In this video we take a closer look at the standard bore and standard shaft fit system. It starts with bores with the basic dimension A, which has the largest positive dimensions. In alphabetical order, the dimensions then become smaller and smaller. Up to and including the basic dimension H, the tolerance fields are always in the positive range. The tolerance fields with the basic dimensions J and K vary around the zero line and thus have a positive upper dimension and a negative lower dimension. From the basic dimension with the letter M, the tolerance fields are completely in the negative range. In fact, specific numerical values ​​are hidden behind these letters. The basic dimension is generally defined as the dimension that is closest to the zero line. For the tolerance fields A to H of bores, this corresponds to the lower dimension. For the tolerance fields K to Z, the upper dimension is the basic dimension. The nominal dimension J is a special case. The basic dimensions for shafts are essentially structured in the same way as for bores. In comparison to the positions of the tolerance fields of the holes, the tolerance fields of the shafts can essentially be imagined as mirrored on the zero line. With the exception of the tolerance fields from the letter j, the basic dimensions of holes and shafts are in principle relatively symmetrical to the zero line. Drilling with a twist drill is usually not sufficient for holes to produce basic tolerance levels smaller than IT10. The hole is therefore reamed to the desired size with a reamer. The dimensional accuracy is then checked with a limit plug gauge. Limit plug gauges are designed for a nominal size and a tolerance class. They have a go-side, the diameter of which is manufactured very precisely to the minimum size, and a reject side, which is manufactured accordingly to the maximum size. The reject side is usually provided with a red plastic ring or a red marking. The go-side is used to check whether the hole is below the minimum size. To do this, the go-side must fit into the hole without great pressure. This ensures that the hole does not fall below the minimum size. The reject side is used to check whether the hole exceeds the maximum size. The reject side must not fit into the hole. This ensures that the hole does not exceed the maximum dimension. Fits for shafts can usually be produced using normal lathes and turning tools. An outside micrometer can be used to check dimensional accuracy. Similar to limit plug gauges, there are limit snap gauges for shafts. They also have a go side and a reject side. In this case, however, the reject side is manufactured to the minimum dimension and the go side to the maximum dimension. The go side is therefore used to check whether the shaft is below the minimum dimension. To do this, an attempt is made to move the shaft through the reject side without exerting too much force. If this were the case, the shaft would obviously be smaller than the minimum dimension. In this case, the shaft could not be reworked and would therefore be reject. The shaft must therefore not fit through the reject side, because only then is the shaft larger than the minimum dimension. The go side is then used to check whether the shaft does not exceed the maximum dimension. To do this, the shaft must fit through the go side without exerting too much force. Only then does the shaft not exceed the maximum dimension. If the shaft were larger than the maximum dimension, the shaft would not fit through the go side. In this case, however, the shaft can still be reworked to correct the dimension. For reasons of economy, it is sensible not to allow all basic dimension combinations, but to decide on a basic dimension that is as uniform as possible and to use this as a priority. For bores, this basic dimension, which should preferably be used uniformly, is the basic dimension with the letter H. This is then also referred to as the standard bore fitting system. The basic dimension of the shaft is selected accordingly depending on whether a clearance fit, an interference fit or a transition fit is to be produced with the shaft to be joined. In addition to the standard bore fitting system, there is also the standard shaft fitting system. Analogous to the standard bore, in this case the uniform basic dimension h is specified for the shaft. The basic dimension of the bore is then selected accordingly depending on whether a clearance fit, an interference fit or a transition fit is to be produced with the bore to be joined. DIN 7157 further restricts the possible combinations of tolerance classes.