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Add Geometric Tolerances

May 20, 2011

   Geometric tolerances in a drawing show acceptable deviations of form, profile, location, orientation, and runout of a feature.

   You can create geometric tolerances with or without leaders depending on you use the LEADER or the TOLERANCE command.


Overview

      Each feature control frame consists of at least two compartments. The first compartment contains a geometric tolerance symbol that indicates the geometric characteristic to which the tolerance is applied, such as location, orientation, or form. For example, a form tolerance may indicate the flatness or roundness of a surface. The geometric tolerance symbols and their characteristics are shown in the following.

  • Position\:Location

  • Concentricity or coaxiality :Location

  • Symmetry\ :Location

  • Parallelism\ :Orientation

  • Perpendicularity\ :Orientation

  • Angularity\ :Orientation

  • Cylindricity \:Form

  • Flatness \:Form

  • Circularity or roundness \:Form

  • Straightness \:Form

  • Profile of a surface \:Profile

  • Profile of a line \:Profile

  • Circular runout \:Runout

  • Total runout \:Runout

 
     The tolerance value can then be followed by primary, secondary, and tertiary datum reference letters, along with the material conditions of each datum. Datum reference letters are generally used as reference tolerances to one of up to three perpendicular planes from which a measurement is made, although datum reference letters can also indicate an exact point or axis.

      A feature control frame contains symbols that represent location, orientation, form, profile and runout that apply to a tolerance. Form tolerances control straightness, circularity, flatness and cylindricity; profiles control surface and line.

       Most of editing commands can change feature control frames, and these feature control frames can be snapped to with the object snap. You can also use grips to edit geometric tolerances. Unlike dimensions and leaders, geometric objects are not associated with geometric objects.
 

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Material Conditions

       The second compartment displays tolerance values. Depending on control type, you can add a diameter symbol prior to the tolerance value, and specify a material condition symbol behind this value.

       You can apply material conditions to features that can vary in size, shown as follows:

  • At maximum material condition (symbol M, also known as MMC), a feature contains the maximum amount of material stated in the limits. At MMC, a hole has minimum diameter, whereas a shaft has maximum diameter.

  • At least material condition (symbol L, also known as LMC), a feature contains the minimum amount of material stated in the limits. At LMC, a hole has maximum diameter, whereas a shaft has minimum diameter.

  • Regardless of feature size (symbol S, also known as RFS) means a feature can be any size within the stated limits.


Datum Reference Frames

        A datum reference consists of values and modifying symbols. A datum is a theoretically exact geometric reference that measures and verifies a theoretically exact point, axis, or plane. The system provides up to three datum reference letters and relative symbols followed by the tolerance value. Two or three mutually perpendicular planes perform best when you applying these datum references. They are jointly called datum reference frame.

      The following illustration shows a datum reference frame that consists of three perpendicular planes.

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Projected Tolerance Zones

       Projected tolerance zones control the height of the fixed perpendicular part of the extension area, and control tolerance accuracy by positional tolerances.

      Before you set the symbol to projected tolerance zones, you set a height value to specify the minimum projected tolerance zone. The following illustration shows the projected tolerance zone height and symbol appearing in a frame.

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Composite Tolerances

        A composite tolerance consists of two tolerances that applied to the same geometric characteristic of a feature or for features that have different datum requirements. One tolerance is in relation to a pattern of features and the other tolerance to each feature within the pattern. The pattern tolerance is less restrictive than the individual feature tolerance.

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     In the illustration above, the axis formed by the point where datums A and B intersect called the datum axis. The position of the pattern is calculated from that axis.

      Before you create a dimension and add a composite tolerance for a drawing, you have to specify the first line of a feature control frame and then choose the same geometric characteristic symbol for the second line of the feature control frame. The specified geometric symbol compartment is extended over both lines. Then you can create a second line of tolerance symbols.

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Adding geometric tolerances

      Geometric tolerances indicate the maximum allowable variations in the geometry defined by a drawing. ZWCAD draws geometric tolerances using a feature control frame, which is a rectangle divided into compartments.


To add a geometric tolerance

  1. Choose Dimensions > Tolerance.

  2. To display the geometric tolerance symbols, on the first line, click the Sym box.

  3. Click to select a geometric tolerance symbol.

  4. Under Tolerance 1, click the Diabox to add a diameter symbol.

  5. In the field, type the first tolerance value.

  6. To display the material condition symbols, click the other line box in Tolerance 1.

  7. Click to select a material condition.

  8. Under Tolerance 2, repeat steps 4 through 7 to add a secondary tolerance value, if appropriate.

  9. Under Datum 1, type the primary datum reference letter.

  10. To display the material condition symbols, click the other line box in Datum 1.

  11. Click to select a material condition.

  12. Repeat steps 9 through 11 to add secondary and tertiary datum, if appropriate.

  13. In the second row, repeat steps 2 through 12 to add composite tolerances, if appropriate.

  14. In the Height box, type a projected tolerance zone height value, if appropriate.

  15. To insert a projected tolerance zone symbol, click the Projected Tolerance Zone box, if appropriate.

  16. Click OK.

  17. In the drawing, specify the location of the feature frame.

Dimension toolbar\

Command line TOLERANCE


Controlling dimension tolerance

       You can create dimensions as either tolerance or limits dimensions. When creating a tolerance dimension, you can control the upper and lower tolerance limits as well as the number of decimal places of the dimension text. The image tile on the right side of the Dimension Settings dialog box shows the appearance of tolerance and limits dimensions based on the current dimension style settings.

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Tolerance dimension. Limits dimension.


To create a tolerance dimension

  1. Choose Dimensions > Style.

  2. In the Dimension Style Manager, select the style you wish to change and choose Modify.

  3. Click the Tolerances tab.

  4. Make your selections.

  5. Click OK.

  6. Choose Close to exit.

Dimension toolbar

Command line DIMSTYLE


Commands Reference

LEADER: Creates a line that connects annotation to a feature

TOLERANCE: Creates geometric tolerances

System Variables Reference

None


 

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