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What is a Machine Bolt?

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  • Written By: James Back
  • Edited By: Jenn Walker
  • Last Modified Date: 19 September 2016
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    Conjecture Corporation
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A machine bolt, also called a machine screw bolt, is a cylindrical fastener used to clamp two pieces of metal together. The bolt consists of a threaded shaft with a square- or hexagon-shaped head at one end. A hand or socket wrench that fits the head of machine bolt 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 onto the protruding threads of the bolt. Proper clamping pressure is applied by tightening the nut to a specified torque. In some applications, a nut is not used, as the threads of the bolt engage a threaded hole in the second piece of metal.

Most machine bolts are manufactured from low or medium carbon steel, although bolts made from stainless steel and silicon bronze are available as well. Stainless steel and silicon bronze bolts resist corrosion much better then carbon steel ones, and are typically used in applications where they will be exposed to the elements. Aluminum and brass machine bolts are available for specialty applications in the aerospace and marine industries. Machine head bolts are available in both standard and metric sizes, and in left and right-handed threads. Coarse- and fine-thread varieties are available as well.

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The cold-forming process is the most commonly used method of manufacturing machine bolts. A cold-forming machine typically has a series of three or four dies that progressively shape a metal blank to the desired final shape and dimensions. A thread rolling machine cuts threads into the shaft of the unfinished machine bolt once the cold-forming operations are finished. The machine bolts are usually heat-treated to obtain the desired hardness and tensile strength. Finally, the finished bolts may undergo a final zinc plating or hot-dipping process as a corrosion inhibitor.

A machine bolt can be categorized into one of several grades based on tensile strength and material composition. The three most commonly used grades in the US are grades two, five and eight. Higher-grade bolts have higher minimum tensile strengths and hardness, and are generally used where stronger fasteners are required. Metric grade 8.8 and 10.9 machine head bolts are roughly equivalent to grade five and grade eight bolts, respectively. A stainless steel machine bolt is classified according to the type of steel used.

In order to obtain the maximum clamping pressure and assure a good joint, a machine bolt should always be tightened to the proper torque using a torque wrench. Torque charts are widely available online or from the bolt manufacturer for a range of bolt diameters and thread pitches. Under tightening can potentially lead to the bolt working loose, causing joint failure. Over tightening can stretch the machine bolt, potentially leading to fatigue or breakage over time.

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