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Geometric Dimensioning and Tolerancing
Geometric Dimensioning and Tolerancing
 

Common Geometric Symbols and Terms

Form Tolerances

Profile Tolerances

Orientation Tolerances

Runout Tolerances

Location Tolerances

Common Geometric Symbols and Terms

Tolerances are categorized into five types. These tolerance types include form, profile, orientation, runout, and location. This section will discuss primarily those tolerances that relate to die casting. Table 1 identifies common geometric characteristics, their symbols, their type, and the features to which they apply.

Table 1 Common geometric symbols and terms.

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Form Tolerances

Form tolerances include flatness, straightness, circularity, and cylindricity. These tolerances relate to individual features only.

The flatness tolerance is symbolized by a parallelogram and specifies a tolerance zone confined by two parallel planes within which the entire surface must lie. Figure 1a demonstrates how the flatness symbol is applied, and interpreted.

Fig. 1a Flatness symbol on a drawing.

Fig. 1b Isometric view interpretation.

Fig. 1c Side view interpretation.

In the example shown in figure 1b and 1c, the entire top surface must lie between two parallel planes separated by a tolerance zone of .03 inches. A flatness tolerance establishes limits a feature must exist within. Exceeding these limits may cause the part not to fit or function properly. Die cast parts are subject to warping due to shrinking, cooling, and other thermal factors (thermal gradients) that may influence flatness. For flatness specifications, refer to Coordinate Dimensioning Tolerances in Engineering and Design, section 4.

Straightness is very similar to flatness, except that the tolerance is applied between two parallel lines instead of two parallel planes.Straightness is symbolized by a short horizontal line as shown in figure 2a.

Fig. 2a Straightness symbol on a drawing.

Fig. 2b Side view explanation.

Straightness is defined by points on a line lying between two parallel lines separated by a tolerance. Points on the line that cross one of the parallel lines is unacceptable. See figure 2b. In the example shown in figure 2, the top line must lie between two parallel lines separated by a tolerance zone of .005 inches.

3a Circular symbol on a drawing.

3b Circular interpretation.

Circularity is the third form tolerance and is defined as all points constrained to lie within two concentric circles of specific diameter. See table 2 for diameter and radius symbology. Any points lying outside the larger circle or inside the smaller circle are unacceptable. See figure 3b. Circularity is symbolized by a circle as shown in figure 3a. In figure 3, the tolerance is .04 inches.

Table 2 Additional symbols.

Cylindricity is essentially the combination of both circularity and straightness. It is defined as all points constrained to lie within two circles and extend the length of the cylinder. Cylindricity is symbolized by a circle with a tangent line on the left and right side as shown in fig 4a. In figure 4, the tolerance is .04 inches.

Fig. 4a Cylindrical symbol on a drawing.

Fig. 4b Cylindrical interpretation.

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Profile Tolerances

Profile tolerances include two characteristics; profile of a line and profile of a surface. Profile tolerances can describe single features or related features. They are used to relate one feature to another or one feature to a datum. Profile tolerances and their symbols are identified in table 1.

Profile of a line is symbolized by an open semi-circle and typically refers to irregular lines. Profile of a line is defined as the amount of deviation allowed from a given line. Figure 5 demonstrates how the profile of a line is applied and interpreted.

In figure 5, each line element of the surface between C and D must lie between two profile boundaries .015 inches apart in relation to datum planes A and B. See figure 5b.

Surface profile is symbolized by a closed half-circle, flat side down and typically refers to irregular planes. Surface profile is defined as the amount of deviation allowed from a given surface. Profile of a surface can be applied to a figure in four different ways; bilateral tolerance as shown in figure figure 5a, unilateral tolerance (inside) as shown in figure 5b, unilateral tolerance (outside) as shown in figure 5c, and bilateral tolerance unequal distribution as shown in figure 5d. Thefour applications of profile of a surface are demonstrated in figure 5. When the type of tolerance zone is not specified, the tolerance zone is assumed to be bilateral.

Fig. 5a Bilateral Tolerance
Fig. 5b Unilateral Tolerance (Inside)
Fig. 5c Unilateral Tolerance (Outside)
Fig. 5d Bilateral Tolerance unequal distribution

Fig. 5e Bilateral tolerance.

Fig. 5f Unilateral tolerance (inside).

Fig. 5g Unilateral tolerance (Outside).

