Design for Manufacturing (continued)

• Table summarizes machined features that are relatively easy to provide on nonrotational workpieces in increasing order of cost.

Design for Manufacturing (continued)

(continued)

Design for Manufacturing (continued) • Table presents machined features that would be more expensive to provide and should be

avoided when possible. • Again it should be remembered that the relative difficulty of providing these features depends

on the proportions of the component.

Design for Manufacturing (continued) (continued)

Assembly of Components • Most machined components must eventually be assembled, and the designer

should give consideration to the assembly process. • One important requirement is that it should be possible to assemble the

components. • Fig. shows some impossible assembly situations.

Assembly of Components

• For example, where flanges on castings are to be bolted together, the area around the bolt holes should be machined perpendicular to the hole (spot-faced for example) to provide proper seating for the bolt heads, nuts or washers.

• Also internal corners should not interfere with the external corner on the mating component.

• Figs. shown are examples of how this interference can be avoided.

• Finally incorrect specification of tolerances can make assembly difficult or even impossible.

•A further requirement is that each operating machined surface on a component should have a corresponding machined surface on the mating component.

Design for Manufacturing

Accuracy and Surface finish • A designer will not generally want to specify an accurate surface with a rough

finish or an inaccurate surface with a smooth finish. • When determining the accuracy and finish of machined surfaces, it is

necessary to take into account the function intended for the machined surface.

• The specification of too-close tolerances or too-smooth surfaces is one of the major ways a designer can add unnecessarily to manufacturing costs.

• Such specifications could, for example, necessitate a finishing process, such as cylindrical grinding after rough tunring, where an adequate accuracy and finish might have been possible using the lathe that performed the rough turning operation.

• Thus the designer should specify the widest tolerances and roughest surface that will give acceptable performance for operating surfaces.

Design for Manufacturing

Accuracy and Surface finish (continued) • As a guide to the difficulty of machining to within required tolerances it can

be stated that • 1. Tolerances from 0.127 to 0.25 mm are readily obtained. • 2. Tolerances from 0.025 to 0.05 mm are slightly more difficult to obtain and

will increase production costs. • 3. Tolerances 0.0127 mm or smaller require good equipment and skilled

operators and add significantly to production costs.

Design for Manufacturing

Accuracy and Surface finish (continued) • Fig. illustrates the general range of surface finish that can be obtained in

different operations. • It can be seen that any surface with a specified surface finish of 1 µm

arithmetical mean or better will generally require separate finishing operations, which subsequently increases costs.

• Even when the surface can be finished on the one machine, a smoother surface requirement will mean increased costs.

Design for Manufacturing

Accuracy and Surface finish (continued)

• For many applications, a smooth, accurate surface is essential.

• This smooth accurate surface can most frequently be provided by finish grinding.

• When specifying finish grinding, the designer should take into account the accessibility of the surfaces to be ground.

• In general surfaces to be finish ground should be raised and should never intersect to form internal corners.

• Fig. shows the types of surfaces that are most readily finish ground using standard shaped abrasive wheels.

Design for Manufacturing

Summary of Design Guidelines Standardization 1. Utilize standard components as much as possible. 2. Preshape the workpiece, if appropriate, by casting, forging, welding, etc. 3. Utilize standard pre-shaped workpieces, if possible. 4. Employ standard machined features wherever possible. Raw materials 5. Choose raw materials that result in minimum component cost (including

cost of production and cost of raw material). 6. Utilize raw materials in the standard forms supplied. Component design a. General 7. Try to design the component so that it can be machined on one machine

tool only. 8. Try to design the component so that machining is not needed on the

unexposed surfaces of the workpiece when the component is gripped in the work-holding device.

Design for Manufacturing

Component design (continued) 9. Avoid machined features the company the company is not equipped to

handle. 10. Design the component so that the workpiece, when gripped in the work

holding device is sufficiently rigid to withstand the machining forces. 11. Verify that when features are to be machined, the tool, toolholder, work,

and work-holding device will not interfere with one another. 12. Ensure that auxiliary holes or main bores are cylindrical and have L/D

ratios that make it possible to machine them with standard drills or boring tools.

13. Ensure that auxiliary holes are parallel or normal to the workpiece axis or reference surface and related by a drilling pattern.

14. Ensure that the ends of blind holes are conical and that in a tapped blind hole the thread does not continue to the bottom of the hole.

15. Avoid bent holes or dogleg holes.

Design for Manufacturing

Component design (continued) b. Rotational components

16. Try to ensure that cylindrical surfaces are concentric, and plane surfaces are normal to the component axis.

17. Try to ensure that the diameters of external features increase from the exposed face of the workpiece.

18. Try to ensure that the diameters of internal features decrease from the exposed face of the workpiece.

19. For internal corners on the component, specify radii equal to the radius of a standard rounded tool corner.

20. Avoid internal features for long components. 21. Avoid components with very large or very small L/D ratios.

Design for Manufacturing

Component design (continued) c. Nonrotational components

22. Provide a base for work holding and reference. 23. If possible, ensure that the exposed surfaces of the component consists

of a series of mutually perpendicular plane surfaces parallel to and normal to the base.

24. Ensure that the internal corners normal to the base have a radius equal to a standard tool radius. Also ensure that for machined pockets, the inetrnal corners normal to the base have as large radius as possible.

25. If possible, restrict plane surface machining (slots, groovers, etc.) to one surface of the component only.

26. Avoid cylindrical bores in long components. 27. Avoid machined surfaces on long cylindrical components by using

workmaterial preformed to the cross section required. 28. Avoid extremely long or extremely thin components.

