機械制造技術基礎雙語第6章課件

1、2022/7/24河南科技大學機電工程學院16.1 Introduction 概述 Mechanical products are generally composed of many workparts and assemblies. Generally, assembly is the last link in machine manufacturing. It mainly includes such jobs as assembling, adjusting, inspecting, and testing etc. The machine quality is finally ens

2、ured by assembly. If the assembling is inappropriate, even the manufacturing quality of the workpart is qualified, it would not be able to assemble the qualified product. On the contrary, even the manufacturing quality of the workpart is not very good, provided the appropriate processing measures ar

3、e used in the assembling procedure, the assembled product can also meet the prescribed requirements Therefore, mechanical assembly plays a very important role in product manufacturing process. 2022/7/24河南科技大學機電工程學院26.1.1 Concept of assembly accuracy 1.Assembly All machines are composed of workparts,

4、 combined members, subassemblies, and assemblies. According to the requirements for assembly accuracy, some related workparts, subassemblies and assemblies are put together by necessary connecting and fitting to compose a qualified machine product. This process is called assembly. In order to carry

5、out assembling work successfully, machine is generally divided into several independent parts which are called assembly unit. There are five classes of assembly unit, i.e. workpart, combined part, subassembly, assembly, and machine.6.1 Introduction 概述2022/7/24河南科技大學機電工程學院36.1.1 Concept of assembly a

6、ccuracy 1.AssemblyWorkpart is the most basic cell to compose a machine. Combined member is the permanent connection (welding, riveting etc.) of some workparts or the combination of assembling one or two components on a datum part. 6.1 Introduction 概述 1. Base part; 2. rivet; 3. Gear Fig. 6-1 gear ass

7、embly2022/7/24河南科技大學機電工程學院46.1.1 Concept of assembly accuracy 1.AssemblySubassembly is composed of combined members and workparts, which are mounted on a datum part. Take the spindle in headstock of machine tool for example. Gears, sleeves, washers, keys and bearings all are assemblied on a datum pa

8、rtspindle, which is called spindle subassembly.Assembly is composed of subassemblies, combined members and workparts, which are mounted on a datum part. The headstock is a example of assembly, in which the box body is the datum part.6.1 Introduction 概述2022/7/24河南科技大學機電工程學院56.1.1 Concept of assembly

9、accuracy 1.AssemblyMachine is composed of assemblies, subassemblies, combined members and workparts, which are mounted on a datum part. Take the lathe for example, 6.1 Introduction 概述2. Assembly accuracyRelative position accuracy. Relative motion accuracy. Fit accuracy. Fit accuracy is the basis of

10、the distance accuracy and the relative position accuracy, and the relative position accuracy is the basis of the relative motion accuracy.2022/7/24河南科技大學機電工程學院66.1.2 Factors of influencing on the assembly accuracy Dimension, form and position errors of mating parts;Poor finish of mating surface;Inac

11、curate location of assembled machine elements;Poor fit and adjustment of machine mating parts; Non-observance(不遵守) of assembly process requirements (non-uniform tightening of screw fasteners, misalignment and deformation of parts in the pressing process and other operations, etc.) Geometric errors o

12、f assembly equipment;Thermal distortions of assembly system;Poor adjustment of assembly equipment.6.1 Introduction 概述2022/7/24河南科技大學機電工程學院76.1.3 The relationship between the part accuracy and assembly accuracy6.1 Introduction 概述 Since any workpart has certain machining error, the error accumulation

13、would affect the machine assembly accuracy. If the accumulated errors exceed the allownce, unqualified products would appear. From the view of the assembling process, it is expected that the assembling process is just a simple connecting processwithout any repair or adjustment, and still can meet th

14、e requirements for machine. The machining accuracy of a workpart is limited not only by the technological conditions, but by the cost. Even giving no thought to the production cost in improving machining accuracy of workparts, the final assembly accuracy can not be attained yet. 2022/7/24河南科技大學機電工程學

15、院86.1.3 The relationship between the part accuracy and assembly accuracy6.1 Introduction 概述 Therefore, the assembly accuracy does not only depend on the accuracy of mating parts, but also on the assembly technology to some extent, especially for the machines with higher accuracy and small-lot produc

16、tion. In the long-term assembly practice, people have created many ingenious assembly methods according to the different machines, different production types and conditions. In the different assembly methods, there are different relations between part accuracy and assembly accuracy. In order to anal

