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在机械加工中我們常要用各种面來定位;在精密另件的加工,常以原始基準面來定位。所謂原始基準面,也就是尺寸从它開始的一个面或點。原始基準面可以是真实的,也可以是假設的。举例來說,如[图一a]所示,假設的原始基準面为點O,而尺寸B需根据它來確定。圖中C面为真实的原始基準面,尺寸A根据它確定。零件装夾時,往往以別的尺寸來决定原始基準面与加工面的位置,这个尺寸叫做基準尺寸。[圖—b]表示銑一平面之例子,此平面須保持尺寸Q,如圖所示,基準尺寸为外徑D及內徑d。
In machining we often use a variety of surface to locate; in precision parts processing, often with the original datum to locate. The so-called primitive plane, which is the size of a face or point from the beginning. The original datum can be real or hypothetical. For example, as shown in Figure 1 a, the assumed original datum is point O, and dimension B is to be determined from it. Figure C is the true original reference plane, the size A is determined by it. When the parts are clamped, the dimensions of the original reference plane and the machined plane are often determined by other dimensions. This dimension is called the reference dimension. [Figure-b] shows an example of milling a plane, the plane to maintain the size Q, as shown, the reference size of the outer diameter D and inner diameter d.