DimensioningDimensioning 零件图的尺寸标注零件图的尺寸标注 Datum featuresDatum features 尺寸基准的选择尺寸基准的选择 Direct specifications of important dimensionsDirect specifications of important dimensions 重要的尺寸直接注出重要的尺寸直接注出 Avoiding close dimension chainsAvoiding close dimension chains 避免出现封闭的尺寸链避免出现封闭的尺寸链 Producing dimensions in accordance with machining sequenceProducing dimensions in accordance with machining sequence 标注尺寸应符合加工顺序标注尺寸应符合加工顺序 Only one link allowed between machined and non-machined surfacesOnly one link allowed between machined and non-machined surfaces 同一个方向只能有一个非加工面与加工面联系同一个方向只能有一个非加工面与加工面联系 Exercises Exercises 练习题练习题 请点击相应标题显示其内容请点击相应标题显示其内容请点击鼠标左键显示后面内容请点击鼠标左键显示后面内容Datum features 尺寸基准的选择尺寸基准的选择When selecting a datum, one must consider the part position, When selecting a datum, one must consider the part position, function, assembling, position setting and measurement. Therefore, function, assembling, position setting and measurement. Therefore, datum features should be determined according to the designs datum features should be determined according to the designs requirements, machining conditions, and measurements.requirements, machining conditions, and measurements.在选择尺寸基准时在选择尺寸基准时, , 必须考虑零件在机器或部件中位置必须考虑零件在机器或部件中位置、作用作用、零件之间的零件之间的 装配关系以及零件在加工过程中的定位和测量等要求。因此装配关系以及零件在加工过程中的定位和测量等要求。因此, , 基准应根据设计基准应根据设计 要求要求、加工情况和测量方法确定。加工情况和测量方法确定。Design datumDesign datum This class includes datum planes or datum lines used to locate a given part inside other parts. 设计基准设计基准 是用于确定零件是用于确定零件在部件中工作位置的基准面或线。在部件中工作位置的基准面或线。Process datumProcess datum This class includes datum plane or datum line used to machine or measure the parts. 工艺基准工艺基准 是零件在加工、测量时的基准面或线。是零件在加工、测量时的基准面或线。 Determining datum featuresDetermining datum features 尺寸基准的确定尺寸基准的确定 请请 点点 击击 鼠鼠 标标 左左 键键 显显 示示 后后 面面 内内 容容Datum features 尺寸基准的选择尺寸基准的选择This section focuses on some general issues of dimensioning and some commonly This section focuses on some general issues of dimensioning and some commonlyused structures so that the dimensions will not only meet the design requirements but used structures so that the dimensions will not only meet the design requirements but also other requirements to facilitate machining, measurement and inspection.also other requirements to facilitate machining, measurement and inspection. 使所标注的尺寸既能满足设计要求使所标注的尺寸既能满足设计要求, ,又符合生产实际又符合生产实际, , 便于加工、测量和检验。便于加工、测量和检验。Select the bottom surface D as theSelect the bottom surface D as the datum for the dimension of the centerdatum for the dimension of the center height of the brackets axle hole.height of the brackets axle hole. 标注支架轴孔的中心高，应以底标注支架轴孔的中心高，应以底 面面D D为基准为基准The dimensions of the two The dimensions of the two threaded holes select datumthreaded holes select datumface B face B、C.C. 底板两螺孔的底板两螺孔的 定位尺寸定位尺寸, ,以以B B、C C为基准为基准It is easy to It is easy to use datum Euse datum E to measure to measure the depth of thethe depth of thethreaded hole. threaded hole. 以以E E为基准测为基准测 量螺孔的深量螺孔的深 度比较方便度比较方便Process datumProcess datum 工艺基准工艺基准DesignDesign datumdatum 设计基准设计基准: :B B、C C、D DLength datum Length datum长度基准长度基准 B BD Height datum高度基准zzzzWidth datum 宽度基准 C CProcess datum Process datum工艺基准工艺基准: :E EBracketBracket支架支架The dimensions of theThe dimensions of the threaded holes selectthreaded holes selectdatum face C. datum face C. 螺孔的螺孔的 定位尺寸以定位尺寸以C C为基准为基准请请 点点 击击 鼠鼠 标标 左左 键键 显显 示示 后后 面面 内内 容容Direct specifications of important dimensions 重要的尺寸直接注出重要的尺寸直接注出Important dimensions include the ones used to mount other parts, key relative Important dimensions include the ones used to mount other parts, key relative positions and any other information related to the function of the part. All of them positions and any other information related to the function of the part. All of them should be directly specified when drawing parts.should be directly specified when drawing parts. 重要尺寸是指与其他零件相配合的尺寸、重要的相对位置尺寸、影响零件重要尺寸是指与其他零件相配合的尺寸、重要的相对位置尺寸、影响零件 使用性能的其他尺寸，这些尺寸都应在零件图上直接注出。使用性能的其他尺寸，这些尺寸都应在零件图上直接注出。BracketBracket支架支架The center height of the axle hole is an important The center height of the axle hole is an important dimension. So it should be directly specified.dimension. So it should be directly specified.轴孔的中心高是重要尺寸应直接注出轴孔的中心高是重要尺寸应直接注出The distance between theThe distance between theThe distance between the centers of the two The distance between the centers of the two holes on the bottom board should be directly holes on the bottom board should be directly specified for mounting purposes.specified for mounting purposes. 底板上两孔底板上两孔 的中心距离应直接注出以方便装配的中心距离应直接注出以方便装配AppropriateAppropriate 标注合理标注合理InappropriateInappropriate 标注不合理标注不合理As shown in Fig, dimensions As shown in Fig, dimensions A, B ,CA, B ,C and and L L form a close form a close dimension chain. dimension chain. Since Since L=A+B+CL=A+B+C, , if dimension if dimension L has a constant error, L has a c