Even a part that looks perfect at a macro level will reveal imperfections at a micro view. As long as an imperfection will still allow the part to function as designed, that given part does not necessarily have to be scrapped. We dictate how much variation we can accept by using tolerances. ISO 13920:2023—Welding – General Tolerances For Welded Constructions – Dimensions For Lengths And Angles, Shape And Position details general tolerances for linear and angular dimensions and for shape and position of welded structures to ensure product quality.
What Is Meant by General Tolerance?
Tolerance is the total amount a dimension (i.e., the numerical value that defines the size, shape, location, surface texture, or geometric characteristic of a feature) may vary. It is the difference between the upper (maximum) and lower (minimum) limits. Because it is impossible to make everything to an exact size, tolerances are used on production drawings to control the parts. For example, when a 100.00 mm metal rod is machined, all the bars are intended to be machined into the same shape. Not all of the bar’s manufacturing accuracy, however, can be machined to exactly 100.00 mm due to bars’ size and orientation. Although the design and fabrication sites have been working to reduce such deviations, they still cannot be controlled to zero. In short, tolerance is the deviation of parts in the process of processing; is the acceptable range of dimensional variation that still allows for proper function. Standard tolerances are hence critical to the manufacturing and engineering industries.
What Is ISO 13920?
ISO 13920:2023 specifies general tolerances for linear and angular dimensions and for shape and position of welded structures in four tolerance classes, based on customary workshop accuracy (ranging from fine, mean, approximate, and very approximate). Welded structures can include rail vehicles, motor vehicles, crane construction, DAR construction, steel construction, and machine construction, and shipbuilding; these structures should adhere to certain specified accuracy classes.
Instead of specifying individual tolerances, the tolerance classes according to this standard can be used. The main criterion for selection of a particular tolerance class in ISO 13920:2023 is based on the functional requirements, such as those for straightness, flatness, and parallelism. The specifications are based on the independency principle of ISO 8015, according to which the dimensional and geometrical tolerances apply independently of each other.
The general tolerances specified in this standard are applicable for weldments, welding assemblies, welded structures, etc., and are always those that are stated in the drawing. Specifically, ISO 13920:2023 covers these general tolerances:
- Tolerances for linear dimensions
- Tolerances for angular dimension
- Straightness, flatness, and parallelism tolerances
Welding Was Done Before Electricity
Welding was done before the invention of electricity. The first examples of welding found by archaeologists date back 2,000 years ago to small gold circular boxes that were made during the Bronze Age (3300 BC to 1200 BC). The ancient process of welding typically involves hammering two pieces of metal together under heat until they joined. Conventional welding as we know it today was invented in 1836 when English chemist Edmund Davy discovered acetylene gas, and, in 1885, William W Jacques invented a welding procedure called resistance welding that used electricity as its main source of heat.
Check out ANSI Z49.1-2021: Safety In Welding to learn more about the history of welding techniques.
Testing & Measuring Devices for Welding
Testing and measuring devices used in ISO 13920:2023 should be suitable and accurate for their intended purpose:
- Graduated steel straightedges
- Tape measures
- Vernier calipers (in accordance with ISO 13385-1 or ISO 13385-2)
- Dial gauges (in accordance with ISO 463)
ISO 13920:2023—Welding – General Tolerances For Welded Constructions – Dimensions For Lengths And Angles, Shape And Position is available on the ANSI Webstore.