公差，这两种标注，表示意义有什么区别的？
这两种标注，表示意义有什么区别的？ 本帖最后由 HC小丁 于 2014-5-23 20:22 编辑基本尺寸不一样，实际加工尺寸可能是一样的，但设计尺寸不同 前一种标法，尺寸两侧分别加工成型，如铣或磨削两个侧面；后一种标法一般用于一个刀具完成的加工成型，如钻孔，砂轮磨槽。 对于我们加工的来说：左图我们会尽量把尺寸做到20.45至20.5之间
右图我们会尽量把尺寸做到20.4至20.45之间
便于装配 是不是根据入体原则来的？ 长见识了，我原先以为是个人的习惯问题， 一般是另一个部件的基本尺寸一样的 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
as +1.500 +.010/-.004 ). If we look at the length of the shaft, we see that there are several different ways we
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,
all of these methods perform the same function. They give a boundary within which the dimension is
acceptable.
The 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
falsely 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
stated on the drawing and the process follows the normal distribution. In this example, if the manufacturer
aimed 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
manufactured 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.
1. 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.)
2. Subtract the lower limit from the upper limit to get the total tolerance band. (3.031-3.019=.012)
3. 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
may force manufacturing to use nonstandard tools.In these cases, we should not use equal bilateral
tolerances.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
Æ.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.
As we will see later, when we convert dimensions to equal bilateral tolerances, we don’t need to keep
track of which tolerances are “positive” and which tolerances are “negative” because the positive toler-
ances are equal to the negative tolerances. This makes the analysis easier. Table 9-1 converts the neces-
sary dimensions and tolerances to mean dimensions with equal bilateral tolerances.
"Dimensioning and Tolerancing Handbook, by Paul J. Drake, Jr."
我来看看高手们怎么说，我对这些太不熟悉了，这几天看书貌似看懂了，其实还是不懂 左图，尺寸尽量避开20.4
右图，尽量把尺寸避开20.5