Fig. 5h Bilateral tolerance unequal distribution

Interpretations of these tolerances are explained in figure 5e, figure 5f, figure 5g, and figure 5h. A bilateral tolerance zone, as demonstrated in figure 5e, means that the .006 tolerance is equally distributed (.003) on both sides of the true profile. A unilateral tolerance zone is one, which is entirely on one side of the true profile. In figure 5f, the entire .006 tolerance zone is on the inside of the true profile. In figure 5g, the entire 0.006 tolerance zone is on the outside of the true profile. Figure 5h demonstrates a bilateral tolerance of unequal distribution, which means the tolerance zone, is unequally distributed on one side of the true profile.

The profile of a surface can also be applied to the entire surface all around the entire part. A circle drawn on the leader line leading from the feature control frame as shown in figure 6a means “all around.”

Fig. 6a Profile of a surface all around symbol on a drawing.

Fig. 6b Profile of a surface all around interpretation.

In the example shown in figure 6b, the surface all around the object shown are constrained by two parallel boundaries separated by a tolerance. In figure 6b, all surfaces around the entire object must lie between two parallel boundaries 0.02 inches apart and perpendicular to datum plane F.

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Orientation Tolerances

Three orientation tolerances are used when dimensioning die cast parts, parallelism, perpendicularity, and angularity. These tolerances establish the orientation of features to each other. These tolerances and their symbols are identified in table 1.

Parallelism (symbolized by two parallel lines slanting slightly to the right) is a condition of a surface or axis where all the points on a surface are equidistant to a given datum surface and within a specified tolerance. Figure 7a illustrates the parallelism symbol followed by its interpretation, figure 7b.

Fig. 7a Parallelism symbol on a drawing.

 

Fig. 7b Parallelism interperatation.

The tolerance zone is established parallel to the datum plane “D”. Parallelism tolerance, when applied to a plane surface, establishes flatness.

Perpendicularity is symbolized by an upside down “T” and requires that all points on a specified feature form a 90° angle with the datum plane or axis. Figure 8a illustrates the perpendicularity symbol followed by its interpretation, figure 8b.

Fig. 8a Perpendicularity symbol on a drawing.

Fig. 8b Perpendicularity symbol interpretation.

Fig. 9a Angularity symbol on a drawing.

Fig. 9b Angularity interpretation.

In this example, the surface must lie between two parallel planes 0.005 apart. These two planes form a 90° with the datum plane F. The tolerance zone is 0.005 as shown in figure 8.

Angularity is symbolized by a small angle opening to the right as demonstrated in figure 9a. Angularity requires that all points specified on a feature form an angle with a datum reference. This tolerance is similar to perpendicularity only the angle measures something other than 90°.

In figure 9b, the surface must lie between two parallel planes .02 apart. These two planes are inclined at 35° to datum plane C.

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Runout Tolerances

Runout tolerances include circular runout and total runout and are used to control the functional relationship of one feature to another or a feature to a datum axis. Runout can be used to detect and control concavity and convexity. Runout tolerance is applicable to rotating parts where the composite surface criteria is based on the part function and design requirements. Refer to table 1 for runout tolerance characteristics and their symbols.

Circular runout is symbolized with an arrow pointing upward and slightly to the right. When dealing with three-dimensional objects, circular runout is defined as the amount that is allowed to deviate from the central axis at one cross section. Figure 10 demonstrates circular runout. To better visualize circular runout, imagine a record on a turntable. If you were to place your finger lightly on the edge of the record, and turn the record one full revolution, the distance your finger moved back and forth would be the circular runout for the record.

Fig. 10a Circular runout symbol on a drawing.

Fig. 10b Circular runout interpretation.

In this example, the circular element of the surface must be within the specified tolerance. The indicator should not move more than the specified limit of 0.005 when the part is rotated 360° about the datum axis. Circular runout controls only one circular element of the part, and not the total circular surface. Circular runout controls composite variations of circularity and cross-sectional form variations of the surface at each circular element around the datum axis. For example, circular runout may be applied to a car tire to detect bumps, bulges, and ‘out of round’ conditions.

Total runout is symbolized by two arrows pointing upward and slightly to the right and connected on the bottom with a horizontal line. See figure 11a. Total runout is a composite tolerance used to establish the functional relationship of one or more features of a part to a datum axis. When dealing with threedimensional objects, total runout is defined as that amount which is allowed to deviate from a central axis. This is demonstrated in figure 11b.

