## Geometric tolerance classes

The table in Figure 5.13 has shown the various classes of geometrical tolerance. These are only a selection of the most commonly used ones. The full set is given in ISO 1101:2002.

Row 1 in the table in Figure 5.13 refers to 'GTs of straightness'. The symbol for straightness is a small straight line as is seen in the final column of the table. An example of straightness is seen in Figure | Interpretation |

At the periphery of the section, run-out is not to exceed 0,15 measured normal to the toleranced surface over one revolution

Possible centre-

s I That part of the axis of the ¿1 ~ part that Is toleranced is to lie in a cylindrical tolerance zone of 00,1

Figure 5.17 Examples of straightness and runout geometrical tolerancing

| Interpretation |

At the periphery of the section, run-out is not to exceed 0,15 measured normal to the toleranced surface over one revolution

Possible centre-

I Interpretation!

s I That part of the axis of the ¿1 ~ part that Is toleranced is to lie in a cylindrical tolerance zone of 00,1

Figure 5.17 Examples of straightness and runout geometrical tolerancing 1 Drawing |

1 Drawing |

Toteranctttne. two plana* 0,3 apart symmetrically

Toteranctttne. two plana* 0,3 apart symmetrically

Figure 5.18 Examples of flatness and symmetry geometrical tolerancing

| Interpretation |

The median plane of the tongue is to lie between parallel planes 0,03 apart that are symmetrical 1 about the median plane of the 20 section j Interpretation!

The 20 x 25 surface is to lie between two parallel planes 0,02 apart.

Figure 5.18 Examples of flatness and symmetry geometrical tolerancing

5.16. This refers to the straightness of any part of the outline. A straight line rotating about a fixed point generates a cylindrical surface and a GT referring to this is seen in the example of the headed part in Figure 5.17. This is the straightness of the centre axis of the 20mm diameter section. This is the straightness of the axis of a solid of revolution and in this case the tolerance zone is a cylinder whose diameter is the tolerance value, i.e. in this case lOOum.

Row 2 in the table in Figure 5.13 refers to GTs of 'flatness'. The symbol for flatness is a parallelogram. This symbol meant to represent a 3D flat surface viewed at angle. This GT controls the flatness of a surface. Flatness cannot be related to any other feature and hence there is no datum. An example of this is shown in the inverted tee component in Figure 5.18. In this case, the tolerance zone is the space between two parallel planes, the distance between which is the tolerance value. In the case of the example in Figure 5.18, it is the 20um space between the two 20 x 25 mm planes.

Row 3 in the table in Figure 5.13 refers to GTs of 'circularity'. Circularity can also be called roundness. The symbol for circularity is a circle. Circularity GTs control the deviation of the form of a circle in the plane in which it lies. Circularity cannot be related to any other feature and hence there is no need for a datum. For a solid of revolution (a cylinder, cone or sphere) the circularity GT controls the roundness of any cross-section. This is the annular space between two concentric circles lying in the same plane. The tolerance value is the radial separation between the two circles. In the case of the example in Figure 5.16, it is the roundness deviation of the 10mm diameter cylinder given by the 20um annular ring at any cross section.

Row 4 in the table in Figure 5.13 refers to GTs of 'cylindricity'. The symbol for cylindricity is a circle with two inclined parallel lines touching it on either side. Cylindricity is a combination of roundness, straightness and parallelism. Cylindricity cannot be related to any other feature and hence there is no datum. The cylin-

f Interpretation^

The axis of the right hand 012 cylinder is to be contained in a cylindrical tolerance zone 0,05 diameter that is coaxial with the datum axi s of the left hand 9(20 cylinder f Interpretation^

The axis of the right hand 012 cylinder is to be contained in a cylindrical tolerance zone 0,05 diameter that is coaxial with the datum axi s of the left hand 9(20 cylinder

| Interpretation]

The cu rved su rface of the cylinder is to lie between two coaxial surfaces 0,02 apart radially

I Drawing |

Figure 5.19 Examples of cylindricity and concentricity geometrical tolerancing

I Drawing |

| Interpretation]

The cu rved su rface of the cylinder is to lie between two coaxial surfaces 0,02 apart radially

Figure 5.19 Examples of cylindricity and concentricity geometrical tolerancing Figure 5.20 Examples of parallelism and line profile geometrical tolerancing

[Interpretation!

The actual profile is to be contained within two equidistant lines given by enveloping circles of diameter 0,05mm, the centres of which are situated on the line of the theoretically exact radius.

[Interpretation!

The actual surface is to be contained between two parallel planes 0,05mm apart which are parallel to the datum face C.

Figure 5.20 Examples of parallelism and line profile geometrical tolerancing dricity tolerance zone is the annular space between two coaxial cylinders and the tolerance value is the radial separation of these cylinders. In the case of the example in Figure 5.19 it is the 20umX 15mm annular cylinder of the 20mm diameter section.

