PGS. TS. NGUYN VN YN

BI GING

DUNG SAI LP GHP

NNG - 2007

50H

7

50n

6

1

MC LC

TrangBi m u

1- Gii thiu 2- Mt s k hiu thng dng trong Dung sai Lp ghp

Chng 1: Nhng vn c bn trong Dung sai Lp ghp 1.1. Kch thc 1.2. Lp ghp 1.3. Tnh i ln chc nng

Chng 2: Dung sai lp ghp tr trn 2.1. Dung sai kch thc 2.1.1. Quy nh v dung sai kch thc 2.1.2. Min dung sai v sai lch c bn 2.1.3. Dung sai ca kch thc 2.2. Quy nh lp ghp

2.2.1. H thng l, h thng trc 2.2.2. Kiu lp u tin 2.2.3. K hiu kiu lp tr trn trn bn v

Chng 3: Dung sai cc yu t hnh hc 3.1. Sai lch hnh dng v v tr tng i gia cc b mt

3.1.1. Biu din dung sai hnh dng v v tr tng i 3.1.2. Chn dung sai hnh dng v v tr tng i

3.2. nhm b mt Chng 4: Dung sai cc lp ghp in hnh

4.1. Dung sai lp ghp then bng, then bn nguyt 4.2. Dung sai lp ghp then hoa 4.3. Dung sai lp ghp ln 4.4. Dung sai lp ghp ren 4.5. Dung sai truyn ng bnh rng

4.5.1. Sai s gia cng bnh rng 4.5.2. chnh xc truyn ng bnh rng 4.5.3. Ghi cp chnh xc v dng khe h mt bn

Chng 5: Chui kch thc v cch ghi kch thc 5.1. Chui kch thc

5.1.1. Cc khi nim c bn 5.1.2. Gii bi ton thun 5.1.3. Gii bi ton nghch theo i ln chc nng hon ton 5.1.4. Gii bi ton nghch theo i ln chc nng khng hon ton

3 4 5 7 8 9 9 9

11 11 12 12 13

15 15 17 18

20 20 22 23 25 25 25 27

29 29 30

30

31

2

5.1.5. Gii bi ton nghch theo phng php tnh xc sut

5.2. Ghi kch thc cho bn v c kh 5.2.1. Nhng nguyn tc ch yu cn m bo khi ghi kch thc 5.2.2. Chn phng n ghi kch thc chiu di cho bn v chi tit my

32 33

33

34

3

BI M U

I- Gii thiu

Dung sai - Lp ghp l mn hc c s trong chng trnh o to k s c kh. Mn hc trang b cho sinh vin kin thc c bn thit k cc chi tit my, cc mi ghp v thit lp cc bn v c kh.

Hc phn Dung sai Lp ghp c 02 n v hc trnh. im hc tp ca sinh vin c nh gi qua bi kim tra gia hc k, bi thi kt thc hc phn v im chuyn cn. Hnh thc thi t lun. im chuyn cn c nh gi qua vic hon thnh cc bi tp trong qu trnh hc v thi gian c mt trn lp ca sinh vin.

Gio trnh s dng hc mn hc ny l sch Dung sai v lp ghp ca tc gi Ninh c Tn, do Nh xut bn Gio dc xut bn nm 2006 (Ti liu [1]).

Trc khi hc mn Dung sai Lp ghp, sinh vin cn c trang b cc kin thc v V k thut c kh, C kh i cng, C s thit k my.

Ni dung ca mn hc Dung sai Lp ghp c trnh by trong 05 chng:

Chng1: Nhng vn c bn trong Dung sai Lp ghp Trnh by khi nim sai s gia cng v dung sai. Nhng nguyn nhn

ch yu dn n c sai s trong qu trnh gia cng ct gt. p dng l thuyt Xc sut Thng k kho st kch thc gia cng. Trn c s s chn c phng php gia cng hiu qu nht, hoc c th a ra phng n iu chnh my hp l hn ch ph phm.

Trnh by cc khi nim v lp ghp: mi ghp c di, mi ghp c khe h, kiu lp cht, kiu lp lng, kiu lp trung gian.

Trong chng ny cn cp n vn tnh i ln chc nng ca chi tit my. Chng 2: Dung sai lp ghp tr trn

Nghin cu dung sai kch thc dng trc v kch thc dng l ca chi tit my. Chn sai lch gii hn trn, sai lch gii hn di ca kch thc trc, kch thc l, m bo mi ghp c c tnh theo yu cu. Cch ghi kiu lp trn bn v k thut.

Chng 3: Dung sai cc yu t hnh hc ca chi tit my Trnh by cc sai lch v hnh dng: phng ca mt phng; thng

ca ng thng; trn, cn, tr ca mt tr. Cc sai lch v v tr tng quan: song song, vung gc, ng tm, giao nhau, i

4

xng, o mt u, o hng knh. Sai lch cht lng b mt, nhm ca b mt. Hng dn cch chn gi tr cho php ca cc sai lch trn, v cch biu din cc sai lch, gi tr cho php ca sai lch trn bn v k thut.

Chng 4: Dung sai cc lp ghp in hnh Tp trung nghin cu mi ghp then bng, mi ghp then hoa, mi ghp

ln ln trc v ln bc; mi ghp ren. Hng dn cch chn kiu lp ghp, v biu din kiu lp trn bn v. Trong chng ny cn quan tm n dung sai truyn ng bnh rng. Nguyn nhn dn n cc sai s, cc thng s nh gi sai lch trong truyn ng bnh rng, cch chn gi tr sai lch cho php, v cch biu din dung sai truyn ng bnh rng trn bn v.

Chng 5: Chui kch thc v cch ghi kch thc Trnh by cch ghi kch thc trn bn v lp b phn my v bn v

chi tit my, m bo thun li cho vic gia cng. Thit lp cc chui kch thc v tnh ton dung sai cho cc khu trong chui kch thc. II- Mt s k hiu thng dng trong mn hc Dung sai Lp ghp

Ai l kch thc ca khu th i trong chui kch thc d l kch thc ca khu dng trc D l kch thc ca khu dng l ei l sai lch di ca kch thc dng trc es l sai lch trn ca kch thc dng trc EI l sai lch di ca kch thc dng l ES l sai lch trn ca kch thc dng l IT l dung sai ca kch thc N l di ca mi ghp S l khe h ca m ghp T l sai lch ca kch thc.

5

CHNG 1

NHNG VN C BN TRONG DUNG SAI LP GHP 1.1. Kch thc

- Kch thc ca chi tit my l khong cch gia hai im, hai ng, hoc hai mt thuc chi tit my, n v o dng trn bn v c kh l mm. - Kch thc c phn thnh 02 nhm: + Kch thc nhm trc, khi ct gt thm s lm gim kch thc, k hiu l d (Hnh 1.1). + Kch thc nhm l, khi ct gt thm s lm tng kch thc, k hiu l D (Hnh 1.2). - Gi tr ca kch thc c xc nh bng cch o. S dng dng c o chnh xc, phng php o thch hp, o nhiu ln s nhn c kt qu o vi chnh xc cao. - Sai s khi gia cng chi tit my: Khi gia cng khng th t c gi tr ng nh mong mun, do c cc nguyn nhn sau: + My khng chnh xc. + Dao khng chnh xc. + G t khng chnh xc. + H thng cng ngh: MGDC b bin dng.

+ Rung ng do lc ct thay i. + Gin n khng u do nhit thay i.

V d: cn gia cng 100 chi tit trc c ng knh 20 mm, ta s nhn c cc chi tit c kch thc ng knh dao ng trong khong 20,03 mm n 19,99 mm.

Nh vy lot chi tit c kch thc ddn = 20 mm

dmax = 20,03 mm, dmin = 19,99 mm,

dmax c gi l kch thc gii hn trn. dmin l kch thc gii hn di. dmax - dmin c gi l khong phn b kch thc (hay sai lch ca kch thc), k hiu l T. ddn l kch thc danh ngha. (dmax + dmin)/2 l kch thc trung bnh, k hiu l dm.

Sai lch kch thc ln, tc l gia cng c chnh xc thp.

