Position and Form Tolerances in Engineering Drawings

Indicating the position and form tolerances on technical drawings is very important. They show all the details about the part and all kinds of tolerances. These tolerances are essential in the production of these parts. While indicating the position and form tolerances, we are using these elements in general;

• Basic dimensions
• Datum identification symbols
• Other additional symbols
• Form tolerances
• Datum references

And here, we will show you how to use these important elements in engineering drawings. Before reading here, it is very important to learn GD&T symbols for engineering drawings.

How to Use Position and Form Tolerances in Technical Drawings?

You need to be able to know and use the features about position and form tolerances. You can use them to obtain professional technical drawings in engineering.

Showing Basic Dimensions

Dimensions are a very important part of technical drawings. We indicate them by adding tolerances in general. But in the position and form tolerance, we are giving the exact theoretical size of the dimensions.

The indication difference of this dimension is, that we type the dimension inside a square shape. So, we can give the tolerances related to this basic dimension in control frames. Also, check the example above. We gave a dimension which is 35 and we typed it in a square.

Datum Identification Symbols

These identifier letters are very important parts of engineering drawings. We are using these elements to show the surface feature of entities. And we can use these symbols to define form and position tolerances in control frames.

If you look at the examples above, the identifier letter is A. So, we can show these letters with the form and tolerance symbols in the control frames. Readers of this technical drawing will understand that tolerance values in control frames are related to which geometric feature.

We position the triangles on the surfaces. And we show the letters above and connect them with these triangles by letters.

Showing Additional Symbols in Technical Drawings

Additional symbols or modifying symbols in technical drawings are very important to show additional features. For example, LMC and MMC are very important notes and we are using them to show the ‘least material condition’ and ‘maximum material condition’ features of surfaces.

You can define these symbols in the control frames of engineering drawings.

Giving Form Tolerances in Engineering Drawings

It is very simple to indicate the form tolerances for features. You just need to indicate the required form symbol and you need to tolerance value besides this symbol. You need to separate the symbol and the value with the vertical line.

For example, we define the roundness tolerance on the control frame on a technical drawing. And we assigned 0.05 for this form tolerance.

How to Reference Datums in Engineering Drawings?

Above, you can see the indications of datums on technical drawings. So, you need to properly use the references on the control frames.

Firstly, you need to define the tolerance symbol on the control frame. And then, you need to give the tolerance value as we show above. After them, you can add the datum reference letters respectively.

If you want to add modifier symbols for datums, you need to define them directly beside the letters. Check the example below.

For instance, we would like to give position tolerance of multiple entities on the part. Assume that we use reference datums on these surfaces, A, B, and C respectively. As we explained above, we put the position symbol at the beginning of the control frame. After that, we indicated the tolerance value. And then, we used A, B, and C datum references to define this tolerance value for them. In addition, we defined a ‘maximum material condition’ for the ‘A’ datum.

It is very simple like this.

Combining Different Symbols on Control Frames

As you see above, there are different forms and tolerancing elements. You can use these combinations on the control frames to fully define the geometrical features. You can see an example below.

For example, we indicated a perpendicularity tolerance value for A and B datum references on technical drawings. And also, we defined a ‘projected tolerance zone’ symbol for the datum B.

Also, right below this tolerance, we defined a flatness tolerance for all the surfaces and entities of the part on the technical drawing.

Conclusion

So, it is very simple to use datums, reference datums, form and positioning tolerances, and modify symbols in technical drawings. Also, they are very effective to show all the details about the part on the engineering drawing.

Above all, do not forget to leave your comments and questions below about the form and positioning tolerances in technical drawings.

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