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Sheet metal Deep drawing technology is one of the most challenging process in manufacturing. Successful deep drawing process depends on many factors.



deep drawingThe sheet metal deep drawn technology is one of the most challenging process in manufacturing. The sizes, shapes, thickness and deep drawn metal used to produces sheet metal deep drawn parts cover a diverse range of variables. Individual variables should be evaluated carefully to determine the optimum manufacturing method.

sheel metal deep drawRegardless of the many factors involved affecting the draw quality, the most important element to a successful sheet metal deep drawing operation is the smoothness of sheet metal flow. The following are key elements affecting metal flow during deep drawing process and each of them should be considered when designing or troubleshooting sheet metal deep drawing stamping tools:

Type of material used and its thickness. = In deep drawing process, thicker materials are stiffer, they can be gripped better during deep drawing. Thicker materials also have more volume, so they can stretch longer distances during deep drawing process ( deeper draw ).

Tool surface finish & type of Lubricant used = Die surface finishes and lubricants are important to reduces the friction between tool surfaces and metal been drawn, it allowing materials to flow through tools more easily. Die temperatures can affect the viscosity of lubricants. Slower deep drawing speed gives better metal flow.

Blank size and shape = Blank sizes and shapes that are too large can restrict metal flow, and the geometry of parts affects the ability of metal to flow during deep drawing process.

Punching speed = sufficient punching speed allows time for materials to flow through the tool. Corner cracking will always occur if press speed is tool fast in deep drawing process.

Draw radii = Radius on the draw die where the material flow through, too big the radii will result wrinkling where too small will creates cracking at bottom radius of drawn part.

Draw ratio = Refers to the ability of a material to flow or draw. Affecting the draw depth and size per any single draw.

Draw bead height and shape = To control metal flow and gripping pressure in deep drawing process, draw bead height and shape can cause materials to bend and unbend to creates restrictive forces going into a tool. Increasing pressure will exerts more force on a material, creating more restraint on material going into the tool.

Causes of common problems in the deep drawing.



Metal fracture during deep drawing process
1. Deep drawing radii too small, creating resistance to metal flow to cause undue thinning or fracture. Correct by increasing die radius or by changing die entrance to conical or elliptical shape.
2. Clearance between punch and die too little, too great or uneven.
3. Blank holder pressure too great.
4. Lubricant inadequate or unsuitable.
5. Punch nose radius too small.

Wrinkles on top edge or flange
1. Blank holder pressure too tight.
2. Draw radius too large.
3. Punch nose radius too large.
4. Wrinkles on the side of cup or flange caused by burr on blank or by an unbalanced blank holder pressure.

Uneven top rim or flange
1. Nicks or burr along the periphery of the blank.
2. Punch die or blank locator not concentric. Too much material pulled into the die by off center forming punch preventing forming of an even cup depth or flange.
3. Blank holder exerting an unbalanced force on the blank, permitting the material flow unevenly into the die.

Flange wrinkled, Puckered.
1. Blank thickness wrong or out of tolerance.
2. Draw radius too large.
3. Clearance between punch & Die too large for the stock thickness.
3. Depth of previous drawn too short or redraw operation too deep. Or side wall of drawn cup being tinned down.

Fracture ay bottom of redrawn cup
1. Draw radius and blank holding surface scratched, nicked or galled. Restricting metal flow into the die.
2. Punch nose and draw radius too small.
3. Diameter Reduction too great.

Fracture in or at Flange of drawn cup.
1. Punch and die clearance too small.

Excess material at top of deep drawn shell
1. Material thickness too great or punch die clearance too small or uneven.
2. Punch nose radius larger than cup bottom radius, causes excess
sheet metal to flow up side wall.
3. Draw radius too large, permitting wrinkles to start that enlarge during redraw of cup.

Deep drawn process computation




By definition, a draw piece part which requires only one draw operation is considered to be a shallow draw, and a draw piece part which require more than one drawing operation is consider to be deep draw. In the nutshell, the number of draws in deep drawing process can be estimated by table A.

Table A number of draw
 
H/D
P
 
 
< 0.75
1
 
 
0.75 - 1.5
2
 
 
1.5 - 3.0
3
 
 
3.0 - 4.7
4
 
draw ratio H = Inside height
D = mean Diameter
N > 4 Thickness
P = Number of draws


 

Deep drawn reduction percentages
When a sheet metal parts draw piece is drawn from a flat blank, the diameter of the draw piece is smaller than the diameter of the flat blank. The difference in diameter can be express as a percentage of the original 1st blank diameter.
Material = Steel
Sheet metal
Thickness
( inch )
1st draw
2nd draw
3rd draw
4th draw
< 0.063
40-48
20-25
18-20
16-18
0.063 - 0.125
40-48
15-18
14-15
13-14
0.125 - 0.187
40-48
12-15
11-12
10-11
0.187 - 0.25
40-48
10-13
9-10
8-9
> 0.25
40-48
8-10
7-8
6-7
Material = Brass
< 0.063
44-50
20-29
18-23
18-20
0.063 - 0.125
44-50
16-23
15-17
12-15
0.125 - 0.187
44-50
14-17
12-14
11-13
0.187 - 0.25
44-50
10-14
9-11
8-10
> 0.25
44-50
9-11
8-9
7-8