20

100

Hnh 1.1: Cc kch thc dng trc

15

22

Hnh 1.2: Cc kch thc dng l

d min

ddn

d m

d max

Hnh 1.3: Sai lch ca kch thc gia cng

6

- Ngoi ra, kch thc cn c phn thnh nhng loi sau: + Kch thc thc, + Kch thc o c, + Kch thc thc theo k thut (kch thc o c + dung sai ca dng c o. + Kch thc danh ngha, + Kch thc gii hn,

- Biu din kch thc v sai lch kch thc trn s (Hnh 1.3).

nh gi mc chnh xc gia cng chi tit my, ngi ta tin hnh gia cng mt lot N chi tit (s lng N khng t hn 60 chic), sau o, xc nh cc kch thc gii hn, xc nh tn sut xut hin cc gi tr kch thc, v ng cong phn b tn sut, v ng cong phn b mt xc sut. - ng cong phn b tn sut Hnh 1.4. Trong di l gi tr kch thc nm trong khong dmin dmax), di/N l tn sut xut hin kch thc th i (Khi N ln c th xem y l xc sut p ca kch thc th i).

- ng cong phn b mt xc sut ca kch thc gia cng, phn b chun Gauss (Hnh 1.5). Trong x = di - dm , y = dp/dx, l sai lch bnh phng trung bnh

2 = NxNi

i /1

2=

Theo l thuyt xc sut, c 99,73% kch thc ca lot chi tit nm trong khong xmax - xmin = 6.

dm dmax

Hnh 1.4: S phn b tn sut kch thc

Tn sut di/N

Kch thc

dminMin phn b kch thc

dm 6

Hnh 1.5: S phn b mt xc sut kch thc

Trung tm phn b y

x

7

Khi thit k, m bo cho chi tit my c kh nng lm vic, ngi thit k phi xc nh sai lch cho php ca kch thc d (cn gi l dung sai, k hiu l IT), n nh kch thc ln nht v nh nht c th chp nhn: dmax = d + es dmin = d + ei IT = es - ei es gi l gi tr sai lch trn cho php ei l gi tr sai lch di cho php

Nu ta chn phng php gia cng khng hp l, min phn b 6 khng nm trong min dung sai IT, s c ph phm. Nu min phn b 6 qu nh hn IT, c ngha l chng ta gia cng chnh xc cao hn so vi yu cu, lm tng gi thnh ca chi tit. Phng php gia cng hp l nht (m bo khng c ph phm, v gi gia cng r), khi m 6 = IT v min phn b kch thc trng vi min dung sai.

Dng xc sut kho st sai s gia cng kch thc ch c th s dng trong trng hp sn xut hng lot. Sau khi kho st lot chi tit gia cng u tin, ta c th chn c phng php gia cng thch hp hn, hoc iu chnh my loi b cc chi tit ph phm.

1.2. Lp ghp - Mi ghp: Lp chi tit trc vo chi tit bc s c mt mi ghp. Trn hnh 1.6 biu din mi ghp tr trn.

+ Mi ghp c di gi l mi ghp cht. di k hiu l N, N = d - D. + Mi ghp c khe h gi l mi ghp lng. Khe h k hiu l S, S = D - d

- Kiu lp: Khi xem xt cc mi ghp ghp ca lot chi tit bc A vi lot chi tit trc B, ngi ta phn bit:

+ Kiu lp cht: Tt c cc mi ghp ca lot chi tit lp ghp vi nhau u c di. Ly bt c chi tit bc no lp vi chi tit trc u c mi ghp cht. + Kiu lp lng: Tt c cc mi ghp ca lot chi tit lp ghp vi nhau u c khe h. + Kiu lp trung gian: Mt s mi ghp trong lot c di, s khc c khe h. Ly mt chi tit bc lp vi mt trc bt k s c mi ghp c th cht, cng c th lng.

d

D

Hnh 1.6: Mi ghp tr trn

8

Biu din kiu lp, min dung sai ca kch thc trc, ca kch thc l trn Hnh 1.7. Kch thc trc, l c sai lch, nn di v khe h cng c sai lch.

Sai lch ca di v khe h: Nmax = dmax - Dmin = es - EI Nmin = dmim - Dmax = ei - ES Smax = Dmax - dmin = ES - ei Smin = Dmim - dmax = EI - es

Cn c vo yu cu lm vic ca mi ghp, khi thit k chng ta phi chn cc gi tr cho php [Nmax], [Nmin] hoc [Smax], [Smin]. Mi ghp t yu cu, khi di hoc khe h nm trong gii hn cho php.

1.3. Tnh i ln chc nng - Yu cu ca kiu lp: khe h nm trong khong t [Smax] n [Smin]. Nu lot chi tit trc B lp vi lot chi tit l A c Smax [Smax] v Smin [Smin]. C ngha l ta ly bt c chi tit trc no trong lot A lp vi mt chi tit trong lot B u c mi ghp tho mn yu cu. Ta ni cc chi tit my trong lot A v B c tnh i ln chc nng hon ton. Chng c th thay th cho nhau, m vn m bo chc nng lm vic.

- Nu Smax > [Smax] hoc Smin < [Smin], lc cc chi tit trong lot A v B khng c tnh i ln chc nng. Tc l mt mi ghp ang tho mn yu cu, nu thay th mt chi tit trc khc lp vo bc ang c, c th nhn c mt mi ghp khng t yu cu.

- Khi thit k, ngi ta c gng chn dung sai kch thc ca chi tit my mt cch hp l, chi tit my tho mn tnh i ln chc nng.

- Trong sn xut hng lot, nu mi chi tit ca lot u t tnh i ln chc nng th lot chi tit t tnh i ln chc nng hon ton. Nu c mt hoc mt s chi tit trong lot khng t tnh i ln chc nng, th lot chi tit t tnh i ln chc nng khng hon ton.

Hnh 1.7: Min dung sai ca kch thc trc, kch thc l

Sai lch ca kch thc l

Sai lch ca kch thc truc

Kc

h th

c da

nh n

gha

es

ei ES

EI

9

CHNG 2

DUNG SAI LP GHP TR TRN

2.1. Dung sai kch thc 2.1.1. Quy nh v dung sai kch thc

Dung sai ca kch thc c chn tu thuc vo chnh xc yu cu v ln ca kch thc

T = ai Trong i l n v dung sai. n v dung sai c th tnh theo cng thc:

i = 0,45 3 d + 0,001d i vi kch thc t 1 dn 500 mm i = 0,004d + 2,1 i vi kch thc trn 500 n 3150 mm Tuy nhin, n gii cho vic lp tiu chun, TCVN quy nh gi tr i

c th cho cc khong kch thc. Kch thc t 1 n 500 mm c th phn thnh 13 25 khong, tu theo c tnh ca tng loi lp ghp. a: l h s ph thuc vo yu cu mc chnh xc ca kch thc.

TCVN 2244-1999 c quy nh 20 cp chnh xc kch thc: cp 01 ; 0 ; 1 ; 2 ; 3 ; .... ; 18. Trong cp 01 chnh xc cao nht, cp 18 t chnh xc nht. Cp 5 n cp 11 c dng trong thit k cc my thng dng. Gi tr ca a c th chn nh sau:

Cp chnh xc 4 5 6 7 8 9 10 11 12 Gi tr ca a 5 7 10 16 25 40 64 100 160 Dung sai kch thc do TCVN quy nh c k hiu l IT. Gi tr ca IT c chn theo Bng 4.2 trang 24 ti liu [1]. Theo Bng 4.2, ng vi mi cp chnh xc v tng khong kch thc, kch thc cng ln, chnh xc cng thp, th dung sai cng ln.

2.1.2. Min dung sai v sai lch c bn V d, to mi ghp tr trn t chi

tit trc v bc (Hnh 2.1) c kch thc danh ngha d = D = 60 mm, chnh xc cp 7. Tra bng 4.2 ta c

ITd = ITD = 30m = 0,03mm. Mun c mi ghp cht ta phi b tr min dung sai ca trc mn pha trn min dung

60

Hnh 2.1: Mi ghp tr trn

10

sai ca l (Hnh 2.2). Mun c mi ghp lng ta phi b tr min dung sai ca trc nm pha di min dung sai ca l (Hnh 2.3).

TCVN quy nh 27 cch b tr min dung sai (so vi kch thc danh ngha) cho kch trc, v 27 min dung sai cho kch thc l.

Mim dung sai ca kch thc trc c k hiu bng ch in thng: a, b, c, cd, d, e, ef, f, g, h, j, js, k, m, n, p, r, s, t, u, v, x, y, z, za, zb, zc

Min dung sai ca kch thc l c k hiu bng ch in hoa: A, B, C, CD, D, E, EF, F, G, H, J, JS, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC V tr ca min dung sai c xc nh bi sai lch c bn (Hnh 2.4).

