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48. SHEET METAL FABRICATION

•Sheet metal typically begins as sheets, but after undergoing cutting, bending, stamping and welding operations it takes on useful engineering forms.

•Sheet metal has become a significant material for,

-automotive bodies and frames,

-office furniture

-frames for home appliances

-

• Sheets are popular materials because the sheets themselves are easy to produce, and the subsequent operations can be performed easily. The major operations typically include,

bending - an angle is used to create non-parallel faces

punching/shearing/blanking - a major portion of the material is cut off by putting the material in shear.

forming - spinning - stamping - embossing -

48.1 SHEET METAL PROPERTIES

•The properties of sheet metal determine how well it can be stretched or bent.

•The various properties include,

-Formability - a larger strain rate exponent ‘n’ relates to longer deformation

-Uniform Necking - the higher the strain rate sensitivity ‘m’, the less localized the necking

-Uniform Elongation - when the yield point has upper and lower points the material may deform in bands - giving long depressions in work surface called Leuder’s bands. These may occur in low carbon steels and aluminum/magnesium alloys.

-Anisotropy - if the material properties have no directionality deformation will be even.

-Small Grains - finer grains are preferred for better metal properties and surface finish.

48.2 SHEARING

• A shear force is applied that will cut off part of a sheet. The cut off ‘blank’ becomes the workpiece.

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• To find the shear force for a cut we can go back to the basic mechanics of materials (with one adjustment factor).

F

w

t

F

F = 0.7twUTS

where,

F = force needed to shear t = thickness of sheet

w = width of sheet

UTS = Ultimate Shear Strength of material

• The basic terms used in shearing are,

Punching - a small section of material is sheared out of a larger piece and discarded. Blanking - outside/surrounding material is cut off a smaller piece and discarded.

Die Cutting - small features are cut into the sheet, such as series of holes, notches (adjacent material removed), lancing out tabs (no material removed), parting to cut the sheet into smaller pieces.

Fine Blanking - dies are designed that have small clearances and pressure pads that hold the material while it is sheared. The final result are blanks that have extremely close tolerances.

Slitting - moving rollers trace out complex paths during cutting (like a can opener).

Steel Rules - soft materials are cut with a steel strip shaped so that the edge is the pattern to be cut.

Nibbling - a single punch is moved up and down rapidly, each time cutting off a small