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GO does not GO and NOGO does GO

Internal Screw Thread

My GO screw thread working plug gauge does not GO but my NOGO screw thread work plug gage screws in just fine. Now in what logical universe can that happen?

Here is the situation. In setting up the screw thread cutting operation we kept increasing the size until the GO screw thread work plug gage screwed into the part. We then applied the NOGO screw thread working plug gage to the part and it easily screwed into the part. It even felt somewhat loose.

Thinking that we had done something wrong, we set-up another part. This time we monitored with both the GO and NOGO screw thread working plug gages as the size increased. The NOGO actually screwed into the part before the GO screw thread work plug gauge. So now we have a NOGO that GOs and a GO that NOGOs (say that three times fast).

Why did this happen?

The most likely reason for this situation is that the cutting is worn or damaged. The area of the cutting tool that gets the most wear is the point. It sees the most linear feet of travel. It has the most amount of heat generated. It breaks down the easiest because it is only a small point. It might chip off a corner. The corners will wear to round. The result is that the screw thread root is not cut as deeply as it should be cut.

The Pitch Diameter of the GO screw thread work plug is smaller than the NOGO Pitch Diameter. The Major Diameter of the GO screw thread work plug gage is larger than the NOGO Major Diameter. The inverse relationship between the Pitch Diameter and the Major Diameter is by design. One of the Taylor's Principals states that for GO/NOGO gages the GO gage checks all aspects of the shape, fit and function and the NOGO only checks one aspect at a time. Thus, the GO screw thread working plug gage is designed to check the maximum material condition. For a screw thread plug gage all dimensions are positioned at the maximum material condition, or otherwise stated, the small end of the tolerance zone. The NOGO screw thread work plug gage is designed only to check the pitch diameter position. Because of its intended function, the major diameter is reduced in size so that it does not engage the test product at all.

How do we fix this? The most common fix is to replace or sharpen the cutting tool. I have also heard of one person who had a CNC threading 'canned cycle' error which produced an incorrect thread form. This error was caught on visual inspection of the thread form under magnification.

More reading on the major diameter relationship between the part, the GO gage and the NOGO gage.


External Screw Thread

My GO screw thread ring gauge does not GO but my NOGO screw thread ring gage screws on just fine. Now in what logical universe can that happen?

Here is the situation. In setting up the screw thread cutting operation we kept decreasing the size until the GO screw thread ring gage screwed onto the part. We then applied the NOGO screw thread ring gage to the part and it easily screwed onto the part. It even felt somewhat loose.

Thinking that we had done something wrong, we set-up another part. This time we monitored with both the GO and NOGO screw thread ring gages as the size decreased. The NOGO screw thread ring gage actually screwed onto the part before the GO screw thread ring gauge. So now we have a NOGO that GOs and a GO that NOGOs (say that three times fast).

Why did this happen?

The most likely reason for this situation is that the cutting is worn or damaged. The area of the cutting tool that gets the most wear is the point. It sees the most linear feet of travel. It has the most amount of heat generated. It breaks down the easiest because it is only a small point. It might chip off a corner. The corners will wear to round. The result is that the screw thread root is not cut as deeply as it should be cut.

The Pitch Diameter of the GO screw thread ring gage is larger than the NOGO Pitch Diameter. The Major Diameter of the GO screw thread ring gage is smaller than the NOGO Major Diameter. The inverse relationship between the Pitch Diameter and the Major Diameter is by design. One of the Taylor's Principals states that for GO/NOGO gages the GO gage checks all aspects of the shape, fit and function and the NOGO only checks one aspect at a time. Thus, the GO screw thread ring gage is designed to check the maximum material condition. For a screw thread ring gage all dimensions are positioned at the maximum material condition, or otherwise stated, the large end of the tolerance zone. The NOGO screw thread ring gage is designed only to check the pitch diameter position. Because of its intended function, the major diameter is increased in size so that it does not engage the test product at all.

How do we fix this? The most common fix is to replace or sharpen the cutting tool. I have also heard of one person who had a CNC threading 'canned cycle' error which produced an incorrect thread form. This error was caught on visual inspection of the thread form under magnification.

More reading on the major diameter relationship between the part, the GO gage and the NOGO gage.

Originally posted: 9/10/2010
Revised:5/23/2012
If you have a pressing need for this information contact Wayne at:gageguy@gagecrib.com

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