ITd

ITD

d dn

Hnh 2.2: Mi ghp cht

ITd

ITD

d dn

Hnh 2.3: Mi ghp lng

R h

H

F

TD

Ddn

, ddn

Hnh 2.4: H thng sai lch c bn theo TCVN

A

TD

B

TDTD

TD

J S

Td

j s Td

Td

a

f

Td b

Td

TD

Td

r

Td z

c Z

C

TD

11

Tr s ca cc sai lch c bn ca trc c cho trong Bng 4.3 trang 27; tr s ca cc sai lch c bn ca l c cho trong Bng 4.4 trang 29 ti liu [1].

2.1.3. Dung sai ca kch thc Kch thc trc, kch thc l v dung sai ca n c ghi trn bn v nh sau (Hnh 2.5): - Ghi kch thc danh ngha, sai lch c bn v chnh xc kch thc

V d: 50n7 ; 150K7; 230h8 ; 130JS7 - Ghi kch thc danh ngha, km theo sai lch trn v sai lch di V d: 042,0 017,050++ ; 012,0 028,0150+ ; 230-0,072 ; 02,0130 Nu sai lch trn, hoc sai lch di bng 0 th khng ghi s 0; nu sai lch trn v sai lch di c gi tr tuyt i bng nhau, th ghi ln pha trn cng vi du . - Hoc ghi c sai lch c bn, cp chnh xc v sai lch trn, sai lch di )(750 042,0 017,0++ n ; 230h8 (-0,072)

Tr s sai lch gii hn ca kch thc l c cho trong Bng 1 trang 133 ti liu [1]. Tr s sai lch gii hn ca kch thc trc c cho trong Bng 2 trang 138 ti liu [1].

Trc khi ghi dung sai kch thc ln bn v, ngi thit k phi tnh ton, hoc la chn cp chnh xc cho kch thc, dng sai lch c bn, hoc cc sai lch gii hn ca kch thc. Cc sai lch gii hn c th nhn c thng qua bi ton gii chui kch thc, hoc xut pht t yu cu ca mi ghp, hoc ly theo kinh nghim c thng k trong cc bng tra. Khi chn cp chnh xc gia cng cc kch thc cng cn n kh nng t c chnh xc gia cng ca my, ca cc nguyn cng gia cng trn my.

2.2. Quy nh lp ghp: p ng yu cu sn xut, ngi ta phi quy nh cc kiu lp vi

nhng c tnh khc nhau. Yu cu tt c cc mi ghp phi c di (chn kiu lp cht), yu cu tt c cc mi ghp c khe (chn kiu lp lng); hoc yu cu mi ghp c di cng c m c khe h nh cng c (chn kiu lp trung gian).

50n

7

1300,02

Hnh 2.5: Ghi dung sai

12

2.2.1. H thng l, h thng trc Khi lp nhiu trc trn mt chi tit bc, ngi ta dng h thng l c bn. Min dung sai ca l c nh, c sai lch c bn kiu H. c c tnh khc nhau ca cc mi ghp, ta thay i min dung sai ca kch thc trc (Hnh 2.6).

Khi lp nhiu bc trn mt chi tit trc, ngi ta dng h thng trc c bn. Min dung sai ca trc c nh, c sai lch c bn kiu h. c c tnh khc nhau ca cc mi ghp, ta thay i min dung sai ca kch thc l (Hnh 2.7).

2.2.2. Kiu lp tiu chun, kiu lp u tin

Khi thit k mt mi ghp vi c tnh cho trc, chng ta c th chn mt min dung sai ca trc kt hp vi mt min dung sai ca l c di, hoc khe h ng theo yu cu. Song thun tin cho vic gia cng, o kim tra kch thc ca chi tit trc v l, chng ta nn chn cc kiu lp tiu chun c TCVN quy nh. Cc kiu lp tiu chun theo h thng l c cho trn Bng 4.5 trang 32 ti liu [1]. Cc kiu lp tiu chun theo h thng trc c cho trn Bng 4.6 trang 33 ti liu [1]. Cc kiu lp trong khung bi en l kiu lp u tin.

Nh vy, vi cc gi tr gii hn ca khe h, hoc di cho trc, chng ta la chn cc kiu lp u tin trc; nu khng tm c kiu lp tho mn c tnh cho ca mi ghp th chn cc kiu lp khng khng u tin trong tiu chun; nu cng khng tho mn c tnh cho ca mi ghp, ta c th dng kiu lp kt hp gia h thng l v h thng trc theo

Td

TD1

TD2

TD3

d dn

Hnh 2.7: Lp theo h thng trc c bn

Hnh 2.8: Ghi mi ghp tr trn

6087

eH

TD

Td1

Td2

Td3

Ddn

Hnh 2.6: Lp theo h thng l c bn

13

Bng 2- 1: i chiu tiu chun TCVN 2245-77 vi tiu chun lp ghp c Mim dung sai ca trc cho kch thc t 1 mm n 500 mm

TCVN c

TCVN 2245-77

TCVN c

TCVN 2245-77

TCVN c

TCVN 2245-77

TCVN c TCVN 2245-77

T41 T31 T21 T11

L11 = B1 L21 L31 T4 T3

n5 m5 k5 jS5 h5 g5 f6 n6 m6

T2 T1

L1 = B L2 L3 L4 L5 L6 T43

k6 jS6 h6 g6 f7 e8 d8 c8 n7

T33 T23 T13

L13 = B3 L33

L14 = B4 L34 L54

L15 = B5

m7 k7 jS7 h7 f8

h8, h9 f9, e9

d9 jS10

L16 = B6 L36 L46 L56

L17 = B7 L37

L18 = B8 L19 = B9 L110=B10

h11 d11

b11, c11 a11, b11

h12 b12 h14 h15 h16

tiu chun; nu vn khng tm c kiu lp tho mn c tnh cho ca mi ghp, lc ta c th kt hp min dung sai bt k ca trc v l c mi ghp c c tnh ng theo yu cu.

2.2.3. Ghi k hiu kiu lp tr trn trn bn v Sau khi xc nh c tnh ca mi ghp, chn chnh xc cho kch

thc trc, kch thc l, chn h thng lp ghp, chn dng sai lch c bn (hay min phn b dung sai) cho kch thc trc, kch thc l, chng ta ghi kiu lp ln bn v.

Kiu lp bao gm kch thc danh ngha, sai lch c bn v cp chnh xc ca kch thc l ghi trn t s, sai lch c bn v chnh xc ca trc ghi di mu s (Hnh 2.8).

Trong nhiu ti liu thit k, cng nh trong cc tp bn v chi tit my, cc kiu lp c ghi theo tiu chun TCVN ban hnh nm 1963 (TCVN c). V d: - Kiu lp cht C1, C2, C3, C4, - Kiu lp trung gian T1, T2, T33, T41 , - Kiu lp lng L1, L2, L3, L4, L5, L6.

n nay cc tiu chun ny c thay th bng TCVN 2245-77, trong tiu chun mi, cch ghi kiu lp c quy nh khc vi tiu chun c. ngi thit k tin s dng cc ti liu c trong vic thit lp cc bn v, trn Bng 2-1 v Bng 2-2 ghi s chuyn i t tiu chun c sang tiu chun mi ca mt s kiu lp thng dng.

14

V d: - Kiu lp ghi theo kiu c

2TA , c thay th bng kiu ghi mi

67

kH ,

- Kiu lp BL2 , c thay th bng kiu ghi mi

67

hG ,

Bng 2-2: i chiu tiu chun TCVN 2245-77 vi tiu chun lp ghp c Mim dung sai ca l cho kch thc t 1 mm n 500 mm

TCVN c

TCVN 2245-77

TCVN c

TCVN 2245-77

TCVN c

TCVN 2245-77

TCVN c

TCVN 2245-77

T41 T31 T21 T11

L11 = A1 L21 L31 T4 T3

N6 M6 K6 JS6 H6 G6 F7 N7 M7

T2 T1

L1 = A L2 L3 L4 L5 T43 T33

K7 JS7 H7 G7

F7, F8 E8 D8 N8 M8

T23 T13

L13 = A3 L14 = A4

L34 L54

L15 = A5 L16 = A6

L36

K8 JS8 H8

H8, H9 F9, E9

D9, F10 H10 H11 D11

L46 L56

L17 = A7 L37 A8 A9 A10

B11, C11 A11, B11

H12 B12 H14 H15 H16

15

CHNG 3

DUNG SAI CC YU T HNH HC

3.1. Sai lch hnh dng v v tr tng i ca cc b mt Trong qu trnh gia cng c, khng nhng kch thc ca chi tit my

c sai s, m hnh dng hnh hc ca b mt, v tr tng quan gia cc b mt cng c sai lch so vi l thuyt.

