Sheet Metal Work
Overview
Sheet Metal Work is a core topic in ITI Fitter Semester 3 (NCVT). It covers the selection and working of thin metal sheets using hand and machine tools to produce useful articles. Topics include types of sheet metals, measuring and marking tools, cutting and joining methods, development of surfaces, and common sheet metal joints. Understanding these concepts is essential for fabrication, maintenance, and repair tasks performed by a Fitter in industries.
Typed Notes
Sheet Metal — Types and Gauges
Sheet metal refers to metal formed into thin flat pieces. Thickness is measured using Standard Wire Gauge (SWG). A higher SWG number indicates a thinner sheet. Commonly used sheet metals in ITI Fitter trade include Galvanised Iron (GI) — most widely used due to rust resistance; Black Iron — used where painting is done; Aluminium — lightweight and corrosion-resistant; Copper — good conductor, used in electrical work; Tinplate — mild steel coated with tin, used for food cans; and Stainless Steel — used in kitchen equipment. Sheets below 3 mm thickness are generally classified as sheet metal.
Marking and Cutting Tools
Marking tools used in sheet metal work include: Steel rule, Try square, Scriber, Divider, Dot punch (not centre punch), and Trammel. Cutting tools include: Straight snips (for straight cuts), Curved snips (for curved cuts), Bench shear (lever-type, for straight cuts up to 1.5 mm), and Guillotine shear (power-operated, for long straight cuts). Files used are flat, half-round, and smooth files. A mallet (wooden or rubber) is used for bending and shaping to avoid damaging the sheet surface. Aviation snips are colour-coded: Red = left cut, Green = right cut, Yellow = straight cut.
Sheet Metal Joints and Seams
Joints in sheet metal work are used to join two pieces together. Lap Joint: one sheet overlaps the other; simplest joint. Butt Joint: edges placed end-to-end and soldered. Grooved Seam (Locked Seam): both edges folded and interlocked; very strong, used in cylindrical articles like buckets. Knocked-up Seam: used at corners of rectangular boxes. Pittsburgh Seam: used in HVAC duct work. Riveting, soldering, and spot welding are also used for joining. Seam allowance (extra material for folding) must be added while developing the pattern. Grooved seam requires 3× the seam width as allowance on each side.
Development of Surfaces and Soldering
Development (Pattern/Blank Development) means drawing the flat shape of a 3D object before cutting. Three methods: Parallel Line Development (for prisms and cylinders — e.g., pipe, box), Radial Line Development (for cones and pyramids — e.g., funnel, hopper), and Triangulation Development (for transition pieces — e.g., square-to-round duct). Soldering is the process of joining metals using a filler metal (solder) with melting point below 450 °C. Soft solder = 60% tin + 40% lead (melting point ~183 °C). Flux removes oxide film; Zinc Chloride for GI, Resin for electronics/tinplate. Soldering iron tip is made of copper.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Sheet Metal Types and Gauges
- Standard Wire Gauge (SWG) system and thickness values
- Common metals: GI, Aluminium, Copper, Tinplate, Stainless Steel
- Selection of sheet metal based on application
- Difference between sheet, plate, and foil by thickness
A GI sheet of SWG 20 has thickness 0.914 mm and is commonly used to fabricate air-conditioning ducts in a factory.
02Marking and Measuring Tools
- Scriber, divider, trammel for layout marking
- Dot punch use on sheet metal (not centre punch)
- Steel rule and try square for checking squareness
- Odd-leg caliper for marking parallel lines from edge
A dot punch is used at 50 mm intervals along a scribed line on a 0.8 mm aluminium sheet to mark rivet hole positions before drilling.
03Cutting and Forming Tools
- Straight snips, curved snips, and aviation snips (colour-coded)
- Bench shear capacity up to 1.5 mm sheet thickness
- Mallet (wooden/rubber) for bending without surface damage
- Hatchet stake and beak iron stake for forming shapes
A green aviation snip is used to cut a right-hand curve on a 1 mm GI sheet when making a circular elbow for a ventilation pipe.
04Sheet Metal Joints and Seams
- Lap joint, butt joint — basic joining methods
- Grooved seam (locked seam) for cylindrical articles like buckets
- Pittsburgh seam used in HVAC rectangular ducts
- Seam allowance calculation: 3× seam width per side for grooved seam
A grooved seam with 6 mm seam width requires 18 mm extra allowance on each edge of the GI sheet when making a 200 mm diameter water bucket.
05Surface Development and Soldering
- Parallel line development for cylinders and prisms (pipes, boxes)
- Radial line development for cones and pyramids (funnels, hoppers)
- Triangulation development for transition pieces (square-to-round ducts)
- Soft soldering: 60% tin + 40% lead solder; flux selection by metal type
A conical funnel with base diameter 150 mm and slant height 200 mm is developed using radial line method; the flat pattern is a sector of a circle with radius 200 mm.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Sheet metal thickness is measured by Standard Wire Gauge (SWG) — higher SWG number means thinner sheet; SWG 10 = 3.25 mm, SWG 20 = 0.914 mm.
- 02Common sheet metals used: GI (Galvanised Iron), Black Iron, Aluminium, Copper, Stainless Steel, and Tinplate.
- 03Soldering flux used for GI sheet is Zinc Chloride (killed spirit); for tinplate, resin flux is used.
- 04Allowance added to flat pattern for bending is called Bend Allowance; it depends on metal thickness and bend radius.
- 05Types of sheet metal joints: Lap joint, Butt joint, Grooved seam, Knocked-up seam, and Pittsburgh seam.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is Standard Wire Gauge (SWG) and how does it relate to sheet metal thickness? Give two examples of SWG numbers with their thickness values.
Name the three methods of surface development used in sheet metal work and state which method is used for a funnel and which for a rectangular duct.
What is a grooved seam? Where is it used and how much seam allowance is required on each edge?
Name the flux used for soldering GI sheet and for soldering tinplate/electronic components. What is the composition of soft solder?
What are aviation snips? Explain the colour-coding system and state the cutting capacity of a bench shear.
Sheet Metal Work in ITI Fitter Semester 3 covers SWG-based thickness measurement, types of sheet metals (GI, aluminium, copper, tinplate), marking and cutting tools (snips, shears), types of joints and seams (grooved, lap, Pittsburgh), surface development methods (parallel, radial, triangulation), and soft soldering techniques. Mastery of these topics enables a Fitter to fabricate ducts, funnels, boxes, and containers accurately in industrial settings.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which SWG number represents a THINNER sheet metal?
Q2.Which flux is used when soldering a Galvanised Iron (GI) sheet?
Q3.Radial line development method is used for which of the following sheet metal articles?
Q4.What is the correct colour of aviation snips used for making a LEFT-HAND curved cut?
Mark karo — dashboard par progress dikhega.