Dimensioning of Detail Drawings (零件图的尺寸标注)

Dimension is one of the major contents of a part drawing and provides important information for machining and inspection. Therefore, it is a must to represent all dimensions correctly, completely, clearly and reasonably on apart drawing. The previous sections have introduced the requirements for correctness, completeness and clarity. This section focuses on some general issues of dimensioning and some commonly used structures so that the dimensions will not only meet the design requirements but also other requirements to facilitate machining, measurement and inspection. 

尺寸标注是零件图的主要内容之一,是制造加工和检验零件的重要依据。因此,在零件图中,必须正确、完整、清晰、合理地标注零件的全部尺寸。对于正确、完整、清晰的要求,前面相关章节已经作了介绍,本节着重讨论合理标注尺寸的一些基本问题和常见结构的尺寸注法,使所标注的尺寸既能满足设计要求,又符合生产实际,便于加工、测量和检验。

Selection of datumfeatures(尺寸基准的选择)

As mentioned before, a datum provides important reference for dimensioning. In order to produce reasonable dimensions, one should appropriately select datum.When selecting a datum, one must consider part position,function,assembling position setting and inspection.Therefore, datum features should be determined according to the requirements of design,machining conditio ns,and inspection methods.According to the application datums can be divided into design datum and process datum, andthey can be divided into primary datum and secondary datum according to their importance. The design and process datum are discussed below. 

尺寸基准是标注尺寸的起点。要做到合理地标注尺寸,首先必须选择好尺寸基准。在选择尺寸基准时,必须考虑零件在机器或部件中的位置、作用、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,基准应根据设计要求、加工情况和测量方法确定。按基准的用途可分为设计基准、工艺基准等。按基准的主次可分为主要基准和辅助基准。下面只讨论设计基准和工艺基准。

(1) Design datum   This class includes datum planes or datum lines used to locate a given part inside other parts. As shown in Fig. 8-24, select the bottom surface D as the datum for the dimension of the center height 40±0.02mm of the bracket's axlehole.

(1) 设计基准  设计基准是用于确定零件在部件中工作位置的基准面或线。如图8-24所示,标注支架轴孔的中心高40±0.02mm,应以底面D为基准注出。

A shaft is supported by two brackets, therefore, the centers of the two axle holes shall be kept in the same axial line, in order to keep the axial line horizontal. When reflecting the dimensions of the two holes, select surface B as the lengthwise datum to keep the symmetry between the two holes and the axle hole. select the back surface C as the widthwise datum. Thus, B and C are the design datum. 

因为一根轴要用两个支架支承,为了保证轴线的水平位置,两个轴孔的中心应在同一轴线上。标注底板两孔的定位尺寸,长度方向以对称面B为基准,以保证两孔与轴孔的对称关系。宽度方向以其后表面C为基准,故B、C为设计基准。

(2) Process datum    This class includes datum plane or datum line used to machine or measure the part.As shown in Fig. 8-24,the top surface E of the dummy club on the upper position is a process datum, so it can be easily used to measure the depth of the threaded hole. 

(2) 工艺基准  工艺基准是零件在加工、测量时的基准面或线。在图8-24中,上部凸台的顶面E是工艺基准,以此为基准测量螺孔的深度比较方便。

According to their importance, the design and process datum can be divided into primary datum and secondary datum, but there must bea link between the two, like the height of 58mm shown in Fig. 8-24. The part must have a primary datum in each direction—length, width, height, shown as B, C, and D in Fig. 8-24. 

根据基准的重要性,设计基准和工艺基准又分为主要基准和次要基准。两个基准之间应有联系尺寸,如图8-24中的高度尺寸58mm。零件在长、宽、高三个方向都应有一个主要基准,如图8-24中的B、C、D。

 Direct specifications of important dimensions(重要的尺寸直接注出)

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 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、影响零件使用性能的其他尺寸,这些尺寸都应在零件图上直接注出。

The center height of the axle hole shown in Fig. 8-25 is an important dimension. If the dimension is specified according to Fig. 8-25b, the accumulation error of dimensions b and c will make it impossible for the center height of the axle hole to meet design requirements. In addition, for mounting purposes, the distance between the centers of the two holes on the bottom board shown in Fig. 8-25a should also be directly specified.Indirectly determining l by calculating dimensions from e cannot meet matching requirements, as shown in Fig. 8-25b. 

图8-25轴孔的中心高是重要尺寸。若按图8-25b标注,则尺寸c和b的累积误差,会使得孔中心高不能满足设计要求。另外,为装配方便,图8-25a中底板上两孔的中心距离也应直接注出。图8-25b中由标注e间接确定的l则不能满足装配要求。

Avoiding closedimension chains(避免出现封闭的尺寸链)

As shown in Fig. 8-25b, dimensions a, b and c form a close dimension chain. Since a=b+c,if dimension a has a constant error,dimensions for b and c must have very little error, which may lead to difficulties in machining. Therefore, close dimension chains should be avoided and the dimension with less importance, i.e., c in this example, should be removed. 

图8-25b中的尺寸a、b、c构成一个封闭的尺寸链。由于a=b+c,若尺寸a的误差一定,则b、c两个尺寸的误差就要定得很小。这样,就会使加工困难。所以应当避免封闭的尺寸链,例如在此应将一个不重要的尺寸c去掉。

Producing dimensions in accordance with machining sequence       (标注尺寸应符合加工顺序)

1) Dimensions should also be specified in accordance with technical requirement for machining. Fig.8-26 shows the technical requirement for machining sequence of a shaft.

① Determine the location of the undercut according to the dimension 36mm,and then machine the undercut(Fig. 8-26a).

②Turn the external diameter φ18mm and chamfer the end of the shaft (Fig.8-26b). Dimensioning shown in Fig. 8-26c is appropriate, but that of Fig. 8-26d is not.

1)标注尺寸应符合加工工艺要求。图8-26所示轴的加工工艺要求是:

①按尺寸36mm确定退刀槽的位 置,并加工退刀槽(图8-26a)。

②车φ18mm的外圆和轴端倒角(图8-26b)。图8-26c的尺寸标注合理,图8-26d的尺寸标注不合理。

2) Dimensions should be specified for easy inspection. Fig. 8-27 shows the axial dimension of the sleeve. Dimensioning A and C shown in Fig. 8-27a is convenient for measurement, while dimension C in Fig. 8-27b is not. 



2)标注尺寸应便于测量。图8-27所示为套筒轴向尺寸的标注。按图8-27a标注尺寸,A、C便于测量,若按图8-27b标注尺寸C,则不便于测量。


Only one link allowedbetween 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)

As shown in Fig. 8-28a,there are three non-machined surfaces along the height line: B, C,and D,in which only surface B and machined surface A have a link of dimension 8mm,which is appropriate. If dimensions are specified as shown in Fig. 8-28b with three non-machined surfaces linked to A,it would be difficult to ensure the precision of linked dimensions 8mm,34mm and 42mm while machining the surface. 

在图8-28a中,沿铸件的高度方向有三个非加工面B、C和D,其中只有B面与加工面A有尺寸8mm的联系,这是合理的。如果按图8-28 b所示标注尺寸,三个非加工面B、C和D都与加工面A有联系,那么在加工A面时,就很难同时保证三个联系尺寸8mm、34mm和42mm的精度。