Nguyn nhn c cc sai lch trn cng do h thng cng ngh MDGC bin dng, do my khng chnh xc, lc ct khng n nh, nhit thay i, dng c ct khng chnh xc. Cc sai lch hnh dng thng gp:

- phng ca mt phng - thng ca ng thng - tr ca mt tr - trn ca mt tr - cn ( lm, trng) ca mt tr

Cc sai lch v tr tng i thng gp: - song song ca hai b mt - vung gc ca b mt - ng tm gia hai mt tr - i xng ga hai b mt - giao nhau gia hai ng - o hng knh ca mt tr so vi ng tm - o mt u so vi ng tm

3.1.1. Biu din dung sai hnh dng v v tr tng i trn bn v

Dung sai hnh dng v v tr tng i ca cc b mt chi tit my c ghi trn bn v nh Hnh 3.1.

Dung sai hnh dng c biu din trong hai hnh ch nht. th nht ghi k hiu dng sai lch, th hai ghi gi tr sai lch ln nht cho php:

- Dung sai thng, c tnh theo mt on thng no , v c k hiu l " ", gi tr sai lch ghi trn bn v c n v l mm. - Dung sai phng, tnh cho mt phng c gii hn bi mt ng bao, c k hiu l " ", gi tr sai lch c ghi trn bn v l mm.

16

Hnh 3.1: Sai lch hnh dng v v tr tng quan ca cc b mt

A

0,06 AB

B

0,03 0,05 0,03 C 0,05 C

C

0,05

0,02

0,1/100 A

A

0,05

0,06 B

B

0,06 A

0,2 B

B A

- Dung sai trn, tnh cho mt ng trn, k hiu l " ", gi tr sai lch ghi trn bn v c n v l mm. - Dung sai cn ( lm, trng) c tnh trn mt mt ct dc trc ca mt tr, c k hiu l " ", gi tr sai lch ghi trn bn v c n v l mm. - Dung sai tr, tnh cho mt tr c di nht nh, c k hiu l " ", gi tr ca sai lch ghi trn bn v l mm.

Dung sai v tr tng i c biu din trong ba hnh ch nht. th nht ghi k hiu dng sai lch, th hai ghi gi tr sai lch ln nht cho php, th ba ghi mt hoc ng chun dng so snh. Cc dung sai v tr tng i gm c:

- Dung sai song song, quy nh song song gia 2 mt phng, ng tm vi mt phng, gia 2 ng tm vi nhau, c k hiu l "// ", gi tr sai lch song song ghi trn bn v c n v l mm.

17

- Dung sai vung gc, quy nh vung gc gia hai mt phng vi nhau, gia hai ng tm vi nhau, gia ng tm vi mt phng, trn bn v k hiu l " ", gi tr vung gc ghi trn bn v c n v l mm. - Dung sai ng tm, quy nh s lch ng tm cho php ca hai mt tr theo l thuyt c chung ng tm, k hiu l " ", gi tr ng tm ghi trn bn v c n v l mm. - Dung sai i xng, quy nh sai lch cho php ca hai mt i xng thc ca hai phn t c chung mt i xng, k hiu l " ", gi tr ghi trn bn v c n v l mm. - Dung sai ct nhau, quy nh khong cch nhau cho php gia hai ng tm ca hai mt tr, theo danh ngha n phi ct nhau, k hiu l " ", gi tr ghi trn bn v l mm. - Dung sai o hng knh - cn gi l o hng tm - ca mt mt tr i vi ng tm danh ngha hoc ng tm ca mt tr chun no , c o trn mt mt ct ngang; k hiu l " ", gi tr ghi trn bn v c n v l mm. - Dung sai o mt u - cn gi l o mt mt - i vi ng tm danh ngha hoc ng tm ca mt tr chun no , c o trn ng bin ca mt u; k hiu l " ", gi tr ghi trn bn v c n v l mm.

3.1.2. Chn gi tr dung sai hnh dng v v tr tng i

Trc khi ghi dung sai hnh dng v v tr tng quan ln bn v, chng ta phi chn gi tr dung sai hp l cho cc b mt v v tr tng i gia cc b mt.

Dung sai hnh dng v v tr tng i ca cc b mt c chn tu thuc vo chnh xc yu cu ca chi tit my, m bo chi tit my c kh nng lm vic. ng thi cng phi m bo c th d dng gia cng ch to chi tit my.

Tiu chun Vit Nam TCVN 384-1993 quy nh 16 cp chnh xc cho yu t hnh dng v v tr tng i ca b mt. Cp chnh xc ca yu t hnh dng c chn tng ng vi cp chnh xc kch thc ca chi tit my, theo Bng 5.2 trang 76 ti liu [1]. Cp chnh xc ca yu t hnh dng v v tr tng quan tng ng vi cc nguyn cng ct gt c cho trong Bng 5.5 trang 80 ti liu [1]. Sai lch gii hn ca cc yu t hnh dng v v tr tng quan c chn theo Bng 8 n Bng 11 Ph lc 2 trang 147 ti liu [1].

18

3.2. nhm b mt

B mt chi tit my sau khi gia cng khng bng phng mt cch l tng, m c nhng mp m (Hnh 3.2).

Nhng mp m ny l do lp b mt b bin dng do khi ct gt, l do nh hng ca cc chn ng khi ct, l vt ca li ct li trn b mt gia cng, vv..

Theo tiu chun c ca Vit Nam (v tiu chun OCT ca Lin X c) nhm b mt c chia lm 14 cp (cp 1 n cp 14), cp 1 l nhm nht v cp 14 l bng nht. Trn bn v nhm c k hiu 1 n 14 - hoa 1 n hoa 14 (Hnh 3.3).

Theo quy nh mi ca Vit Nam TCVN 2511-1995, nhm trn b mt chi tit my c nh gi bng mt trong hai thng s Ra hoc Rz (Hnh 3.4). Nhng s c km ch Rz l gi tr ca Rz, nhng s khng km ch l gi tr ca Ra, n v l m.

- Sai lch trung bnh s hc ca prfil Ra, c o bng m. L trung bnh s hc cc gi tr tuyt i ca prfil (hi) trong khong chiu di chun (L). Ch tiu Ra thng dng nh gi nhm b mt cp 6 n cp 12.

- Chiu cao trung bnh ca prfil Rz, m. L tr s trung bnh ca tng cc gi tr tuyt i ca chiu cao 5 nh cao nht (ti) v chiu su ca 5 y thp nht (ki) ca prfil trong khong chiu di chun (L). Ch tiu Rz thng dng nh gi nhm b mt cp 1 n cp 5 v cp 13, 14.

tin s dng cc ti liu thit k c xut bn trc y, trn Bng 3-1 ghi gi tr ca Ra v Rz tng ng vi 14 cp nhm.

6 5

Hnh 3.3: Ghi nhm theo quy nh c

2,1 Rz15

Hnh 3.4: Ghi nhm theo quy nh mi

L

ki

hi

Hnh 3.2: Bin dng ca b mt chi tit

ti

19

Cp nhm ca b mt chi tit my cng c chn tng ng vi cp chnh xc kch thc, cp chnh xc hnh dng ca chi tit my. ng thi cng phi tng ng vi kh nng gia cng ca cc nguyn cng gia cng c.

Gi tr Ra v Rz ca cc b mt c chn theo Bng 5.5 trang 80 v Bng 5.6 trang 82 ti liu [1]. Cp

nhm Ra, m Rz, m 1 2 3 4 5 6 7 8 9 10 11 12 13 14

T 80,0 40,0 di 40 20 di 20 10 di 10 5 di 5 2,5 di 2,5 1,25 di 1,25 0,63 di 0,63 0,32 di 0,32 0,16 di 0,16 0,08 di 0,08 0,04 di 0,04 0,02 di 0,02 0,01 di 0,01 0,005

T 320 160 di 160 80 di 80 40 di 40 20 di 20 10 di 10 6,3 di 6,3 3,2 di 3,2 1,6 di 1,6 0,8 di 0,8 0,4 di 0,4 0,2 di 0,2 0,1 di 0,1 0,05 di 0,05 0,025

Bng 3.1: nhm b mt chi tit my

20

CHNG 4

DUNG SAI CC LP GHP IN HNH 4.1. Dung sai lp ghp then bng, then bn nguyt

Mi ghp then bng, then bn nguyt thng c dng lp bnh rng, bnh vt, bnh ai, a xch, khp ni trn trc.

