Manufacturing Considerations during Designing

In this tutorial, you will learn about the various factors which are taken into consideration when designing a product from a manufacturing point of view. You will be informed about the manufacturing methodologies prevalent in the industries and what is expected from a design being prepared for it.

Contents:

  1. Manufacturing and Designing as a Single Unit
  2. Selection of Manufacturing Method
  3. Designing for a Casting Process
  4. Design Considerations in Forging
  5. Design Considerations in Machining
  6. Hot and Cold Working
  7. Considering a Welding Assembly
  8. Design for Manufacture and Assembly

Manufacturing and Designing as a Single Unit

Manufacturing of the product is an important link in the chain of events that begins with the concept of a probable product and ends with a competitive product in the marketplace. Product design, selection of materials, and processing of the materials into finished components are closely related. The general guidelines to be followed when designing a product are

  • The cost of a product is proportional to the number of components it contains. The cost of a product is usually reduced by reducing the number of parts in it. Combining two or more elements into an integrated design helps reduce the total number of elements.
  • Minimizing the number of different components saves manufacturing costs, improves part quality, and minimizes inventory requirements.
  • The cost of standard parts is always cheaper than the cost of custom-made parts. As a result, standard parts should be employed in a product as much as feasible.
  • The manufacturing method should be chosen such that the part’s final shape may be achieved with the fewest amount of cost-effective operations possible. Grinding, lapping, honing, and other finishing procedures should be avoided if at all feasible.
  • The components should be designed in such a manner that they can be manufactured with the fewest possible steps.

Selection of Manufacturing Method

Manufacturing can be considered as processing the available material into useful components of the product, like converting a mild steel sheet into a car body, converting a billet of cast iron into a machine tool bed, or converting a steel bar into a transmission shaft. The manufacturing processes can be broadly classified into three categories. The following figure shows the grain structure for the three processes.

Grain Structures of Components
  • Casting Processes – In these processes, molten metals such as cast iron, copper, aluminum, or non-metals like plastic are poured into the mold and solidified into the desired shapes like housing of gearbox, flywheel with rim and spokes, machine tool beds, and guides.
  • Deformation Processes – In these processes, a metal, either hot or cold, is plastically deformed into the desired shape. Forging, rolling, extrusion, press working are examples of deformation processes. The products include connecting rods, crankshafts, I-section beams, car bodies, and springs.
  • Material Removal or Cutting Processes – Here, the material is removed using sharp cutting tools. Turning, milling, drilling, shaping, planing, grinding, shaving, and lapping are examples of material removal processes. The products include transmission shafts, keys, bolts, and nuts.
  • In addition, there are joining processes like bolting, welding, and riveting. They are essential for the assembly of the product.

Designing for a Casting Process

Complex parts, which are otherwise difficult to machine, are made by the casting process using a sand mold. Poor shaping of a cast iron component can adversely affect its strength more than the composition of the material. The general principles for the design of casting are as follows.

advertisement
advertisement
  • Always Keep the Stressed Areas of the Part in Compression Cast iron has more compressive strength than its tensile strength. The balanced sections with equal areas in tension and compression are not suitable for cast iron components. The following figure shows the correct way to place a component in casting.
  • Placing Component for Casting
  • Rounding all External Corners has two advantages—it increases the endurance limit of the component and reduces the formation of brittle chilled edges.
  • Wherever possible, the section thickness throughout should be held as uniform as compatible with overall design considerations Abrupt changes in the cross-section result in high-stress concentration. If the thickness is to be varied at all, the change should be gradual.
  • Avoid very thin sections. In general, if the thickness of a cast iron component is calculated from strength considerations, it is often too small. In such cases, the thickness should be increased to certain practical proportions.
  • Shot blast the parts wherever possible. The shot blasting process improves the endurance limit of the component, particularly in the case of thin sections.
  • Oval-shaped holes are preferred with larger dimensions along the direction of forces. Patterns without a draft make a mold difficult and costly.

Design Considerations in Forging

Forged components are widely used in the automotive and aircraft industries. They are usually made of steel and non-ferrous metals. They can be as small as a gudgeon pin and as large as a crankshaft.

  • While designing a forging, advantage should be taken in the direction of fiber lines. The grain structure of a crankshaft is manufactured by the three principal methods, casting, machining, and forging. It is only in the case of forged parts that the fiber lines are arranged in a favorable way to withstand stresses due to external load.
  • The forged component should be provided with an adequate draft. The draft angle is provided for easy removal of the part from the die impressions. The following figure shows a draft introduced in the forging design.
    Introducing a Draft in Component
  • The parting line should be in one plane as far as possible, and it should divide the forging into two equal parts. A parting line that divides the forging into two halves ensures the minimum depth to which the steel must flow to fill the die impressions.
  • The forging should be provided with adequate fillet and corner radii. A small radius results in folds on the inner surface and cracks on the outer surface. A large radius is undesirable, particularly if the forged component is to be machined, during which the fiber lines are broken. Sharp corners result in increasing difficulties in filling the material, excessive forging forces, and poor die life.

