Chapter Six The Process Planning of Mechanical Manufacturing

1. Chapter Six The Process Planning of Mechanical Manufacturing

2. Instructional objectives At the end of this lesson, the student would be able to : (1) Understand the process of product and the process of part, master the composition of the process of product. (2) Understand the content and steps of the machining process planning, and focus on the main problems to be solved when formulating the machining process planning. (3) Understand machining metal removel and its influencing factors, master the determination of process size and tolerance. (4) Understand the definition and characteristics of dimensional chains, master its composition and the basic calculation formula of it. Master the analysis and calculation of dimensional chains with emphasis.

3. Introduction The purpose of mechanical machining is to turn a kind of raw material into the finished parts that meet the requirement of the produce. Under given manufacture conditions, how to apply both an economical and efficient method, as well as a rational process roution, to achieve the desaired parts is the concern of this chapter.

4. 6.1 Manufacturing Process A.Important Concepts 1. Process of product: Manufaturing is the utilization andmanagement of materials,people,equipment, and money to produce products.For successful adn ecomomical manufacturing to be achieved, lanning must start at the design stage and continue through the selection of materials, processes,equipment, and the scheduling of production. 2. Process of Part: The Process by which the shape, dimensions and performance of raw material or rough casting are changed directly by mechanical manufacturing into desiged parts is called component manufactured part production,or process route.

5. 3. Operations: When one operator (or a group of operators) works in a definite position (one machine tool or clipping table ) to achieve the continous manufactruing of one or several parts simultaneously. An operation is the basic element of process procedure as well as of the production schedule and of cost estimation. Example1: Fig.6.1 the engneering drawing of a shaft

6. Analysis: If the number of the parts in a batch is small, the manufacturing process can be completed through 5 operations, which are shown in table 6-1. For the manufacturing of the same part, if the number of part is large, the operation 1 in table 6-1 can be divided into 2 operations: a) facing and centering one end in an engine lathe by one operator. b) transferring to another engine lathe for facing and centering the another end by another operator. table 6-1 the process of shaft manufacturing

7. 4. Operation Step: An operation step is the basic element of an operation. One operation step is defined as an operation that is completed under the condition when the surface to be manufactured, the cutting parameters and cutting tool remain the same. When any of the cutting parameters is changed, it will become another operation step. 5. Cutting pass: When the amount of metal to be removed is large, several cutting passes may need to be carried out. A cutting pass is a kind of operation step that is completed by a single type of tool to cut the same surface under the same cutting parameters.

8. 6. Operation position: In order to reduce the number of setups, a rotational worktable or fixture is usually used to make sequential machining in different positions possible from a single setup. The job performend in one position is called one operation position. Demontration of operation, operation step, position and cutting pass.

9. B.Production Type and its impact on process planning 1. Production type: refers to the specialized classification of an enterprise in respect of a workshop, manufacturingline or group, work place, and so on. Generally, it can be classified into 3 types, which are, a)Single Production: The number of workpieces with different structure or different dimensions is only one at a time, or only in small quantity. For example: heavy-duty machines, specialized machines and new products. b)Batchs: The number of the same products in one batch. Including: small batch, large batch and medium batch.

10. c) Large quantity production:The quantity of each part being produced is large, and many work places often manufacture the same product, carrying out the same operations on each example of the product repeatedly. For example: the manufacturing of vehicles, trucks and bearings. 2. Impact on process planning Process planning are highly affected by the produciton type. When the produciton type changes, the production organization and management, the layout of workshop and machine tools, the process method of the raw material, the manufactruing equipment,such as kinds of cutting tool, fixture and inspeciton tools and the technical level of the operators will also varied addordingly.

11. ●the characteristics of different production types are summarized in the following table:

12. 6.2 The Function and Design Method of Process Planning File A.The Format of process sheet The term process route is the process planning file in which the process and operation method are specified. In another word, the aligned sequence of each operation, the part dimensions, tolerance and technical requiremnt, technical equipment and measures, cutting parameters, standard produciton time, and levels of opertors' skill are all included in the process planning file.

14. Before products are manufatured, technical and production preparation can be made beforehand according to the demands of process planning sheet.(table 6-4) According to the process planning sheet, the kinds, type and number of machine tools, the production area needed, the plan arrangement of equipment, the number, type and class of operators required can all be dicided. Then, the planning of factory preparation, enlarging or rebuilding will be decided accordingly.

