# Chapter-11| Basics of Screw thread Metrology

According to ASTME (The American Society of Tool and Manufacturing Engineers) defines a screw thread as screw thread is the helical ridge produced by forming a continuous helical groove of a uniform section on the external or internal surface of a cylinder or cone.

We need to measure inter-related geometric aspects such as pitch diameter, lead, helix, and flank angle, among others

In contrast to measurements of geometric features such as length and diameter, screw thread measurement is more complex

Flank angle

It is the angle formed between a flank of the thread and the perpendicular to the axis of the thread that passes through the vertex of the fundamental triangle.

Pitch

It is the distance between two corresponding points on adjacent threads, measured parallel to the axis of the thread.

It is the axial distance moved by the screw when the crew is given one complete revolution about its axis

It is the angle made by the helix of the thread at the pitch line with the plane perpendicular to the axis.

Helix angle

It is the angle made by the helix of the thread at the pitch line with the axis. This angle is measured in an axial plane.

Major diameter

In case of external threads, the major diameter is the diameter of the major cylinder (imaginary), which is coaxial with the screw and touches the crests of an external thread.

For internal threads, it is the diameter of the cylinder that touches the root of the threads.

Minor diameter

In case of external threads, the minor diameter is the diameter of the minor cylinder (imaginary), which is coaxial with the screw and touches the roots of an external thread. For internal threads, it is the diameter of the cylinder that touches the crests of the threads. It is also called the root diameter.

The effective diameter or pitch diameter

It is the diameter of the pitch cylinder, which is coaxial with the axis of the screw and intersects the flanks of the threads in such a way as to make the widths of threads and the widths of spaces between them equal. In general, each of the screw threads is specified by an effective diameter as it decides the quality of fit between the screw and a nut.

In case of a single-start thread, the lead is equal to the pitch. Therefore, the axial distance moved by the screw equals the pitch of the thread.

In a multiple-start thread, the lead is an integral multiple of the pitch. Accordingly, a double start will move by an amount equal to two pitch lengths for one complete revolution of the screw.

## Measurement of Screw Thread Element

In the following sections, we’ll discuss the methods for measuring:

• Major diameter, • Minor diameter, • Effective diameter • Pitch • Angle • Form of threads.

### Measurement of Major Diameter

The simplest way of measuring a major diameter is to measure it using a screw thread micrometer

• However, for a more precise measurement, it is recommended to use a bench micrometer

• A major advantage of a bench micrometer is that a fiducial indicator is a part of the measuring system.

• It is thus possible to apply a pressure already decided upon by referring to the fiducial indicator.

• The machine is essentially used as a comparator.

### Measurement of Minor Diameter

• The best way of measuring a minor diameter is to measure it using a floating carriage micrometer

• Minor diameter is measured by a comparative process, wherein small Vpieces that make contact at the root of the threads are used.

• The selection of V-pieces should be such that the included angle of a V-piece is less than the angle of the thread.

### Measurement of Effective Diameter

Thread measurement by wire method is a simple and popular way of measuring an effective diameter.

• There are three methods of using wires:

1. One-wire method

2. Two-wire method

3. The Diameter of best-size wire

## Classification of thread gauges based on their forms:

(a) Plug screw gauges (b) Ring screw gauges

• It may be remembered that according to Taylor’s principle, a GO gauge should check both size and geometric features, and thus be of full form. • On the other hand, a NOT GO gauge should check only one dimension at a time.

Plug Screw Gauges • A straight thread plug gauge assures the accuracy of the basic elements of a screw thread, namely major diameter, pitch diameter, root clearance, lead, and flank angle.

• A taper thread plug gauge checks, in addition to these elements, taper of the screw as well as length from the front end to the gauge notch.

Ring Screw Gauges • Ring gauges are used to check external thread forms like bolts.

• Similar to plug gauges, a system of limit gauges is provided by the full-form GO gauge and NOT GO gauge to check the effective diameter.