Forming and bending are essential operations in the metal fabricator (fitter) trade. These processes transform sheet metal or metal profiles through plastic deformation, without removing material. Unlike machining, forming changes the geometry of the workpiece by bending, rolling, or curving it to precise radii and angles.
For the Red Seal exam, you must master the physical principles of deformation, material formability limits, development calculations (flattened length), correction factors (K-factor, bend allowance), and safe operating procedures for equipment.
Fundamental Principles of Plastic Deformation
Yield Strength and Ultimate Strength
When a force is applied to metal, it first undergoes elastic deformation (reversible). If the force exceeds the yield strength (σe), the metal enters plastic deformation (permanent). Bending exploits this plastic deformation without reaching the ultimate tensile strength (σr).
The relationship between applied stress and strain is illustrated by the material's stress-strain curve. For forming, the critical properties are:
Modulus of Elasticity (E): material stiffness (in MPa or GPa)
Yield Strength (σe): threshold of permanent deformation
Elongation at Break (%): deformation capacity before fracture
Hardness: resistance to penetration, often correlated with strength
Neutral Fibre and Neutral Axis
During bending, the cross-section of the sheet metal undergoes unequal stresses:
Outer fibres (convex side) are in tension (elongation)
Inner fibres (concave side) are in compression (shortening)
The neutral fibre is the imaginary line where stress is zero and length does not change
For thin sheets (thickness < 3 mm), the neutral fibre is approximately at the centre of the thickness. For thicker sheets, it shifts toward the inside of the bend, at approximately 0.33 to 0.45 of the thickness from the inner surface.
K-Factor and Neutral Fibre Position
The K-factor (k) is the ratio between the distance from the neutral fibre to the inner surface and the sheet thickness:
k = t / e
Where:
t = distance from the neutral fibre to the inner surface (mm)
e = sheet thickness (mm)
Typical K-factor values:
Inner bend radius r < e: k ≈ 0.33
Inner bend radius r = e: k ≈ 0.40
Inner bend radius r > 2e: k ≈ 0.50
Thickness (mm)
Inner Radius (mm)
K-Factor
1.5
1.5
0.40
3.0
3.0
0.40
3.0
6.0
0.45
6.0
12.0
0.48
6.0
3.0
0.35
Calculating Developed (Flattened) Length
General Principle
The developed length (Ld) is the total length of the sheet before bending, measured along the neutral fibre. It is calculated by adding the lengths of the straight segments and the lengths of the bend arcs.
Ld = L1 + L2 + ... + Ln + A1 + A2 + ... + An
Where:
Ln = length of straight segments (mm)
An = length of bend arcs (mm)
Length of a Bend Arc
The length of a bend arc is calculated using the formula:
A = (π × R × α) / 180
Where:
A = arc length (mm)
R = neutral fibre radius = r + k × e (mm)
α = bend angle in degrees
π ≈ 3.1416
Complete Calculation Example
Problem: Bend a 3 mm thick mild steel sheet to 90°, with an inner radius of 3 mm. The straight segments measure 50 mm and 80 mm.
Solution:
49.K-factor for r = e: k = 0.40
50.Neutral fibre radius: R = 3 + 0.40 × 3 = 3 + 1.2 = 4.2 mm
For quick shop calculations, bend allowances (BA) for a 90° angle are often used:
Thickness (mm)
Inner Radius (mm)
Bend Allowance (mm)
1.0
1.0
1.7
1.5
1.5
2.6
2.0
2.0
3.5
3.0
3.0
5.2
4.0
4.0
7.0
5.0
5.0
8.7
6.0
6.0
10.5
The bend allowance represents the amount to subtract from the sum of the outside dimensions to obtain the developed length.
Bending Operations
Press Brake Bending
The press brake is the most common equipment for sheet metal bending. It consists of a punch (upper die) and a die (lower V). Bending is achieved by the punch penetrating into the die.
Setup Parameters:
Bend angle: controlled by the depth of punch penetration
Springback: the metal partially returns to its original shape after release. The bend angle must be slightly less than the desired angle to compensate for this.
Die opening (V): typically 6 to 8 times the sheet thickness
Bend radius: depends on the die opening and the punch radius
Springback Formula (approximation):
Δα = α × (σe / E) × 180 / π
Where:
Δα = springback angle (degrees)
α = bend angle (degrees)
σe = yield strength (MPa)
E = modulus of elasticity (MPa)
For mild steel: E = 200,000 MPa, σe ≈ 250 MPa → Δα ≈ 0.07° per degree of bend.
Roll Bending
Roll bending (or three-roll bending) is used to form cylinders, cones, and large-radius arcs. The three rolls (two fixed lower rolls, one movable upper roll) apply progressive pressure to the sheet.