17、yze this relation quantitatively, dimension chain theory is often used in the assembly process. 2022/7/24河南科技大學機電工程學院96.2.1 Assembly dimension chain1. Concept of assembly dimension chain6.2 Assembly methods In machine assembly relation, the dimension chain composed of relative part dimension or mutu

18、al position relation is called assembly dimension chain. The closed link of assembly dimension chain is just the assembly accuracy or technical requirement to be ensured in assembly. The assembly accuracy (closed link) is the dimension and position relation formed after the parts and subassemblies a

19、re assembled. All the dimensions and position relations which have a direct influence on the assembly accuracy are component links.2022/7/24河南科技大學機電工程學院106.2.1 Assembly dimension chain1. Concept of assembly dimension chain6.2 Assembly methods Fig 6-2 shows the assembly relation between a shaft and a

20、 hole. The clearance A0 between shaft and hole must be ensured after assembling. The clearance A0 is the closing link of this assembly dimension chain. It is formed after fitting shaft (dimension A2) into hole(dimension A1). A0 increases with the increase of A1, and decreases with the increase of A2

21、. Therefore, A1 is increasing link, and A2 is decreasing link. The dimension chain equation is A0=A1A2.2022/7/24河南科技大學機電工程學院116.2.1 Assembly dimension chain2. Method to establish assembly dimension chainMethod of finding out assembly dimension chain Firstly to determine closed link according to asse

22、mbly accuracy. Then start from an end of any part at two ends of closing link, march along the position direction required by assembly accuracy, take assembly datum surface as contact clue , search respectively the related parts affecting assembly accuracy up to the same datum surface.6.2 Assembly m

23、ethods Fig. 6-3 Assembly chain of centerlines with equal height of lathe 2022/7/24河南科技大學機電工程學院126.2.1 Assembly dimension chain2. Method to establish assembly dimension chainAttentions in establishing assembly dimension chainSimplification of assembly dimension chainShortest route (least links) princ

24、ipleThe number of component links should be equal to the number of related parts and assemblies, i.e. “one component one link”. The axial clearance dimension chain formed in gear assembly (Fig. 6-4) reflects the “one component one link” principle.6.2 Assembly methods Fig.6-4 Axial clearance dimensio

25、n chain formed in gear assembly2022/7/24河南科技大學機電工程學院136.2.1 Assembly dimension chain3. Calculation method of assembly dimension chain Assembly method has a close relation to the calculation method of assembly dimension chain. The calculation method of assembly dimension chain varies with assembly me

26、thod for the same assembly accuracy.6.2 Assembly methods Fig.6-5 Gear box assembly dimension chain2022/7/24河南科技大學機電工程學院146.2.2 Interchangeable methods1. Strict interchangeable methodThe mating parts are manufactured strictly following the design requirements on the blue print. When assembling, take

27、any workpart into assembly without any repairing and adjustment. And the assembly accuracy still can be ensured. T0 6.2 Assembly methods This is“Max-Min method” to solve the assembly dimension chain to find Ti.2022/7/24河南科技大學機電工程學院156.2.2 Interchangeable methods1. Strict interchangeable methodWhen c

28、ounter-calculating assembly dimension chain, “Intermediate calculation method” (“Interdependent dimension tolerance method”) is often used. The method means that the tolerance of some component links, which are difficult to machine or whose tolerance are inappropriate to change (standard element), a

29、re predetermined, very a few or only one component link is determined as “coordination link”, which is easy to machine, or confined by fewer conditions in production, or measured by means of general-purpose measuring tool. 6.2 Assembly methods 2022/7/24河南科技大學機電工程學院166.2.2 Interchangeable methods1. S

30、trict interchangeable methodThe “coordination link” is also called “interdependent dimension” “So called “interdependent dimension” means the dimension is dependent on closing link and other component links. The tolerance of “interdependent dimension” should be calculated by the dimension chain calc

31、ulation formula. Its calculation procedure is as follows:To establish assembly dimension chain and determine the “coordination link”.To determine the tolerance of component links.6.2 Assembly methods 2022/7/24河南科技大學機電工程學院176.2.2 Interchangeable methods1. Strict interchangeable methodThe average tole

