9.2.3 Converting Dimensions to Equal Bilateral Tolerances, Z% W6 x0 M3 z) f
In Fig. 9-2, there were several dimensions that were toleranced using unilateral tolerances8 d- J9 Y- D, ~/ D9 X( w, j) |
(such as .375 +.000/-.031, 3.019 +.012/-.000 and .438 +.000/-.015) or unequal bilateral tolerances (such
: h" n4 K3 j: e" c% l7 Vas +1.500 +.010/-.004 ). If we look at the length of the shaft, we see that there are several different ways we7 i: Q& C4 J6 ^7 n( d; C" x" t1 K/ y- ]
could have applied the tolerances. Fig. 9-4 shows several ways we can dimension and tolerance the length
* ~0 O9 \# H5 v1 S; w* a9 Eof the shaft to achieve the same upper and lower tolerance limits (3.031/3.019). From a design perspective,+ d; c, ]- b& j. O
all of these methods perform the same function. They give a boundary within which the dimension is
( L* [: }2 h! o7 W! Hacceptable.
1 M9 B+ z) p6 U+ K( c* H; |
8 n/ t& G( \6 y) sThe designer might think that changing the nominal dimension has an effect on the assembly. For' Z5 ]6 K$ i5 L; q% z' f6 V
example, a designer may dimension the part length as 3.019 +.012/-.000. In doing so, the designer may! ~3 h/ |- A- V
falsely think that this will help minimize the gap for Requirement 1. A drawing, however, doesn’t give
) h) K! e8 J9 ]" b( o* s, ypreference to any dimension within the tolerance range.3 C, c1 t. Q/ t
Fig. 9-5 shows what happens to the manufacturing yield if the manufacturer “aims” for the dimension6 V' a0 H. |$ m& ~
stated on the drawing and the process follows the normal distribution. In this example, if the manufacturer# V# X0 X$ _7 [9 H. C* l( t
aimed for 3.019, half of the parts would be outside of the tolerance zone. Since manufacturing shops want0 x1 N4 {4 U* p( o2 ~- ]$ M
to maximize the yield of each dimension, they will aim for the nominal that yields the largest number of6 c% ?" ^3 N: s6 R0 w4 ]4 I/ k6 t
good parts. This helps them minimize their costs. In this example, the manufacturer would aim for 3.025.: T& A: e7 n4 h; s; Q: ]( [$ K+ V+ H
This allows them the highest probability of making good parts. If they aimed for 3.019 or 3.031, half of the: i/ r( o/ I7 Q- I5 c2 B
manufactured parts would be outside the tolerance limits.
6 Y( o; W" H B: T. IAs in the previous example, many manufacturing processes are normally distributed. Therefore, if we
! k) _9 T9 Z+ n# }; p) H6 \put any unilateral, or unequal bilateral tolerances on dimensions, the manufacturer would convert them to
/ S' w4 P) E" w, Ca mean dimension with an equal bilateral tolerance. The steps for converting to an equal bilateral tolerance; |$ ]3 N3 ]: z& o3 v; @* _
follow.
3 q9 f8 J0 X% j6 W) X: p) }
) R, D6 z2 E4 z
, U2 o3 m& ? _1. Convert the dimension with tolerances to an upper limit and a lower limit. (For example, 3.028 +.003/9 w0 H4 W& x0 G. i+ f
-.009 has an upper limit of 3.031 and a lower limit of 3.019.)$ y; L' t+ b% l( F! U, B4 R4 a
2. Subtract the lower limit from the upper limit to get the total tolerance band. (3.031-3.019=.012)( J1 v3 i [* ?
3. Divide the tolerance band by two to get an equal bilateral tolerance. (.012/2=.006)
; w4 E9 @" u" S M4. Add the equal bilateral tolerance to the lower limit to get the mean dimension. (3.019 +.006=3.025).9 ^" w u( n) f; [& f# P7 Q/ }. r
Alternately, you could subtract the equal bilateral tolerance from the upper limit. (3.031-.006=3.025)+ ]+ C* K5 }3 S$ g$ C6 }
2 [2 D: A" ~* e) H4 D) R" mAs a rule, designers should use equal bilateral tolerances. Sometimes, using equal bilateral tolerances
: `. v. x0 P% Y+ j, A: m2 [5 a; pmay force manufacturing to use nonstandard tools. In these cases, we should not use equal bilateral
: F3 k3 k: u, N% j, d6 B9 ctolerances. For example, we would not want to convert a drilled hole diameter from Æ.125 +.005/-.001 to- ~( x% ^4 A' \; H
Æ.127 ±.003. In this case, we want the manufacturer to use a standard Æ.125 drill. If the manufacturer sees
# {9 A4 @; ~! G% x# ZÆ.127 on a drawing, he may think he needs to build a special tool. In the case of drilled holes, we would
/ c9 t; q; U2 u$ y6 B0 [: Dalso want to use an unequal bilateral tolerance because the mean of the drilling process is usually larger
) N1 A6 L* }& t3 r* rthan the standard drill size. These dimensions should have a larger plus tolerance than minus tolerance.
6 b. u6 e* a& Q# XAs we will see later, when we convert dimensions to equal bilateral tolerances, we don’t need to keep
/ k. b# u# N5 P' _! o" O6 y6 gtrack of which tolerances are “positive” and which tolerances are “negative” because the positive toler-! {% p8 {/ {' q* h: t3 y. s
ances are equal to the negative tolerances. This makes the analysis easier. Table 9-1 converts the neces-8 [& {, k. J$ Q+ Z$ w: b3 S
sary dimensions and tolerances to mean dimensions with equal bilateral tolerances.1 d$ G6 N5 @8 ] H. R5 ]3 p0 h3 V
4 T& p6 B( H7 N5 u; E5 J" Q
6 j5 ]. S5 K' M* T, {0 [+ P
"Dimensioning and Tolerancing Handbook, by Paul J. Drake, Jr."
8 ^5 U7 a! [# H0 ]: e8 c |
發(fā)表于 2014-5-23 20:05:11
QQ好友和群
QQ空間
