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Tolerances and fits.

1 The concept of interchangeability.

Interchangeability refers to taking any one of the same parts from a batch and fitting them into a machine or component without repair, and meeting the performance requirements of the product.

Interchangeability significance: the interchangeability of parts is conducive to organizational cooperation and specialized production, and is of great significance to ensure product quality, reduce costs, facilitate assembly and maintenance.

Overview of Dimensional Tolerances:

Because the parts are affected by many factors such as processing and measurement in the actual production process, there is always a certain error in the actual size of a batch of parts, in order to ensure the interchangeability of the parts, the size of the parts must be controlled within the allowable change range, and the allowable size change is called the dimensional tolerance, referred to as the tolerance.

Terms and definitions for dimensional tolerances.

2) Actual size - the size obtained by measurement after the part is made.

3) Limit size - two limit values that allow the actual size of the part to change, of which the larger one is called the maximum limit size, and the smaller one is called the minimum limit size.

4) Dimensional deviation (abbreviated deviation) - the algebraic difference of a certain size minus the basic size. Dimensional deviations include upper deviation, lower deviation (collectively referred to as limit deviation) and actual deviation.

Upper Deviation Maximum Limit Dimension - Basic Dimension.

Lower Deviation Minimum Limit Dimension - Basic Dimension.

The axis shown in the image above:

Upper deviation (lower deviation (

The national standard stipulates that the code ES and ES are used to indicate the upper deviation of the hole and the shaft respectively; The code names EI and EI are used to indicate the hole and shaft, respectively.

of the lower deviation. The deviation can be positive, negative, or zero.

The algebraic difference between the actual size minus the base size is called the actual deviation. The actual deviation of the part size is acceptable between the upper and lower deviations.

5) Dimensional tolerance (referred to as tolerance) - the amount of allowable dimensional variation.

That is: Tolerance Maximum Limit Dimension - Minimum Limit Dimension.

Or: Tolerance Upper Deviation - Lower Deviation.

6) Dimensional Tolerance Zone (referred to as Tolerance Zone) – A tolerance zone is an area that represents the size of the tolerance and its position relative to the zero line.

Standard tolerances and fundamental deviations.

The national standard "Tolerance and Fit" stipulates that the tolerance zone is composed of two elements: standard tolerance and basic deviation.

The standard tolerance determines the size of the tolerance zone, while the fundamental deviation determines the position of the tolerance zone.

1) Standard Tolerance (IT).

The value of the standard tolerance is determined by the basic dimensions and the tolerance class. The tolerance class is the grade that determines the degree of dimensional accuracy. Standard Male.

The difference is 20 levels, i.e., IT01, IT0, IT1, ,...，iti8。Its dimensional accuracy decreases sequentially from IT01 to ITI8. The specific values of the standard tolerances can be found in the table.

2) Fundamental deviations.

The basic deviation generally refers to the deviation that is close to the zero line of the upper and lower deviations. That is, when the tolerance zone is above the zero line, the basic deviation is the lower deviation; When the tolerance zone is below the zero line, the basic deviation is the upper deviation, as shown in the figure above.

The national standard specifies 28 different basic deviations for both the bore and the shaft. The basic deviation code is indicated in Latin letters, with uppercase letters for holes and lowercase letters for axes. The figure below is a series of 28 basic deviations for holes and shafts.

This kind of problem, you should read the book of tolerance fit and technical measurement by yourself, others give you a few or dozens of hundreds of questions, you still don't understand it, so how do you work?

Specific Questions Please put your questions on it.

What can help.

Fit tolerance is the sum of the tolerances of the holes and shafts that make up the fit. It is the amount of movement that allows for a gap or interference. The size of the tolerance zone and the position of the tolerance zone of the bore and shaft form the fit tolerance.

The magnitude of the hole and shaft fit tolerance indicates the fit accuracy of the hole and shaft. The size and position of the bore and shaft fit tolerance zone indicate the fit accuracy and fit nature of the hole and shaft. The size of the fit tolerance The size of the tolerance zone; Fit Tolerance Zone Size and Position Fit Nature.

