大连机械加工的零件都是由若干外貌组成 ，各外貌之间有一定的尺寸和相互位置要求 。零件外貌间的相对位置要求包括两方面：外貌间的距离尺寸精度和相对位置精度（如同轴度、平行度、笔直度和圆跳动等）要求 。研究零件外貌间的相对位置关系离不开基准 ，不明确基准就无法确定零件外貌的位置 �；季推湟话阋庖謇唇� ，就是零件上用以确定其他点、线、面的位置所依据的点、线、面 �；及雌渥饔貌畋� ，可分为设计基准和工艺基准两大类 。
1、设计基准
在零件图上用以确定其他点、线、面的基准 ，称为设计基准 ，就活塞来说 ，设计基准指活塞中心线和销孔中心线 。
2、工艺基准
零件在加工和装配历程中所使用的基准 ，称为工艺基准 。工艺基准按用途差别 ，又分为定位基准、丈量基准和装配基准 。
The parts processed by Dalian Machinery are composed of several surfaces, with certain dimensions and mutual positioning requirements between each surface. The requirements for the relative position between the surfaces of parts include two aspects: the accuracy of distance dimensions between surfaces and the accuracy of relative position (such as coaxiality, parallelism, perpendicularity, and circular runout). The study of the relative positional relationship between the surfaces of parts cannot be separated from a reference, and without a clear reference, the position of the part surface cannot be determined. In its general sense, a benchmark refers to the points, lines, and surfaces used on a component to determine the position of other points, lines, and surfaces. Benchmarks can be divided into two categories based on their different functions: design benchmarks and process benchmarks.
1. Design Basis
The reference used on the part drawing to determine other points, lines, and surfaces is called the design reference. For pistons, the design reference refers to the centerline of the piston and the centerline of the pin hole.
2. Process benchmark
The reference used in the processing and assembly of parts is called the process reference. Process standards are divided into positioning standards, measurement standards, and assembly standards according to different purposes.
