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6.3 Nguyn tc 6 im khi nh v chi tit gia cng6.3.1 Nguyn tc 6 im Mt vt rn tuyt i trong khng gian c su bc t do chuyn ng, khi ta t n trong h ta -cc (khng gian ba chiu) l: - 3 bc tnh tin dc 3 trc ta , k hiu:

X Y Zhiu:

- tnh tin dc trc X - tnh tin dc trc Y - tnh tin dc trc Z

- 3 bc quay quanh 3 trc ta , k

X Y Z

- quay quanh trc X - quay quanh trc Y - quay quanh trc Y

V d: Khi t mt khi lp phng trong h ta -cc (hnh 6.7), c th thy cc chuyn ng trn c khng ch nh sau: - Mt phng XOY khng ch ba bc t do. im 1 khng ch bc t do tnh tin dc trc Z ; im 2 khng ch bc t do quay quanh trc X ; im 2 khng ch bc t do quay quanh trc

Y.

- Mt phng YOZ khng ch hai bc t do. im 4 khng ch bc t do tnh tin dc trc X ; im 5 khng ch bc t do quay quanh trc Z . - Mt phng XOZ khng ch mt bc t do. im 6 khng ch bc t do tnh tin dc trc Y . Cn ch rng mi mt phng u c kh nng khng ch 3 bc t do, nhng nhng mt phng YOZ v XOZ ch cn khng ch hai v mt bc t do v c nhng bc t do mt ny c th khng ch th mt XOY khng ch ri. Nh vy 6 bc t do chuyn ng ca vt th rn tuyt i c khng ch hay ni cch khc ta xc nh c v tr duy nht ca vt th rn trong khng gian v ch mt v tr m thi. Nu ch cn cho vt th c chuyn ng theo mt bc t do no th vt th s c v s v tr v do khng c v tr c nh trong khng gian. 6.3.2 ng dng nguyn tc 6 im khi nh v chi tit gia cng Ngi ta dng nguyn tc 6 im trn nh v chi tit gia cng. Khi coi chi tit nh mt vt rn tuyt i v cng t n trong h ta cc.

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V vy nguyn tc su im khi nh v chi tit c th pht biu nh sau: nh v hon ton phi (hoc chi tit) trong g cn phi to su im t b tr trn cc mt chun ca phi (hoc chi tit) khng ch 6 bc t do chuyn ng (3 tnh tin v 3 quay) trong h ta -cc. Trong thc t khng phi lc no ngi ta cng nh v ht c su im m ty theo yu cu gia cng tng nguyn cng m s bc t do nh v c th t 1 n 6. V d: - Ch cn hn ch 1 bc t do: trong cng ngh mi bi cu (hnh 6.8).

- Ch cn hn ch 2 bc t do: trong cng ngh mi bi a (hnh 6.9). mi Hnh 6.9 D Bi a ,

- Ch cn hn ch 3 bc t do: phay mt phng B t kch thc H mt phng A (hnh 6.10).

0,1

v song song vi

-

Ch cn hn ch 4 bc t do: phay rnh then sut dc chi tit tr, m bo kch thc h v i xng qua tm (hnh 6.11).

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- Ch cn hn ch 5 bc t do: phay bc sut dc chi tit , m bo kch thc M v N (hnh 6.12).

S im nh v cn ph thuc vo kch thc ca b mt c nh v, vo cc mi lp gia b mt nh v ca chi tit vi cc b mt ca nh v. V d: - Mt mt phng tng ng 3 im (khng ch 3 bc t do) (hnh 6.13a, mt A) - Mt khi V ngn (L D, L = chiu di tip xc ca khi V vi mt tr chun ca chi tit; D = ng knh ca mt tr chun) tng ng 4 im (hnh 6.14).

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- Mt khi V ngn tng ng 1 im (hnh 6.15, mt s 1). - Mt cht tr ngn (L