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Today, the bulk of the written record about controlling threading operations relates to external threads. Unfortunately, no similar body of information exists to guide the manufacturer of an internally threaded product. How to analyze the geometric relationship between an internal thread's size, position, and form remains somewhat of a mystery.
A primary cause of this situation is the failure of traditional thread inspection practices-such as GO/NOT-GO plug gage techniques-to provide data suitable for use in process control. Plug gages can't provide data that engineers can use to analyze this geometric relationship; in fact they provide no quantitative data at all. This lack of quantitative information discourages many efforts to make internal-thread manufacturing as well understood as external thread production.
Further, the tooling and the process used to form internal threads combine to slow the development of real process control for internal threads. Why? Well, consider one real-world example. Many people believe new taps will produce similar, perfectly tapped holes. It's a simple operation, right? When threads begin to cause problems, you can change the tap and eliminate the problem.
Not so, unfortunately. In fact, tapping a hole involves a range of considerations, including:
* Tap geometry,
* Feeds and speeds,
* Coolant chemistry, cleanliness and effectiveness,
* Workholder rigidity or looseness under load,
* Workdriver characteristics,
* Material properties (hardness, presence of inclusions, "gumminess," etc.).
Interaction between some or all of these factors can change thread size, position, or form in an unpredictable fashion.
Manufacturers give products with nonconforming internal threads a special name: scrap. Unfortunately, few-if any-tooling-inspection programs can accurately predict the specific point in the manufacturing process where a tap begins to produce nonconforming threads.
Differential gaging enables manufacturers to end this dearth of quantitative internal thread data. An economical and cost-effective method of measuring and inspecting internal product screw threads, differential gaging is one of the most helpful tools manufacturers possess for controlling the tooling and processes used to produce internal threads.
Industry needs to examine the basic concepts involved with process control of internal screw threads, and also to address and eliminate the widespread confusion relating to simple screw-thread technology. A good place to begin is with an approach to process control that takes advantage of modern techniques. Differential...