When choosing screws for a project, it is important to choose the right size for your job. Screw diameters must match the corresponding hole sizes to ensure the screws will fit properly.
When identifying screw sizes, two key numbers are used: the major diameter and the thread pitch. Understanding these terms will help you quickly find the right screw for your project.
Major Diameter
The major diameter of a screw is its largest dimension and measures across the widest part of the thread. This measurement can be taken with a caliper or a thread micrometer. The major diameter is also the first number used in a thread designation, such as 1/2-10 Acme.
Most screw threads are not perfectly sharp, but instead truncated, with the height of a thread’s crest or root being a fraction of its pitch value. Both UTS and ISO standards codify the amount of truncation and tolerance ranges.
The minor diameter, on the other hand, is a measurement of a thread’s roots and their depths. It can be calculated as the following formula:
Minor Diameter
The minor diameter of a screw is the narrowest dimension of the thread’s root, which is not as critical as the major diameter. Its minimum requirement ensures that the sharp-V form of the thread flanks are not truncated and that the pitch surface is properly formed.
The lead displaystyle l of a screw determines its mechanical advantage: the smaller the lead, the greater the force it can exert for a given applied torque. This is not a universal rule, however: some screws work on completely different physical principles.
In general, the female threads on a fastener should have a smaller minor diameter than their male counterparts, in order to guarantee that they will fit together correctly. This is a fundamental requirement that must be specified in every standard for screw threads, and that is why it is usually stated in tables of sizes together with the major diameter. Nevertheless, the precise measurement of this is difficult, and requires a specialized tool such as a thread gauge or a caliper.
Pitch Diameter
The pitch diameter is the largest material diameter of a thread where the medium area of the thread or gear tooth feature contacts the associated screw or nut. It is not directly measurable (it is obstructed by the threads themselves) but may be tested using go/no-go gauges.
Unlike major and minor diameters, which are easily measured using common calipers, thread pitch is very difficult to measure because of the minute spacing involved. It can be figured by using one of the many available calculators online but requires considerable expertise due to the accuracy requirements.
Screw sizes are commonly listed with three figures – the gauge, threads per inch, and length either in inches or millimeters (mm). The first number is the screw gauge and is determined by the basic size of the head. This is followed by the threads per inch and then the length. This allows you to select the correct screw for your application.
Helix Angle
The helix is a smooth 3-dimensional curve that spirals around a linear axis. You can see this shape in a screw, helical gear or worm gear – all common mechanical components. When unwound, this curve forms a straight line known as the helix angle. This angle is a geometric complement to the lead angle, which references a line perpendicular to the helix.
Helix angle is an important consideration for machinists when selecting the appropriate tool for a job. A helix angle that is too high can shorten the life of a tool, while a helix angle that is too low will cause chattering and poor finish quality.
Generally speaking, higher helix angles improve chip management by enlarging the cutting area and ejecting chips at an angle. However, this can also increase the risk of deflection, which may cause the tool to bend under pressure and compromise finish quality. Therefore, machinists should balance the trade-offs when selecting the helix angle of a tool.