Then l mt thanh hnh tr c tit din ch nht (i vi then bng) v hnh bn nguyt (i vi then bn nguyt). Trn trc v trn bc c lm cc rnh hnh tr c tit din tng ng vi tit din ca then.

Mi ghp then bng trn trc bao gm hai mi ghp tr trn: Mi ghp gia then vi rnh trn trc v mi ghp gia then vi rnh trn bc (Hnh 4.1).

Min dung sai ca kch thc b ca then, ca rnh then trn trc v trn bc c chn theo Bng 4.11 trang 39 ti liu [1]. Dung sai chiu rng b ca then dng kiu h9. Dung sai chiu rng b ca rnh trn trc thng dng kiu H9, N9, P9. Dung sai chiu rng b ca rnh trn bc thng dng kiu D10, JS9, P9.

Kiu lp ca mi ghp then c th chn nh sau: - Trong sn xut hng lot ln, then lp vi trc theo kiu N9/h9, v lp vi bc theo kiu JS9/h9. - Trong sn xut n chic v lot nh, then lp trn trc v trn bc theo kiu P9/h9. - Khi chiu di then ln (l > 2d), then nn lp vi bc theo kiu D10/h9. - i vi then dn hng, cn di trt chi tit bc dc trc, then lp trn trc theo kiu N9/h9 v lp vi bc theo kiu D10/h9.

Sai lch gii hn ca cc kch thc trong mi ghp c th tra Bng 1, Bng 2 Ph lc 1 trang 133, 138 ti liu [1]. 4.2. Dung sai lp ghp then hoa

Mi ghp trc then hoa vi bc then hoa thc hin lp ghp theo hai trong ba kch thc: Chiu rng b, ng knh ngoi D, v ng knh trong d (Hnh 4.2).

Lp ghp then hoa c th thc hin nh sau: - Lp ghp theo yu t kch thc D v b khi lm ng tm theo D.

b

Hnh 4.1: Mi ghp then bng

21

- Lp theo yu t kch thc d v b khi lm ng tm theo d. - Lp theo yu t kch thc b khi lm ng tm theo b.

Min dung sai kch thc ca trc then hoa v bc then hoa c quy nh trong TCVN 2324-1978. Khi thit k mi ghp c th chn min dung sai theo ch dn trn Bng 4.12 v Bng 4.13 trang 41 ti liu [1].

Kiu lp ca mi ghp then hoa c th chn nh sau: - Khi nh tm theo kch thc D + Lp ghp theo kch thc D c th chn kiu H7/f7 hoc H7/js6 + Lp ghp theo kch thc b c th chn kiu F8/f7 hoc F8/js7 - Khi nh tm theo kch thc d + Lp ghp theo kch thc d c th chn kiu H7/f7 hoc H7/g6 + Lp ghp theo kch thc b c th chn kiu D9/h9 hoc D9/js7

Ghi k hiu lp ghp then hoa trn bn v: - C th ghi ring tng mi ghp theo ba kch thc trn bn v. - Cng c th ghi chung thnh mt dy s (Hnh 4.3). Trong d biu th nh tm theo b mt tr ng knh d. S 8 ch ra c 8 then trn trc. Tip theo l kch thc v kiu lp ca ng knh d, kch thc v kiu lp ca ng knh D, kch thc v kiu lp ca chiu rng b.

Hnh 4.3: Ghi kiu lp cho mi ghp then hoa

d-83677

fH 40

1112

aH 7

99

hD

D d

d1

Hnh 4.2: Mi ghp then hoa

b

22

Sai lch ca D, cp chnh xc 0, m

Sai lch ca D, cp chnh xc 6, m

Sai lch ca d, cp chnh xc 0, m

Sai lch ca d, cp chnh xc 6, m

ng knh danh ngha ca : d, mm

es ei es ei ES EI ES EI n 18 trn 18 30 trn 30 50 trn 50 80 trn 80 120 trn 120150 trn 150180 trn 180250 trn 250315 trn 315400 trn 400500

0 0 0 0 0 0 0 0 0 0 0

-8 -9

-11 -13 -15 -18 -25 -30 -35 -40 -45

0 0 0 0 0 0 0 0 0 0 0

-7 -8 -9

-11 -13 -15 -18 -21 -25 -28 -33

0 0 0 0 0 0 0 0 0 0 0

-8 -10 -12 -15 -20 -22 -25 -30 -35 -40 -45

0 0 0 0 0 0 0 0 0 0 0

-7 -8 -10 -12 -14 -16 -18 -21 -25 -30 -35

Bng 4.1: Sai lch gii hn ca kch thc ln

4.3. Dung sai lp ghp ln

Mi ghp ln c biu din trn Hnh 4.4. ln l chi tit my c tiu chun ha cao, c ch to trong nh my chuyn mn ha. Dung sai ca ln c quy nh trong tiu chun v ln, nh my ch to ln gia cng ng theo tiu chun.

Khi thit k chng ta ch tnh chn kiu , c ln v cp chnh xc ca , khng cn quy nh dung sai cho .

Tiu chun quy nh 5 cp chnh xc ca ln: cp 0, cp 6, cp 5, cp 4 v cp 2. Trong cp 0 l cp chnh xc bnh thng, cp 2 l chnh xc cao nht.

Cc ln thng dng trong hp gim tc c cp chnh xc 0, trng hp s vng quay ca trc qu ln hoc yu cu chnh xc ng tm ca trc cao, c th dng ln cp chnh xc 6. Bit k hiu ca ln chng ta s bit dung sai ca , do khng cn ghi k hiu dung sai ca ln trn bn v lp.

V d: ghi kiu lp gia bi vi trc v gi (Hnh 4.4):

K hiu 40k6 biu th: ng knh trc l 40mm,

Hnh 4.4: Ghi kiu lp cho ln

40k

6

68G

7

23

p r

d1 =D1d2 =D2

d = D

ai c

bu lng

Hnh 4.5: Kch thc ca mi ghp ren

Mim dung sai ca trc l k6, ng knh l vng trong ca d=40mm, min dung sai ca kch thc d do nh my ch to quy nh.

K hiu 68G7 biu th: ng knh l ca gi l 68mm, Mim dung sai ca l l G7, ng knh vng ngoi ca D=68mm, min dung sai ca kch thc D do nh my ch to quy nh.

Tuy nhin, khi lp ghp ln vi trc v gi , thng mt trong hai vng lp c di. di s lm cho vng bin dng c th dn n kt . tnh ton dn n ca cc vng , kim tra h hng tm sau khi lp ghp ln, chng ta cn bit dung sai kch thc ng knh ngoi D, ng knh trong d ca . Gi tr dung sai kch thc D, v d c ghi trn Bng 4.1.

Kiu lp ghp ln vi trc v v hp c chn tu thuc vo kt cu ca , iu kin s dng , c tnh tc dng ca ti trng v dng ti trng ca cc vng ln.

C ba dng ti trng tc dng ln ln: Ti trng cc b, ti trng chu k v ti trng dao ng.

i vi vng chu ti trng cc b v dao ng, thng chn kiu lp c h di tc dng ca va p v chn ng, vng b x dch, thay i im chu lc, lc ln tng c tui bn.

i vi vng chu ti chu k, thng chn kiu lp c di duy tr tnh trng chu lc ng u cu .

Tham kho Bng 4.7 v Bng 4.8 trang 37 ti liu [1] chn kiu lp hp l cho vng trong v vng ngoi ca . 4.4. Dung sai lp ghp ren

Mi ghp ren gm c chi tit bu lng ghp vi chi tit ai c. Bu lng c ren ngoi, ai c c ren trong. y chng ta ch quan tm n mi ghp ren h Mt, c tit din ren tam gic.

