"Allowable deviation" and "tolerance" are two terms with different meanings. In daily production and work, there are often terms that confuse the two terms. If the user disputes the size discrepancy, it is often said that the thickness of a product is "tolerance" out of tolerance, and some production technicians will say that when the steel plate is delivered according to the theoretical total, it is rolled with "negative tolerance" and so on. The following is a brief introduction and description of the meaning and use of these two terms. The size of steel products is the common concern of manufacturers and users. The size of products that users want to buy is the size of the order, but in fact the products processed according to a certain production process will always deviate from the order size. That is, the positive and negative deviations. Of course, users always hope that the smaller the deviation, the better, that is, the higher the accuracy, the better. In actual production, the higher the dimensional accuracy control, the greater the difficulty of production. In order to ensure the interests of both parties, in the product standards There is a prescribed range for the deviation of the size, that is, the "allowable deviation" of the size.
Take the Baosteel enterprise standard Q / BQB401-1999 cold-rolled steel plate and steel strip size, shape, total amount and allowable deviation in thickness as an example: If the user wants to order thickness 1.00mm, width <1200mm, according to ordinary accuracy For the supplied steel strip, the allowable deviation is ± 0.07mm, that is, the steel strip with a thickness in the range of 0.93 to 1.07mm is in line with the order thickness requirements, that is, steel with a positive deviation of 0.07mm and a negative deviation of 0.07mm The belt is qualified. The tolerance is 0.14mm. Of course, the higher the dimensional accuracy, the smaller the allowable positive and negative deviations, and the smaller the tolerance value. For the definition, please refer to the relevant information.
When the steel plate is supplied, if the weight is calculated based on the theoretical weight, the manufacturer will set the target rolling thickness to be slightly less than the order thickness, and the rolling precision control is stricter (because the standard value cannot be exceeded). If the ordering thickness of the steel plate is 1mm, the target rolling thickness is set to 0.97mm, and the allowable deviation is ± 0.04mm, so that the rolled steel plate meets the standard requirements (also meets User requirements), and can save materials. This kind of rolling is called "negative partial rolling" instead of "negative tolerance rolling". Remember! The deviation is positive or negative. The "size deviation" only has "positive deviation deviation" or "negative deviation deviation", while the tolerance is not positive or negative, it is an absolute value.
tolerance
The allowable variation of the actual parameter value. Parameters include not only geometric parameters in mechanical processing, but also parameters in disciplines such as physics, chemistry, and electricity. So tolerance is a concept with a wide range of uses. For mechanical manufacturing, the purpose of formulating tolerances is to determine the geometric parameters of the product and make its variation within a certain range, so as to meet the requirements of interchange or cooperation.
The tolerances of geometric parameters include dimensional tolerance, shape tolerance, position tolerance, etc.
①Dimensional tolerance. Refers to the allowable size variation, which is equal to the absolute value of the algebraic difference between the maximum limit size and the minimum limit size.
②Shape tolerance. Refers to the total amount of variation allowed for the shape of a single actual element, including 6 items of straightness, flatness, roundness, cylindricity, line profile and area profile.
③Position tolerance. Refers to the total amount of variation allowed by the position of the associated actual element from the reference, which limits the mutual positional relationship between two or more points, lines, and areas of the part, including parallelism, perpendicularity, inclination, and coaxiality , Symmetry, position, round beating and full beating 8 items. Tolerances represent the manufacturing accuracy requirements of parts and reflect the difficulty of processing.
Tolerance levels are divided into IT01, IT0, IT1, ..., IT18, a total of 20 levels, the levels decrease in sequence, and the tolerance values increase in sequence. IT stands for international tolerance. The basic principle for the selection of the tolerance level or the tolerance value is: the comprehensive economic effect of the manufacturing cost and use value of the machine parts should be the best, generally IT5 ~ IT13 for the matching size, IT2 ~ IT5 for the special precision parts, and IT12 for the non-matching size ~ IT18, IT8 ~ IT14 for raw materials.
