公差，這兩種標注，表示意義有什么區別的？

wucaida

這兩種標注，表示意義有什么區別的？

HC小丁

/ U7 b3 U! \, p# J基本尺寸不一樣，實際加工尺寸可能是一樣的，但設計尺寸不同

tedwu

前一種標法，尺寸兩側分別加工成型，如銑或磨削兩個側面；后一種標法一般用于一個刀具完成的加工成型，如鉆孔，砂輪磨槽。

hb406863722

對于我們加工的來說：左圖我們會盡量把尺寸做到20.45至20.5之間
                              右圖我們會盡量把尺寸做到20.4至20.45之間
                便于裝配

SYZQ1991

是不是根據入體原則來的？

深奧的幸福

長見識了，我原先以為是個人的習慣問題，

aaxin74

一般是另一個部件的基本尺寸一樣的

茉莉素馨

9.2.3 Converting Dimensions to Equal Bilateral Tolerances
In Fig. 9-2, there were several dimensions that were toleranced using unilateral tolerances
(such as .375 +.000/-.031,  3.019 +.012/-.000 and .438 +.000/-.015) or unequal bilateral tolerances (such
  V$ l  w7 p  k2 M/ t9 {as +1.500 +.010/-.004 ). If we look at the length of the shaft, we see that there are several different ways we
8 p, O- N6 x3 T. K2 {could have applied the tolerances. Fig. 9-4 shows several ways we can dimension and tolerance the length
of the shaft to achieve the same upper and lower tolerance limits (3.031/3.019). From a design perspective,
7 \  n4 d8 Q. m: \$ Oall of these methods perform the same function. They give a boundary within which the dimension is
acceptable.

  K) U. W# ^# z7 @$ dThe designer might think that changing the nominal dimension has an effect on the assembly. For
example, a designer may dimension the part length as 3.019 +.012/-.000. In doing so, the designer may
% G( W, W2 A. g+ j0 ^5 i; v, H- cfalsely think that this will help minimize the gap for Requirement 1. A drawing, however, doesn’t give
preference to any dimension within the tolerance range.
Fig. 9-5 shows what happens to the manufacturing yield if the manufacturer “aims” for the dimension
: w* K8 \4 r" b2 sstated on the drawing and the process follows the normal distribution. In this example, if the manufacturer
9 U3 r; U( y" F* f4 Jaimed for 3.019, half of the parts would be outside of the tolerance zone. Since manufacturing shops want
to maximize the yield of each dimension, they will aim for the nominal that yields the largest number of
good parts. This helps them minimize their costs. In this example, the manufacturer would aim for 3.025.
This allows them the highest probability of making good parts. If they aimed for 3.019 or 3.031, half of the
% \2 J7 d1 ^0 G7 Q, Pmanufactured parts would be outside the tolerance limits.
As in the previous example, many manufacturing processes are normally distributed. Therefore, if we
put any unilateral, or unequal bilateral tolerances on dimensions, the manufacturer would convert them to
a mean dimension with an equal bilateral tolerance. The steps for converting to an equal bilateral tolerance
follow.
% D7 j4 u: `9 `7 x9 p( Q# x

) p  Q" n. j, s0 I$ c& _% p( k5 h& q1. Convert the dimension with tolerances to an upper limit and a lower limit. (For example, 3.028 +.003/
-.009 has an upper limit of 3.031 and a lower limit of 3.019.)
  `$ t0 e  v& z; P2. Subtract the lower limit from the upper limit to get the total tolerance band. (3.031-3.019=.012)
4 S* f1 a) G) W: q: s) s3. Divide the tolerance band by two to get an equal bilateral tolerance. (.012/2=.006)
4. Add the equal bilateral tolerance to the lower limit to get the mean dimension. (3.019 +.006=3.025).
Alternately, you could subtract the equal bilateral tolerance from the upper limit. (3.031-.006=3.025)

As a rule, designers should use equal bilateral tolerances. Sometimes, using equal bilateral tolerances
1 E: l/ e- x. b" U; ?1 imay force manufacturing to use nonstandard tools.  In these cases, we should not use equal bilateral
. U' ~  p, r1 ^4 etolerances.  For example, we would not want to convert a drilled hole diameter from Æ.125 +.005/-.001 to
Æ.127 ±.003. In this case, we want the manufacturer to use a standard Æ.125 drill. If the manufacturer sees
; y( a; r1 l$ v* O! yÆ.127 on a drawing, he may think he needs to build a special tool. In the case of drilled holes, we would
also want to use an unequal bilateral tolerance because the mean of the drilling process is usually larger
than the standard drill size. These dimensions should have a larger plus tolerance than minus tolerance.
5 V% R  y& p! {/ qAs we will see later, when we convert dimensions to equal bilateral tolerances, we don’t need to keep
! U8 Y; L/ U7 W. h6 Q& D' Itrack of which tolerances are “positive” and which tolerances are “negative” because the positive toler-
# w- S  _/ E: `/ ~' a# kances are equal to the negative tolerances. This makes the analysis easier. Table 9-1 converts the neces-
0 u. w2 N7 `/ d: [' `4 tsary dimensions and tolerances to mean dimensions with equal bilateral tolerances.
* ]2 n; b; H6 R5 A, K- l( M
# |* g" h& ~/ }5 ^3 @% s
% b5 Q& i% W4 |" X, b3 C4 }"Dimensioning and Tolerancing Handbook, by Paul J. Drake, Jr."

亂試佳人

我來看看高手們怎么說，我對這些太不熟悉了，這幾天看書貌似看懂了，其實還是不懂

檳城6號

左圖，尺寸盡量避開20.4
右圖，盡量把尺寸避開20.5