Rows 5 and 6 in the table in Figure 5.13 refer to 'line profile' and 'area profile' GTs. The former applies to a line and the latter to an area. The symbol for a line profile GT is an open semicircle and the symbol for an area profile GT is a closed semicircle. These are similar to the straightness (row 1) and flatness (row 2) GTs considered above except that the line and area will be curved in some way or other and defined by some geometric shape. Line or area profiles cannot be related to any other feature and hence there is no datum. The two lines that envelop circles define the line profile tolerance zone. The diameter of these circles is the tolerance value. The centres of the circles are situated on the line having the theoretically exact geometry of the feature. This is to be the case for any section taken parallel to the plane of the projection. An example of a line profile GT is seen in the cam component in Figure 5.20. In this case, the line profile GT means that the profile of any section through the 18mm radius face is to be contained within two equidistant lines given by enveloping circles of 50um diameter about the theoretically exact radius. In the case of area profile GTs, the tolerance zone is limited by two surfaces that envelop spheres. The diameter of these spheres is the same as the tolerance value. The centres of the spheres are situated on the surface having the theoretically exact geometry as the feature referred to.

The remaining rows (7 to 13) in the table in Figure 5.13 are GTs of orientation, location and runout. All these relate to some other feature and hence all require a datum.

Row 7 in the table in Figure 5.13 refers to the first of the GTs that require a datum. These are GTs of 'parallelism'. The symbol for parallelism is two inclined short parallel lines. The toleranced feature may be a line or a surface and the datum feature may be a line or a plane. In general, the tolerance zone is the area between two parallel lines or the space between two parallel planes. These lines or planes are to be parallel to the datum feature. The tolerance value is the distance between the lines or planes. In the case of the cam in Figure 5.20, the left-hand cam face is to be contained within two planes 50um apart, both of which are parallel to the right-hand face.

Row 8 in the table in Figure 5.13 refers to GTs of 'perpendicularity'. Perpendicularity is sometimes referred to as squareness. The symbol for perpendicularity is an inverted capital T. Note that a perpendicularity GT is a particular case of angularity which is referred to in the next row in the table (row 9). With respect to angularity or squareness, the toleranced feature may be a line, a surface or an axis and the datum feature may be a line or a plane. The tolerance zone is the area between two parallel lines, the space between two planes or, as in the case of Figure 5.15, the space within a cylinder perpendicular to the datum face or plane. In the case of the example in Figure 5.15, the dowel will not assemble with the upper plate if its axis is not within the 30um diameter x 15mm cylindrical tolerance zone which is perpendicular to the upper surface (A) in the lower plate.

Row 9 in the table of Figure 5.13 refers to GTs of'angularity'. The symbol for angularity is two short lines that make an angle of approximately 30° with each other. As with perpendicularity, the toleranced feature may be a line, a surface or an axis and the datum feature may be a line or a plane. The tolerance zone is the area between two parallel lines, the space between two planes or the space within a cylinder that is at some defined angle to the datum face or plane. There is no example of angularity in the figures since it is the general case. One could say an example of angularity has been given in Figure 5.15; it just so happens that the angle referred to is 90°.

Row 10 in the table in Figure 5.13 refers to GTs of 'position'. The symbol for position is a 'target' consisting of a circle with vertical and horizontal lines. The position tolerance zone limits the deviation of the position of a feature from a specified true position. The toleranced feature may be a circle, sphere, cylinder, area or space. In the case of Figure 5.14, it is the position of the centre of the hole with respect to the two datum faces 'A' and 'B' of the corner of the plate.

Row 11 in the table in Figure 5.13 refers to GTs of'concentricity'. Concentricity is also referred to as coaxiality. The symbol for concentricity is two small concentric circles. This is a particular case of a positional GT (row 10 in the table) in which both the toleranced feature and the datum feature are circles or cylinders. The tolerance zone limits the deviation of the position of the centre axis of a toleranced feature from its true position. An example of this is shown in Figure 5.19. This refers to the concentricity of the smaller 12mm diameter with respect to the larger 20mm diameter section. The centre axis of the 12mm diameter section is to be contained in a cylinder of 50um diameter that is coaxial with the axis of the 20mm diameter section.

Row 12 in the table in Figure 5.13 refers to GTs of 'symmetry . The symbol for symmetry is a three bar 'equals' sign in which the middle bar is slightly longer than the other two. A symmetry GT is a particular case of a positional GT in which the position of feature is specified by the symmetrical relationship to a datum feature. In general the tolerance zone is the area between two parallel lines or the space between two parallel planes which are symmetrically disposed about a datum feature. The tolerance value is the distance between the lines or planes. In the case of the tee block in the example in Figure 5.18, the median plane of the 10mm wide tongue is to lie between two parallel lines, 30um apart, which are symmetrically placed about the 20mm wide section of the tee block.

Row 13 in the table in Figure 5.13 refers to GTs of 'runout'. The symbol for runout is a short arrow inclined at approximately 45°. Runout GTs are applied to the surface of a solid of revolution. Runout is defined by a measurement taken during one rotation of the component about a specified datum axis. A dial test indicator (DTI) contacting the specified surface typically measures it. The tolerance value is the maximum deviation of the DTI reading as it touches the specified surface at any position along its length. An example of a runout GT is shown in the headed shaft in Figure 5.17. In this case, the centre axis of the largest diameter (30mm) is the axis of rotation. The DTI touches the chamfer at any point along its length and, as the component is rotated, the DTI deviation must be within the 150um-tolerance value.

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### Responses

• marigold gamgee
How should a geometric tolerance be shown on an engineering drawing?
8 years ago
• Claudio Romani
How to represented consentric circle and datum in a engineering drawings?
8 years ago