Tu theo yu cu s dng, mi ghp ren cng c thit k theo kiu lp cht,

24

kiu lp lp lng, hoc kiu lp trung gian. Cc kch thc ch yu ca mi ghp ren c trnh by trn Hnh 4.5. Mt ren l b mt xon vt, chnh xc to hnh ca n do ba thng s

(ng knh trung bnh d2 = D2, bc ren pr, v gc nh ren ) quyt nh. Tuy nhin vic o kim tra kch thc pr v tng i kh khn,

ngi ta thng quy nh dung sai cho cc kch thc d, d2, D2, D1 m bo chnh xc ca mi ghp ren.

Cp chnh xc ca cc kch thc ch yu ca mi ghp ren c chn theo Bng 7.1 trang 91 ti liu [1].

Sai lch c bn, min dung sai ca mi ghp ren theo kiu lp lng c chn theo Bng 7.2 v Bng 7.3 trang 91 ti liu [1].

Sai lch c bn, min dung sai ca mi ghp ren theo kiu lp trung gian c chn theo Bng 7.4 v Bng 7.5 trang 92 ti liu [1].

Sai lch c bn, min dung sai ca mi ghp ren theo kiu lp cht c chn theo Bng 7.6 v Bng 7.7 trang 93 ti liu [1].

Dung sai kch thc v sai lch gii hn ca cc kch thc trong mi ghp ren c chn trong Bng 15 n Bng 21 Ph lc 4 trang 152 ti liu [1].

Biu din mi ghp ren trn bn v, v d nh trn Hnh 4.6. Trong : - M20 biu th ren tam gic h Mt, c ng knh thn bu lng d = 20mm, - S 1,5 biu th dng ren bc nh pr=1,5 mm (nu dng bc ren ln th khng cn ghi), - K hiu 2(Pl) ch ren 2 u mi (nu ren 1 u mi th khng cn ghi, - Ch LH ch ren tri (nu ren xon phi th khng cn ghi), - K hiu 4H6H l mim dung sai ca l ai c, ng knh trung bnh D2 c cp chnh xc 4 sai lch c bn kiu H; ng knh trong ca ai c D1 c cp chnh xc 6 sai lch c bn kiu H (nu kch thc D2 v D1 cng min dung sai th ch ghi mt ln). - K hiu 4j6g biu th min dung sai ng knh trung bnh d2 ca bu lng, cp chnh xc 4 sai lch c bn kiu j, (d2 = (d+d1) / 2); ng knh ngoi ca bu lng d c cp chnh xc 6 sai lch c bn kiu g. Nu kch thc d2 v d cng cp chnh xc v min dung sai th ch ghi mt ln.

Hnh 4.6: Ghi kiu lp mi ghp ren M20 1,5 2(Pl) LH - 4H6H /4j6g

d1

d

25

4.5. Dung sai truyn ng bnh rng

4.5.1. Sai s gia cng bnh rng

B truyn bnh rng c gia cng v lp t chnh xc s thc hin chuyn chuyn ng m, s vng quay n2 ca trc b dn khng dao ng, trong qu trnh n khp cc mt rng tip xc tt vi nhau, khng xy ra chn p nhau. Thc t khi gia cng bnh rng, c nhiu yu t lm sai lch hnh dng v kch thc ca rng, ca bnh rng, dn n b truyn lm vic khng tt.

Cc sai s khi gia cng bnh rng c chia thnh bn loi: - Sai s hng tm, gy ra s dch chuyn bin dng rng theo hng

knh, bin dng thc t gn tm quay hoc xa tm quay hn so vi v tr l thuyt.

- Sai s hng tip tuyn, lm cho bin dng rng dch chuyn qua li so vi v tr l thuyt theo phng tip tuyn ca vng trn chia.

- Sai s hng trc, lm cho bin dng rng dch chuyn sai vi v tr l thuyt theo hng dc trc.

- Sai s hnh dng bin dng rng, lm cho bin dng rng khng ng vi ng thn khai ca vng trn.

Cc sai s gia cng bnh rng cng c chia lm hai nhm: - Sai s tn s thp, l sai s xut hin mt ln sau mi vng quay ca

bnh rng. Cc sai s ny gn lin vi phi v bn my mang phi ch to bnh rng. Sai s tn s thp c k hiu bng ch F.

- Sai s tn s cao, l sai s xut hin nhiu ln sau mi vng quay ca bnh rng. Cc sai s ny thng gn lin vi dao v bn my mang dao gia cng bnh rng. Sai s tn s cao c k hiu bng ch f.

4.5.2. chnh xc truyn ng bnh rng

chnh xc ca truyn ng bnh rng c nh gi thng qua bn chnh xc thnh phn:

- chnh xc ng hc, l mc dao ng ca s vng quay trn trc b dn.

- chnh xc n khp m, l mc gy nn rung ng, va p, ting n trong qu trnh b truyn lm vic.

- chnh xc tip xc, l kh nng tip xc nhiu hay t ca i rng n khp trong qu trnh chu ti trng.

- chnh xc khe h mt bn ca i rng, l kh nng khng gy chn p gia cc rng trong qu trnh n khp. chnh xc ng hc ca b truyn bnh rng c phn chia thnh

12 cp, cp 1 c mc chnh xc cao nht, cp 12 c mc chnh xc thp nht. chnh xc ng hc c nh gi qua cc thng s:

26

- o hng tm ca vnh rng Fr, l s thay i ln nht khong cch t tm quay n ng chia ca rng, sau mt vng quay.

- Sai s ng hc ca bnh rng Fi, l sai lch ln nht v gc quay ca bnh rng sau mt vng quay, khi n n khp vi bnh rng mu chnh xc.

- dao ng khong cch tm o sau mt vng Fi, l s thay i ln nht ca khong cch tm gia bnh rng c sai s (bnh rng o) v bnh rng mu chnh xc n khp kht vi nhau, khi quay bnh rng o i mt vng.

- Sai s tch lu bc rng FP, l hiu i s ln nht ca cc gi tr sai s tch lu k bc rng, vi tt c cc gi tr k t 2 n z/2 (z l s rng ca bnh rng).

- Sai s ln rng Fc, l sai s ln nht v gc quay gia bnh rng gia cng v dng c ct rng (dao phay rng).

- Dao ng khong php tuyn chung Fvw, l s dch chuyn bin dng rng theo hng tip tuyn trong phm vi mt vng quay ca bnh rng.

chnh xc lm vic m cng c chia lm 12 cp . Mc chnh xc cao hay thp, tu thuc vo gi tr ca cc thng s sau:

- Sai s ng hc cc b fi, l sai lch ln nht v gc quay ca bnh rng sau khi quay i mt rng, khi n n khp vi bnh rng mu chnh xc.

- Sai lch khong cch tm o sau mt rng fi, l s thay i ln nht ca khong cch tm gia bnh rng c sai s (bnh rng o) v bnh rng mu chnh xc n khp kht vi nhau, khi quay bnh rng o i mt rng.

- Sai s bin dng rng ff, l khong cch ln nht gia hai bin dng danh ngha p vi bin dng thc.

- Sai lch bc rng fpt, l hiu ca sai lch trn v sai lch di ca bc rng pt.

- Sai lch bc c s fpb, l hiu gia bc c s thc v danh ngha.

chnh xc tip xc cng c 12 cp . nh gi mc chnh xc tip xc thng qua cc thng s:

- Vt tip xc, quan st c khi ta bi sn ln rng ca mt bnh rng v cho n khp vi bnh rng khc, c ti trng. T s gia din tch ca vt tip xc v din tch b mt lm vic ca rng cng ln, th chnh xc cng cao.

- Sai s hng rng F, l khong cch ln nht gia ng thng hoc ng xon p vi mt rng v ng l thuyt.

- Sai s hnh dng v v tr ng tip xc Fk, l khong cch gia hai ng thng p vi mt rng thc, song song vi ng tip xc ca i rng.

27

Mc h mt bn c nh gi qua h mt bn Jn. Jn c o trn ng php tuyn vi bin dng rng. Jn dao ng nhiu th chnh xc thp v ngc li.

i vi cp bnh rng khng iu chnh c v tr tm bnh rng, th h mt bn c dnh gi thng qua sai lch khong cch tm fa.

i vi bnh rng iu chnh c v tr tm, th h mt bn c nh gi thng qua dch chuyn ph nh nht ca bin dng gc EH.

kim tra mc chnh xc ch to bnh rng ta dng mt b thng s bao gm nhng thng s v nhng cp thng s nh gi mc chnh xc ng hc, n khp m, tip xc v h mt bn (xem Bngg 8.1 trang 102 ti liu [1]). Vic chn b thng s no l tu thuc vo cp chnh xc ca bnh rng v iu kin sn xut, kim tra ca c s sn xut. B thng s c chn cn kt hp sao cho vic kim tra l n gin nht, s dng c s dng l t nht. V d, khi chn thng s nh gi chnh xc ng hc l Fi, th s dng ngay thng s fi nh gi chnh xc n khp m.

