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amount of material. This allows a simple die to cut complex slots.
Nesting - a sheet can be used more effectively (reduce scrap) if part patterns are closely packed in before shearing.
•Dies used in shearing typically have small clearances between the punch (moving part) and Die (non-moving backing). If this gap is too great the parts will have rough edges and excess shear force will be required. Clearances that are too small lead to premature wear. Typical design issues for clearances are given below,
-for softer materials the clearances are generally smaller
-thicker sheets require larger clearances
-typical clearance values range from 2-8% of sheet thickness
-extreme clearances range from 1-30% of sheet thickness
•Typical dies will come in a number of forms,
-bevel/double bevel/convex shear dies - these have an angle on the punch or die so that the shear starts at one point and then moves, much like cutting with scissors.
-compound dies - a die has multiple punches and dies that operate on the piece at the same time
-progressive dies - a single die contains a number of die slots. A part will stop at each die inside the progressive die before it is complete. This type of dies allows slow working of parts.
-transfer dies - a sequence of dies in one or more presses will operate on a piece - this is basically a scaled up progressive die.
48.2.1 Progressive and Transfer Dies
• These have dies with stations that will
48.2.2 DRAWING
• Material is pulled into the die.
48.3 DEEP DRAWING
•Commonly the process is,
1.A blank is clamped over a die so that it is not free to move.
2.A punch is advanced into the material, forcing it into the die and permanently deforming it.
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3. The punch is removed, the part removed from the die, and the excess blank is trimmed off.
• Typical applications for this process include pots, cups, etc.
48.4 SPINNING
•Basically,
1.A mandrel (or die for internal pieces) is placed on a rotating axis (like a turning center).
2.A blank or tube is held to the face of the mandrel.
3.A roller is pushed against the material near the center of rotation, and slowly moved. outwards, pushing the blank against the mandrel.
4.the part conforms to the shape of the mandrel (with some springback).
5.The process is stopped, and the part is removed and trimmed.
•This process can form very large items well over 10’ in diameter.
•Items that can be produced are,
-buckets
-pots
-satellite dishes
-inlet rotated parts
48.5 MAGNETIC PULSE FORMING
•Basic operation,
1.A large current discharge is directed through a coil. The coil has been placed inside another shape.
2.The discharging current creates a magnetic field. In the nearby sheet of metal an opposing magnetic field is induced. The result is that the two magnetic fields oppose and a force moves the sheet away from the coil.
3.Over a period of time the part is deformed, often to the shape of a mandrel, or other form.
•Applications,
-fittings for ends of tubes
-embossing
-forming
•Capacitor banks are used to accumulate charge for larger discharges.
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•The part is formed to a mandrel that has a negative image of the part.
•The method generates pressures up to 50 Kpsi creating velocities up to 900 fps, production rates can climb to 3 parts a second.
•Applications,
-ball joint seals
-fuel pumps
-baseball bats
•Generally there are three methods of magnetic forming,
-swaging
-expanding
-embossing and blanking
•Swaging - An external coil forces a metal tube down onto a base shape (tubular coil).
•Expanding - an inner tube is expanded outwards to take the shape of an outer collar (tubular coil).
•Embossing and Blanking - A part is forced into a mold or over another part (a flat coil) - This could be used to apply thin metal sheets to plastic parts.
Forming Coil
Sheet
Mold
•Advantages,
-easy to control
-allows forming of metals to any material
-no contact eliminates many requirements such as lubricants, heat dissipation, surface repair, etc.
-parts are uniform
-no tool wear
-minimal operator skill
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-very strong joints
-energy efficient
-easy installation
-high production rates (typically a few seconds)
•Disadvantages,
-complex shapes not possible
-no pressure variations over work
-limits forming pressures
48.6 HYDROFORMING
•Basic process,
-A metal sheet is placed over a male punch.
-Fluid is on the other side of the metal sheet.
-The punch advances and the metal sheet is forced into the shape of the punch. The hydraulic chamber acts as a mate for the punch.
•The basic operation is,
1.The metal is placed between the fluid chamber and the punch bed.
2.The fluid is encased behind a wear pad, and this wear pad is brought into contact with the sheet with pressures up to 5 Kpsi.
3.The punch is advanced with pressures up to 15 Kpsi causing the metal to take the shape of the punch.
4.The pressures are released, the punch withdrawn, the fluid chamber pulled back to remove the metal part.
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-fewer anneals required
-only rotational parts possible with spinning
•Methods permissible,
-punch forming - for large drawing depths
-negative punch forming - allows recessed features
-cavity die forming
-male die forming
-expansion forming
•Advantages,
-any type of sheet material can be used
-thicknesses of 0.1 to 16mm
-tools can be used for more than 1 metal thickness
-flexible and easy to operate
-less expensive tooling
-tolerances down to 0.002”
-reduced setup times
-less thinning
-reduced die wear
•Disadvantages,
-sharp corners difficult to control
-high equipment cost
-no holes in surface
-incorrectly set pressures may lead to buckling and tearing for high pressures
•Design points
-the metal springback should be considered in design, or the size of the punch/die changed through trial and error experiments.
-a draft (taper) of 1-2° will prolong tool life.
-the minimum part radius should be 2-3 times the sheet thickness.
•Applications,
-cups/kitchenware