Other Dimensions: The Phantom Feature

April 29, 2009
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I stand in awe of the lowly screw thread because of the engineering that goes into it. A cursory look at a typical screw thread standard will show you what I mean. Despite their innocent appearance, screw threads are difficult to measure, which causes the ongoing battles between those who make them and those who use them. This is because of their most critical, unseen feature-the pitch diameter.

Pitch diameter is an imaginary cylinder approximately halfway between the major and minor diameters of the thread form. I say approximately because there are truncations and root relief applied to these features, meaning that from a functional point of view, pitch diameter won’t be located exactly halfway. So we’re trying to measure a diameter that can’t be seen but is physically defined by the sloping surfaces of the thread flanks, which may or may not be as accurate as they should be.

To make matters more interesting, the flanks are advancing in a helical path. This means that little or nothing is straight or directly accessible with the usual measuring tools when it comes to finding and measuring the pitch diameter.

When there is a phantom feature to be measured there will be a lot of gadgets that claim to make it easy, but what they really do is enable you to get a “reading” easily. Getting an accurate measurement is another story. Here’s a brief look at two popular methods.

Thread Micrometers.
Often referred to as pitch mikes, even that name doesn’t tell you what they measure. Because the conical spindle contacts over the thread flanks rather than at the pitch line, thread micrometers, or pitch mikes, do not give a correct reading of pitch diameter. And they don’t measure the pitch of the thread either.

I have seen situations in which readings of what people consider pitch diameter are off the mark by several thousandths of an inch.

Thread micrometers are great for setting up a machine or monitoring production, but not for qualifying a product. They perform best when used as a comparator set to a thread gage of the same size and pitch of the product being measured.

Micrometer and Thread Wires.
Thread wires are made to specific sizes so they’ll contact the thread at the pitch line and protrude above the major diameter, where they can be contacted using a micrometer. This is the method usually found in standards covering threads and ensures that pitch diameter is measured as a single element of the thread.

This method works well unless you are dealing with very coarse pitch threads. In these situations the micrometer anvils may not span the two wires on the one side of the thread.

One item in the U.S. standards for pitch diameter measurement that these methods tend to ignore is measuring force, and that can be substantial. The typical handheld micrometer cannot provide the force needed. This must be accounted for, particularly when your measurements indicate that the pitch diameter is close to product limits.

You must always remember that a measurement of pitch diameter only partially qualifies the thread. Thread form, linear pitch, and major and minor diameters also have to be measured.

Most threads are checked using fixed limit or go/no go gages for speed and economy. But when the gages reject a product or two different sets of gages give different answers, everyone reaches for the measuring instruments to sort things out. And the first feature they measure, or try to measure, is the phantom-pitch diameter.

Things can get messy because the typical instrument will be measuring a single feature, such as pitch diameter, while the fixed limit gages are a check incorporating all elements of the thread combined. If there are variations in the pitch of the thread or helical path, their effects are magnified from a functional point of view, which is what the fixed limit gages inspect. I won’t completely ruin your day by bringing up roundness problems at this time.

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