SHEET METAL WORKING
1. Introduction
Sheet metal is simply
metal formedinto thin and flat pieces. It is one of the fundamental forms used in
metalworking,and can be cut and bent into a variety of different shapes. Countless everydayobjects are constructed of the material. Thicknesses can vary significantly,although extremely thin thicknesses are considered
foilor
leaf,and pieces thicker than 6 mm (0.25 in) are considered
plate.
2. Sheet metal processing
The raw material for sheet metal manufacturing processes isthe output of the rolling process. Typically, sheets of metal are sold as flat,rectangular sheets of standard size. If the sheets are thin and very long, theymay be in the form of rolls. Therefore the first step in any sheet metalprocess is to cut the correct shape and sized ‘blank’ from larger sheet.
3. Sheet metal forming processes
Sheet metal processes can be broken down into two majorclassifications and one minor classification
· Shearing processes -- processes which apply shearing forces to cut, fracture, or separatethe material.
· Forming processes -- processes which cause the metal to undergo desired shapechanges without failure, excessive thinning, or cracking. This includes bending and stretching.
· Finishing processes -- processes which are used to improve the final surfacecharacteristics.
3.1 Shearing Process
- Punching: shearing process using a die and punch where the interior portion of the sheared sheet is to be discarded.
- Blanking: shearing process using a die and punch where the exterior portion of the shearing operation is to be discarded.
- Perforating: punching a number of holes in a sheet
- Parting: shearing the sheet into two or more pieces
- Notching: removing pieces from the edges
- Lancing: leaving a tab without removing any material
Fig.1ShearingOperations: Punching, Blanking and Perforating
3.2 Forming Processes
· Bending: forming process causes thesheet metal to undergo the desired shape change by bending without failure.Ref fig.2 & 2a
· Stretching: forming process causes the sheet metal to undergo thedesired shape change by stretching without failure. Ref fig.3
· Drawing: forming process causes the sheet metal to undergo thedesired shape change by drawing without failure. Ref fig.4
· Roll forming: Roll forming is a process by which a metal strip isprogressively bent as it passes through a series of forming rolls. Ref fig.5
Fig.2 Common Die-Bending Operations
Fig.2a
Various Bending Operations
Fig.3 Schematicillustration of a stretch-forming process.
Fig.4 Schematic of the Drawing process.
Fig.5 Eight-rollsequence for the roll forming of a box channel
3.3Finishing processes
Material properties, geometry of the starting material, andthe geometry of the desired final product play important roles in determiningthe best process
4. Equipments
Basic sheet forming operationsinvolve a press, punch, or ram and a set of dies
4.1 Presses
- Mechanical Press - The ram is actuated using a flywheel. Stroke motion is not uniform. Ref fig.6
- Hydraulic Press - Longer strokes than mechanical presses, and develop full force throughout the stroke. Stroke motion is of uniform speed, especially adapted to deep drawing operations. Ref fig.7
Fig.6 Mechanical Press
Fig.7 Hydraulic Press
4.2 Dies andPunches
· Simple-single operation with a single stroke
· Compound-two operations with a single stroke
· Combination-two operations at two stations
· Progressive-two or more operations at two or more stations with each press stroke, createswhat is called a strip development
Fig 8 Progressive dies Punches
5. Tools and Accessories
The various operations such as cutting, shearing, bending,folding etc. are performed
by these tools.
5.1 Marking andmeasuring tools
- Steel Rule - It is used to set out dimensions.
- Try Square - Try square is used for making and testing angles of 90degree
- Scriber – It used to scribe or mark lines on metal work pieces.
- Divider - This is used for marking circles, arcs, laying out perpendicular lines, bisecting lines, etc
Fig.9 Marking and measuring tools
5.2 Cutting Tools
· Straight snip - Theyhave straight jaws and used for straight line cutting. Ref fig.10
· Curved snip - Theyhave curved blades for making circular cuts. Ref fig.10a
Ref fig.10 Straight snip
Ref Fig.10a Curved Snip
6. Striking Tools
· Mallet - It iswooden-headed hammer of round or rectangular cross section. The striking faceis made flat to the work. A mallet is used to give light blows to the Sheetmetal in bending and finishing. Ref fig.11
Fig.11 Types of Mallets
6. Merits
· High strength
· Good dimensional accuracy and surface finish
· Relatively low cost
7. Demerits
· Wrinkling and tearing are typical limits to drawing operations
· Different techniques can be used to overcome these limitations
o Draw beads
o Vertical projections and matching grooves in the die and blank holder
· Trimming may be used to reach final dimensions
8. Applications
· Roofings
· Ductings
· Vehicles body buildings like 3 wheelers, 4 wheelers, ships, aircraftsetc.
· Furnitures, House hold articles and Railway equipment
9. Questions:
Part A
- What is sheet metal work?
- Write down any four sheet metal characteristics
- What is meant by clearance?
- What is stretching?
- Define the term “spring back”
- How force exerted on the form block is calculated
- What are the formability test methods?
- What is super plasticity of metals?
- What is metal spinning process?
- What is sheet metal?
Part B
- What are Punching, Nibbling, Blanking, Piercing, tools/machines are needed for these processes?
- What is deep drawing? Provide a few examples of products/parts made using deep drawing operations.
- What is progressive die stamping?
- Describe shearing operations in a sheet metal work with a neat sketch
- Describe various types of bending operations with its neat sketches
- Explain any one method of stretch forming operation with a neat sketch
- Explain hydro forming process with its neat sketches. State their advantage and applications
- Explain the power spinning process with a neat sketch .give their applications
- How magnetic pulse forming process is carried out on sheet metal?Explain peen forming process with a neat sketch
- What is super plastic of metal? How this process is carried out on sheet metals?
END
11. References
Book
- Manufacturing Technology by Hajra choudry
- Sheet metal working by Robert cook
