Screw threads are among the most widely used machine elements. The effective (pitch) diameter of a screw thread is one of the most critical parameters governing thread fit, assembly performance, load distribution, and interchangeability. Accurate measurement of this parameter is therefore essential for quality control and compliance with international standards. Current measurement techniques, including the three-wire and two-wire methods, provide reliable results but require specialized accessories, careful setup, skilled operators, and significant measurement time. These limitations reduce productivity, particularly in high-volume manufacturing environments and shop-floor inspection applications.

This project introduces a novel method and portable device for rapid and accurate measurement of screw thread pitch diameter. The proposed approach simplifies the measurement process while maintaining the accuracy required for industrial inspection.

WORKING PRINCIPLE:

As we know, the effective diameter of a screw thread is defined as the diameter of an imaginary co-axial hollow cylinder which intersects the flanks of the screw thread such that, at the point of intersection the width of thread and the thread spaces are equal, each being half of the thread pitch.

Now, the measuring instrument should be constructed in such a way that with the given half pitch, it should be able to measure the distance between one point on the pitch circle (effective diameter circle) and another point on the major diameter circle, both lying on a common line intersecting the axis of the screw thread, so that the value of effective diameter can be calculated mathematically.

CONSTRUCTION:

The experimental setup consists of five main components:
I. Thread pitch gauge
II. Disc micrometer
III. Slip gauges
IV. Clamping magnets
V. Screw thread for measurement

The pitch of the given screw thread is determined using the thread pitch gauge and the pitch value is divided by two to give the value of half thread pitch.

The clamping magnets are placed on the fixed anvil of the disc micrometer and the required slip gauge (with thickness equal to the value of the half thread pitch) is clamped in between the pair of magnets. The given screw thread is placed between the rotating spindle and the slip gauge such that the slip gauge rests in the depression (valley) of the helical groove on the screw thread, touching its flanks at two points.

Now, as the slip gauge having value equal to the half pitch was chosen, the two points on the flanks of the screw thread that the slip gauge would remain in contact with, must lie on the effective diameter circle of the given screw thread. This reading is noted down as (X) and would be obtained by subtracting the value obtained when only the slip gauge would be placed in between the micrometer spindles, without the given screw thread.

The major diameter of the screw thread would be determined by placing the given screw thread in between the spindles of the micrometer and this value would be noted down as (MD).

The Effective diameter (ED) can now be found using equation derived in illustrations.