The Relationship Between Dimensional Tolerances, Geometric Tolerances, And Surface Roughness Values

It is not difficult to see from the numerical relationship between size, shape and position and surface roughness that the numerical relationship of the three should be coordinated during design, and the tolerance value should be marked on the drawing: the roughness value of the given same surface should be less than Its shape tolerance value; and its shape tolerance value should be less than its position tolerance value; each position difference value should be less than its size tolerance value.

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1. Numerical relationship between shape tolerance and dimensional tolerance

When the dimensional tolerance accuracy is determined, the shape tolerance has an appropriate value, that is, generally about 50% of the dimensional tolerance value is used as the shape tolerance value; about 20% of the dimensional tolerance value in the instrument industry is used as the shape tolerance value; heavy industry Approximately 70% of the dimensional tolerance value is used as the shape tolerance value. This shows that. The higher the dimensional tolerance accuracy, the smaller the proportion of the shape tolerance in the dimensional tolerance. Therefore, when designing the size and shape tolerance requirements, except for special circumstances, when the dimensional accuracy is determined, the 50% dimensional tolerance value is generally used as the shape tolerance value. , Which is not only good for manufacturing but also good for ensuring quality.

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2. Numerical relationship between shape tolerance and position tolerance

There is also a certain relationship between shape tolerance and position tolerance. From the perspective of the cause of the error, the shape error is caused by machine tool vibration, tool vibration, spindle runout, etc.; while the position error is caused by the non-parallelism of the machine tool rails, the non-parallel or non-perpendicular tool clamping, and the effect of clamping force. As a result, from the definition of tolerance zone, the position error includes the shape error of the measured surface. For example, the parallelism error contains the flatness error, so the position error is much larger than the shape error. Therefore, under normal circumstances, when there is no further requirement, the position tolerance is given, and the shape tolerance is no longer given. When there are special requirements, the shape and position tolerance requirements can be marked at the same time, but the marked shape tolerance value should be less than the marked position tolerance value, otherwise, the parts cannot be manufactured according to the design requirements during production.

3. The relationship between shape tolerance and surface roughness

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Although there is no direct connection between shape error and surface roughness in numerical and measurement, there is a certain proportional relationship between the two under certain processing conditions. According to experimental research, in general accuracy, the surface roughness accounts for the shape tolerance 1/5 to 1/4 of. It can be seen that in order to ensure the shape tolerance, the maximum allowable value of the corresponding surface roughness height parameter should be appropriately limited.

In general, the tolerance value between dimensional tolerance, shape tolerance, position tolerance, and surface roughness has the following relationship: dimensional tolerance>position tolerance>shape tolerance>surface roughness height parameter

It is not difficult to see from the numerical relationship between size, shape and position and surface roughness that the numerical relationship of the three should be coordinated during design, and the tolerance value should be marked on the drawing: the roughness value of the given same surface should be less than Its shape tolerance value; and its shape tolerance value should be less than its position tolerance value; each position difference value should be less than its size tolerance value. Otherwise, it will bring all kinds of troubles to manufacturing. However, the most involved in the design work is how to deal with the relationship between dimensional tolerances and surface roughness and the relationship between various matching accuracy and surface roughness.

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