Tiu chun Vit Nam TCVN 1067-84 quy nh 12 cp chnh xc cho chnh xc ng hc, n khp m, tip xc, vi cp 1 l chnh xc cao nht. TCVN 1067-84 cng quy nh 6 dng khe h mt bn: A, B, C, D, E, H. Trong dng A c sai lch c bn ln nht v dng H c sai lch c bn bng khng (Jnmin = 0). Tiu chun cng quy nh 8 cp chnh xc ca h mt bn ca rng, k hiu x, y, z, a, b, c, d, h. Trong cp x c dung sai ln nht v cp h c dung sai nh nht.

Chn cp chnh xc cho truyn ng bnh rng, phi da vo iu kin lm vic c th ca b truyn, v nhng yu cu v truyn ng. Xc nh cp chnh xc c th bng tnh ton hoc da theo kinh nghim, theo cc bng ch dn (Bng 8.2. trang 105 ti liu [1]). Cc chnh xc ca truyn ng bnh rng c th chn cc cp khc nhau. Theo kinh nghim: cp chnh xc n khp m cao hn khng qu 2 cp v thp hn khng qu 1 cp so vi cp chnh xc ng hc, cp chnh xc tip xc c th cao hn hoc thp hn 1 cp so vi cp chnh xc n khp m. 4.5.3. Ghi k hiu cp chnh xc v dng khe h mt bn trn bn v

Cp chnh xc v dng khe h mt bn ca b truyn bnh rng c ghi bng dy s v ch. V d: 8-7-6 Ba TCVN 1067-1984. Trong ch s u tin ch cp chnh xc ca chnh xc ng hc, s th hai ch cp ca chnh xc n khp m, s th ba ch cp ca chnh xc tip xc, ch in hoa ch dng khe h cnh rng, ch in thng ch chnh xc ca dng khe h.

28

Trng hp c bit, nu cp chnh xc ca ba chnh xc nh nhau, th ch cn ghi mt ch s (v d 8 Ba TCVN 1067-1984). Nu cp chnh xc ca dng khe h cnh bn trng vi dng khe h th khng phi ghi ch in thng km theo (v d 8-7-6 B TCVN 1067-1984).

29

CHNG 5

CHUI KCH THC V CCH GHI KCH THC 5.1. Chui kch thc 5.1.1. Cc khi nim c bn - nh ngha:

Chui kch thc l tp hp cc kch thc c lin quan vi nhau v to thnh mt vng kn (Hnh 5.1).

Cc kch thc tham gia trong chui gi l cc khu. - Phn loi: + Chui kch thc chi tit: Tt c cc khu trong chui thuc mt chi tit. + Chui kch thc lp ghp: Cc khu trong chui thuc cc chi tit khc nhau trong mi ghp. + Chui ng thng: Cc khu trong chui song song vi nhau. + Chui mt phng: Cc khu trong chui khng song vi nhau, nhng nm trong cng mt mt phng (Hnh 5.2). + Chui khng gian: Cc khu trong chui khng song vi nhau, v nm trong cc mt phng khc nhau. + Khu thnh phn: Kch thc ca khu thnh phn do qu trnh gia cng quyt nh v khng ph thuc vo cc khu khc. Cc khu thnh phn c k hiu l Ai, vi i = 1 n+m. + Khu khp kn (hay khu tng): Kch thc ca n hon ton ph thuc vo cc khu thnh phn. Trong mi chui ch c mt khu khp kn, c k hiu l A. + Khu tng: Khi tng kch thc ca khu tng, th kch thc ca khu khp kn cng tng v ngc li. S lng khu tng trong chui c k hiu l m. Khu tng c tnh cht ging nh kch thc l, nn sai lch gii hn trn ca khu tng k hiu l ES, sai lch gii hn di l EI. + Khu gim: Khi tng kch thc ca khu gim, th kch thc ca khu khp kn gim v ngc li. S lng khu gim trong chui k hiu l n. Khu gim c tnh cht nh kch thc dng trc, nn sai lch gii hn ca khu gim k hiu l es v ei. - Cc bi ton gii chui kch thc, c hai bi ton:

Hnh 5.1: Chui kch

A1

A2

A3 A

2 A2

A3

Hnh 5.2: Chui mt phng

A1

A

30

+ Bi ton thun: Cho kch thc v sai lch gii hn ca cc khu thnh phn, tnh kch thc v sai lch gii hn ca khu khp kn. + Bi ton nghch: Cho kch thc ca cc khu trong chui, cho sai lch gii hn ca khu tng, tnh sai lch gii hn ca cc khu thnh phn.

5.1.2. Gii bi ton thun Xt chui kch thc mt phng c n khu gim v m khu tng. Dung

sai ca khu thnh phn th i l Ti. Sai lch trn ca khu tng l ESi, sai lch di l EIi (i=1m). Sai lch ca khu gim l esj v eij, vi j=1n. Cc n s ca bi ton c xc nh nh sau: - Kch thc ca khu tng c tnh theo cng thc:

A = +=

mn

iii A

1

Trong i l h s nh hng ca khu thnh phn n khu tng. i vi khu tng i = cosi, i vi khu gim i = -cosi. Vi i l gc lm bi phng ca khu i v phng ca khu tng. - Dung sai ca khu khp kn bng tng dung sai ca cc khu thnh phn:

T = imn

ii T+

=1

- Sai lch trn ca khu khp kn c tnh theo cng thc:

ES = = =

m

i

n

jjji eiE

1 1iS

- Sai lch di ca khu khp kn c tnh theo cng thc:

EI = = =

m

i

n

jjji esE

1 1iI

- Sai lch trung bnh ca khu khp kin k hiu l Em, sai lch trung bnh ca khu khp kn bng tng sai lch trung bnh ca khu tng tr i tng sai lch trung bnh ca khu gim:

Em = = =

m

i

n

jmjji eE

1 1mi

5.1.3. Gii bi ton nghch theo i ln chc nng hon ton n gin ta xt chui ng, c m+n khu thnh phn, v mt khu

tng. bit kch thc ca cc khu, bit sai lch trn v sai lch di ca khu tng. Tnh sai lch trn v sai lch di ca cc khu thnh phn. Bi ton c 2(n+m) n s l ESi v EIi vi i = 1 m+n. Bi ton c gii nh sau: - Gi s cc khu thnh phn c cng chnh xc, tc l cc khu c cng h s chnh xc am. - Xc nh h s kch thc i ca cc khu thnh phn, dng Bng 9.1 trang 114 ti liu [1].

(5.1)

(5.2)

(5.3)

(5.4)

(5.5)

31

- Tnh am: T cng thc T = +=

nm

iiT

1= +

=

nm

jjmia

1= amij , suy ra

am = T/ij - Xc nh chnh xc ca cc khu: Dng Bng 4.1 trang 24 ti liu [1] xc nh chnh xc ca cc khu thnh phn. Nu am trng vi gi tr a trong bng, ta ly chnh xc ca cc khu l nh nhau. Nu am nm gia hai gi tr a trong bng 4.1, ta ly mt s khu c chnh xc cao hn am, mt s khu c chnh xc thp so vi am. - Chn mt khu k li lm khu b, sai lch ca khu ny c tnh ton. - Tra sai lch gii hn ca cc khu tng theo sai lch c bn kiu H, sai lch gii hn ca cc khu gim theo sai lch c bn kiu h. Ta s tm c m+n-1 sai lch gii hn trn v m+n-1 sai lch gii hn di. - Tnh sai lch ca khu Ak tm hai n s cn li: Dung sai ca khu Ak c tnh theo cng thc:

Tk = T - +=

1

1

nm

iiT

Trng hp Ak l khu tng

Emk =

==+ mm

imi

n

jmj EEe

1

11

ESk = Emk + Tk/2 EIk = Emk - Tk/2

Trng hp Ak l khu gim

emk =

== mn

jmj

m

imj EE

1

11E

esk = emk + Tk/2 eik = emk - Tk/2

Gii chui theo phng php i ln chc nng hon ton s thun li cho vic t chc sn xut, lp rp my, c bit thun li cho vic sa cha thay th cc chi tit my b hng. Nhng trong trng hp dung sai ca khu tng nh, s lng khu thnh phn ln, lm cho dung sai ca khu thnh phn qu nh, kh khn cho vic gia cng, tng gi thnh gia cng. Lc ngi ta phi gii chui theo phng php i ln chc nng khng hon ton.

