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UNIT 8: Sheet Metal Forming and Poweder Metallurgy Manufacturing Engineering Unit 8 Copyright © 2012 MDIS. All rights reserved. 1
Sheet metal forming All processes have to deal with deforming sheets, material removal from sheets and basically dealing with a 2 dimensional raw form to take up a final 3 dimensional form. Most operations are called Press work or press forming. The machines used are invariably called presses Low carbon steels are the comonly used raw materials here. Aluminium is another very commonly used material in the sheet metal presses. Aerospace materials like titanium are also encountered. 8-2Unit 8 Copyright © 2012 MDIS. All rights reserved.
Shearing A blank of suitable dimensions is removed from a raw sheet. The cut is made using shearing stresses using a punch and a die. It starts with formation of cracks on the top and bottom surface of the work-piece. These crack propagate to meet each other inside the material Die cutting Perforation- punching a number of holes in a sheet parting- shearing into two or more pieces notching- Removing pieces from edges lancing- leaving a tab without material removal 8-3Unit 8 Copyright © 2012 MDIS. All rights reserved.
Fine Blanking Smooth and square edges are produced in fine blanking Slitting Using a pair of circular blades, one can trace a path of any shape like a can opener. This slit edge can be folded over by flattening in rolls Nibbling A sheet is fed through a gap between a straight punch and a die. Making many overlapping holes one can cut any desrired shape in a sheet. 8-4Unit 8 Copyright © 2012 MDIS. All rights reserved.
Dies in Shearing Straight punch and die Compound dies- where several operations on the same sheet can be performed in one stroke Progressive dies- here the set of punch and dies are lined up in sequence to perform complex successive operations on the same sheet as it progresses through these synchronized dies Transfer dies- complex operations are achieved by transferred a sheet from one die to another by physical transfer in the line. 8-5Unit 8 Copyright © 2012 MDIS. All rights reserved.
Sheet metal characteristics and formability Elongation High uniform elongation is a desirable property Necking causes a weakness in the material which is exploited in this process, so diffuse necking is desirable. Yield point elongation Yield point elongation in typical materials results in strech marks which may be undesirable This is avoided by subjecting materials to cold rolling Anisotropy Preferred grain orientation in the sheet metal due to thermo mechanical processes in its making result in anisotropy. 8-6Unit 8 Copyright © 2012 MDIS. All rights reserved.
This causes the effects of the same forces in punch and die to have differing effects at different places Grain size The grain size of microstructure causes stronger materials with smaller grains Grain sizes which are finer are preferred in sheet metal operations 8-7Unit 8 Copyright © 2012 MDIS. All rights reserved.
Sheet bending It gives the sheets a usable 3 dimensional shape from the otherwise planar raw sheet form. Bending is done by subjecting the sheets to high deformation forces over a bend edge with reasonably blunt dies. These cause a permanent deformation about the bend line, and associated bend radius, which depends on sheet thickness and material characteristics. Each material has a given minimum bend radius below which the material on outside of the bend would develope cracks. 8-8Unit 8 Copyright © 2012 MDIS. All rights reserved.
Spring back is the characteristic of the material to end up in a final position that springs back from final bending in die. This causes great problems in the overall dimensional stability of the products. One has to compensate for this effect in the design of the bending dies Unit 8 Copyright © 2012 MDIS. All rights reserved. 7-9
Press brake forming press brake forming bend are: channel forming- where a sheet can be bent to include a channel Joggle- the surface can be made to bend in successive planes creating offset planes Hemming- the creation of a 180 degree edge bend that can be interlaced to form excellent sheet metal joints. Offset forming- the creation of a complex series of closely placed bends Unit 8 Copyright © 2012 MDIS. All rights reserved. 7-10
Roll Bending Plates can be bent into radius using a set of rolls Adjusting the distance between and the location of rolls also provides for radius variations needed in a sheet metal bend Beading The edge of the sheet is bent over within the die. This gives the sheet a greater moment of inertia and very good appearance Flanging Bending edges of a sheet to 90 degrees over a feature desired is called flanging. Unit 8 Copyright © 2012 MDIS. All rights reserved. 7-11
Roll forming Sheets can be converted from planar to varied spatial forms like C sections, angles, etc Deep drawing Creating a deep shaped object from a plane sheet of steel is called deep drawing The process involves a super stretch of the sheet metal into the thickness direction of the sheet The sheet is held in a blank holder. A punch is pushed into the blank, causing the stretching and deformation of the sheet in its direction of draw. Unit 8 Copyright © 2012 MDIS. All rights reserved. 7-12
Metal Spining A sheet is placed against a spindle and then a spin forming tool is pressed against the sheet This causing the sheet to deform along the spinning blank surface. Unit 8 Copyright © 2012 MDIS. All rights reserved. 7-13
Powder metalurgy The use of powdered form of metals in various combinations by way of sintering instead of treating it in traditional molten or solid states for forming. Today there is a wide range of metal powder combinations that are used in engineering applications. Examples include the tungsten filaments of bulbs, pre lubricated bearings, surgical implants etc. Unit 8 Copyright © 2012 MDIS. All rights reserved. 7-14
Metal Powders Production Atomization Molten metals streaming through an orifice is broken and cooled by jets of inert gas, air or water. The particle size produced depends on the temperature of molten metal, flow rate, nozzle size etc. Reduction the reduction of metal oxides uses gases like hydrogen and carbon monoxide. Fine metal oxides are reduced to metallic states. Powders thus produced are spongy and porous with uniform sized spherical shapes. Unit 8 Copyright © 2012 MDIS. All rights reserved. 7-15
Electrolytic deposition The deposition of the metals on electrodes of an electrolytic cell produces metal globules that are tiny and uniform. Mechanical Alloying Powders of two or more pure metals are mixed in a ball under impact of hard balls, the fracture of powder particles causes them to bond together by diffusion, giving alloy powders. Unit 8 Copyright © 2012 MDIS. All rights reserved. 7-16
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