I have been emphasising the importance of Manufacturing processes in the whole Product Design process. (Read my post on HOW TO BECOME A GREAT DESIGN ENGINEER)
In this article, I will briefly, explain and advise the correct method of designing casted parts.
Yes!!! There will be few changes in your CAD Model depending upon the manufacturing process.
Product design, selection of material and processing the materials into finished components are closely related to one another,
One of the easiest methods to convert the raw material into a finished component is casting.
Modification in Design for Casted parts.
Some components are very complex to machine or they may require a series of complicated operations. Consider manufacturing a Cylinder Block (which is cast) would otherwise require a series of operations. Other examples are pumps, gear box housing etc.
Cylinder block image.
1. Keep the Stressed Areas of the component in Compression.
Almost every metal, including cast Iron, is stronger against Compressive stress than the tensile (Due to Microstructure of metal). Therefore, you must design your component carrying a load, in such a way that the area under stress is always under the state compression.
2. Round all the corners
It has two advantages- it increases the endurance limit of the component and reduces the formation of brittle chilled edges.
When the metal in the corner cools faster than the metal adjacent to the corner, brittle chilled edges are formed.
Appropriate fillet radius reduces the stress concentration:
3. No sudden change in the Section Thickness
Due to abrupt changes in cross section, high-stress concentration region is formed at the edges. Try to maintain a gradual cross-sectional area. It should be held as uniform as possible (keep in mind it should not affect the overall component function).
4. Avoid concentration of metal at the junctions
In the casting process, metal solidifies faster at the surface than at the centre. And therefore the central portion remains at the molten stage, with the result that a shrinkage cavity or blowhole may appear at the centre. There are two ways to avoid the concentration of metal. One is to provide a cored opening in webs and ribs. Alternatively, one can stagger the ribs and webs, as shown in figure
6. Draft angle
All the casted parts, need to have a certain draft angle. And therefore one must include certain draft to the component.
1º for internal draft (min)
0.5º for external (min)
min 0.1º for internal holes. (min)
Surfaces perpendicular to the direction of ejection can damage the product and the mould. The metal formed will stick to the mould and an ejection by force and pressure causing an immeasurable damage. Draft angle if certain degree ensures better surface finish and prevents broken corners.
Note: Casting can be done without draft angles but it will be a costly process.
7. Add material where surface finish is needed.
As we know cast parts have a very rough finish. And require further machining for better finishing. So, it’s strongly recommended to use the extra material on the face of the component where you require better finishing. As further machining will result in material removal. Which may cause a mismatch component.
Read my post: What will make you a Professional Cad Designer.
Design of machine elements, V B Bhandari