2024 Sheet metal forming PJS (2)

Lecture Overview

  • Title: Sheet Metal Forming

  • Instructor: Patrick J. Smith

  • Institution: The University Of Sheffield

Lecture Schedule

  • Week 1: 15 Oct - Introduction to Manufacturing, 14 Oct, 1:00 PM, Diamond LT 3

    • Topics: Disposable Metal Framed Mound Casting, Adjustable Wrench

  • Week 2: 22 Oct - Sheet Metal Forming, Diamond LT 3, 2:00 PM

  • Week 3: 24 Oct - Plastic Forming & Shaping, Diamond LT 3, 3:00 PM

Sheet Metal Forming

  • Definition: The process of shaping metal sheets into specific forms for various products.

  • Applications: Beverage cans, cookware, filing cabinets, metal desks, appliances, car bodies, trailers, aircraft fuselages.

  • Historical Context: Dating back to 5,000 B.C. with the creation of household utensils and jewellery.

  • Advantages: Lightweight, versatile shape, and cost-effective using low-carbon steel, which has good strength and formability.

Shearing Process

  • Definition: The initial step in forming where a blank is cut from a larger sheet.

  • Method: Utilizes shear stresses via a punch and die.

  • Characteristics: Involves plastic deformation followed by crack formation; resulting surfaces vary (rough fractures vs. smooth burnished).

Shearing Parameters

  • Critical Factors:

    • Shape of punch and die

    • Speed of operation

    • Lubrication techniques

    • Clearance between punch and die

  • Effects of Clearance: Greater clearance increases roughness of the sheared edge, making secondary operations potentially necessary to smooth edges.

Edge Quality Improvement

  • Enhancements:

    • Higher punch speed leads to better edge quality, reducing roughness and improving formability.

    • Cold working due to high shear strains can work-harden edges, affecting ductility in subsequent operations.

Common Shearing Operations

  • Types:

    • Punching: Produces scrap

    • Blanking: Produces usable blanks

    • Mnemonic: "Punch out what you don’t need; Blanking produces a blank!"

Important Metal Characteristics for Sheet Forming

  • Elongation: Measures capability of the sheet to stretch without necking.

  • Yield Point Elongation: Can cause surface depressions.

  • Anisotropy: Results in different deformation behaviors along planar directions.

  • Grain Size & Residual Stresses: Affect surface quality and can lead to part distortion during sectioning.

  • Spring Back: Causes part distortion post-bending.

  • Wrinkling: Can be objectionable depending on its severity.

Bending Operations

  • Definition: A common industrial forming operation for shaping sheets, plates, and tubes.

  • Applications: Includes automobile bodies, and paperclips.

  • Stiffness: Bending increases the moment of inertia, improving stiffness without additional weight.

    • Bend Allowance: Length of the neutral axis used to determine the length of the blank needed.

Bend Allowance Formula

  • Formula: Lb = a(R + kT)

    • Where:

      • Lb = bend allowance

      • a = bend angle in radians

      • T = sheet thickness

      • R = bend radius

      • k = constant (0.33 to 0.5)

Minimum Bend Radius and Strain

  • Minimum bend radius: The point at which cracks initially appear when bending a sheet.

  • Strain on fibers during bending can lead to material failure if strain increases beyond a certain threshold.

Spring Back in Bending

  • Definition: Elastic recovery that occurs after load removal.

  • Factors Influencing Spring Back:

    • R/T ratio (bend radius to thickness)

    • Yield stress and elastic modulus of the material.

  • Compensatory Techniques:

    • Overbending, bottoming punch, and stretch bending.

Bending Force Equation

  • Bending Force: P = kYLT^2

    • Where:

      • P = bending force

      • Y = yield stress

      • T = sheet thickness

      • k = factor based on die type

Deep Drawing Process

  • Definition: A process for producing cylindrical or box-shaped products through localized forming of a sheet-metal blank.

  • Applications: Pots, containers, sinks, fuel tanks, etc.

  • Illustration of Process:

    • Steps include Blanking, Deep Drawing, Redrawing, Ironing, Doming, Necking.

Spinning Operations

  • Definition: A forming process to create axisymmetric parts over a mandrel using tools and rollers.

  • Types: Conventional spinning, also known as shear spinning, power spinning, etc.

Summary of Sheet Metal Forming

  • Versatility: Sheet-metal forming is versatile for workpieces with high surface-to-thickness ratios.

  • Key Material Parameters:

    • Quality of the sheared edges

    • Ability to stretch uniformly

    • Grain size and yield-point elongation of low-carbon steels.

  • Challenges: Managing spring-back, buckling, and wrinkling during processing.

  • Developments: Includes superplastic forming for aerospace applications and predictions for formability related to various properties.