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https://academia.edu/103626052/INTRODUCTION_TO_MODERN_INDUSTRIAL_ENGINEERING_Version_3_0
Lesson 251 of IEKC Industrial Engineering ONLINE Course Notes.
Engineering in Industrial Engineering - Machine work study or machine effort improvement, value engineering and design for manufacturing and assembly are major engineering based IE methods. All are available as existing methods.
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.
Picture Source: BHARTIYA INDUSTRIES produces various types of components such as :• Brackets, • Cover, • Spring seats, • Hinges, • Caster Wheel Bracket, • Steel Cap, • Metal Inserts, • Frame Parts.
Head Office & Manufacturing Plant 1: Plot No.268, Digvijay Industrial Estate, Upvan, Pokhran Road No. 1, Thane - 400 606, Maharashtra, India.
Design for Sheet Metalworking - Design Rules
In the design of sheet metal stampings the first consideration is the shape of the external perimeter. For parts that are to be manufactured with dedicated dies, it is advantageous to design the outer profile with parallel straight edges defining the part width. To allow for satisfactory shearing in cut-off or part-off operations, the end profiles should meet the straight edges at angles no less than 15 degrees. The profile shape should not contain narrow projections or notches that will require narrow weak sections in either punches or die plates.
Similar considerations for the avoidance of weak tool sections apply to internal punched holes. That is, small holes or narrow cut-outs that will require fragile punches should be avoided. In addition, internal punched holes should be separated from each other, and from the outside edge, with sufficient clearance to avoid distortion of narrow sections of the workpiece material during punching. The accepted rule of thumb is that both feature dimensions and feature spacings should be at least twice the material thickness. All profile radii, are subjected to the same rule of thumb. In this case the concern is the associated corner radii in the die plate. Radii equal to at least twice the gage thickness will minimize the corner stress concentrations in the die plate, which may lead to crack formation and failure.
It is good practice to incorporate relief cut-outs at the ends of proposed bend lines that terminate at internal corners in the outer profile. These circular relief cut-outs will be part of the die profile for blanking or will be punched before the adjacent outer profile in turret press working.
If for any reason holes that intersect the outer profile must be punched later, then the diameter should be at least three times the gage thickness to accommodate the offset loading to which the punch will be subjected. When formed features are being considered, the principal design constraint is the maximum tensile strain the material can withstand; this is usually called the material ductility. Typical ductility values are given in books. Thus, if a lanced and formed bridge is to be incorporated into a component the ratio of L to H can be calculated as follows. Assume that the transition or ramp from the surface to the top of the bridge is 45 degrees. The length along the bridge from end to end is approximately
Bridge length = L-2H/ tan(45) + 2H/ sin(45)
= L + 0.82H
Assuming uniform stretching of the bridge, the tensile strain along the bridge is thus
maximum permissible strain in tension e = 0.82H/L
There is a rule of thumb quoted in the literature that the length of bridges should be greater than 4 times their height. However, such rules are frequently based on experience with press working of annealed low-carbon steel. For different materials or varying geometries, such as changing the ramp angles in the preceding example, the tensile strains must be estimated and compared to the permissible maximum value. A common example of a lanced and formed feature in sheet metal parts is the louver. Louvers are often formed as groups of parallel slots in the sides of sheet metal enclosures for air circulation and cooling purposes. The length of the front edge of the louver must be greater than a certain multiple of the louver opening height H, determined by the material ductility and the end ramp angles exactly as in the bridge calculation.
However, stretching also occurs at right angles to the louver edge where the material is stretched upward into a circular arc. This will not cause material failure, since the front edge of the louver will be pulled backward as the tensile stress develops in the surface.
Another type of feature, which involves stretching along a sheared edge, is the hole flange. Hole flanging is often carried out to provide increased local thickness for tapping of screw threads or for assembly with self-tapping screws. The hole flange is formed by pressing a taper-nosed punch of diameter D into a smaller punched hole of diameter d. The tensile strain around the top edge of the formed flange is thus = (D- d)/D and this value must be less than the permissible material ductility. The limit of the ratio D/d, due to limited ductility, limits the amount of material displaced and in turn the height of hole flanges that can be produced. Typical values of flange height in sheet steel components, for example, range between 2 and 3 times the material gage thickness. In the design of beads or ribs used to stiffen open surfaces of sheet metal parts, the cross-sectional geometry is important. Ribs may be circular in section as shown, or are sometimes V-shaped. In any case, for a required height, H, the width and shape of the rib must be chosen so that the required amount of stretching across the rib does not exceed the material ductility. The radius at the base of the rib must also be greater than a certain value to prevent overstraining the material on the underside of the part. This may result from the bending effect along the sides of the rib and will be considered next. The maximum tensile strain in bending is in the outer fibers of the sheet on the outside of the bend and is governed by the ratio of inside bend radius, r, to sheet gage thickness, h. For a bend through any angle ø, the length of the outer surface is Ls
= (r + h)ø
and the length of the surface in the center of the sheet, on which lies the neutral axis of bending, is
L0 = (r + h/2)ø
Hence the strain on the outer surface is e = (Ls- Lo)/Lo = 1/(1 + 2r/h)
Radius r is defined precisely by the profile radius of the bending tool: either the convex radius of the die block for a wiper die or the convex radius of the punch in a v-die. In any case the minimum acceptable radius value can be obtained from Eq.(9.34) and the ductility of the material to be bent. For example, for low-carbon, commercial-quality steel with ductility 0.22, above Eq. gives
e = 0.22= 1/(1 + 2r/h)
or
r = 177h
A rule of thumb often quoted in the literature is that the inside bend radius should be greater than or equal to twice the sheet thickness. This is, in fact, the limiting value for a material with 20% ductility.
