Correct way to check minor diameter on external threads

[Larry1958]

Im hoping someone can shed some light on an issue Im having. Im trying to do research on the proper way to check the minor diameter on a metric series M26 x 1 4h/4h 4 start thread. Im using thread tech software for the specs and get "Max flat form minor diameter @24.917, Max flat width at minor Diameter @.25, Min. flat width at minor diameter @.144, Min rounded form minor diameter (ref) @24.917. The issue is how to check max minor and min. minor (or the correct way to check them). Hoping there are some Quality Control people here that could shed some light. The issue is that my minor Diameter the way we are checking it is undersize. We are checking the Bottom of the Radius at the root. I am using a standard 1.0mm full profile ISO insert to machine the thread and pitch, major diameters are fine. Thanks in Advance.

[PinkertonD]

Hi Larry,

Not an expert on QC and the like, but checking the root radius of a thread is of no real value. The load bearing portion of the thread will define the degree of actual fit when engaged with the other part (nut/bolt) of the thread.

Do some research on thread measuring wires (three wire), go/no-go gauges and also comparators. There is a great deal of theory involved in these measurements and not something that can be answered with a simple "do this." It is very dependent on how much accuracy and consistency that you require. You may also want to chase down some NIST information on the topic.

Depending on your requirements, the go/no-go gauges may suffice. Certainly the most simple.

[zeke]

Forgive my ignorance, but why would anybody need this data since all my experience indicates that the threads will never engage there.
Why wouldn't standard wire methods be sufficient to assure the fit?

[Kelly_Bramble]

First, I'm assuming that you’re talking about an external thread as an internal thread minor diameter can be measured with a gauge pin.

Second, verify that you actually need to physically verify the minor diameter.

Typically, thread go and no-go gauges are used to check the thread pitch and MMC condition of the major (external thread) and minor (internal thread) diameters. The thread gauge check is adequate in most applications.

When the thread root is of sufficient size, one can verify with a CMM if the stylus is smaller in diameter then the thread root - just measure many minor diameter locations preferably as far apart as reasonable.

Assuming that there is not a special widget sold for specifically checking external thread minor diameters… let’s invent.

There is a well know method of checking the pitch diameter using three gauge pins.

One could use three gauge pins as my images shows, measure using calipers then…

Using gauge pins, find the largest diameter that can slide between the larger gauge pins and the thread root. Using simple math, subtract and one has the minor diameter at that thread location. Measure many or some locations as required.

minor-thread-measure.gif

[Kelly_Bramble]

Another method is if one has an optical comparator with sufficient magnification capabilities…. Simply support the external threaded feature, measure the minor diameter…

[Kelly_Bramble]

Here's another way to measure the minor diameter on an external thread.

First, measure the major diameter at some location. Then, using a "Pit Depth Gage" of sufficient depth, measure from the major diameter to the thread root. Measure both sides, and substract.

Measure many locations...