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Lesson 254 of IEKC Industrial Engineering ONLINE Course Notes.
Product Design for Manufacture and Assembly, Third Edition
Geoffrey Boothroyd, Peter Dewhurst, Winston A. Knight
CRC Press, 08-Dec-2010 - Technology & Engineering - 712 pages
https://books.google.co.in/books/about/Product_Design_for_Manufacture_and_Assem.html?id=W2FDCcVPBcAC
Note: It is important to read the books by Boothroyd to understand the full method of DFMA. The DFMA method is to be combined with Value Analysis and Engineering to do product industrial engineering. In the note only attempt is made to make readers aware of issues raised and solutions proposed by DFMA method.
DESIGN RULES FOR SAND CASTINGS
A study states that the cost of a casting can easily be doubled due to designs that do not take cost drivers into consideration.
The following design rules are to be followed to avoid the expense of increased scrap and possibly large increases in production cost.
Avoid Sharp Angles and Multiple-Section Joints
Metal structure is affected by the shape of the casting section. Solidification of the molten metal begins at the mold face, from which crystals grow into the casting at right angles. A straight section of constant thickness results in uniform cooling, which will in turn produce uniform material properties. On the other hand, sharp angles can cause large temperature variations in the casting, which often lead to casting defects. Hot spots result where the free cooling of the casting is interrupted as parts of the sand are loaded with more energy than other areas. Also, chilled areas arise on external corners from being exposed to two cooling planes. The resulting grain structure is not homogeneous and, in particular, weak areas in the casting are created in the areas where the cooling rate is excessive. A well-designed casting brings the minimum number of sections together at intersections and avoids acute angles. Wherever a number of sections converge, the appropriate solution is to create a large hole like the center of a web.
Examples of good and bad section configurations are shown in a figure in the book.
Design Sections of Uniform Thickness
Design the casting so that all of the section thicknesses are as consistent as possible. This promotes even cooling of the casting, reducing the likelihood of defects. If larger masses of metal are unavoidable, the designer should make them accessible for feeding either directly or with a riser.
Designing for uniform thickness also reduces the amount of material in a casting, saving weight and reducing machining, and results in a stronger casting.
However, if section thicknesses are too small, then feeding problems may occur.
The increased cost of scrap caused by incomplete feeding (caused by metal freezing and blocking the section from being completely filled) will normally be higher than the material savings in a lighter casting. The economical minimum section thicknesses of different metals to be sand cast are available.
Proportion Inner Wall Thickness
Inner sections in a casting cool more slowly than a section exposed to the mold face. If a complex geometry is necessary, the designer should reduce the inner section thickness to 80% of the outer wall thickness. Also, core section thicknesses should always be greater than the section thickness of the surrounding metal. If the core is too small, it will become overheated and slow down the solidification rate of the surrounding metal, leading to the possibility of defects.
Consider Metal Shrinkage in the Design
Almost all alloys shrink as they solidify. While the patternmaker is the one affected by the shrinkage, the designer must still compensate for it in the design. In a good design, the section thicknesses decrease as the distance from the feed system or riser increases. In order to accomplish this, the designer must be familiar enough with the casting process to be able to visualize how the casting will be fed and adjust the casting's dimensions to assist the metal flow. The greater the shrinkage of the metal, the more the designer must consider it when designing the casting. Tables are available in handbooks regarding the shrinkage of several of the commonly cast alloy groups. The amount of shrinkage depends upon the precise carbon content for irons and steels and varies over the ranges shown.
Use a Simple Parting Line A flat plane, known as a straight parting line, separating the two mold halves, results in more economical casting than a tiered or contoured separating surface.
More complex parting lines often result in fewer parts per mold, more costly patterns, less accuracy, and increased scrap. Also, the parting line should be positioned so that it has minimal effect on the functional characteristics of the part.
Locating the parting line in less critical parts of the casting is desirable for two main reasons. First, dimensions around the parting line are the hardest to control.
Additionally, flash occurs at the parting line. If the surface around the parting line is not critical, then flash removal costs will be lower.
