24. 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,
• 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.
• 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.
• A shear force is applied that will cut off part of a sheet. The cut off ‘blank’ becomes the workpiece.
• To find the shear force for a cut we can go back to the basic mechanics of materials (with one adjustment factor).
• 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 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.
24.2.1 Progressive and Transfer Dies
• These have dies with stations that will shape a part with multiple operations. The parts normally travel through the machine on a web.
• Material is pulled into the die.
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.
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.
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,
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.
• Capacitor banks are used to accumulate charge for larger discharges.
• 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.
• Generally there are three methods of magnetic forming,
• 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.
allows forming of metals to any material
no contact eliminates many requirements such as lubricants, heat dissipation, surface repair, etc.
high production rates (typically a few seconds)
no pressure variations over work
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.
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.
• Compared to conventional forming,
asymmetrical parts made in on pass
many high strength alloys can be formed, for example stainless steel
only rotational parts possible with spinning
punch forming: for large drawing depths
negative punch forming: allows recessed features
any type of sheet material can be used
tools can be used for more than 1 metal thickness
sharp corners difficult to control
incorrectly set pressures may lead to buckling and tearing for high pressures
the metal spring-back 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.
• Basic process: some alloys can be slowly stretched well beyond their normal limitations at elevated temperatures. This allows very deep forming methods to be used that would normally rupture parts.
• Some materials developed for super plastic forming are,
aluminum-lithium (2090, 2091, 8090)
• In general the alloys should have a grain size below 5-8 microns and be equip-axed. The grain size must not increase if kept at temperatures 90% of melting for a few hours.
• Strain rates are generally low, approx. 10**-4/sec.
• Conventional forming techniques compared to SPF,
require multiple annealing and forming steps
have lower accuracy and repeatability
rate of 10**-4 to 10**-2 per second
temperature must be carefully maintained
cavitation (voids) can occur in the aluminum if pressure is not applied to both sides of the sheet: a different pressure still causes motion.
• Parts are less expensive because only half of the tooling is required.
• The typical process is like,
24.7.1 Diffusion Bonding
• Diffusion bonding is used with SPF to create more complicated shapes.
Problem 24.1 TRUE / FALSE: Electroforming removes metal from a mandrel.
Problem 24.2 TRUE / FALSE: Titanium can undergo superplastic deformation to be fully shaped by mating molds.
Problem 24.3 The part below is to be made from sheet metal. The dimensions specified are for the final part. The aluminum sheet metal has a thickness of 1/16”, and all radii are 1/16”.
a) Using allowances for bend radii, determine the actual size of the blank for this piece. Draw a dimensioned sketch.
b) Create a complete process plan for this part, from sheet metal on a roll, to a final part.
c) What is the largest force required to a) bend the part? b) to shear the part? State your assumptions.
d) Give an example (and short reason) of a part best suited for production by, Hydroforming, Spinning, Magnetic Pulse Forming, Stamping, Superplastic Forming, Powder Metallurgy, Stereolithography, Solid Ground Curing
Problem 24.4 If we can reduce sheet thickness by rolling, could this also be done by stretching the sheet? State the relative benefits of each method.
Problem 24.5 Spreading in rolled sheets increases as friction decreases, the material becomes thicker relative to width, and the roller radius becomes smaller. Describe why all three cases are true.
Problem 24.6 We are given a 1020 steel strip that is 1.0m wide, and 6mm thick. It is rolled to a thickness of 4mm. If the rollers are rotating at 300rpm and have a radius of 15cm, what is the roll force?
Problem 24.7 A sheet is being rolled in a set of tandem rollers. The original sheet thickness is 5mm, at the following stages it is rolled to 3.5mm, 2.8mm, 2.4mm and finally to 2.1mm. If the speed of the sheet entering the first rollers is 20m/s, calculate the drafts for each set of rollers, and the velocity of the sheet after each set of rollers.
Problem 24.8 Name 5 components that would be suitable to manufacturing with spinning.
Problem 24.9 a) Derive the equation for the tensile stress in the outermost fibers of a sheet of thickness ‘T’ that has been bent to a radius ‘r’.
Problem 24.10 Sheet metal is bent to have the profile below. The steel sheet thickness is 1/16”, and the bend radii are both 1/8”. (Note: Assume K = 0.5)
a) Do appropriate calculations to determine the length of the sheet metal before bending.
b) If the steel cannot be elongated more than 10%, can this part be made?
Problem 24.11 List and briefly describe 10 different processes for working and forming sheet metals.
Problem 24.12 From your own experiences describe a part that would be good to make with super plastic forming.
Problem 24.13 What is unique about diffusion bonding?
Problem 24.14 Design a die for hydroforming a barbecue propane tank.
Problem 24.15 How can you visually determine if a thread has been rolled or cut on a lathe?
Problem 24.16 We are rolling a 72” wide 9/64” thick aluminum sheet to 7/64” thick. If the roller has a diameter of 10”, and is rotating at 500RPM, What is the roll force and torque?
Problem 24.17 Show with figures how the mannesman process creates a hole in the center of a round piece.
Problem 24.18 What is the difference between punching and blanking?
Problem 24.19 What manufactured components could be made by spinning?
Problem 24.20 Step 1. Locate a sheet metal part of reasonable complexity. Show the part to me to verify that it is a good complexity.
Step 2. Develop a process plan for the part. The plan should consider a reasonable operation sequence, as well as suitable machines. This must include estimation of press tonnage, bend allowances, spring-back allowances, punch/shear clearances.
Step 3. Make a mock-up of the part by drawing the part on a computer. Print the drawing, and glue it to a thicker paper or plastic backing. Bend/shear/punch/etc the backing to get the desired shapes.
Problem 24.21 How could a long piece with cross section shape shown below be made by rolling?
Problem 24.22 Describe three different methods for making the following round part with sheet metal. Recall that we saw a similar part at MetalFlow.
Problem 24.23 The following piece has been made with 1/16” sheet steel and has the given dimensions.
a) What are the dimensions of the unbent piece? Draw a simple sketch.
b) What will the maximum strain be on the outermost fibers?
c) Is the piece likely to break?
d) What force will be required to shear the four sides of the piece (one at a time) if the UTS of the material is 300MPa?
• The forging process typically involves,
1. Having material in a bulk form such as billet, bar, ingot, etc. The material may be heated.
2. Developing dies for the final part.
3. The material is placed between dies, and the dies are closed with force.
4. As the dies close the material reforms.
5. The material may be repositioned, or placed between another die set for continued shaping.
6. The final part is trimmed and prepared for use.
1. A collapsible/removable metal mandrel is placed in an electrolyte solution (this will be the cathode).
2. A conductive bar of pure metal is put in the solution (this will be the anode).
3. Current is applied, and atoms liberated from the bar coat the mandrel.
4. The part is removed when enough metal has built up.
5. Rinse the part and strip it from the mandrel.
• The mandrel should be created to have a negative impression of the part to be made.
• agitating the electrolyte speeds deposition.
very good reproduction of mandrel
production of 0.001-2” per hour
exterior surfaces hard to control
thin walled products preferred
edges, deep recesses and corners not suited to electroforming.
generally the part is a male or female mate that lifts off easily.
a tapered shape makes parts easy to remove
these mandrels often have undercuts that stop a part from sliding off
the mandrel can be dissolved, broken, etc.
an example is aluminum mandrels that can be dissolved in sodium hydroxide with no effect on a nickel part.
a collapsible reusable mandrel that a part is formed about.
If the mandrel is made from a material such as PVC, it must have a conductive coating applied before every use.
complex piping (thin seamless pipe)