The tolerance of the base hole refers to the selection of H, the tolerance level can be any IT0 to IT18, the tolerance of the shaft can be any A to Z, and the tolerance level can also be any It0 to It18. For example, H0 A0, H1 D4, H7 A0, H1 F4, H5 E4, H3 F4 and so on are all base hole systems. The tolerance h is characterized by a lower deviation of 0, e.g. 12h7

The tolerance of the base shaft refers to H, the tolerance level can be any IT0 to IT18, the tolerance of the hole can be any A to Z, and the tolerance level can be any IT0 to It18. For example, A1 H0, B2 H5, C3 H6, D4 H7, E5 H9, F1 H8, etc. are all base hole systems. The tolerance h is characterized by an upper deviation of 0, e.g. 12h7 0

The tolerance fit is generally the fit of the hole and the shaft, of course, sometimes it will evolve into the fit of the groove and the block, generally in this fit, the hole is biased towards the upper difference, and it is generally according to the positive value, and it is processed according to the larger size in the actual processing; The shaft or block is generally biased towards the lower difference, according to the negative value, in the actual processing according to the smaller size of the processing. There are three types of fits, namely transition fit, interference fit and gap fit.

Transition fit means that the tolerance zone of the two sides of the fit is partially overlapped, and the difference between the hole and the shaft is about, which is slightly difficult to assemble, and it is assembled with force or with the help of tools;

Interference fit means that the tolerance zones of the two sides of the fit are completely overlapped, and the difference between the hole and the shaft is about, which is difficult to assemble and must be assembled with the help of tools;

Clearance fit means that the tolerance zone of the two sides of the fit does not overlap at all, and the difference between the hole and the shaft is about above, which is easy to assemble and can be assembled by hand without force.

There are three types of mating terms: gap fit, transition fit, interference fit. Three kinds of fit are commonly known as loose fit, slip fit, tight fit.

A gap fit is one in which the tolerance zones of the two mating sides do not overlap at all. In this case, it is easy to assemble and can be assembled by hand without force.

A transitional fit is when the tolerance zones of the two sides of the fit partially overlap. In this case, it is slightly difficult to assemble, either by force or with the help of tools.

A gap fit is when the tolerance zones of the two mating sides overlap completely. In this case, it is difficult to assemble and must be assembled with the help of tools.

Tight fit, slippery fit, loose fit, these three should know what it means when you look at the first word.

In order to prevent the accumulation of measurement errors and improve the measurement accuracy, in the actual measurement work, it is necessary to follow the principle of "from the whole to the part" in the layout, "control first and then break the part" in the program, and "from advanced to low" in the accuracy. These principles should be followed both in the survey area and within the mining area.

For the measurement of geometric quantities, the measurement method is based on the characteristics of the measured parameter, such as tolerance value, size, weight, material, quantity, etc., and analyzes and studies the relationship between the parameter and other parameters, and finally determines the operation method of how to measure the parameter.

First high-level, then low-level, first global and then local.

(1) Determine the tolerance class of the bore shaft: tf=| 80-(-35)|=45um;The tolerance between the bore and the shaft is about 45 2=; Check the standard tolerance value table: d=40mm, IT6=16um; IT7=25um, and when high-precision fit, the hole is one level lower than the accuracy level of the shaft, so the hole is 7 grades, and the shaft is 6 grades;

2) Under normal circumstances, the base hole system is selected, so the basic deviation code of the hole is H; And the lower deviation of the hole ei=0, then es=0+25=25um;

3) Interference fit, axis ei= es-ymin=25+35=60um; Check the basic deviation value table of the axis, and the lower deviation coded U is +60um; Fit: 40h7 u6

4) Check: axis es=ei+t=76 "given ymax=80; So meet the requirements.

There are no special requirements, so the base hole system is preferred, and the basic deviation of the hole is coded H. Therefore, the deviation under the hole ei=0 to get ymax=ei-es=-80, es=+80, "ymax -80um, ymin -35um" y indicates that it is an interference fit. tf=|ymax-ymin|=|80um-（-35um.

45um。Another tf = td + td. Get 45=|es-ei|+|es-ei|Another ymin=es-ei=-35 gets ei=es-80, es=ei-35, that's all I can do, sorry!

Maximum Interference = Maximum Limit Size of Shaft - Minimum Limit Size of Hole Minimum Limit Size of Hole Minimum Limit Size of Hole = Maximum Limit Size of Hole = Minimum Limit Size of Hole + Tolerance Maximum Limit Size of Hole = Minimum Interference = Maximum Interference - Fit Tolerance Minimum Interference = Minimum Interference = Smallest Shaft - Largest Hole Smallest Shaft Smallest Shaft =