Design Considerations in Machining

Machined components are widely used in all industrial products. They are usually made from ferrous and non-ferrous metals. They are as small as a miniature gear in a wristwatch and as large as a huge turbine housing.

  • Machining operations increase the cost of the component. Components made by casting or forming methods are usually cheaper. Therefore, as far as possible, the designer should avoid machined surfaces.
  • The secondary machining operations like grinding or reaming are costly. Therefore, depending upon the functional requirement of the component, the designer should specify the most liberal dimensional and geometric tolerances. The closer the tolerance, the higher is the cost.
  • Sharp corners result in stress concentration. Therefore, the designer should avoid shapes that require sharp corners.
  • Using stock dimensions eliminates machining operations. For example, a hexagonal bar can be used for a bolt, and only the threaded portion can be machined. This will eliminate the machining of hexagonal surfaces.
  • Shoulders and undercuts usually involve separate operations and separate tools, which increases the cost of machining.

Hot and Cold Working

The temperature at which new stress-free grains are formed in the metal is called the recrystallization temperature. There are two types of metal deformation methods, namely, hot working and cold working.

  • Metal deformation processes that are carried out above the recrystallization temperature are called hot working processes. Hot rolling, hot forging, hot spinning, hot extrusion, and hot drawing are hot working processes.
  • Metal deformation processes that are carried out below the recrystallization temperature are called cold working processes. Cold rolling, cold forging, cold spinning, cold extrusion, and cold drawing are cold working processes.

Considering a Welding Assembly

Welding is the most important method of joining the parts into a complex assembly. The guidelines are as follows

advertisement
  • Select the material with high weldability. In general, low carbon steel is more easily welded than high carbon steel.
  • Uneven expansion and contraction in the weld area result in distortion. When distortion is prevented by clamping fixtures, residual stresses are built up in the parts. Since distortion always occurs in welding, the design should involve a minimum number of welds and avoid over the welding. It will not only reduce the distortion but also the cost.
  • The designer should specify standard sizes for plates, bars, and rolled sections. Non-standard sections involve flame cutting of plates and additional welding.
  • If at all a stiffener is required to provide rigidity to the plate, it should be designed properly with minimum weight. The use of a separate stiffener involves additional welding increasing distortion and cost.
  • The welded joint should be in an area where stresses and deflection are not critical. Also, it should be located in such a place that the welder and welding machine has unobstructed access to that location.
  • The designer should consider the sequence in which the parts should be welded together for minimum distortion. This is particularly important for a complex job involving several welds.

Design for Manufacture and Assembly

The design effort makes up only about 5% of the total cost of a product. However, it usually determines more than 70% of the manufacturing cost of the product. Therefore, at best, only 30% of the product’s cost can be changed once the design is finalized and drawings are prepared. The best strategy to lower product costs is to recognize the importance of manufacturing early in the design stage.

  • Assembly operation should be carried out in clear view. This is important for manual assembly. It also decreases the chances of manufacturing defects slipping past the inspector.
  • Design products so that they can be assembled from the bottom to top along the vertical axis. This allows simple robots and insertion tools because gravity is used to assist the assembly process.
  • For both manual and automated assembly, symmetric parts are easier to handle and orient. As the assembly rate increases, symmetry becomes more important. Features should be added to enhance symmetry.
  • Fasteners are a major obstacle to efficient assembly and should be avoided wherever possible. They are difficult to handle and can cause jamming, if defective.
  • Part compliance is the ability of one part to move so that it is seated properly with another. The product should be designed for part compliance. Features such as added chamfers on both parts and adequate guiding surfaces make assembly faster and more reliable.
  • The number of processes needed to assemble a product should be kept to a minimum. Processes that are difficult to control, such as welding or brazing, should be avoided.

Key Points to Remember

Here is the list of key points we need to remember about “Manufacturing Considerations during Designing”.

  • The product lifecycle plays an important role in deciding the manufacturing process and the subsequent design expectations of the design.
  • Any manufacturing process falls under three major categories, casting processes, deformation processes, and machining processes.
  • When designing a product that needs to be cast, considerations like draft, bore diameter, fillets, die placement, etc. must be considered to ensure a well-casted product.
  • When designing for a deformation process like forging, care must be taken for the flow of material during the process and a draft must be provided to ensure easy removal at completion.
  • Machining components is a costly process and must only be undertaken if other manufacturing methods are not feasible.
  • The temperature at which new stress-free grains are formed in the metal is called the recrystallization temperature.
  • The best way to decrease product cost is to understand the importance of manufacturing early in the design stage.

advertisement

If you find any mistake above, kindly email to [email protected]

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & discussions at Telegram SanfoundryClasses.