標題: 國外對于公差的定義和解釋（Tolerances: A Brief Introduction） [打印本頁] 作者: 小師妹_c937B 時間: 2016-8-20 16:43 標題: 國外對于公差的定義和解釋（Tolerances: A Brief Introduction） 什么是尺寸公差？為什么工程師要設定公差？公差的大小是由何決定的？下面這篇文章可以給這些問題提供一些解答的思路。
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6 |+ o( \! F" N/ b6 n' R8 qManufacturing Tolerances1 ?. q* p4 v( }
Dimensional tolerances are needed because no two parts are identical, and no single part feature is perfect. To function correctly, there is a limited dimensional “zone” where a part is good enough to work as intended and mate correctly with other parts. Every feature has this “zone”, and is the designer’s job to make sure that it is defined correctly, in the form of a tolerance. - R! D6 h3 i0 w. ^( q ) d4 r3 y$ {; Q9 {+ V: y- AThe tighter the tolerance, the closer the part will be to the design intent. However, problems may start to arise or parts will be too difficult to manufacture when tolerances are made too tight. * A1 z2 @/ J- k, ?8 L$ _! t, v8 p- E- n, b( N9 K: w0 f
Normally dimensional tolerancing is done on a 2-d print or in 3D CAD model. For example, a hole is located from an edge by a certain amount (a linear dimension), plus or minus a smaller amount (the tolerance). On most engineering drawings, there is a general tolerance block indicating the standard tolerance, in addition to the tolerances specified on particular dimensions. 5 z6 e8 A( l" r! _5 v4 y
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Sounds easy, right? In fact, a lot goes into determining the correct tolerance for a feature; the product material, the manufacturing process and capability, the function of the feature, the geometry of mating parts, and the cost restrictions of the product are all factors that influence the tolerance. A clearance hole in a plastic part will not require the same tolerances as a press-fit hole in a metal part. Likewise, a part that is 10 inches long may not require the same tolerance as a part that is .100 inches long.5 N" L7 \& T% K; K7 \; F" X0 n
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+ s1 A: c6 s, P$ j8 y( {4 Q7 S: MWhy does the product material matter? Plastics generally require looser tolerances than metal, due to their tendency to move more with temperature changes. During injection molding, machining and fabrication activities, the material is heated and then cooled, resulting in a final part size different than the initial (warm) measurement would suggest.+ b2 N8 N+ `) `+ X6 u
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Why is manufacturing method a factor? The capability of the manufacturing process will often dictate which tolerances are economically feasible. For a thermoplastic welded part, .001 may be possible to achieve, but only after scrapping a significant quantity of product; .030 is a more typical tolerance for this type of part. If it is vital to have an extremely tight tolerance, changing the manufacturing method (machining a part rather than welding it, for example) may create parts within specification with less expense throughout the supply chain. ! A4 M+ ^+ q3 H' F8 X 6 c0 M# b) K4 o9 n6 B+ P/ E0 O c% D
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% u) K: Z' W3 ]6 uThe function of the feature is important because not all parts need the same tolerance. An external feature that doesn’t mate with another part may have a much larger tolerance than an intricate gear series that requires every component to fit with the others. A clearance hole for a fastener will require a larger tolerance than the location of a threaded hole. A machining or assembly fixture generally needs to have a much tighter tolerance (typically 10 times tighter) than the product which will be machined or assembled.0 M3 ?8 B6 m/ X9 n
/ t0 M, u3 q, o6 g4 U" DDimensional tolerancing is also used to assure functional interchangeability between pieces. Tighter tolerances in general create a higher cost product, so using the loosest tolerance that still enables correct functional operation will provide the lowest cost product. 6 S0 i. B4 w( f# X$ s* U+ b8 q% @; f. [- O 作者: 龍游河畔 時間: 2016-8-20 17:04
對看懂和理解國外圖紙有幫助。作者: albert.tang 時間: 2016-8-21 09:49
的確，影響公差定義的因素很多：+ ?7 K2 L9 H" C/ J0 N1 k
1、產品的材料；2、加工工藝和加工能力；3、零件特征的功能；4、相配合零件的幾何形狀；5、產品加工成本的控制。作者: 軒轅天一 時間: 2016-8-25 22:24
樓主能說一下您是從哪看到這個的 嗎？或者是從哪本書里截取下來的？沒有不進的意思，只是希望能跟多的了解公差，感覺這篇文章說的挺好的，不想國內的教材比較死板作者: 軒轅天一 時間: 2016-8-26 13:28