5.1.4. Gii bi ton nghch theo i ln chc nng khng hon ton Thc cht ca phng php ny l m rng dung sai ca cc khu thnh

phn cho d gia cng, h gi thnh sn phm. Chng ta c th dng mt trong bn cch sau y: - Phng php tnh xc sut: Chp nhn c mt s lng nh ph phm (khng m bo yu cu ca khu tng) ln trong sn phm. S lng ph

(5.6)

32

phm thng c khng ch di 5%. Lng dung sai m rng thm c tnh theo xc sut. - Phng php lp sa: Chng ta chn ra mt khu Ak c th d dng ct gt, co sa trong khi lp (Ak c gi l khu b, hay khu bi thng). Dung sai ca cc khu thnh phn cn li s c chn theo chnh xc v kh nng gia cng kch thc thun li nht. c tnh ca khu khp kn c m bo bng cch co sa khu Ak trong qu trnh lp ghp. Dung sai ca cc khu thnh phn phi m bo cho khu Ak c lng d ln co sa. Phng php ny tn rt nhiu thi gian cho qu trnh lp ghp. c bit kh khn, khi cn phi thay th chi tit my b hng. - Phng php lp iu chnh: Chng ta chn ra mt khu Ak lm khu iu chnh, c th iu chnh c kch thc ca khu ny trong qu trnh lp rp. Dung sai ca cc khu thnh phn khc c chn theo chnh xc v kh nng gia cng kch thc. c tnh ca khu khp kn c m bo bng cch iu chnh khu Ak trong qu trnh lp ghp. Dung sai ca cc khu thnh phn phi m bo cho khu Ak c khong iu chnh khng qu ln. Phng php ny tn t thi gian hn phng php lp sa, nhng kt cu my phc tp. - Phng php lp chn: M rng dung sai ca mi khu thnh phn ln hai, ba hoc bn ln. Sau mi khu c chia thnh cc nhm tng ng vi s ln tng dung sai. V d tng 3 ln, thi ta chia lm 3 nhm. Khi lp ghp, phi ly chi tit ca cc nhm tng ng lp vi nhau. V d nhm 1 lp vi 1, nhm 2 lp vi 2. Nh vy s m bo c c tnh ca khu khp kn. Phng php ny mt thi gian phn nhm, phin phc trong lp ghp v bo qun cc nhm.

5.1.5. Gii bi ton nghch theo phng php tnh xc sut Khi gii theo yng php i ln chc nng hon ton, ta thy khu

khp kn A c gi tr ln nht khi tt c cc khu tng c gi tr ln nht v cc khu gim c gi tr b nht, v ngc li. Song xc sut kch thc nh nht v kch thc ln nht ca mi khu thnh phn l qu b, nn xc sut kch thc nh nht v kch thc ln nht ca khu tng, c th coi nh bng khng. Vi l lun nh vy, ta c th m rng dung sai ca mi khu thnh phn ln mt lng, th xc sut cc kch thc ca khu tng nm ngoi min dung sai cng tng ln khng ng k, c th chp nhn c.

p dng nh l ca xc sut cho chui kch thc ta c:

+=

=nm

ii

1

22 Nu ly Ti = 6i v T = 6, , thay vo cng thc (5.7), ta c

+=

=nm

iiTT

1

22 hay

(5.7)

33

+=

=nm

iiTT

1

2

Chn cp chnh xc ca cc khu thnh phn nh nhau, Ti = amii , ta c: +

= =

nm

iim iaT

1

2 suy ra

am = T / +=

nm

iii

1

2

Gi tr ca am tnh theo cng thc (5.9) ln hn nhiu so vi tnh theo cng thc (5.6). C ngha l chnh xc ca kch thc gim i, dung sai kch thc c tng ln.

C c gi th am chng ta tip tc thc hin cc bc gii bi ton tng t nh gii bi ton theo phng php i ln chc nng hon ton.

5.2. Ghi kch thc cho bn v c kh Sau khi tnh ton kch thc ca cc chi tit my theo cc iu kin bn,

iu kin cng. Ta thit lp bn v lp cc b phn my, v tch ring tng chi tit my. Tin hnh ghi kch thc cho bn v lp b phn my, bn v chi tit my. Ghi kch thc cho bn v lp mt cch hp l, s thun li cho vic lp rp, d dng m bo cht lng ca my, thun li cho vic gii chui kch thc xc nh dung sai kch thc ca cc khu. Ghi kch thc cho bn v chi tit my mt cch hp l, chn ng chun kch thc, chn ng khu khp kn, s m bo cho chi tit my c kh nng lm vic v d dng cho qu trnh gia cng ch to chi tit my. Kch thc trn bn v c ba loi: Kch thc chiu di, kch thc ng knh v kch thc gc. a s cc kch thc chiu di tham gia vo chui kch thc, chng ta s quan tm nhiu hn n loi kch thc ny.

5.2.1. Nhng nguyn tc ch yu cn m bo khi ghi kch thc Khi ghi kch thc cho bn v lp, bn v chi tt my cn m bo mt s nguyn tc sau: - Kch thc ca mi phn t trn bn v phi c xc nh mt cch duy nht. Hoc c trc tip trn bn v, hoc c tnh ton qua cc kch thc cc khu thnh phn ca chui. - Khng ghi kch thc cho khu khp kn trong chui kch thc. Nu ghi th phi c du hiu ch r l khu khp kn. - Trn bn v mi kch thc ch c ghi mt ln.

(5.9)

(5.8)

34

- Tt c cc kch thc u phi c sai lch trn v sai lch di cho php. - S khu trong mt chui cng t, cng tt. - Mt kch thc c th tham gia nhiu chui kch thc. S chui kch thc, mt khu tham gia, cng t cng tt. - Mi kch thc c mt chun xc nh (mc tnh). Ngi lp quy trnh cng ngh c th dng chun khc vi ngi thit k. C gng dng cng t chun cng tt. Chun thit k nn trng vi chun cng ngh.

5.2.2. Chn phng n ghi kch thc chiu di cho bn v chi tit my Kch thc chiu di ca chi tit my c th ghi theo nhiu phng n

khc nhau. Cc phng n u th hin y kch thc mi phn t thuc chi tit my. Song chng ta phi chn ra phng n tt nht, ph hp vi phng php gia cng v quy m sn xut chi tit my. C ba cch ghi kch thc chiu di:

- Ghi kch thc thnh mt chui, cc kch thc ni tip nhau (Hnh 5.3). Mi kch thc dng mt chun khc nhau. Mi khu thnh phn c dung sai nh, nhng dung sai ca khu khp kn rt ln. Phng php ny ph hp vi dng sn xut nh, thc hin trn cc my vn nng. - Ghi kch thc theo mt chun thng nht (Hnh 5.4). S chui kch thc nhiu, khu khp kn nhiu. Dung sai ca mt s khu khp kn c th vt qu gi tr yu cu. Kiu ny thng dng trong sn xut hng lot, dng nhiu dao gia cng trong mt nguyn cng.

Hnh 5.3: Ghi lin tip cc kch thc thnh mt chui

A1 A2 A3 A4 A5

Hnh 5.3: Ghi kch thc theo chun thng nht

A1 A2

A3A4

A5

35

Hnh 5.3: Ghi kch thc kt hp c hai cch

A1 A2

A3

A4

A5

- Dng phi hp hai cch trn (Hnh 5.5). Dng chun thng nht gim s khu. Trong khi nhng kch thc cn chnh xc cao, c dung sai nh, khng dng lm khu khp kn trong chui kch thc. Phng php ny c dng nhiu trong thc t sn xut.

Mc lcBi m uC1 : Nhng vn c bn trong dung sai lp ghpC2 : Dung sai lp ghp tr trn C3 : Dung sai cc yu t hnh hcC4: Dung sai cc lp ghp inhnh C5 : Chui kch thc v cch ghi kch thc