Determining tolerances can be a tedious task while a part is in the drawing phase. Do you know that there is a certain relationship among the three values of dimensional tolerance, shape tolerance and surface roughness? If not, that’s exactly what we’ve come up with in this article, let’s take you through the relationship between them!

Numerical relationship between shape tolerance and dimensional tolerance

When the dimensional tolerance accuracy is determined, the shape tolerance has an appropriate value corresponding, that is, generally about 50% of the dimensional tolerance value is used as the shape tolerance value; about 20% of the dimensional tolerance value is used as the shape tolerance value in the instrument industry; the heavy industry About 70% of the size tolerance value is used as the shape tolerance value.
It can be seen that the higher the dimensional tolerance accuracy, the smaller the proportion of shape tolerance to dimensional tolerance. Therefore, when designing dimension and shape tolerance requirements, except for special circumstances, when the dimensional accuracy is determined, generally 50% of the dimensional tolerance value As a shape tolerance value, this is beneficial both for manufacturing and for ensuring quality.

Numerical relationship between shape tolerance and position tolerance

There is also a certain relationship between shape tolerance and position tolerance.
From the cause of the error, the shape error is caused by the vibration of the machine tool, the vibration of the tool, the beating of the spindle, etc.; the position error is caused by the non-parallel of the guide rails of the machine tool, the non-parallel or non-perpendicular tool clamping, and the action of the clamping force. As a result, from the definition of the tolerance zone, the position error includes the shape error of the measured surface, such as the parallelism error contains the flatness error, so the position error is much larger than the shape error.
Therefore, in general, 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 smaller than the marked position tolerance value, otherwise, the parts cannot be manufactured according to the design requirements during production.

The relationship between shape tolerance and surface roughness

Although there is no direct relationship between shape error and surface roughness in terms of value and measurement, there is a certain proportional relationship between the two under certain processing conditions. According to experimental research, in general precision, surface roughness accounts for the shape tolerance. 1/5~1/4 of that. 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 values among dimensional tolerance, shape tolerance, position tolerance and surface roughness have the following relationship: dimension tolerance > position tolerance > shape tolerance > surface roughness height parameter

It is not difficult to see from the numerical relationship between size, shape and surface roughness that the numerical relationship between 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; while the shape tolerance value should be less than its position tolerance value; each position difference 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 fit accuracy and surface roughness.
In general, it can be determined according to the following relationship:

1. When the shape tolerance is 60% of the size tolerance (medium relative geometric accuracy), Ra≤0.05IT;
2. When the shape tolerance is 40% of the size tolerance (higher relative geometric accuracy), Ra≤0.025IT;
3. When the shape tolerance is 25% of the size tolerance (high relative geometric accuracy), Ra≤0.012IT;
4. When the shape tolerance is less than 25% of the size tolerance (ultra-high relative geometric accuracy), Ra≤0.15Tf (shape tolerance value).
   The simplest reference value: the dimensional tolerance is 3-4 times the roughness, which is the most economical.

Hope the above content can help you!