Typical product requirement

In the example shown in Figs 23.2 and 23.3, the hole axis must lie within the cylindrical tolerance zone fixed by the true-position dimensions.

Some advantages of using this method are:

1 interpretation is easier, since true boxed dimensions fix the exact positions of details;

2 there are no cumulative tolerances;

3 it permits the use of functional gauges to match the mating part;

4 it can ensure interchangeability without resorting to small position tolerances, required by the coordinate tolerancing system;

5 the tolerancing of complicated components is simplified;

6 positional-tolerance zones can control squareness and parallelism.

The following examples show some typical cases where positional tolerances are applied to engineering drawings.

4 Holes 0 10

Case 1 (Figs 23.4 and 23.5). The axes of the four fixing holes must be contained within cylindrical tolerance zones 0.03 diameter.

4 Holes 0 10

Diameter Tolerances Engineering Drawing
Fig. 23.5 Case 1: Drawing instruction

0.04

Fig. 23.6 Case 2: Product requirement

Fig. 23.4 Case 1: Product requirement

4 holes 0 10

0.04

0.02

Engineering Tolerance Symbols

Fig. 23.7 Case 2: Drawing instruction

Case 2 (Figs 23.6 and 23.7). The axes of the four fixing holes must be contained within rectangular tolerance zones 0.04 x 0.02.

In cases 3 and 4, the perpendicularity and co-axial symbols shown, are constituents of the position characteristic, and could have been indicated with the position symbol equally as well.

Fig. 23.7 Case 2: Drawing instruction

Case 3 Figure 23.8 shows a component where the outside diameter at the upper end is required to be square and coaxial within a combined tolerance zone with face A and diameter B as the primary and secondary datums.

Component Engineering Drawing

Fig. 23.10 Case 5: Product requirement

Case 4 In the component illustrated in Fig. 23.9, the three dimensioned features are required to be perfectly square to the datum face A, and also truly coaxial with each other in the maximum material condition.

Fig. 23.10 Case 5: Product requirement

/ /

///i kl

//

20.01 020.00

40.1 0 40.0

5.018

6 holes 0 5.000 Equispaced

5.018

6 holes 0 5.000 Equispaced

Dimensioning Equispaced Holes

Fig. 23.11 Case 5: Drawing instruction

Case 5 (Figs 23.10 and 23.11). The six boltholes on the flange in Fig. 23.10 must have their centres positioned within six tolerance zones of 00.25 when the bolt holes are at their maximum material condition (i.e. minimum limit of size).

Note in Fig. 23.11 that all the features in the group have the same positional tolerance in relation to each other. This method also limits in all directions the relative displacement of each of the features to each other.

Case 6 (Figs 23.12 and 23.13). The group of holes in Fig. 23.12, dimensioned with a positional tolerance, is also required to be positioned with respect to the datum spigot and the face of the flange.

Note in Fig. 23.13 that the four holes and the spigot are dimensioned at the maximum material condition. It follows that, if any hole is larger than 12.00, it will

4 cylindrical tolerance zones 0.25 dia. in true position relative to Datum face B and Datum axis A

4 cylindrical tolerance zones 0.25 dia. in true position relative to Datum face B and Datum axis A

0 80

Fig. 23.12 Case 6: Product requirement

0 80

Fig. 23.12 Case 6: Product requirement

Datum face B

Axis of datum A

have the effect of increasing the positional tolerance for that hole. If the spigot is machined to less than 50.05, then the positional tolerance for the four holes as a group will also increase.

12.25

4 holes 0 12.00 equispaced

12.25

4 holes 0 12.00 equispaced

Curved Building Floor Plan
Fig. 23.13 Case 6: Drawing instruction

Case 7 Figure 23.14 shows a drawing instruction where the group of equally spaced holes is required to be positioned relative to a coaxial datum bore.

Engineering Drawing Dimensioning Pattern

Fig. 23.14 Case 7

Case 8 Figure 23.15 shows a drawing instruction where a pattern of features is located by positional tolerancing. Each specific requirement is met independently. The product requirement in Fig. 23.16 shows that the axis of each of the four holes is required to lie within a cylindrical tolerance of 00,01. The positional tolerance zones are located in their theoretically exact positions to each other and perpendicular to datum A.

In Fig. 23.17, the axis of each of the four holes must lie within the cylindrical tolerance zone of 00,2 and the cylindrical tolerance must lie perpendicular to datum A and also located in their theoretical exact positions to each other and to datums B and C.

rh

0 0,2

A

B

C

0 0,01

A

Typical Engineering Drawing 308

Fig. 23.15

Note that in product requirement drawings, Fig. 23.16 and Fig. 23.17, simulated datums A, B and C are numbered 1,2 and 3.

Further information may be obtained with reference to BS EN ISO 5458.

Fig. 23.16

0 0,01 k TT"

T T

Li i

15

30

Typical Engineering Drawing 308

Fig. 23.17

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  • grady
    How to draw diameter on a technical drawings?
    8 years ago

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