32、rance of each component link is firstly determined according to “equal tolerance” rule. Then TAy=TA0 Where Aycoordination link; TAy、TA0、TAi-tolerance of coordination link, closing link and other component links separately.6.2 Assembly methods 2022/7/24河南科技大學機電工程學院186.2.2 Interchangeable methods1. St

33、rict interchangeable methodTo determine the limit deviation If the coordination link is increasing link, then6.2 Assembly methods If the coordination link is decreasing link, then2022/7/24河南科技大學機電工程學院196.2.2 Interchangeable methods1. Strict interchangeable method Example 6-1. Fig. 6-6 shows the asse

34、mbly relation of shaft to gear and washers. The axial clearance between gear and a washer should be kept in 0.1-0.35mm. Given that A1=30mm, A2=5mm, A3=43mm, A4=3mm (standard part), A5=5mm. When interchangeable assembly method is used, try to determine the tolerance and limit deviation of every compo

35、nent link. 6.2 Assembly methods Fig. 6-62022/7/24河南科技大學機電工程學院206.2.2 Interchangeable methods1. Strict interchangeable method Example . Solution:To establish assembly dimension chain and verify the basic dimension of each link A0= mm A0=A3-(A1+A2+A4+A5)=43-(30+5+3+5)=0 As washer A5 is easy to machine

36、 and measure, so A5 is selected to be coordination link.To determine the tolerance and the limit deviation of component links6.2 Assembly methods 2022/7/24河南科技大學機電工程學院216.2.2 Interchangeable methods1. Strict interchangeable method Example . Shield ring A4 is a standard part, A4= mm, TA4=0.05mm. In t

37、he light of the size and machining difficulty, the tolerance of other component links is determined as following: TA1=0.06mm, TA2=0.02mm, TA3=0.1mm，(The tolerance grade of each component link is about IT9). The limit deviations of A1 and A2 are determined following Basic Shaft System: A1= mm，A2= mm，

38、 and A3 is determined following Basic Hole System: A3= mm.6.2 Assembly methods 2022/7/24河南科技大學機電工程學院226.2.2 Interchangeable methods1. Strict interchangeable method Example . To calculate the tolerance and the limit deviation of coordination link TA5=TA0-(TA1+TA2+TA3+TA4) =0.25-(0.06+0.02+0.1+0.05)=0

39、.02 mm EIA0=ESA3-(EIA1+EIA2+EIA4+EIA5) 0.35=0.1-(-0.06-0.02-0.05+EIA5) EIA5=0.12 mm ESA5=TA5+EIA5=0.02+(0.12)=0.10 mm A5= A1= mm，A2= mm，A3= mm， A4= mm，A5= mm。6.2 Assembly methods 2022/7/24河南科技大學機電工程學院236.2.2 Interchangeable methods2. Partial interchangeable assembly method For the most of machine pr

40、oducts, even though their components are directly assembled into machines without being selected, repaired and adjusted, the assembly accuracy still can be met. But a small number of waste products may be produced. This method is called partial interchangeable assembly method. Its essence is to enla

41、rge the tolerance of components (larger than that of interchangeable method). As the method is based theoretically on probability, it is also named as probability method. According to the distribution of component links, there are two instances to be discussed.6.2 Assembly methods 2022/7/24河南科技大學機電工

42、程學院246.2.2 Interchangeable methods2. Partial interchangeable assembly method Normal distribution The formula used to solve assembly dimension chain in probability method can be derived as follows: A0M= TA0= Non- normal distribution Similarly, the formula used to solve dimension chain in probability

43、method under the condition of non-normal distribution can be derived as follows: TA0S= For the sake of calculation, set k1=k2=kn-1 =k，then the approximate formula can be got as follows: TA0E=k (k=1.21.6) 6.2 Assembly methods 2022/7/24河南科技大學機電工程學院256.2.2 Interchangeable methods2. Partial interchangea

44、ble assembly method Exam. 6-2 Use partial interchangeable assembly method to solve Exam.6-1, try to determine the tolerance and deviation of component links.Solution: Considering that A3 is difficult to machine, A3 is selected to be coordination link. Therefore, its dimension and tolerance are found

45、 lastly.To determine the tolerance and the limit deviation of component links TAiM= mm6.2 Assembly methods 2022/7/24河南科技大學機電工程學院266.2.2 Interchangeable methods2. Partial interchangeable assembly method A4 is the dimension of standard part, its tolerance has been fixed. The tolerance of other compone