Define Appropriate Machining Allowances
The machining allowance is material added to the casting to compensate for dimensional and surface variations in the as-cast part. The amount of stock added is a function of the size of the surface to be machined and to a lesser degree the machining method and the final accuracy required. Minimal additional material is needed if only flatness, possibly with some unmachined surface areas, is desired. A larger allowance is required if the full surface is to be machined without any imperfections. Normal machining allowances vary from 0.25cm for small castings (< 15 cm) to as much as 2.5 cm for large castings (>250 cm).
Use Economical Tolerances
The tolerances achievable by a foundry vary depending on the types of processes employed at the facility. For example, automated molding machines are capable of producing molds with tighter tolerances than might be produced by hand.
Conservative tolerances, which are readily achievable by most foundries and are therefore the most economical, are used in the following discussion.
Tighter tolerances may be obtained by machining, which significantly increases the cost of the casting.
The most basic tolerance is the linear tolerance. It refers to how precisely the distance between two points can be produced. Linear tolerances of ±1.0 mm are readily achievable for small castings. An additional factor of ±0.03 mm should be added for every centimeter over 15 cm for larger parts. An additional tolerance must be added to the linear tolerance of a dimension that passes through or originates from the parting surface. These additional tolerances reflect variations caused by expansion and contraction of the mold, the metal during solidification, patternmaking tolerances, and vibration of the pattern during removal from the mold. The size of the additional tolerance depends upon the projected area of the casting at the parting surface. The typical tolerance assignment is ±0.25 mm for each 10cm2 of projected area.
Cores create tolerance variation because of the clearance that is necessary for their placement into the mold. The features produced by the core surface can be held to a tighter tolerance than the features produced by the mold surface, because cores are stronger and able to be produced to tighter tolerances than the mold.
However, the surface produced by the core may be displaced from the surface created by the mold because of core shift. The additional tolerance for core shift varies with the protected area of the core normal to the dimension being considered. The recommended value is the same as for the additional parting line tolerance given above.
https://dawangcasting.com/sand-casting-design-considerations/
REFERENCES
1. Brown, J.R., Foseco Foundryman's Handbook, 10th ed. Butterworth-Ffeinemann, 1996.
2. Wukovich, N., Evaluating Side Risers and Necks, Part 1, Modern Casting, December 1988, p. 42. Design for Sand Casting 547
3. Wukovich, N., Evaluating Side Risers and Necks, Part 2, Modern Casting, January 1988, p. 49.
4. Wukovich, N, Evaluating Side Risers and Necks, Part 3, Modern Casting, February 1988, p. 56.
5. American Society of Metals Casting, Vol. 15, Metals Handbook, 9th ed. ASM International, Metals Park, OH, 1988, pp. 577-597.
6. Suschil, T., Designing Gates and Risers in an Artful Compromise, Modern Casting, March 1989, pp. 27-29.
7. Wieser, PR, Steel Castings Handbook, 5th ed., Steel Founders Society of America, 1980.
8. Bralower, P.M., Sand Molding: From Hand Ramming to Near Net Shape Castings, Modern Casting, May 1989, pp. 53-58.
9. Burditt, M.F., Designs and Operation of Melting Furnaces Differ Markedly, Modern Casting, August 1989, pp. 51-55.
10. Burditt M.F. and P.M. Bralower, Good Pouring Practice Contributes to Quality Castings, Modern Casting, 1989, pp. 59-63.
11. Mrdjenovich, R., Shakeout: Separating the Casting from Its Mold, Modern Casting, October 1989, pp. 45-47.
12. Luther, N., Cleaning and Finishing: Getting the Casting Ready for Shipping, Modern Casting, November 1989, pp. 53-58.
13. Kobrak, G., Design and Early Cost Estimation of Sand Castings, M.S. Thesis, University of Rhode Island, Kingston, 1993.
14. Mietrach, D., AGARD Handbook on Advanced Casting, AGARD-AG-299. North Atlantic Treaty Organization Advisory Group for Aerospace Research and Development, Bremen, Germany, p. 9.
15. Casting Engineering and Foundry World, Continental Communications, Inc., Bridgeport, CT.
16. Customers Foundry Orientation Manual, Robinson Foundry, Alexander City, AL.
17. Bralla, G.B., Handbook of Product Design for Manufacturing, McGraw-Hill, New York, 1986.
Casting Design Issues and Practices
H.W. Stoll
https://books.google.co.in/books?id=vQAj5iB4KY4C&pg=PA1#v=onepage&q&f=false
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