Screw threads fall into a few different categories: normal, square and ACME. These thread types different in shape, strength, and usage. You are probably familiar with normal screw threads like the ones below.
Fig. 1 Normal thread examples
Normal threads are used in thousands of different applications and products. You can tell from the image that the thread is triangular, thin, and sharp. Unlike normal threads, ACME and square threads are blunt and used in very specific ways.
ACME threads are trapezoidal, which means that they are not sharp like a normal thread, but the base of the thread is slightly wider than the top, unlike a square thread.
Fig. 2 ACME thread example
Fig. 3 Square thread example
ACME thread form is commonly used for applications that involve high accuracy or very large loads. They are often used for lead screws (which are screws that convert turning motion into linear motion) and in lathes and vises, but are also found in many other applications and products like jacks and aircraft flaps. The square thread form was originally used for these applications, but the trapezoidal or ACME form has mostly replaced it because it is easier to manufacture, and it is stronger than the square form.
Types of ACME Threads
ACME thread forms have a thread angle of 29°, and the similar trapezoidal metric thread form has a thread angle of 30°. Both are commonly used, but the ACME thread is the most prolific throughout the world, even in countries that use the metric system. ACME threads and gages come in two main types with the respective classes of fit.
Because ACME threads are often used for precise manufacturing machines, it is vital to routinely inspect and calibrate the threads. An Acme thread ring gage is used to check tolerances; it will feature a GO and a NO GO member. If the threads are within tolerance, the GO member should completely screw on, and the NO GO member should not. WESTport Corporation offers ACME GO/NO GO thread ring gages for external threads in classes 2G, 3G, 4G, 2C, 3C, and 4C. These top-quality gages comply with ASME B1.5-1997. Acme thread set plug gages are used to insure that the thread ring gage is calibrated properly and are compliant to ASME B1.5-1997.
Written by K. Hansen, Edited by Robert Forbes – WESTport Corporation – February 23, 2015