Setup Parameters:
Spacing of lower rolls: determines the bend radius
Position of the upper roll: controls the depth of penetration
Number of passes: several successive passes to avoid excessive deformation
Roll Bending Radius Calculation (approximation):
R = (L² + 4 × h²) / (8 × h)
Where:
R = bend radius (mm)
L = distance between lower rolls (mm)
h = deflection (depth of upper roll penetration) (mm)
Tube and Profile Bending
Bending tubes and profiles requires specialized equipment:
Mandrel bender: for small-diameter tubes, with an internal mandrel to prevent crushing
Roll bender: for large-diameter tubes
Induction bender: for thick tubes, localized heating by electromagnetic induction
Tube Bending Limits:
Minimum radius: typically 2 to 3 times the outside diameter of the tube
Maximum flattening: 5 to 8% of the diameter (depending on the standard)
Ovality: difference between the maximum and minimum diameter after bending
Tube Type
Minimum Radius (× diameter)
Maximum Flattening (%)
Mild steel
2.5
8
Stainless steel
3.0
5
Aluminum
3.5
5
Copper
2.0
10
Cold Forming and Hot Forming
Cold Forming
Cold forming is performed at room temperature. It is used for:
Thin and medium sheets (up to approximately 12 mm)
Mild steels and aluminum alloys
Parts requiring good dimensional accuracy
Advantages:
High dimensional accuracy
Clean surface finish
No thermal distortion
Low energy cost
Disadvantages:
Work hardening: the metal hardens and becomes more brittle
Formability limits for brittle materials
Risk of cracking for radii that are too small
Hot Forming
Hot forming is performed above the recrystallization temperature of the material. It is used for:
Thick sheets (> 12 mm)
High-strength steels
Large-dimension parts
Typical Temperatures:
Mild steel: 850 °C to 1,100 °C
Stainless steel: 1,000 °C to 1,200 °C
Aluminum: 350 °C to 450 °C
Advantages:
Increased formability (smaller radii possible)
Reduced work hardening
Reduced forming forces
Disadvantages:
Thermal distortion (shrinkage upon cooling)
Surface oxidation (scale)
High energy cost
Safety risks (burns, fumes)
Applicable Canadian Standards
CSA W59 (Canadian Standard for Welded Steel Construction)
The CSA W59 standard (Welded Steel Construction) applies to welded assemblies, but it contains requirements related to the forming of structural members:
Clause 5.4: Fabrication tolerances for bent or rolled members
Clause 5.5: Dimensional tolerances after forming
Clause 7.2: Requirements for hot and cold forming of members
Typical tolerances for bending structural members according to CSA W59:
Maximum angular deviation: ± 2° from the specified angle
Radius deviation: ± 5% of the specified radius
Maximum surface waviness: 3 mm over 300 mm
CSA S16 (Design of Steel Structures)
The CSA S16 standard (Design of Steel Structures) defines requirements for cold-formed members:
Clause 13.5: Resistance of cold-formed members
Clause 21.2: Fabrication requirements for bent members
The standard specifies that cold forming reduces the ductility of the material and may require a stress-relieving heat treatment for heavily deformed members.
CSA B149.1 (Natural Gas and Propane Installation Code)
For bent piping in gas installations, the CSA B149.1 standard requires:
Clause 6.3.2: Minimum bend radius for steel tubes: 3 times the nominal diameter
Clause 6.3.3: Prohibition on bending cast iron tubes
Clause 6.3.4: Ovality tolerance: 8% of the outside diameter
Canadian Electrical Code, Part I
The Canadian Electrical Code, Part I (C22.1) contains requirements for bending electrical conduits:
Rule 12-908: Minimum bend radius for rigid metal conduits: 6 times the nominal diameter
Rule 12-1008: Minimum bend radius for flexible metal conduits: 8 times the nominal diameter
Rule 12-1108: Minimum bend radius for electrical metallic tubing: 5 times the nominal diameter
Forming Equipment and Tools
Press Brake
The press brake is the central piece of equipment in the forming shop. Its main components:
Frame: rigid welded steel structure
Hydraulic cylinders: provide the bending force (50 to 1,000 tons)
Lower bed: supports the V-die
Punch: upper tool that penetrates into the die
Back gauge: positions the sheet with precision
CNC control system: controls depth, angle, and sequence
Required Bending Force (approximation):
F = (1.42 × σr × e² × L) / V
Where:
F = bending force (N)
σr = tensile strength of the material (MPa)
e = sheet thickness (mm)
L = length of the bend (mm)
V = die opening (mm)
Example: Bend a mild steel sheet (σr = 400 MPa) of 6 mm thickness, over 1,000 mm length, with a V-die = 48 mm:
The three-roll bending machine (or roll former) is used for rolling sheets into cylinders or cones. Its setup parameters:
Spacing of lower rolls: adjustable according to thickness and desired radius
Upper roll pressure: controls deformation
Rotation speed: affects surface quality
Usage Precautions:
Pre-bend the ends to avoid flat zones
Perform several progressive passes
Check the radius with a template after each pass
Measurement and Inspection Tools
To verify forming quality, the metal fabricator uses:
Protractor: measures bend angles (± 0.5°)
Radius gauge: checks inner and outer radii
Calipers: measures thicknesses and dimensions
Spirit level: checks the plumb of parts
Crack detector (magnetic particle or dye penetrant): inspects surface defects
Forming Defects and Remedies
Cracking
Cracking occurs when deformation exceeds the material's capacity. Possible causes:
Bend radius too small relative to thickness
Brittle material (hardened, high carbon content)
Bending in the rolling direction (fibres parallel to the bend)
Temperature too low for hot forming
Remedies:
Increase the bend radius
Bend perpendicular to the rolling direction
Preheat the material
Use a more ductile material
Excessive Springback
Springback is the tendency of metal to return to its original shape. Causes:
High yield strength of the material
Large bend radius relative to thickness
Small bend angle
Remedies:
Over-bend (bend to a smaller angle than the desired angle)
Use a die with a tighter angle
Perform a coining bend (pressure maintained at the end of the stroke)
Wrinkling and Buckling
Wrinkling of the inner surface of the bend is caused by excessive compression. Buckling of the flanges occurs when the flange width is too large relative to the thickness.
