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20 B - SHEET METALWORKING: DRAWING


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DRAWING
PUNCH PUSHES BLANK INTO HOLE AND CREATES CUP, the punch with rounded edges pushing a lubricated shit into a die with filleted edges, the blankholder pressing on the top of the blank prevents formation of wrinkles, the punch and the die having rounded corners that allow for forming the blank into a cup, a box, or any other concave shape, such as a beverage cans, ammunition shells, cooking pots, sinks, or automotive parts. STURDY PUNCH FORMS LUBED SHEET INTO FIZZY-POP CANS
*Mechanics of drawing
MECHANICS OF DRAWING operate in the following manner: first the punch bends the blank into the hole, then the blank starts straightening in the hole while the punch compresses the thickening edges of the flange. >>> BENDING, STRAIGHTENING, COMPRESSION AND THICKENING.
*Analysis of drawing
Analysis of drawing focuses on [1] Measures of drawing - how much you can draw a part, [2] Forces - the forces you have to apply to draw a part, and [3] Blank size determination. >>> MilFS: MEASURES, FORCES, SIZE
Drawing ratio
Drawing ratio is the ratio in a drawn cylinder between the blank and punch diameters. $$DR= \frac{D_{blank} }{D_{punch} } \leq 2.0$$ The drawing ratio provides a crude indication to the severity of drawing operation, with the upper limit of the drawing ratio being around 2.0, yet in fact it depends on die and punch corder radii, friction, depth of the draw, and material properties
Reduction
Measures reduction of blank's radius: $$r= \frac{D_{blank} -D_{punch} }{D_{blank} }\leq0.5$$
Thickness to diameter ratio
THICKNESS TO DIAMETER RATIO indicates upon the wrinkling tendency - the smaller the ratio, the higher the chance that the part will wrinkle, thus it has to be greater than 1%, a limitation that sometimes causes a part to be drawn in a few stages, sometimes with annealing between the steps. $$\frac{t }{D_{blank }}\geq 1\% $$
Holding force
Force to hold down the blank is about 1.5% of the metal's yield strength, all that pressure applied to the area that the blank holder actually holds: $$F_{h}=0.015 \sigma_{y} \cdot \pi\left \{ { \scriptstyle D_{b}^{2}- \left(D_{p}+ 2.2t+2R_{d} \right)^{2}} \right\} $$
Blank size determination
DETERMINE BLANK SIZE BY CALCULATING VOLUME - determine the volume of the final part under some assumption of the part's thinning during drawing, deriving the blank's dimensions from calculated volume, thus preventive excess material and waste. CUP VOLUME DETERMINES BLANK DIAMETER.
*Other drawing operations
OTHER DRAWING OPERATIONS include: [1] Redrawing - drawing a part in additional steps, [2] Reverse drawing - flipping drawn part over and drawing it, [3] Square and not round shapes drawing, [4] Without blankholder - drawing without the top thingie that keeps the blank pinned to the die. >>> ReaR SaW: REDRAWING, REVERSE,, SQUARE, WITHOUT_BLANKHOLDER
Redrawing
ADDING ONE MORE DRAWING STEP, stretching metal in a few steps in order to make a form with a high draw ratio, each of the consecutive steps decrease in reduction: [1] First step - 40% reduction, [2] Second step - 30% reduction, [3] Third step - 16% reduction. >>> 40,30,16
Reverse drawing
REVERSE DRAWING OF A FLIPPED OVER CUp, an operation producing less strain hardening than redrawing, since it bends metal in the same direction and redrawing bends it in opposite directions
Non-cylindrical drawing
DRAWING OF NON-CYLINDRICAL SHAPES, such as cones, rectangular boxes, stepped cups, strange curved forms, such as automotive body panels.
Drawing without blankholder
DRAWING WITHOUT BLANKHOLDER can be done when the thickness-to-diameter ration, $t/D_b$ is large enough, since then you decrease the tendency for wrinkling, the tendency that the blankholder reduces in a regular operation. THICK PART DEFORMS WITHOUT BLANKHOLDER AND WRINKLES.
*Defects in drawing
Drawing defects, the common drawing defects occurring in this somewhat complex operation include: [1] Wrinkling of flange - undrawn flange wrinkles due to buckling, [2] Wrinkling of wall - wall of drawn part have wrinkles, they appear when the flange goes in, [3] Tearing - crack in the wall, usually due to metal failure, thinning, and sharp die walls, [4] Earing - irregularities in deep drawn cup due to unisotropy of material, [5] Surface scratches - scratched surface of drawn part due to lack of lubrication of smoothness of punch and die. >>> SWEeT: SCRATCHES, WRINKLES, EARING, TEARING
Scratches
Scratches on drawn part due to lack of lubrication or defects of die and punch.
Wrinkles
Wrinkling of flange or wall of drawn part due to compressive buckling
Earing
Ear shaped irregularities on upper edge of deep drawn part due to unisotropy of material
Tearing
Tearing and cracking of vertical walls due to thinning an material failure.
OTHER SHEET METAL FORMING OPERATIONS
Other sheet metal operations that are performed on conventional presses include: [1] Operations with metal tooling, and [2] Rubber forming processes
*Operations with metal tooling
Operations with metal tooling are the following ones: [1] Ironing - make uniform wall thickness after drawing, [2] Coining and embossing - making raised or indented in part, [3] Lancing - combined cutting and bending operation, [4] Twisting - subjection of metal to torsional deformation, to make propeller blade for example. >>> ICE LooT: IRONING, COINING AND EMBOSSING,,, LANCING, TWISTING.
Ironing
'Irons' the drawn cup to achieve uniform thickness
Coining and embossing
Punch makes indentations in metal, such as lettering and strengthening ribs, the regular applications of coining and embossing.
Lancing
Lancing punch cuts and bends at the same time, producing louvers in sheet metal air vents.
Twisting
Twisting a metal into a propeller blade by subjecting it to torsional loading.
*Rubber forming processes
Rubber forming processes involve using flexible tooling and pressure to achieve forming: [1] Guering Process - thick rubber pad surrounds and forms sheet metal, [2] Hydroforming - pressurized liquid forms part though rubber diaphragm. HuG: HYDROFORMING, GUERING
Guering process
THICK RUBBER FORMS SHEET METAL ON FORM BLOCK, the rubber contained withing the cavity of the punch, the punch descending slowly on the part, creating pressures of up to 10[MPa], pressure sufficient enough to form shallow parts but inadequate to prevent wrinkling on deeply drawn parts, a process well fitted for small production quantities due to low cost of equipment: rubber pad and simple plastic or wood block
Hydroforming
PRESSURED OIL FORMS METAL SHEET THROUGH RUBBER DIAPHRAGM, applying pressures of up to 100[MPa], preventing wrinkling on deeply drawn parts due to uniformly applied pressure, a method of forming that allows even for deeper drawing than a regular punch and die can achieve. OIL AND RUBBER SHEET DRAW DEEPER THAN DIE AND PUNCH.