This is going to get confusing so we will start with the part.
Coarse-Fit Metric Screw Sizes
The metric screw thread specification: ANSI/ASME B1.13M, allows, in some cases, for a thread to be called-out by using just the nominal major diameter. This is allowed when you desire a coarse-pitch and medium class-of-fit. Thus, a correct and accurate screw thread call-out on a drawing could read: M12. What is intended is M12x1.75-6H internal screw thread and M12x1.75-6g external screw thread. That may work if everyone has memorized the specification. In the real-world however, most companies do not even own a copy of the specification. At Gage Crib Worldwide Inc. we prefer to use the full nomenclature in our communications.
Part Tolerance (Class-of-Fit)
As stated above, IF the medium class-of-fit is desired for a metric screw thread size; it does not need to be stated on the part drawing. For internal screw threads the medium class-of-fit is 6H. For external screw threads the medium class-of-fit is 6g. Since our topic is Screw Thread Inserts, we are working with internal screw threads. Standard/common screw thread Inserts and designed to return the medium 6H class-of-fit.
Next we discuss the gages.
In the UN-series of STI screw thread gages the class-of-fit is marked to match the desired part class-of-fit. Thus, a 2B finished part uses a 2B-STI gage, and a 3B finished part uses a 3B-STI gage. It is nice and simple, except that Emhart Catalog states: Class 3B holes provide slightly higher and more consistent selflocking torque when Screw-Lock inserts are used.
When we get to M-series screw threads it gets complicated again. The tolerance of the screw threaded hole is tighter than the target finished hole tolerance. Thus, a 6H finished part uses a 5H-STI gage. The logic stated is: 5H-STI tap hole tolerances are smaller than 6H to allow for any deviations occurring when fitting the thread insert. So the 5H-STI class-of-fit for the common 6H finished screw threaded hole.
And if that is not confusing enough, they offer the 4H5H class-of-fit as an alternative in certain situations. Emhart catalog states: Class 4H5H holes provide slightly higher and more consistent selflocking torque when Screw-Lock inserts are used.
Special Note on 4H5H
Please note the 4H5H class-of-fit has two parts: a 4H and a 5H. All M-series screw threads have these two same parts, but when the designation is the same for both parts, the identification is given only once. What that means is that the standard 6H designation is really 6H6H. The first 6H refers to the Pitch Diameter tolerance. The second 6H refers to the minor diameter tolerance. Because screw thread plug gages do not look at the part minor diameter, the use of the minor diameter tolerance is irrelevant when considering the gage design parameters. The screw thread plug gage does not have to be marked with minor diameter tolerances. When a screw thread plug gage states a class-of-fit of 4H, it is acceptable to be used to test a 4H or a 4H5H tapped hole.
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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: A-A-59158:1998; which is a copyrighted document. To purchase a copy visit an Authorized Reseller.
Original Posting: 5/18/2020
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