Remedies:
Use a die with a larger entry radius
Reduce bending speed
Add lateral supports
Common Defects Table
Defect
Probable Cause
Remedy
Outer crack
Radius too small
Increase the radius
Inner crack
Brittle material
Preheat
Springback
High yield strength
Over-bend
Wrinkling
Excessive compression
Reduce speed
Flange buckling
Excessive width
Add supports
Punch marks
Worn punch
Replace the punch
Reduced thickness at bend
Excessive stretching
Reduce force
Safety Procedures
General Safety Rules
Forming and bending present specific risks:
Crushing: hands and fingers can be caught between the punch and die
Shearing: sheet metal edges are sharp
Projection: metal fragments can be thrown
Burns: during hot forming
Mandatory Rules:
225.Never place hands between the punch and die
226.Use holding devices (tongs, suction cups) for small parts
227.Wear protective gloves, safety glasses, and safety footwear
228.Inspect equipment before use
229.Respect maximum machine capacities (tonnage, length)
Press Brake-Specific Safety
Light curtain: optical device that stops the machine if an object crosses the beam
Foot pedal: must be two-position (safety)
Mechanical stop: limits the punch stroke
Emergency stop: accessible at all times
Hot Forming-Specific Safety
Wear insulating gloves and a leather apron
Use long tongs to handle hot parts
Ventilate the work area to remove fumes
Keep a fire extinguisher nearby
Pitfalls to Avoid
241.Forgetting springback: never bend exactly to the desired angle without compensating for the material's springback.
242.Confusing inner radius and neutral fibre radius: the developed length calculation always uses the neutral fibre radius, not the inner radius.
243.Using an incorrect K-factor: the K-factor varies with the r/e ratio. Do not use 0.5 for all cases.
244.Neglecting the rolling direction: bending parallel to the rolling direction increases the risk of cracking.
245.Ignoring standard tolerances: CSA W59 and CSA S16 impose precise tolerances. Exceeding them results in part rejection.
246.Calculating bending force without considering the actual material strength: use the tensile strength (σr), not the yield strength.
247.Forgetting to pre-bend the ends when roll bending: the ends remain straight if they are not pre-bent.
248.Confusing hot forming temperatures: mild steel is formed between 850 °C and 1,100 °C, not at 500 °C.
249.Not checking the ovality of bent tubes: CSA B149.1 limits ovality to 8% of the diameter.
250.Using worn dies: worn dies produce poor-quality bends and increase the risk of cracking.
Summary
Forming and bending are fundamental operations of the metal fabricator trade. To succeed on the Red Seal exam, remember:
The neutral fibre is the line of constant length during bending. Its position is defined by the K-factor (0.33 to 0.50 depending on the r/e ratio).
The developed length is calculated by adding the straight segments and bend arcs, using the neutral fibre radius.
Springback must always be compensated for by over-bending. Its magnitude depends on the yield strength and modulus of elasticity.
The bending force is calculated using the formula F = (1.42 × σr × e² × L) / V. It increases with the square of the thickness.
Canadian standards (CSA W59, CSA S16, CSA B149.1, Canadian Electrical Code) impose specific tolerances and minimum radii.
Hot forming is reserved for thick or high-strength materials, with temperatures of 850 °C to 1,200 °C depending on the material.
Common defects (cracking, springback, wrinkling) have identifiable causes and known remedies.
Safety is paramount: never place hands between the punch and die, use protective devices, respect machine capacities.
Master these concepts, practice the calculations, and know the standard values. Good luck with your exam preparation!
