Buy Machine Screws Online 2: Thread Coarseness

Machine Screw Thread Coarseness

Machine screws are also judged by the coarseness of their thread, which dictates the potential uses of a machine screw. Finer threads offer a much firmer grip than coarser threads, and they are less likely to work themselves lose under the force of vibration. Coarser threads are much easier to engage, requiring fewer turns of the screwdriver, and they are far less likely to cross-thread, an important factor if you plan to use power tools to engage the screw.

Under the American Standard system, the thread density is measured as threads per inch, a figure that can be checked with a thread gauge, a useful device for measuring thread pitch. To simplify things, the American Standard designates codes to machine screws, nuts, and bolts; coarse (UNC); fine (UNF), and extra fine (UNEF). UNC screws have 24 threads per inch; UNF screws have 32 threads per inch. These useful designations make it much easier to buy machine screws online.

The metric system is different and measures the thread pitch according to the distance between threads, in millimeters.

Machine Screw Length

The final measurement is the length of the machine screw, and this is measured from the base of the head to the tip of the bolt, in inches or millimeters. Inch measurements can be in fractions of an inch or the decimal equivalent, such as ½” or 0.5”
Machine Screw Size Codes

For American Standard machine screws, the dimensions are usually given in the form:

#8-20 x .75

This machine screw is 8 Gauge diameter, has 20 threads per inch, and is three quarters of an inch long.

¼-UNC x ½

The external thread diameter of this screw is 1/4 “, it has a UNC coarse thread, of 24 threads per inch, and it is ½” in length.

For Metric Machine Screws

M3-0.4 x 12

This screw has an external thread diameter of 3mm, there is a distance of 0.4 millimeters between threads, and it is 12mm long.

Machine Screw Thread Classes

Occasionally, you will see the thread class of the screw or nut, which allows you to determine how tightly the nut and screw tighten. The most common thread class for machine screws is 2A – the ‘2’ shows that it is a medium fit, with the much rarer ‘1’ indicating a very tight fit, and ‘3’ indicating a looser mating. The ‘A’ shows that this is an external thread measurement – a ‘B’ is the internal thread measurement, and is reserved for nuts, inserts, and tapped holes.