The buttress thread shape is intended to withstand excessive axial thrust in one direction. This is often a 7-degree angle on the weight-bearing surface and a 45-degree angle on the trailing flank, resulting in a shape with high shear strength. The buttress thread is used on both smooth bore and rough bore holes.
Buttress threads are intended to handle exceptionally high axial force in one direction by utilizing a load-bearing thread face that is perpendicular to the screw axis. Buttress threads can prevent degradation by employing split nuts in addition to their low friction and high shear strength. Buttress threads also employ flutes or lands between each thread which project radially outward from the shaft of the screw.
Buttress threads provide efficient use of space while still allowing two pieces of equipment with buttress threads to be fastened together. This type of thread is often used on screws that hold panels or plates together.
In addition to screws, bolts with buttress heads are also available. These bolts have a head with three distinct parts: a domed center piece, a cylindrical skirt surrounding the dome, and six equally spaced apart slotted holes around the outer edge of the dome. The domed center piece provides a large contact area for combining with the threaded hole of a panel or plate, while the slotted holes allow water to escape if the bolt is submerged.
Bolts are usually made of steel but titanium bolts are available as well. They are used where weight is not an issue and where corrosion resistance is important such as in outdoor applications like bridges and buildings.
Buttress head bolts can also be found on woodworking joints such as biscuit joints and dado joints.
Threads used to reinforce equipment. The load-bearing thread face is perpendicular to the screw axis or slanted slightly (usually no greater than 7 degrees). The other face is slanted, usually at a 45-degree angle. The resultant thread form has the same low friction qualities as a square thread form, but with almost double the shear strength due to the longer thread base. This type of thread is used primarily for reinforcement rather than attachment.
The term "reinforcement thread" may also be applied to threads that have a lower lead than the surrounding material by 0.5 mm or more but which are still attached to a bolt. These threads are commonly found on metal furniture and fittings and on wood if the wood is being used instead as a bearing surface because they provide better resistance to wear. Reinforcement threads do not need to be continuous; sometimes they are made up of several segments with different lead angles as long as the overall effect is to increase the area of contact between the thread and its mating hole or surface.
Reinforcing threads were originally developed for use with steel bolts, but they can also be found on brass and bronze screws. Brass has two main uses for reinforcing threads: first, to attach metals that would otherwise react with each other (such as iron and aluminum) by covering their common interface with a layer of plastic that prevents any chemical reaction between them; second, to attach wood to metal fixtures, where the plastic coating on the brass screw protects the wood from corrosion.
Despite being one of the strongest symmetric thread profiles, the asymmetric buttress thread profile may withstand higher weights when loaded in only one direction, such as vises. The trapezoidal metric thread form is identical to the Acme thread form, with the exception that the thread angle is 30 degrees. DIN 103 is the standard that codifies it. In addition, DIN 103 specifies a maximum allowable depth of thread per unit length-called the "cutting depth"-which allows for some variance in manufacturing accuracy.
Asymmetric threads were originally developed for use on small parts where weight was not a concern. As weight limits have been increased over time, so has the strength of the material used in these parts. Today, asymmetric threads are used on heavy machinery where the cost of replacement parts is high. These threads can be found on milling machines, rock drills, and other heavy equipment.
Buttress threads were first developed around 1870 by Charles Conklin of New York City who called his design "Endless Thread." Buttress threads consist of two parallel helical threads with an intermediate flat section between them. This shape prevents the threads from turning against each other which might cause them to lock up. Instead, they slide past each other smoothly without binding.
Conventional threads first appeared around 1872 and are still widely used today. They are symmetrical: there is an equal number of threads on the inside and outside of the shaft. This design is easy to make and inexpensive to produce.