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Common scale tolerances and basic deviations in machining

In order to facilitate the production, realize the interchangeability of parts and meet the different requirements, the national scale "limit and fit" defines the tolerance zone, which consists of two elements, the scale tolerance and the basic deviation. The scale tolerances determine the size of the tolerance bands, while the base deviations determine the tolerance zones.
1) scale tolerance (IT)
The value of the scale tolerance is determined by the basic dimension and the tolerance grade. The tolerance level is a mark to determine the degree of accuracy of the dimension. Scale tolerances are divided into 20 levels, namely, IT01, IT0, and IT1,... IT18. The size accuracy is reduced from IT01 to IT18 in turn. For detailed values of scale tolerances, see the relevant scales.
2) basic deviation
The basic deviation refers to the upper or lower deviation of the tolerance line relative to the zero line in the limit and fit of the scale, which is generally the deviation near the zero line. When the tolerance zone is above the zero line, the basic deviation is the lower deviation; the opposite is the upper deviation. There are 28 basic deviations. The NC code is represented by Latin letters, capitals are holes, and lower case is shafts. From a series of basic deviation can be seen from the figure that the basic hole deviation of A ~ H and K ~ ZC basic axis deviation for the deviation; the basic deviation of a hole, the basic deviation of K ~ ZC and a ~ H axis deviation, JS and JS of the tolerance zone are distributed symmetrically on the zero line both sides of the hole and shaft the upper and lower deviation respectively are +IT/2 and -IT/2. The basic deviation series only indicates the position of the tolerance zone and does not indicate the magnitude of the tolerance. Therefore, one end of the tolerance strip is the opening, and the other end of the opening is defined by the scale tolerance.
The basic deviations and scale tolerances are defined according to the following formulas for the definition of dimensional tolerances:
ES=EI+IT or EI=ES-IT ei=es-IT or es=ei+IT
The tolerance zone code for holes and shafts consists of the basic deviation code and the tolerance band code.
The relation between the basic dimensions of the same, the combination of holes and the axis tolerance bands is called fit. According to the requirements of use, the hole and shaft between the coordination of loose and tight, so the national standard matching type:
1) clearance fit
When the hole and shaft are assembled, there is clearance (including minimum clearance or zero). The tolerance zone of the hole is above the tolerance band of the shaft.
2) transition fit
Holes and shafts may be fitted with clearance or interference fit. The tolerance zone of the hole and the tolerance band of the axle overlap each other.
3) interference fit
The hole and shaft assembly has interference (including minimum interference, that is zero) coordination. The tolerance zone of the hole is under the tolerance band of the shaft.
Benchmark system:
In the manufacture of mating parts, in which a part as a benchmark, it must have basic deviation, to achieve a variety of properties with the system known as the reference system by changing another basic deviation of non reference parts.  According to the actual needs of production, the national scale defines two benchmark systems.
1) base hole system
Base hole system refers to the basic deviation of the hole tolerance zone, with different basic deviation of the axis of the tolerance zone, forming a variety of coordination of a rail system. The hole made by the base hole is called the reference hole, the basic deviation is H, and the lower deviation is zero.
2) base shaft system
Base shaft system is a system of various tolerances in which the tolerances of the shafts with the basic deviations are fixed and the holes with different basic deviations are formed. The shaft of the base shaft is called the reference shaft, the basic deviation is h, and the deviation is zero.
symbols for fit
The matching code is composed of the hole and the shaft's tolerance band code. It is written in the form of fraction, the tolerance of the hole is the code of the hole, and the denominator is the tolerance band code of the shaft. Numerical control of any molecule containing H as the base hole system, and all the denominator containing H as the base axis system coordination.
For example, 25H7/g6 refers to the basic dimensions of the gap with the diameter of 25, the basic hole system with reference hole tolerance zone is H7, (the basic deviation of H tolerance level 7), the axis of the tolerance zone is G6 (the basic error is g, the tolerance level 6).
For example, 25N7/h6 refers to the coordination of basic size is 25, the axis system transition fit, reference axis tolerances of H6 (basic deviation of H, tolerance level 6), hole tolerance zone is N7 (the basic error is N, the tolerance level 7).
Marking of tolerances and fits on drawings
1) the tolerance and fit shall be marked on the assembly drawing, and the combined injection method shall be adopted.
2) there are three ways of marking the part drawings.
2. form tolerance
After machining the parts, there is not only the size error, but also the geometric shape and the error of the positions. Even when the size of the cylinder is qualified, it may also appear on one end of a large, small or small at the end of the middle, thick ends, etc., and its cross section may not be round, which belongs to the error of shape. After processing the diameter shaft, the shaft may be different axis, which belongs to the position error. Therefore, the shape tolerance refers to the actual shape of the ideal shape of the agreed variable. Position tolerance refers to the allowed change of the actual position to the ideal position. Abbreviated form and position tolerance.
Shape and position tolerances
1) code for shape and position tolerances
National scale GB/T 1182-1996 specifies the form and position tolerances by means of codes. In the actual production, when not marked with symbols of form and position tolerance, the technical requirements of text with promise. Form position tolerance includes various items of form and position tolerance symbols, geometric tolerance and lead frame to form and position tolerances, numerical and other relevant symbols, and reference code etc.. Box font height h and pattern size digital high.
2) dimensional tolerance labeling example
A valve stem. The words added around the position tolerance of the figure are repeated only for the reader's instructions in the actual drawing
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