Sheet Metal Introduction
Sheet metal is one type of the raw material and it is used for creating the parts by the help of sheet metal forming operations. Sheet metal forming is one type of subtractive manufacturing methodology.
Sheet metal parts are also known for their durability, which makes them great for end user applications (e.g. chassis). Because parts are formed from a single sheet of metal designs must maintain a uniform thickness. We need to follow the design requirements and tolerances to ensure parts fall closer to design intent.
Sheet metal Definition:
Sheet metal is defined as the metal that has been rolled into a sheet having a thickness between foil and plate.
- A piece of metal whose thickness is between 0.006(0.15 mm) and 0.25 inches(6.35 mm).
- Anything thinner is referred to as a foil and thicker is considered as a plate.
- Sheet thickness is generally measured in gauge. Greater the gauge number, thinner the sheet of metal.
- Sheet metal can be cut, bent and stretched into nearly any shape.
- Generally two types of operations are performed- forming and cutting
- There are many different metals that can be made into sheet metal, such as aluminum, brass, copper, steel, tin, nickel and titanium.
- Gauge 14: .0781” (1.98) and 18 is 0.0500 (1.27).
- GD&T may not be applicable to the sheet metal
Sheet metal fabrication is the process of forming parts from a metal sheet by punching, cutting, stamping and bending.
General Sheet metals:
1. Steel plate cold-rolled (SPCC): Mainly used for part need painted or electro-plating, cheap and easy to work, thickness usually no more than 3.2mm;
2. Hot rolled steel(SHCC): T≥3.0mm,treated with spraying or electro plating as SPCC, and cheap, but difficult to work, mainly for flat parts;
3. Electro or Galvanized steel (SECC/SGCC): SECC includes N and P type, the former is usually with no surface treatment, high cost, while the latter mainly for spraying parts;
4. Copper plate: Mainly applied for electricity conducting function, surface treated with chrome or nickle plating, or without any finish, the cost is a little high;
5. Aluminum plate: Usually treated with chromate, anodize (conductive anodizing or chemical anodizing), silver or nickle plating cost is high;
6. Aluminum extrusion: Aluminum extrusion are with complex structure from side view, is widely applied in different kinds of electronic equipment’s, its surface can be treated as what aluminum plates do;
7. Stainless steel sheet: Always with no any surface treatments, high cost .
Sheet Metal Characteristics and Effects:
Following are the desired sheet metal characteristics and effects on the form-ability and functioning.
Determines the capability of the sheet metal to stretch without necking and failure
2. Yield-point elongation
Observed with mild-steel sheets; also called Lueder’s bands and stretcher strains; causes flame like depressions on the sheet surfaces; can be eliminated by temper rolling, but sheet must be formed within a certain time after rolling
3. Anisotropy (planar)
Exhibits different behavior in different planar directions;
4. Anisotropy (normal)
Determines thinning behavior of sheet metals during stretching; important in deep-drawing operations.
5. Grain size
Determines surface roughness on stretched sheet metal; the coarser the grain, the rougher the appearance (orange peel); also affects material strength.
6. Residual stresses
Caused by non-uniform deformation during forming; causes part distortion when sectioned and can lead to stress-corrosion cracking; reduced or eliminated by stress relieving.
7. Spring back
Is an elastic recovery of the plastically deformed sheet after unloading; causes distortion of part and loss of dimensional accuracy; can be controlled by techniques such as over bending and bottoming of the punch.
Caused by compressive stresses in the plane of the sheet; can be objectionable or can be useful in imparting stiffness to parts; can be controlled by proper tool and die design.
9. Quality of sheared edges
Depends on process used; edges can be rough, not square, and contain cracks, residual stresses, and a work-hardened layer, which are all detrimental to the form-ability of the sheet; quality can be improved by control of clearance, tool and die design, fine blanking, shaving, and lubrication.
10. Surface condition of sheet
Depends on rolling practice; important in sheet forming as it can cause tearing and poor surface quality
Sheet metal Operations:
There are many sheet metal operation which are used to create the finished part. Majority of the operation are performed on the press brake and laser cutting machine.