An additional consideration with respect to bending is the placing of other features next to bend lines. The rule of thumb in stamping is that the edge of circular holes should preferably be 2 times the sheet thickness from the beginning of a bend. For slots parallel to a bend this clearance should increase to 4 times sheet thickness. The manufacture of small flat sheet metal parts can be performed with a high degree of precision. Blanked parts or punched holes with maximum dimensions up to 10cm can be held to tolerances of approximately ±0.05 mm However, as part size increases, precision is more difficult to control, and for a part with dimensions as large as 50 cm permissible tolerances are in the range of ±0.5 mm.
The requirement for tolerances much tighter than these guideline values may call for features to be machined at greatly increased cost. For formed parts, or formed features, variation tends to be larger and minimum tolerances attainable are in the range of ±0.25 mm for small parts. This includes bending when dedicated bending dies are used. Thus a tight tolerance between punched holes, which are on parallel surfaces separated by bends, would require the holes to be punched after bending at greater expense. If the holes are on nonparallel surfaces, then machining may be necessary to obtain the required accuracy. Finally, in the design of turret press parts to be bent on press brakes, it should be noted that the inaccuracies of this bending process are substantially worse than with dedicated dies. Attainable tolerances between bent surfaces and other surfaces, or features on other surfaces, range from ±0.75 mm for small parts up to ±1.5 mm for large ones.
Finally, an important consideration in the design of any sheet metal part should be the minimization of manufactured scrap. This is accomplished by designing part profiles so that they can be nested together as closely as possible on the strip or sheet. Also, if individual dies are to be used, then the part should be designed if possible for cut-off or part-off operations. The cut-off design lacks the elegance of the rounded end profiles. Nevertheless, the acute sharp corner will be removed during debarring, and for many applications this type of design may be perfectly functional.
https://www.protolabs.com/resources/guides-and-trend-reports/designing-for-sheet-metal-fabrication/
NPTEL - Design for Sheet Metalworking
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https://www.youtube.com/watch?v=k8VskWhx0AY
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Some Simple DFMA Magic for Sheet Metal
Feb. 3, 2023
Engineers can turn out highly functional and easy-to-make sheet metal designs by following Design for Manufacturing and Assembly principles.
DFMA ASPECTS OF THE SHEET METAL PARTS IN A POWER ELECTRONIC
SYSTEM CABINET
Bachelor’s thesis of Jarkko Matikka
2018
39 pages
REFERENCES
1. Zenger, D., and Dewhurst, P., Early Assessment of Tooling Costs in the Design of Sheet Metal Parts, Report No. 29, Department of Industrial and Manufacturing Engineering, University of Rhode Island, Kingston, August 1988.
2. Zenger, D.C., Methodology for Early Material/Process Cost of Estimating, Ph.D.
Thesis, University of Rhode Island, Kingston, 1989.
3. Nordquist, W.N., Die Designing and Estimating, 4th ed., Huebner Publishing, Cleveland, 1955.
4. Eary, D.E, and Reed, E.A., Techniques of Pressworking Sheet Metal, 2nd ed., PrenticeHall, Englewood Cliffs, NJ, 1974.
5. Bralla J.G., Handbook of Product Design for Manufacturing, McGraw-Hill, New York,
1987.
6. Ostwald, P.P., AM Cost Estimator, McGraw-Hill, New York, 1986.
7. Wick, C., Benedict, J.T., and Veilleux, R.E, Tool and Manufacturing Engineers
Handbook, Vol. 2: Forming, Society of Manufacturing Engineers, Dearborn, MI, 1984.
8. Donovan, J.R., Computer-Aided Design of Sheet Metal Parts, M.S. Thesis, University of Rhode Island, Kingston, 1992.
Ud. 2.12.2023
Pub. 2.12.2021
Nice Post Sheet Metal Parts and Assemblies
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