What is unfilled tolerance for sheet metal parts?
Unfilled tolerance generally refers to the tolerances that need not be marked on the drawing. It is also called “unfilled tolerance and free tolerance” in China.
Common standards for tolerance of sheet metal parts
The general tolerances are divided into four tolerance classes: precision f, medium m, rough c, and coarsest v. There are many companies that use GB/T 1804-2000, and some companies use GB/T 15055-2007. Who is wrong and who is right? GB/T 1804-2000 is the tolerance of linear and angular dimensions of general tolerance and unfilled tolerance. This standard is applicable to metal cutting dimensions, and is also applicable to the processing dimensions of general sheet metal parts, or the dimensions can be referred to non-metallic materials and other processing methods. GB/T 15055-2007 specifies the limit deviation of the linear dimension of the blanking part, the linear dimension of the forming part, the linear dimension of the punched corner radius, the linear dimension of the forming fillet radius, the punching angle dimension and the bending angle dimension tolerance. Comparing the two standards, it can be found that the tolerances of GB/T 15055-2007 are relaxed in each size section with the same basic dimensions. In fact, the sheet metal parts that can reach the f-class of the two standards are already precision grade. Personally, it is recommended to use GB/T 1804-2000 for the matching requirements between sheet metal parts and machined parts, and GB/T 15055-2007 for simple sheet metal parts.
From the actual point of view, CNC punching, CNC cutting, laser cutting, water cutting and other blanking methods can now reach the f (precision) level, and most companies generally indicate in the technical requirements of the drawings that the unfilled tolerances are based on the above two. One of the standards is m (medium) level implementation, so for the above modern CNC cutting equipment, it is no longer necessary to study which standard is adopted. Although GB/T 15055-2007 also specifies the limit deviation values for different tolerance grades for different material thicknesses (three thickness intervals of t≤1mm, 1mm<t≤4mm, t>4mm), this is true for that traditional ordinary blanking equipment still has practical value. For the bending process of sheet metal parts, CNC automatic bending machines (such as Salvagnini and TRUMPF) can also reach the precision level. The dimensions are not guaranteed when the ordinary CNC bending machine forms the sheet metal parts of larger sizes (such as larger than 1500mm), which is caused by manual positioning and large size deformation during the bending process.