- Deep drawing
- Stretch forming
- Roll forming
- Metal Spinning
Bending is a metal forming process in which a force is applied to a piece of sheet metal, causing it to bend at an angle and form the desired shape.
V-Bending: The sheet metal blank is bent between a V- shaped punch and die.
Air bending: If the punch does not force the sheet to the bottom of the die cavity, leaving space or air underneath, it is called “air bending“.
Edge or Wipe Bending: Wipe bending requires the sheet to be held against the
wipe die by a pressure pad. The punch then presses against the edge of the sheet that extends beyond the die and pad. The sheet will bend against the radius of the edge of the wipe die.
Shearing is defined as separating material into two parts. It utilizes shearing force to cut sheet metal.
A piece of sheet metal is removed from a larger piece of stock. This removed piece is not scrap, it is the useful part.
The typical punching operation in which a cylindrical punch pierces a hole into the sheet.
Punching away excess material from the perimeter of a part. Such as trimming the flange from a drawn cup.
Separating a part from the remaining sheet, by punching away the material between parts.
Cutting straight lines in the sheet. No scrap material is produced.
Creating a partial cut in the sheet, so that no material is removed. The material is left attached to be bent and form a shape, such as a tab, vent, or louver.
Punching the edge of a sheet, forming a notch in the shape of a portion of the punch.
Punching a close arrangement of a large number of holes in a single operation.
Punching a series of small overlapping slits or holes along a path to cut-out a larger contoured shape.
Certain designs are embossed on the sheet metal. Punch and die are of the same contour but in opposite direction.
Shearing away minimal material from the edges of a feature or part, using a small die clearance. Used to improve accuracy or finish. Tolerances of ±0.025 mm are possible.
Separating a part from the remaining sheet, without producing any scrap. The punch will produce a cut line that may be straight, angled, or curved.
Dinking – A specialized form of piercing used for punching soft metals.
A hollow punch, called a dinking die, with bevelled, sharpened edges presses the sheet into a block of wood or soft metal.
Similar to embossing with the difference that similar or different impressions are obtained on both the sides of the sheet metal.
17. Deep drawing
Deep drawing is a metal forming process in which sheet metal is stretched into the desired shape. A tool pushes downward on the sheet metal, forcing it into a die cavity in the shape of the desired part.
18. Stretch forming
Stretch forming is a metal forming process in which a piece of sheet metal is stretched and bent simultaneously over a die in order to form large bent parts
19. Roll forming
Roll forming is a continuous bending operation in which a long strip of sheet metal is passed through sets of rolls mounted on consecutive stands, each set performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing constant-profile parts with long lengths and in large quantities.
20. Metal Spinning
Pressing a sheet against a rotating mandrel to acquire the mandrel shape.
Used for axis-symmetric shapes.
Sheet Metal Dies:
Made up of tool steel and used to cut or shape material.
1. Simple die
Simple dies or single action dies perform single operation for each stroke of the press slide. The operation may be one of the cutting or forming operations.
2. Compound die
In these dies, two or more operations may be performed at one station.
Such dies are considered as cutting tools since, only cutting operations are carried out.
3. Combination die
In this die also , more than one operation may be performed at one station.
It is different from compound die in that in this die, a cutting operation is combined with a bending or drawing operation, due to that it is called
4. Progressive die
A progressive has a series of operations. At each station , an operation is performed on a work piece during a stroke of the press.
• Clearance, c, between the punch and die typically between 2% and 10% of sheet metal thickness
• As clearance increases, sheared edge becomes rougher and zone of deformation becomes larger – clearances are smaller for softer metals, thinner sheets, or larger holes
• Ratio of burnished to rough edges increases with: increasing ductility, decreasing clearance and thickness
• Faster punch speeds cause narrower sheared zones and less burr formation
• Burr height increases with increasing clearance, ductility, or dull tools
• Maximum Punch Force, F = 0.7 T L (UTS) (product of thickness, sheared edge perimeter, and UTS)