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The metric thread system and the choice of safe fastening
Bolts, screws and nuts are one of the most common assembly methods in metal fabrication precisely because a threaded joint can be taken apart again — unlike a welded or soldered joint. Understanding the thread system and choosing the right clamping element is therefore as much part of the craft as being able to weld a seam.
In Denmark and most of the world the metric thread system is used often denoted by the letter M as defined in the international standard ISO 68-1. The system describes the thread's geometry based on two main sizes: the external diameter and the pitch i.e. the distance between two adjacent threads measured along the thread's axis.
A thread designation like M10 tells you there is a metric thread with an outer diameter of 10 millimetres. The ISO 965 standard sets the tolerances for the different thread pitches and specifies what clearance should be chosen between bolt and nut. If a drawing does not specify a specific tolerance class ISO 6g is typically used for bolt thread and 6H for nut thread as standard.
A bolted joint's strength depends not only on the thread's dimension but also on how much it is tightened — the preload. Too little preload can loosen the joint under load, while too much can overload the bolt material or the overall structure. Many designs therefore specify a particular tightening torque to be observed with a torque wrench, rather than tightening 'by feel'.
Vibrations, temperature fluctuations and repeated stress can over time cause a threaded connection to loosen. For exposed connections, there are therefore a number of safety methods — lock nuts with built-in friction lock, spring washers, thread sealant and in some cases split pins or lock plates. The choice depends on how critical the connection is and what loads it experiences during operation.