The Acme screw thread is the original design of the trapezoidal thread form, and is the most common of this form encountered all over the world. Trapezoidal threading is characterized, as the name suggests, by its trapezoidal outlines. The Acme thread form is identified by its 29 degree thread angle, with the height being half of the distance from the crest of one thread to another (in technical terms, its pitch), and the tops and crests being flat planes.
Trapezoidal thread designs are almost always used as the form for power screws (also known as lead screws), for which its purpose of transmitting energy into translating rotary motions into linear ones requires higher strength and durability. The high strength is afforded by the wide, blunt base of the Acme shape's tooth, which can handle greater loads and allow the use of split nuts. Split nuts are nuts split into two lengthwise in order to move along the screw without having it turn to reduce wearing out the nut.
Due to the constant sliding motion and contact between the parts with power screws, the Acme design was created chiefly for machinery and tools which required controlled movements, however this means that the controlled movements create higher degrees of friction. It is convenient then that the shape of its outlines are easier to cut and make than square or point threading.
The thread still undergoes quite a lot of trauma during its application, as evidenced by its fast wearing properties due to the friction, and therefore requires arguably closer inspection than the other screw types.
Inspection must be done by making use of Acme Thread Gauges, which are used to inspect both external and internal threaded products. The two types of gauges one may choose from are ring gauges or plug gauges.
The plug gauge comes in the style of Go Gauge and No Go Gauge and is used to gauge internal threading of the product. You begin by inserting the Go gauge into the hole. If the gauge does not fit into the hole, it is below its low limit and needs to be made bigger and will be indicated as below minus one on the gauge notch. Likewise, to check for the hole's high limit, insert the No Go gauge. If the gauge is able to enter the hole, it is too big and will indicate a figure higher than plus one.
Acme Thread Set Plug Gauges, unlike general purpose plug gauges, are not used to gauge internal threading, but are instead only to be used for calibrating the American Gauge Design (AGD) which is adjustable by screws within the body.
The thread ring gauges, unlike the plug gauges, are used to inspect the external threading of a product. The gauge is used by screwing the ring tightly around the external thread of the object with the use of your hands until it can go no further and is securely in contact with the end of the thread. The tolerance of the thread is then gauged by a similar indication of plus or minus one on the gauging face.