16. B.The Function of process planning sheet The function of the process planning sheet is as follows: 1. The main technique for giving regulations to guide the production 2. The basis of orgnization and management 3.The main information to guide the building, enlarging or rebuilding of a mechanical factory

17. C.Design procedures of process planning 1. The function, importance and working condition of the part should be clarified. 2. Determination of production type according to production expectation. The expected number of parts can be calculated according to the following formula: Np=N*n*(1+α)*(1+β) Where: Np---the expected number of parts in one year; N-----the expected number of products in one year; n-----the number of parts in one product; α-----percent of spare parts; β-----percent of scrap parts;

18. Table 6-5 can be used as a reference to guide the classification of production type . To calssify the production type, both the expected number and the volume, weight of the part should be taken into account.

19. 3. Determination of raw material type The determination of raw material is related with the part geometrical shape, dimensions, mechanical performance of material and produciton type, current production conditions of the workshop. The main type of raw material including: Casting, Forging, Profiled bar and Welding.etc. In large batch or large quantity production, highly precise and efficient manufacturing methods for raw material are often used. In single or small batch production, sand casting with wooden mould or free deformed forging are often used.

20. 4. Process route planning It includes selection of mounting surface and machining method, classification of process phases, sequencing the operations, determining the degre of operation distribution, arrangement of hot working or inspection and other auxiliary operations. 5. Selection of machine tools, technical equipment ( including cutting tools, fixtures and inspection tools ) and auxiliary tools for each operation. 6. Selection of metal removal, operation dimensions and tolerances for each operation 7. Selection of metal cutting parameters and standard operation times 8. Technical economical analysis

21. 6.3 Selection of Positioning Reference(Datum) A.The Definition positioning reference is used to define the position of other points, lines or surfaces of the part are called the positioning reference datum. B.Classification of positioning reference Positioning reference can be classified into two categories according to application purposes.

22. 1. desige reference The points, lines or surfaces that are used by designers to determine the dimensions or relative positions of other points, lines or surfaces are called the design reference. For example: in Fig.6-2, the design reference of plane 2 and 3 is plane 1, plane 5 and 6's is plane 4, the hole 7's is plane 1 and plane 4, the hole 8's is the center of hole 7 and plane 4 .

23. 2. process reference The reference which is used in the process f operation or assembling is called the process reference or manufacturing reference. According to its iespurpose, it can be further classified into 4 subcategories: (a)、Operation reference (b)、Positioning reference (c)、Inspection reference (d)、Assembling reference

24. C.The Clamping method and means to obtain the desired accuracy 1. The Clamping (1) positioning and clamping by direct location When putting the workpiece directly on the machine bed, the operator can locate the machined surface by using a micrometer inspection instrument, making dishe or straight rules to adjust and find the desired machining position relative to the cutting tool. (2) clamping according to the pre-marked trail of outline This kind of clamping method is widely used in single or small batch type production, and the error of the operation itself exists, the clamping accuracy is low.

25. (3) clamping with a fixture This kind of method is convenient, fast, accurate and stable. It is widely used in batch type production and large quantity production. For some parts in small batches, such as the connecting rod or crank shafts, even though the batch quantity is not large, some special fixtures are still required in order to meet the specific machining requirement. As fig.6-3 shows.

26. D.Selection of locating surfaces

27. 1. The criteria of selecting the accurate datum surfaces When selecting the accurate datum surfaces, some factors, such as convenience, accuracy and stability must be taken into consideration. The following general principles should be followed. (a)、Principle of coincident locating surfaces (b)、Prnciple of locating datum surfaces unchangeable (c)、Principle of being datum surfaces with each other and machining time after time (d)、Principle of self-datum surface (e)、Principle of stable and simple clamping

28. 2. Selection of first datum surface

29. Some examples: 1. In Fig.6-4, the guide way surface is first used as the datum surface, then the gide way surface is machined by using the bottom surface as the datum surface to ensure small and uniform material removal, which is shown in fig.6-4b.If the bottom surface were first used as the datum surface, which is shown in Fig.6-4c, the error of the bottom surface would be fully reflected onto the surface of the guide way so that the metal removal would be less uniform.

30. 2. In Fig.6-5a, in order to ensure the uniform thickness of the flange of the pulley wheel, select the free machining surface1 as the first datum surface to machine the external cylindrical surface. In Fig.6-5b, in order to ensure the uniform wall thickness of the part, the external cylindrical surface A should be selected as the first datum surface to bore the hole.