Fig. 11a Total runout symbol on a drawing.

Fig. 11b Total runout symbol on a drawing.

In this example, the indicator may be placed anywhere along the indicated surface. As the part is rotated 360° about the datum axis, the indicator adds the amount the indicator moves from the desired shape. Suppose a stool leg is to have a total runout not to exceed 0.002. The stool leg is placed on a lathe, and an indicator is placed 2 inches from the end of the stool leg. As the leg rotates 360° about the datum axis on the lathe, the amount the indicator moves from the desired shape is recorded. Next, the indicator may be placed at 3 inches from the end of the leg and then is rotated 360° again. The amount the indicator moves from the desired shape of the leg is recorded and combined with the first measurement. This process can continue as many times as required. The total indicator movement should not measure more than 0.002. This is total runout. Total runout provides composite criteria of all surface elements. It is the composite deviation from the desired form andorientation of a part surface. Total runout is the total deviation of the part as it is rotates 360 degrees on a datum axis. It is used to control cumulative variations of circularity, straightness, angularity, taper and profile of a surface.

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Location Tolerances

Location tolerance is the fifth category of tolerancing and includes position, concentricity, and symmetry. These symbols and their characteristics are identified in table 1.

A position tolerance is the total permissible variation in the location of a feature about its exact true position. True position may be defined by a theoretically exact position. Although it can be applied to single features, true position is especially useful when applied to multiple or mating parts. Position tolerance defines a zone in which the center or axis of the feature is permitted to vary from a true position. This is demonstrated in figure 12.

Fig. 12a True position symbol on a drawing.

Fig. 12b True position interperatation.

A position tolerance is indicated by the position symbol, which is a circle with a horizontal line and a vertical line passing through the center of the circle. The position symbol is accompanied by a tolerance value, and an appropriate datum reference. For cylindrical features (holes and bosses), the position tolerance is the diameter of the tolerance zone within which the axis of the feature must lie. Remember the axis is within a tube of acceptable limits. The center of the tolerance should be at the exact true position. For other features such as slots, tabs etc., the position tolerance is the total width of the tolerance zone in which the center of the feature must lie, the center axis of the zone being at the exact true position. Position tolerance is applied to a part for purposes of function or interchangeability.

Concentricity is symbolized by a circle with a slightly smaller circle inside it. Concentricity is a type of location tolerance that establishes the axis of one or more features in composite, relative to a datum axis. This is demonstrated in figure 13.

Fig. 13a Concentricity symbol on a drawing.

Fig. 13b Out of straightness. Error of parallelism

Fig. 13c Out of circularity. Out of cylindricity.

Concentricity is used for die cast parts with cylindrical shapes, where wall thickness needs to be closely maintained. Special steps for additional control are highly recommended. This is the condition where the axes of all cross-sectional elements of a feature's surface are common to the axis of a datum feature. Concentricity tolerance is the diameter of the cylindrical tolerance zone within which the axis of the features must lie. Imagine that the axis must exist within a tube that establishes theacceptable limits. The most appropriate designshould be selected to both meet the design requirements and provide the most economical manufacturing conditions.

Figure 13a illustrates how concentricity looks on a drawing. Figure 13b demonstrates a part that is out of straightness. The smaller cylinder's axis crosses outside of the larger cylinder's tolerance zone. As the part is rotated about its axis, the smaller cylinder wobbles even through the smaller cylinder may be centered on its own axis; it is not centered on the part's axis. The part's axis limit has been exceeded by the smaller cylinder's axis. The part is not concentric about its axis. The part wobble exceeds 0.001 and is not acceptable.

Figure 13c demonstrates out of circularity and out of cylindricity. The smaller cylinder's axis does not line up with the part's axis. This will cause the part to wobble when it is rotated about the axis. The amount of allowable wobble is 0.001.

Symmetry refers to the feature's own shape about its center. It is symbolized by three horizontal lines with the middle line slightly longer on both ends. See table 1. A point, line, or plane establishes the part's center from which the part may vary in shape. Imagine a bell within a larger bell of the same shape. See figure 14.

Fig. 14 Symmetry.

The area between bells establishes the acceptable tolerance zone for symmetrical part features. The centerline or axis of the bell establishes the reference line from which symmetry is measured. Symmetry tolerance is only applied on a regardless of feature size (RFS).

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