to a point that it would interfere with the mating part.

5. The GO plug gage minor diameter is made as small as possible and is tested to assure that it clears a specific diameter. By design, the GO plug gage minor diameter has no measurement function. The part thread minor diameter is to be measured via alternate measurement methods.

6. The NOGO ring gage major diameter is made as large as possible and is tested to assure that it clears a specific diameter. By design, the NOGO ring gage major diameter has no measurement function.

7. The NOGO ring gage minor diameter is set at or just below the gage pitch diameter. This is to allow the NOGO ring gage to properly measure the pitch diameter and exclude data from the minor diameter. Once the minor diameter passes the GO gage it is considered good.

8. The NOGO plug gage major diameter is set at or just above the gage pitch diameter. This is to allow the NOGO plug gage to properly measure the pitch diameter and exclude data from the major diameter. Once the major diameter passes the GO gage it is considered good.

9. The NOGO plug gage minor diameter is made as small as possible and is tested to assure that it clears a specific diameter. By design, the NOGO plug gage minor diameter has no measurement function.

COMMENTS
The data provided is accurate to the best of my knowledge. Please use it at your own risk. COMMENTS:
Original Posted: 4/8/2008
Last Revised: 10/147/2014
Refer comments/corrections to gageguy@gagecrib.com.

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