31. 6.4 Design of Process Planning A.Selection of machining method 1. the economical accuracy is defined as the accuracy achieved under normal conditions including perfect machine tools, necessary technical equipment, standard skilled operators, standard time and production expenditure.

32. 2. Selection of machining method and scheme Firstly, decide the machining method and scheme according to the technical spcifications of every surface which is to be machined.table 6-6,6-7,6-8 have individually introduced the usual used machining method and scheme for the basic surfaces, as well as the economical accuracy and surface finish for each method. Secondly, the material properties should be taken into consideration. For instance, quenching steel is often machined by grinding, but it is difficult for grinding non-ferrus metal. The later is usually machined by diamond boring or high finish turning.

33. Thirdly, the production type should be taken into consideration when selecting the machining method. Last, selecting the machining method should take into account the present condition of equipment and technology. In addition, selecting machining method should take into account the shape of the workpiece, the weight and the physical mechanical properties of the surface achieved by it. The selection of machining scheme can refer to tables similar to Table 6-6.

37. Example: Application of tables, the accuracy of hole machine is grade H7, roughness Ra=1.6--3.2 μm,try deciding the machining scheme of the hole.

38. B.Classifying the machining phase

39. C.Arrangement of operations 1. Decide the machining sequence The machining sequence of each surface should follow the following principles,which are: perform rough machining before finish machining, perform datum surface machining before that of other surfaces, perform main functional surface machining before less important surface machining, perform plane surface machining before hole surface machining.

40. 2.Arrangement of heat and surface treatment In order to improve the mechanical and cutting performance of the material, heat treatment is often arranged during machining.

42. 3. Arrangement of inspection operation Inspection is the most important operation in ensuring product quality and avoiding rejects.In each operation, the operator should inspect the product himself. On the basis of this, an independent inspection operation should be arranged under the following conditions such ad after all the rough machining, before the finish machining and after delivering to other delivering to other workshop, before and after important operations,and after the final operation. 4. Arrangement of other operations During the operation, some assistant operations such as deburing, demagnetising and rinsing can be arranged after some operations when needed.

43. 6.5 Plan the Metal Removal A.The concept of metal removal

44. B.The factors affecting the rate of metal removal 1. Affected by the surface qualities producing by the preceding operation, which are roughness (Ra ) and tolerance (Ta ) As there will be micro-roughness Ra and defective layer Ta left on the surface by the preceding operation, these have to be removed by the current operation. The size of Ra and Ta is related to the machining method, for instance, tables 6--6,6--7,6--8 show some reference values of Ra, and table 6--9 shows the reference values of Ta.

45. Table 6--9 Values of Ta in various machining method (μm)

46. 2. Affected by the tolerance of the preceding operation The tolerance of δα includes various kinds of geometrical shape error, such as the degree of coning, ellipsing and surface flatness. The size of δα can refer to the achievable tolerance of the selected machining method, which can be found in handbooks and in information booklets provided by metal cutting manufacturers. The relationship between metal removal and operation dimensional tolerance is shown in Fig,6--6.

47. 3. Affected by the relative position error between preceding machined surface The error of ρα includes the degree of hinearity, of parallelism between axes, of perpendicularity between axis and surface, coaxiality between inner and external surfaces, and flatness of surface. These kinds of error should be corrected in the current operation so as to ensure the machining quality. 4. Affected by the relative position error between preceding machined surface The error Δεb includes both the positioning and clamping error, as well as the manufacturing error of the fixture. Its value is the vector sum of the above three variables. It will directly affect the relative position between the surface and the cutting tool.

48. C.Method of planning metal removal 1. Caculation method

49. 2. Method of looking up tables This method is widely used in factories. The table is prepared according to the production practice and experimental research work of the factory, as well as modification after considering the specific machining condition. The appropriate rate of metal removal can be found in handbooks and in information booklets provided by metal cutting manufactures. 3. Experiential Method This method is mainly used in single or small batch type production, and the metal removal rate is estimated by experience, thus it is not very accurate. It causes high reject rates.

50. 6.6 Dimensional Chains A.The concept of a dimensional chains In mechanical design and technical work, it is often necessary to analyze some related dimensions, tolerances and technical specifications so as to ensure the product quality during machining, assembling and service.