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What is the General Tolerance for Sheet Metal Fabrication?

Sheet metal processing is the most commonly used metal processing method nowadays. In the process of parts processing, the size of produced sheet metal parts cannot be absolutely accurate due to various factors such as processing accuracy of machinery and equipment, die wear and tear, processing errors, etc. In order to ensure the availability of the workpiece, we must limit the tolerance of the sheet metal workpiece to the minimum. So what are the common tolerance standards in sheet metal processing? How should they be marked? Today, we will share them with you.

Tolerance Standards of Sheet Metal Fabricaiton

1. Basic dimensions

According to the strength and structural requirements of the parts, determine the dimensions determined in the design. Priority should be given to the standard diameter or length.

2. Actual dimensions

The dimensions obtained by measurement.

3. Limit size

The two thresholds that allow size changes. It is based on the basic size. The larger of the two threshold values is called the maximum limit size; the smaller is called the minimum limit size.

4. Size deviation (referred to as deviation)

The algebraic difference obtained by subtracting the basic size of a certain size. Dimensional deviation is as follows.

Upper limit of deviation = maximum limit size – basic size.

Lower limit of deviation = minimum limit size – basic size

The upper and lower deviation can be called limit deviation, and the upper and lower deviation can be positive, negative or zero.

The national standard stipulates that the upper deviation code of hole is ES and the lower deviation code is EI, and the upper deviation code of shaft is ES and the lower deviation code of shaft is EI.

5. Dimension tolerance (abbreviated as tolerance)

The amount of variation of the allowed dimensions.

Dimensional tolerance = maximum limit size – minimum limit size = upper deviation – lower deviation

Since the maximum limit size is always larger than the minimum limit size, i.e. the upper deviation is always larger than the lower deviation, the dimensional tolerance must be positive.

6. Standard tolerance and standard tolerance level

Standard tolerance is any tolerance listed in the national standard for determining the size of the tolerance zone. Standard tolerance grade is the grade that determines the degree of dimensional accuracy. Standard tolerances are divided into 20 levels, namely IT01, IT0, IT1, IT-18, indicating standard tolerances, and Arabic numerals indicate standard tolerance levels, where IT01 level is the highest, the level is reduced in turn, and IT18 level is the lowest. For a certain basic size, the higher the standard tolerance level, the smaller the standard tolerance value, the higher the accuracy of the size.

7. Basic deviation

Used to determine the upper and lower deviation of the tolerance zone relative to the zero line. Generally refers to the deviation near the zero line

8.The shaft tolerance code and the standard tolerance grade code consist of two parts and are written with the same number.

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