Geometric Attributes of Manufactured Parts.pptx

Introduction chapter for manufacturing engineering

1. Geometric Attributes of Manufactured Parts

2. Manufactured components come in all sizes and in a bewildering variety of shapes How to create order from chaos by group technology ? What machine tools have to do to generate shapes? How to make sure parts will fit together? How to make a drawing that truly expresses design intent? Methods of measuring dimensions statically and ‘on the fly’. What a good surface is and how it can be quantified?    

3. Manufactured components come in all sizes and in a bewildering variety of shapes Shape and dimension are critical to the functioning of the product. In an assembly, many parts need to fit together, and this requires that allowable deviations in dimensions be specified and not excluded. The industrial designer will specify a finish for visible parts but there are also strict technical requirements to be met if two mating parts are to function properly. Therefore objective measures of surface quality must be found.

4. SHAPE AND SHAPE CLASSIFICATION Shape:   The shape of the part is dictated, first of all, by its function. Complexity of this shape often determines what process can be considered for making it, and in the most general sense, increasing complexity narrows the range of applicable processes; increases the cost. Car parts( courtesy:Pacific )

5. Shape Classification :   The groupings of several shapes are designed for identifying the process capabilities. Products of uniform cross section (spatial complexity = 0) are two dimensional and others are three dimensional. With increasing spatial complexity, definition of the shape requires additional geometric parameters; it can be said that the shape has greater information content. A small increment in the information content may have significant manufacturing consequences. For example, moving from solid round shape R1 to hollow roundshape T1 adds only one dimension(diameter), yet it excludes some processes or necessitates extra operations in others.

6. Construction parts ( courtesy:pacific )

7. Electronic parts ( courtesy:pacific )

8. Machine parts ( courtesy:Pacific )

9. Prismatic parts (Courtesy: Precision P rismatic Inc.)

10. Group Technology (GT) GT is a powerful tool in designing for manufacturability. Its essence is the recognition that many problems have similar features and, if these problems are solved together, great efficiency and economy result. In applying the concept to manufacturing, individual parts are analyzed in terms of commonalities of design features as well as manufacturing processes and process sequences. This way, families of parts can be identified and economies assured at 3 levels. Design stage Manufacturing stage Production Planning Stage

11. Group Technology (GT)-Pro s Design stage: The task of repetitive design is eliminated. It has been estimated that 40% of all design is simple duplication, 40% requires only some modifications of existing design and only 20% calls for original design. Manufacturing stage: Programs required for making families of parts can be optimized and retained for the future when the part is to be produced again. Because parts that are geometrically similar often require the same production sequence, GT is the first step in reorganising the production facility.

12. Group Technology (GT)-Pro s Production Planning stage: Production Cycle time estimation is accelerated, workpiece movement is rationalized, process design is simplified, cost estimation is facilitated.   Introduction of the computer has made GT attractive, because programs relating to the design of standard elements such as solid & hollow cylinders, rectangular blocks, cones can be retained in memory and easily combined and modified for a large variety of part configurations.  

13. Part Classification The first step in GT is the classification of parts into families, on the basis of design/manufacturing attributes. 1.Experience based judgement 2.Production Flow Analysis 3.Clasification and Coding 4.Engineering database

14. Part Classification 1.Experience based judgement   2.Production Flow Analysis: Information relating to the sequence of operations in an existing plant is contained in the routing sheets or cards from which the flow of parts through various operations can be extracted. Parts that are made by identical operations form a family. Parts on which some additional operations are performed may also be included in the family.

15. Part Classification A critical examination may also reveal that parts falling outside a family could be made more economically by adopting the production sequence typical of the family. Parts that are made by the same process but in different sequences may still logically be classified into the same family, but the flexibility of the production system has to be greater to allow the return of the part to a previous operational position. Computer aided Proecss Planning(CAPP) creates the basis for PFA.

16. Part Classification   3.Clasification and Coding This is a more formal exercise. Some systems are more suitable for design, others for parts made by specific processes(casting, forging, machining, etc.). They all start from a classification of basic workpiece shapes. Part codes are made up of several (upto 30) digits, which define various geometrical features as well as composition and surface properties. Further digits may be added to define process, process parameters and processing sequences. Some computer based systems facilitate coding by guiding the operator through necessary steps in a conversational mode. Others use the database generated by CAD to help in assigning code numbers.  

17. Part Classification 4.Engineering database The database may contain the information about the part, its use and manufacture, in addtion to all information found on the engineering drawing. When structured as relational databases, they can be searched according to different attributes and thus groups can be formed to satisfy specific criteria.

18. Machine Tool Movement and Control Shape complexity has profound influence on the process controls needed. This is most obvious in machining. When the movement of the tool or work piece is restricted to a single axis, for example when drilling a hole in a clamped work piece,it is one-axis movement or control(usually z-axis). Z axis

19. Machine Tool Movement and Control Table movement requires two axis control(usually x and y axes); programmed movement in the z-axis makes it a three axis machine. When movement in the z-axis is simply on/off and proceeds at preset rate , it is two and a half axis control. Z 3 axis y x

20. Machine Tool Movement and Control Swivelling the tool or table would add the fourth and fifth axes. Every joint in the table or tool holder adds further degree of freedom of movement and permits complex shapes to be made, but by expensive machinery and control . z Several(5) axis x y

21. Machine Tool Movement and Control There are some shape features that immediately set certain limitations: 1.Axial symmetry is the simplest,because a two-dimensional shape can be generated by rotating the part or tool (around z-axis), while moving the tool in a straight path. Two or more outer diameters require a second axis of control. 2.Parts of non-rotational symmetry call for a minimum of two-axis control, and spatial curvature can be followed only with three or more axis control.

22. Machine Tool Movement and Control 3. A surface in line with the tool movement can be made with one axis control, although if the tool is difficult to withdraw, a draft angle may be necessary. straight walled pocket ; other with draft angle to withdraw the tool

23. Machine Tool Movement and Control Undercut shapes require control in more than one axis, and five axis control is frequently encountered. undercut shapes require multi-axis control or complex tooling