A machine bolt is a cylindrical fastener used to secure two pieces of metal together. It consists of a threaded shaft with a square or hex head and is tightened using a socket or hand wrench. Machine bolts are made from low or medium carbon steel, stainless steel, silicon bronze, aluminum, or brass. They are manufactured using the cold forming process and are heat treated for desired hardness and tensile strength. Machine bolts are classified into grades based on their tensile strength and material composition. To ensure a good joint, they should be tightened to the correct torque using a torque wrench.
A machine bolt, also called a machine screw bolt, is a cylindrical fastener used to secure two pieces of metal together. The bolt consists of a threaded shaft with a square or hex head at one end. A socket or hand wrench that fits into the bolt head of the machine is used to tighten or loosen the fastener during use. The shaft of the bolt is inserted through holes drilled in both pieces of metal and a nut is threaded over the protruding threads of the bolt. The correct clamping pressure is applied by tightening the nut to a specified torque. In some applications, a nut is not used, as the bolt threads engage a threaded hole in the second piece of metal.
Most machine bolts are made from low or medium carbon steel, although stainless steel and silicon bronze bolts are also available. Stainless steel and silicon bronze bolts resist corrosion much better than carbon steel bolts and are generally used in applications where they will be exposed to the elements. Aluminum and brass machine bolts are available for special applications in the aerospace and marine industries. Machine head bolts are available in both standard and metric sizes and in left-hand and right-hand threads. Coarse and fine thread varieties are also available.
The cold forming process is the most commonly used method for manufacturing machine bolts. A cold forming machine typically has a series of three or four dies that progressively form a metal blank into the final desired shape and size. A thread rolling machine cuts threads into the unfinished machine bolt shaft once the cold forming operations are complete. Machine bolts are usually heat treated to achieve the desired hardness and tensile strength. Finally, the finished bolts may undergo a final galvanizing or hot dip process as a corrosion inhibitor.
A machine bolt can be classified into one of several grades based on tensile strength and material composition. The three most commonly used grades in the United States are grades two, five, and eight. Higher quality bolts have higher minimum tensile strengths and hardness and are generally used where stronger fasteners are required. Grade 8.8 and 10.9 metric head bolts are approximately equivalent to grade five and grade eight bolts, respectively. A stainless steel machine bolt is classified according to the type of steel used.
To achieve maximum clamping pressure and ensure a good joint, a machine bolt should always be tightened to the correct torque using a torque wrench. Torque charts are widely available online or from your bolt manufacturer for a range of bolt diameters and thread pitches. Undertightening can potentially lead to the bolt coming loose, causing the joint to fail. Overtightening can stretch the machine bolt, potentially causing fatigue or breakage over time.
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