The ACME thread is a specific shape of the threads used on machines like milling machines and bench vises. The ACME threads has the specific shape of a trapezoid with a 29 degree angle for the flanks of the trapezoid. This shape was developed as a way to find a middle ground between the high strength of square threads and the tendency of V threads to slip.
Because the threads have a 29 degree angle they are able to handle heavy loads of force while remaining easy to cut with a lathe. The geometry of the ACME thread is important in that the geometry of the threads determine the distribution of the force that is placed upon them. The ACME thread has flat crests and roots which allow even distribution of the forces across the entire thread.
ACME Threads: What They Are and How They Work
Furthermore, each of the flanks of the threads is at an angle of 14.5 degrees from the vertical, which again allow even distribution of the forces upon the nut. These factors lead to the development of the ACME thread as a type of lead screw that dont require the movement of the lead screw to maintain it’s position. To understand the lead of ACME threads, it is first important to understand the difference between the pitch and the lead of the screw.
The pitch is the distance between the individual threads, but the lead is the distance that the screw move in relation to one full revolution of the screw. The lead of the screw change according to the number of starts that the screw has. For instence, a single start screw will move a distance of one pitch per revolution.
A double start screw will move a distance of two pitches per revolution. A triple start screw will move a distance of three pitches per revolution. Thus, the higher the number of starts, the more greater the distance moved per revolution but the less likely the screw is to remain self-locking.
There are several version of the ACME thread. For instance, there are standard, stub, and centralizing ACME threads. Standard ACME threads have dimensions as specified in reference tables.
The stub ACME thread is similar in that it uses the same 29 degree angle for the flanks, but the depth of the threads is 40% shallower then the depth of an ACME thread. The shallower threads allow the screw to remain intact from bending due to the thicker central core of the screw. Finally, the centralizing ACME thread is a modification of the standard ACME thread where both flanks of the screw is in contact with the nut at the same time.
Reference tables can help to determine the correct specifications for the ACME thread that needs to be use in a particular task. These tables will indicate the minor and pitch diameters for different sizes of screws. The minor and pitch diameter of the thread indicate the amount of material left over after the screw is threaded.
These tables can be used to easily replace an ACME screw. While the efficiency of an ACME thread is less than that of a ball screw, the lower efficiency of the ACME thread can actualy be an advantage for some applications. A well-lubricated ACME screw may have an efficiency of 40-45%.
Ball screws can have an efficiency of 90% or higher. However, ball screws requires a brake or motor to maintain there position. Thus, the lower efficiency of the ACME thread means that the ACME thread is less likely to move without power being supply to the screw, increasing its safety for various applications.
ACME threads require some maintenance. For example, many ACME threads has steel screws and bronze nuts. The bronze nuts wear down faster than the steel screws, so only the bronze nut need to be replaced.
Furthermore, it is necessary to add oil to the ACME thread each day to prevent the threads from experiencing galling. Without regular lubrication the metal components of the ACME thread can experience galling and damage the threads. To cut an ACME thread with a manual lathe, it is necessary to use a cutting tool with a 29 degree angle.
Tools with other angles, such as 60 degrees, will not create the flat crest and flat root that are necessary for an ACME thread. The compound rest can be set to 14.5 degrees so that the tool only cut one flank of the screw at a time. After the cutting is complete, a three-wire measurement or a gage can be used to ensure that the pitch diameter of the cut screw is accurate.
Finally, mathematical calculations can be performed to ensure that the dimensions of the screw are correct. For instance, the pitch can be calculated by dividing 1 by the number of threads per inch. Furthermore, the lead can be calculated by multiplying the pitch of the screw by the number of starts.
