The rapid and accurate measurement of internally threaded holes is one of today’s more difficult measurement problems. QUEST Integrated, Inc., has built an electro-optic based internal/external thread measurement system. It measures internal thread sizes from 9 to 150 mm (3/8 to 6 inches) and may be inserted up to 75 mm (3 inches) deep. This is the first system capable of measuring the simple pitch diameters of internal thread forms. It also measures lead, flank angles, major, and minor diametersThe non-contact method of the QC-30 Thread Measurement System enables in-process inspection by eliminating touch labor. It can measure blind holes, master gages, and production parts. The system works by calibrating to a master gage. It then scans other parts and fits a 3D thread model to the data. Inspection results are presented both numerically and graphically.The most commonly used methods for inspecting threaded parts are based on mechanical contacting gages. The simplest of these is the go/no-go gauge, a ring or plug that checks for assembly. Go/no-go gauges wear quickly and must be set for both the size and the tolerance of each threaded fastener. They have limited repeatability since they depend on the operator’s judgment of the snugness of fit.There are a variety of other indicating gauges all of which rely on multiple point contacts at different locations on the threaded surface. Results from measuring the same parameter on gauges of different types or from different manufacturers may not be consistent. Relatively consistent results can be obtained if the parts to be measured are near their nominal tolerance. However, if there is any form error in manufacturing the part, consistent results are difficult to obtain (Veale et al., 1994). There has been no technology to accurately and traceably measure flank angle and lead directly (Greenslade, 1991).Figure 1 illustrates some of the parameters of internal thread forms. One of the key parameters is pitch diameter, which is defined as the diameter of the pitch cylinder, “an imaginary cylinder of such diameter and location of its axis that its surface would pass through a straight thread in such a manner as to make the widths of the thread ridge and the groove equal and, therefore, is located equidistantly between the sharp major and minor cylinders of a given thread form” (ANSI B1.7M-1984). It is clear that mechanical gages cannot measure the simple pitch diameter, but must produce a compound measurement that depends on pitch diameter, lead and flank angle.The inspection of threaded parts using conventional surface- contacting gauges is costly, time-consuming, and the results can be subject to operator interpretation. A special tool is usually required for each thread parameter and size to be inspected. The tools require periodic certified calibration. There is a wide variety of “standard” thread forms. Any of the diameters may be tapered, the leading and trailing flanks may be asymmetric (i.e., buttress threads) or nonplanar, and the lead may be variable.