Glossary of Fastener Terminology

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Dacromet: A high performance surface coating that can be applied to fasteners. The coating consists of passivated zinc flakes that are stoved onto the metal surface. The coating can be coloured and eliminates the risk of hydrogen embrittlement associated with electroplated metal. DACROMET is a registered trademark of Metal Coatings International, Inc. of Chardon Ohio

Decarburized. A fastener has a decarburized surface when the carbon content of the surface is lower than the carbon content of the core.

Decompression Point: The point at which there is zero pressure at the joint interface as a result of forces applied to the joint. If the applied force is increased beyond the decompression point, a gap will form at the interface. Analytically, a criteria of joint failure is often taken as when the applied force on the joint reaches the decompression point. This is because forces acting on the bolt(s) can dramatically increase at this point. Loading beyond this point can also result in fretting at the interface that will lead to bolt tension loss that will subsequently lower the decompression point. This process can continue until bolt failure does occur. The failure can be by fatigue or other mechanism but the underlying cause was loading of the joint beyond the decompression point. It is for this reason that it is frequently taken as a failure criteria in analysis work.

Design Form of Thread: The design form of an internal or external thread is the thread form in it's maximum metal condition. It is the same as the basic thread profile except that the thread roots are rounded. If either the internal or external thread form exceeds the design form of the thread profile then a potential interference exists.

Design Size. The design size is that size from which the limits of size are derived by the application of tolerances. When there is no allowance the design size is the same as the basic size.

Die. (1) One of a pair of hardened metal blocks for forming, impressing, or cutting out a desired shape. (2) (thread). A tool for cutting external threads. Opposite of tap.

Die Chaser. The separate cutting tools used in die heads, which actually cut the screw threads.

Die Fin. A die fin is a slight amount of excess material or flash which may be visible on the bearing surface and/or body of fasteners made by open die heading.

Direct Tension Indicators: Direct Tension Indicators (DTI's) is a term sometimes used to describe load indicating washers. Projections on the face of the washer (usually on the face abuting the bolt head or nut) that deform under loading as the bolt is tensioned. An indication of the tension in the bolt can be made by measuring the gap between the washer face and the nut or bolt head. The smaller the gap - the greater the tension in the bolt. Commonly used in civil rather than mechanical engineering applications.

Drilling. Drilling is the process of forming holes by means of specialized cutting tools called drills.

Dynamic Friction: Resistance to relative movement of two bodies that are already in motion.

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