Tolerances are basically how accurate a part is to its drawings. If a part is not accurate then it won’t fit with other parts that have been accurately made. However if a part is made with too accurate of tolerances then it can become too expensive to make.

What Tolerance Actually Means

A tolerance is stated as a ‘+/-’ value. So, for example, a 10mm diameter hole with a tolerance of +0.2mm / -0.1mm could be anywhere between 9.8mm and 10.2mm. This would still be within the specified tolerance and therefore accepted on inspection.

Cutting Tolerances

Laser and plasma cutting: Typical tolerance for cutting can vary between ±0.1mm and ±0.5mm depending on the thickness of the material.

Bending Tolerances

When looking at forming a bend in a sheet of metal, you will find that there is more variation in this process than with other processes. The springback of the metal after it has been formed is the main cause of this, and is why a press brake bend is generally quoted to within ±0.5mm to ±1mm.

Hole Position and Flatness

Hole position tolerance – typically ±0.25mm from a datum reference. Flatness tolerance is very dependent on how the cut parts are subsequently handled. Thin materials are easily warped during the laser cutting process.

Thickness Tolerances

Raw material thickness also has a natural tolerance specified in BS EN ISO dimensional tolerances for form and dimension.

Sheet Metal Fabrication is covered in more detail at https://dkmsheetmetal.co.uk/.

When Tighter Tolerances Cost More

Standard tolerances are often sufficient for simple components like brackets, cases and general mechanical parts. However, tighter tolerances can cost a lot of extra time, as well as increased cost for inspection and possible rework.

Knowing which of the tolerances apply to your project is important for your project’s quality and for preventing unnecessary costs.