46、nt links are adjusted as follows: T1=0.14mm，T2=T5= 0.05mm. As A1, A2, A5 all are external dimension, their limit deviation are determined according to “ basic shaft system”. Then A1= mm，A2= mm，A4= mm，A5= mm. To find the tolerance and limit deviation of coordination link6.2 Assembly methods 2022/7/24

47、河南科技大學機電工程學院276.2.2 Interchangeable methods2. Partial interchangeable assembly method 6.2 Assembly methods 0.225+(29.93+4.975+2.975+4.975)=43.08mm Comparing the calculation result with that of Exam.6-1, it can be seen that when the tolerance of closing link is given, Probability method can enlarge p

48、art manufacturing tolerance, thus decreasing part manufacturing cost.2022/7/24河南科技大學機電工程學院286.2.3 Selective assembly method The essence of this method is that the mating parts are manufactured in the light of economic machining accuracy, that is, the tolerances of component links in dimension chain

49、are magnified to the economic and feasible degree. Then proper workpart is selected when assembling to reach stipulated assembly accuracy, This method is used where design tolerances are smaller than production tolerances. A selective assembly involves direct selection of mating parts, sorting these

50、 into size group prior to assembly and a combination of the two techniques. The first one is carried out by the fitter, who picks such parts out of lot as seem to fit “by feeling” or by feeler gauge. This procedure is unacceptable on an assembly line, because it is time-consuming and because the ass

51、embly quality is heavily dependent on the fitter skill. Here the second assembly methodGrouping assembly method is mainly discussed.6.2 Assembly methods 2022/7/24河南科技大學機電工程學院296.2.3 Selective assembly method Grouping assembly method is to magnify the manufacturing tolerances of component links into

52、several times relative to the tolerance found by full interchangeable assembly method, thus making the component links be machined economically. Then the parts are grouped based on the measured size. Assembly is performed between the corresponding groups in order to meet the assembly requirements. A

53、s the parts in the same group can be interchangeable, this method is also named as selective interchangeable method. For the assembly with fewer components and higher assembly accuracy, this method is often used.6.2 Assembly methods 2022/7/24河南科技大學機電工程學院306.2.3 Selective assembly method Example .6-3

54、 Fig. 6-7 shows the assembly relation between piston pin and pin hole. The basic size of mating parts is 28mm. It is required that the interference with 0.0025mm0.0075mm should be met in cool assembling. If the economic machining accuracy of pin and pin hole is 0.01mm, and grouping assembly method i

55、s used, try to find the grouping numbers and grouping dimensions.Solution: TA0= 0.0075-0.0025 =0.0050 mm6.2 Assembly methods 2022/7/24河南科技大學機電工程學院316.2.3 Selective assembly methodExample .6-3 6.2 Assembly methods Fig. 6-7 Assembly relation between piston pin and pin hole2022/7/24河南科技大學機電工程學院326.2.3

56、Selective assembly method Example . 6-3 Fig. 6-7 shows the assembly relation between piston pin and pin hole. If the diameter of pin is 280-0.0025 mm, and is divided into 4 groups, then the other diameters of corresponding holes can be found one by one. In this way, piton pins can be machined by mea

57、ns of centerless grinding machine, and pin hole can be bored with diamond boring machine. Then their diameters are measured by means of precision measuring tool and divided into 4 groups in the light of size. 6.2 Assembly methods 2022/7/24河南科技大學機電工程學院336.2.4 Individual fitting assembly method In job

58、 and small-lot production, this method is often used for the product structures with higher assembly accuracy and more component links. Mating parts can be machined economically. When assembling, the dimension of predetermined component link can be modified by filing, hand scraping, lapping to meet

59、the assembly accuracy. The predetermined component link is named as “fitting link”(修配環(huán)). This assembly method is called “individual fitting method”.6.2 Assembly methods 2022/7/24河南科技大學機電工程學院346.2.4 Individual fitting assembly method Selection of “fitting link” Attentions should be paid as follows: F

60、itting link should not be “public link”; Be easy to dismount and assemble; The surface to be fitted should be simple and with small area. To determine the limit dimension of fitting link As the component links in individual fitting method are machined in the light of economic machining accuracy, the