The ruler

The ruler is a simple measuring tool, relatively inexpensive and extremely common across many trades. It is found both in professional workshops and in the toolkits of amateurs, as it is practical and versatile for taking quick and reasonably precise measurements.
Rulers come in various lengths, generally between twenty centimetres and one metre, although shorter or longer versions can sometimes be found depending on the specific needs of certain trades. Its slimness and ease of transport make it a tool that can easily be carried on the person, in a pocket or on a workbench. In our practice, the ruler is the main measuring instrument, used daily to record dimensions.
Despite its apparent simplicity, the ruler is actually a more complex tool than it seems. Its characteristics can vary considerably depending on its intended use and the level of precision required. The material, often stainless steel but sometimes brass or aluminium, its rigidity, the thickness of the strip, the clarity of the graduations, and the quality of the finish all directly influence the reliability of the measurements obtained.

Different types of measurements
Rulers are most often graduated in millimetres, the base unit used in the metric system for precision work. This allows fine and detailed reading of dimensions, particularly for very small parts or elements requiring precise adjustment.
However, some models use other units, such as American inches, to meet the requirements of specific markets. Rulers with dual graduations, combining millimetres and inches, can sometimes be found.

Units of measurement used
In our practice, we use exclusively rulers graduated in millimetres. This choice ensures consistency throughout our work and prevents conversion errors when taking measurements or manufacturing parts.
In addition to millimetres, rulers very often feature graduations in centimetres, marked every ten millimetres. These markers allow for quicker reading and make it easy to estimate larger lengths. Some models also offer half-millimetre subdivisions, providing greater precision for the most detailed work. These different scales make it possible to adapt the reading according to the required level of precision and the nature of the work being carried out.

Problems encountered with rulers
Several difficulties can arise when using a ruler, often linked to the quality of manufacture, wear, or deformation of the tool.

Manufacturing quality
The quality of manufacture is a crucial factor. Generally, two main categories of rulers are distinguished. On one side are those with graduations simply printed or painted on the surface. On the other side are those with markings engraved into the metal and then filled with paint.
The first type tends to wear quickly. Repeated friction against metal, wood or other surfaces causes the numbers and lines to gradually fade, making reading difficult, even impossible, and considerably reducing the tool’s lifespan. The second type, though more expensive, is much more durable. Engraved graduations retain their clarity over time and ensure better reliability when measuring.

Flatness
The flatness of the ruler is another fundamental criterion. To measure a flat surface correctly, the ruler itself must be perfectly straight. A slightly curved, twisted or deformed ruler, even if almost imperceptible to the naked eye, can distort the reading and lead to errors of several tenths of a millimetre or more. This defect becomes particularly problematic when working on mechanical parts, precise assemblies, or polished surfaces, where the slightest variation can have a significant impact on the final result.

The zero point
Finally, the zero point of the ruler is often an unreliable area. In practice, it is common to start a measurement from the ten-millimetre mark rather than from the exact edge of the ruler. This habit offers several advantages. It makes calculations easier, as it is generally simpler to subtract ten than seven or eight millimetres. It also avoids using the edge of the ruler, which is frequently damaged, imperfectly cut, or slightly worn from repeated knocks.