The thread cannot pass the ring gauge freely, should it be accepted?

The thread cannot pass the ring gauge freely, should it be accepted?

Many people wonder: How to evaluate the fasteners that cannot enter the Go thread ring gauge freely when test if fastener can pass the ring gauge
 
In addition, the word "freedom" is very subjective and can have various meanings to different people. These problems usually occur in threaded products that are electroplated or coated, or have a slight bump to the thread.

Fortunately, there are some international standards that have reached agreement on this issue. These standards are:

ISO 6157-3
DIN 267, part 19
DIN EN ISO 26157-3
 
In addition to these three international standards, the same standards were also adopted in the WA-990 standard issued by Ford Motor Company in 1993.
 
These standards give the same formula for the acceptable calculated torque value. The formula for deriving the maximum allowable Go thread ring gauge torque value is as follows:

These formulas can be applied to threads of any size.

The following chart contains the estimated torque acceptance values for the most commonly used inch and metric thread sizes. Use a calibrated torque wrench to test these parts with a "basic size" (3A for imperial parts and 6h for metric parts) of the Go threaded ring gauge.

If the torque reading produced by the test is lower than the allowable value in the chart above, these parts are considered acceptable.

Many special coatings and coatings that provide high corrosion resistance are usually unevenly applied and slightly thicker than common coatings such as electrogalvanized. Many of these platings and coatings are relatively soft, and even though the parts with these surface treatments may not freely enter the Go threaded ring gauge, they will pass and require a small amount of torque.
 
ISO 965/2 stipulates: For coated threads, unless otherwise specified, tolerances apply to parts before coating. After coating, the actual thread profile must not exceed the maximum material limit of position H or H at any point, respectively.

This means that after plating, the Go size (maximum material limit) is allowed to "grow" to the tolerance position h.

In other words, use 6h Go thread ring gauge after plating.


The threads on the metric bolts starting from M10 and the inch bolts starting from 3/8 inch are more prone to bumps during the process of thread rolling, heat treatment and surface treatment.
 
This is because of the batch processing technology that is usually used, and multiple flips between parts often occur. Even though the slightest notch thread is perfectly functional, some cannot freely enter a Go thread ring gauge.

The torque acceptance test using this Go ring provides a practical and objective method for fastener suppliers and users to evaluate bumped threads and uneven plating applications to determine their acceptability. It also eliminates the subjective factor from the inspection because a maximum value is given.
 
There is some concern: If a fastener requires the maximum torque acceptance value, this may adversely affect the clamping force generated when the bolt is tightened in the application. When these accepted torque values are compared with the recommended tightening torque values, it immediately becomes clear how trivial these values are.

To illustrate this point, for example, M10-10.9 grade hexagon flange bolts:

The recommended tightening torque of the M10-10.9 grade hexagon flange bolt is 55Nm. If the torque required to enter the 6h Go thread ring gauge does not exceed 1Nm, the bolt torque acceptance test is accepted. The accepted torque is 1Nm, which is less than the recommended seat in this example 2% of the value.
 
Considering that under standard assembly conditions, due to all the variables in the assembly process, the clamping force generated in the joint will change by ±25%, this allowable torque value is completely irrelevant.
 
The method of torque evaluation for threads that cannot enter the Go thread ring gauge freely is both practical and reasonable.

Fastener suppliers and users should understand and adopt these established international standards.

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