Design for Manufacturing

Component design (continued) c. Nonrotational components

29. Ensure that in flat or cubic components, main bores are normal to the base and consist of cylindrical surfaces decreasing in diameter from the exposed face of the workpiece.

30. Avoid blind bores in large cubic components. 31. Avoid internal machined features in cubic boxlike components. Assembly 32. Ensure that assembly is possible. 33. Ensure that each operating machined surface on a component has a

corresponding external corner on the mating component. 34. Ensure that internal corners do not interfere with a corresponding

external corner on the mating component.

Design for Manufacturing

Product design for Assembly • Design for Assembly (DFA) should be considered at all stages of the design

process, but especially the early stages. • As the design team conceptualizes alternative solutions, it should give serious

consideration to the ease of assembly of the product or subassembly. • The team needs a DFA tool to effectively analyze the ease of assembly of the

products or subassemblies it designs. • The design tool should provide quick results and be simple and easy to use. • It should ensure consistency and completeness in its evaluation of product

assemblability. • It should ensure that solutions are evaluated logically, enable easy

comparison of alternative designs, identify assembly problem areas, and suggest alternative approaches for simplifying the product structure – thereby reducing manufacturing and assembly costs.

Design for Manufacturing

• By applying a DFA tool, communication between manufacturing and design engineering is improved, and ideas, reasoning, and decisions made during the design process become well documented for future reference.

• The Product Design for Assembly provide systematic procedures for evaluating and improving product design for ease of assembly.

• This goal is achieved by providing assembly information at the conceptualization stage of the design process in a logical and organized fashion.

• This approach also offers a clearly defined procedure for evaluating a design with respect to its ease of assembly.

• In this manner, a feedback loop is provided to aid the designers in measuring improvements resulting from specific design changes.

• Thus the design can be studied and improved at the conceptual stage when it can be simply and inexpensively changed.

Design for Manufacturing • The DFA method accomplishes these objectives by: 1. Proving a tool for the designer or design team which assures that

considerations of product complexity and assembly take place at the earliest design stage. This eliminates the danger of focusing exclusively during early design on product function with inadequate regard for product cost and competitiveness.

2. Guiding the designer or design team to simplify the product so that savings in both assembly costs and piece parts can be realized.

3. Gathering information normally possessed by the experienced design engineer and arranging it conveniently for use by less-experienced designers.

4. Establishing a database that consists of assembly times and cost factors for various design situations and production conditions.

• The analysis of a product design for ease of assembly depends to a large

extent on whether the product is to be assembled manually, with special purpose automation, with general-purpose automation (robots), or a combination of these.

• For example, the criteria for ease of automatic feeding and orienting are much more stringent than those for manual handling of parts.

Design for Manufacturing General design guidelines for manual assembly • The process of manual assembly can be divided naturally into two separate

areas: handling (acquiring, orienting and moving the parts) and insertion and fastening (mating a aprt to another part or group of parts).

• The following design for assembly guidelines specifically address each of these areas.

Design guidelines for part handling • In general, for ease of handling, a designer should attempt to: 1. Design parts that have end-to-end symmetry and rotational symmetry

about the axis of insertion. If this cannot be achieved, try to design parts having the maximum possible symmetry (See Fig.(a))

Design for Manufacturing

Design guidelines for part handling (continued) 2. Design parts that, in those instances where the part cannot be made

symmetric, are obviously asymmetric (See Fig.(b)).

3. Provide features that will prevent jamming of parts that tend to nest or stack when stored in bulk (See Fig.(c)).

Design for Manufacturing Design guidelines for part handling (continued) 4. Avoid features that will allow tangling of parts when stored in bulk (See

Fig.(d)).

5. Avoid parts that stick together or are slippery, delicate, flexible, very small, or very large or that are hazardous to the handler (i.e. parts that are sharp, splinter easily, etc.) (See Fig.).

Design for Manufacturing Design guidelines for insertion and fastening For ease of insertion a designer should attempt to: 1. Design so that there is little or no resistance to insertion and provide chamfers to

guide insertion of two mating parts. Generous clearance should be provided, but care must be taken to avoid clearances that will result in a tendency for parts to jam or hang-up during insertion (See Figs).

Design for Manufacturing Design guidelines for insertion and fastening (continued) 2. Standardize by using common parts, processes, and methods across all models and

even across product lines to permit the use of higher volume processes that normally result in lower product cost (See Fig.).

3. Use pyramid assembly – provide for progressive assembly about one axis of reference. In general, it is best to assemble from above (See Fig.).

Design for Manufacturing Design guidelines for insertion and fastening (continued) 4. Avoid where possible, the necessity for holding parts down to maintain their

orientation during manipulation of the subassembly or during the placement of another part (See Fig.). If holding down is required, then try to design so that the part is secured as soon as possible after it has been inserted.

5. Design so that a part is located before it is released. A potential source of problems arises from a part being placed where, due to design constraints, it must be released before it is positively located in the assembly. Under these circumstances, reliance is placed on the trajectory of the part being sufficiently repeatable to locate it consistently (See Fig.).

Design for Manufacturing Design guidelines for insertion and fastening (continued) 6. When common mechanical fasteners are used the following sequence indicates the

relative cost of different fastening processes, listed in order of increasing manual assembly cost (See Fig.): a) Snap fitting, b) Plastic bending, c) Riveting, d) Screw fastening.

7. Avoid the need to reposition the partially completed assembly in the fixture (See Fig.).