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'Slight' Drag

Slight Drag, can we be more specific that that?

I got a question today: What is the authority for using "slight" drag as compared to medium, or high drag? This question came up after reading my page of instructions on how to use a screw thread truncated set plug gage to set a screw thread ring gauge.

While it is a valid question, the short answer is that it does not make any difference if the drag is light, slight or heavy. The specific amount of drag is really irrelevant. The actual amount of drag will differ from person to person depending on an individual's strength. Variations in drag will cause variations in actual screw thread ring gage size, but similar variations will result from variations in screw thread set plug tolerance. The net result is that the amount of drag between screw thread set plug gage and screw thread ring gauge need not be a defined amount, or otherwise stated, searching for a quantified amount is really a wasted effort. The basic idea is that there is some drag.

When manufacturing screw threads, the idea is to stay at the mean of the product tolerance. If you are having discussions about amount of drag, it is because it is affecting the measurement results. Basically; you are working too close to the product tolerance. What is usually the case is that there are two screw thread ring gages involved, one rejecting the part and one accepting the part. When that happens, people try to ferret out the reason. Too much effort here is also a futile.

First, look at the screw thread ring gages and assure that both are considered: in-tolerance. If that is the case, then ANSI/ASME B1.2 paragraph 2.2.1 becomes relevant. In summation it states: Because of the limits of tolerances in measurement instruments, when two calibrated screw thread gages measure a part, and one screw thread gauge fails the part but the other screw thread gage passes the part, the part is to be considered: GOOD.

If you must have two screw thread ring gauges which are considered 'identical' then there are two things that you can do to make this nearly a reality:
1. Overkill: Order them from the same screw thread gage maker, stipulating that they be made in the same lot, and set on the same screw thread set plug gauge by the same person, on the same day. You should also supply the screw thread set plug gage or purchase one with the screw thread ring gauges.
2. Best: Take existing screw thread ring gages and have them set on the same screw thread set plug gauge by the same person, on the same day.
3. Practical: If they cannot be set on the same set plug gage; then assure that both ring gages are set on Class-W master plug gages.

All that said, but still: By what authority do we use the term slight, and what does it mean?

The term 'light' drag is used in ANSI/ASME B1.2 in Appendix A to describe the feel for solid-style, or master-setting, screw thread ring gauge. The Federal Standard H28, Section 6A uses the term 'snug' fit for setting adjustable screw thread ring gages. In the screw thread gage industry, the term 'slight' drag is defined as the amount of drag which will allow a person to turn the screw thread ring gauge smoothly, but not spin it freely.

This is my personal opinion: The setting of the ring gage to the set plug gage is to be a metal-to-metal fit. The screw thread ring gage should be set on the master plug gage with "grunt". By this I mean, without using tools, having the set plug oiled, apply enough force that the person setting the gage must grab and turn forcefully to move the ring gage along the set plug. This differs from the force needed to test a part with the thread gage which should not exceed finger pressure. When testing the part the GO ring gage does not need to spin onto the part, but can be applied with some drag which can be overcome by a person using two fingers only to turn the screw thread ring gage.

I hope this discussion has helped settle whatever gauge debate you are having. If you need to discuss this further, contact Wayne at:office@gagecrib.com.

This data is provided for general information only. The intention is to provide accurate information; regardless; errors may exist in the supplied information. If accuracy is critical, base your final decisions on the data provided in the root document: ANSI/ASME B47.1; which is a copyrighted document. To purchase a copy visit an Authorized Reseller.

Original Posting: 1/18/2010
Last Revision: 3/7/2015
Error corrections in or comments about the above data can be sent to: office@gagecrib.com

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