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What is Dimensional Tolerancing?

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What is Dimensional Tolerancing?

February 21, 2023
Latest company case about What is Dimensional Tolerancing?

 

 

    The parts or components, without any selection or additional modification, can be installed on the machine and meet its technical performance requirements, which is called interchangeability. Parts or components are interchangeable, which is of great significance to simplify product design, shorten production cycle, improve labor productivity, reduce product cost, and facilitate use and maintenance. In order to achieve the interchangeability of parts, the engineering industry has formulated standards for tolerance and fit, and product manufacturing must comply with the following standards.

 

1. Nominal dimension is dimensioning specifications define the nominal, as-modeled. Generally, designer mark the size data on mechanical drawing.

2. Physical dimension is obtained by measurements. Because of measuring errors, physical dimension do not represent actual dimension value.

3. Limit dimension is the two limit values that allow dimensional changes. The larger of the two limit sizes is called the maximum limit size, and the smaller one is called the minimum limit size. The actual size of the part manufactured is a qualified size between the upper limit size and the lower limit size.

4. Dimension deviation. Lower deviation is the difference between the minimum possible component size and the basic size. Upper deviation is the difference between the maximum possible component size and the basic size.

5. Dimension tolerance is the amount of change in the allowable size. The tolerance is equal to the absolute value of upper limit size minus lower limit size.

6. Tolerance interval

7. Standard tolerance. In the metric system, there are International Tolerance (IT) grades that can also be used to specify tolerances by means of symbols. Any tolerance specified by industry standards to determine the size of the tolerance zone is called standard tolerance. The standard tolerance is determined by two factors, the nominal size and the tolerance class. Tolerance class is a class used to determine the degree of dimensional accuracy. International standards specify 20 tolerance classes, including IT01, IT0, IT1, ... IT18. IT01 grade has the highest accuracy and IT18 grade has the lowest accuracy.

8. Fundamental deviation is the minimum difference in size between a component and the basic size.

9. Engineering fits

1) Clearance fits, the hole is larger than the shaft, enabling the two parts to slide and/ or rotate when assembled. Include loose runnung, free runing and close running.

2) Interference fits, The hole is smaller than the shaft and high force and/ or heat is required to assemble/ disassemble. Include press fit, driving fit and forced fit.

3) Transition fits, The hole is fractionally smaller than the shaft and mild force is required to assemble/ disassemble. Include similar fit and fixed fit.

10. Hole and shaft basis. A fit is either specified as shaft-basis or hole-basis, depending on which part has its size controlled to determine the fit. In a hole-basis system, the size of the hole remains constant and the diameter of the shaft is varied to determine the fit; conversely, in a shaft-basis system the size of shaft remains constant and the hole diameter is varied to determine the fit. International regulations, under normal circumstances, priority use the hole-basis system, which can reduce the quantity of customized cutting tools and measuring tools.

 

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