tailieunhanh - Geometric Dimensioning and Tolerancing for Mechanical Design Part 13

Tham khảo tài liệu 'geometric dimensioning and tolerancing for mechanical design part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | A Strategy for Tolerancing Parts Datums B and C not only control location they also control orientation. If the hole in Fig. 14-2 is controlled with the feature control frame in Fig. 14-3 the hole is to be parallel to datum surfaces B and C within the tolerance specified in the feature control frame. The primary datum controls orientation with a minimum of three points of contact with the datum reference frame. The only orientation relationship between the hole and datums B and C is parallelism. Parallelism can be controlled with the primary datum but in only one direction. The secondary datum must make contact with the datum reference frame with a minimum of two points of contact only two points of contact are required to control parallelism in one direction. If the feature control frame in Fig. 14-3 is specified to control the hole in Fig. 14-2 the cylindrical tolerance zone is located from and parallel to datum surfaces B and C establishing both location and orientation for the feature. Typically the front or back surface or both is a mating surface and the hole is required to be perpendicular to one of these surfaces. If that is the case a third datum feature symbol is attached to the more important of the two surfaces front or back. In Fig. 14-4 the back surface has been identified as datum A. Since datum A is specified as the primary datum in the feature control frame and the primary datum controls orientation the cylindrical tolerance zone of the hole is perpendicular to datum A. When applying geometric dimensioning and tolerancing all datums are identified basic location dimensions are included and a feature control frame is specified. n B C Figure 14-3 A position tolerance locating and orienting the feature to datums B and C. Downloaded from Digital Engineering Library @ McGraw-Hill Copyright 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. A .