Most types of spring are manufactured by plastic deformation of the material. This could possibly be compared with forging, but definitely not with machining. During plastic forming, the material springs back to a certain extent and residual stresses are created. The plastic deformation and the spring back depends both of the accuracy of the tools and there positions and the elastic and plastic properties of the material. Today the accuracy of the tools and there positions are high and therefore depends the geometric tolerances most evenness of the material. The material properties can vary to a greater or lesser extent between material heats, material manufacturing batches and even material coils. The consequence is that tolerances below the capability of the material and process lead to extra costs.
For spring tolerances, we normally comply with the DIN standard for springs since this is the standard that best reflects real life conditions. We regularly use also other standards depending on customer demands.
There are a big number of different types and designs for springs therefore we do not give any tolerance examples here. Lesjöfors follow the material and process capability regularly with statistical methods and we provide knowledge and information about type of distributions, probabilities, Cpk values and other statistics for the factors/measurements in spring specifications.