Technical Drawing Fundamentals and Materials for Automotive Body Construction

Posted on Nov 28, 2024 in Technology

Technical Drawing

Technical drawing is the graphical representation of a piece, clear, correct, and accurate on paper. Every drawing technician should:

  • Be sufficiently clear so as not to give rise to equivocal shapes, dimensions, and other characteristics.
  • Not accumulate unnecessary data.

Drawing Classes

  • Sketch drawings of conception
  • Custom defined drawings/manufacturing drawings

Dimensioning (Cota)

Indicates the actual dimensions. Dimensional cota is expressed by its direct reading and not deducted from others.

Particular Cases of Boundedness

  • Profiles: Defined by successive radii or rectangular folding.
  • Parts with bent or curved lines are drawn with dash-dot lines.

Surface Signs

Their function is to indicate the types of surfaces and their qualities.

Classes of Surfaces

  • Floor area
  • Mechanized surfaces (sup. mecanizadas)
  • Treated surfaces (sup. tratadas)

Tolerance

Tolerances are inaccuracies of shape and dimensions of the pieces since it is impossible to build parts with measures exactly similar to those set. These differences are very small, using the micron unit of measure to express them (1 µm = 0.001 mm).

Glyphs

  • Court
  • Welding: MIG/MAG welding, continuous or interval; MIG/MAG welding butt; Sealed tinned.

Materials in Automotive Body Construction

  1. Steel Plate: Features stiffness, strength, suitability for machining.

    Classes of Steels

    • Conventional cold
    • High yield strength (HHS)
    • Hot-rolled and pickled
    Important High-Strength Steels (HSS)
    • Microalloyed steels
    • Dual phase
    • Refosforados
    • IF
    • Bake hardening
    • TRIP
  2. Aluminum: It is greener and safer than steel, 40% lighter compared to steel.
  3. Plastic: Characteristics: good castability, remarkable weight reduction, non-corrosive, high flexibility.
  4. High-Magnesium: Exceptionally light compared to stainless steel volume. Its price is excessive, used exclusively in the exhaust.
  5. Stainless Steel: Resistance levels higher than other steels, reduce vehicle weight between 40 and 50%.

Metal Properties and Formation

Metals are formed by groups of atoms held together by metallic bonds.

Crystal Structure: Describes how atoms are arranged in the material. This structure determines the properties of the metal.

Grain Structure: The number and size of grains depend mainly on the manufacturing process and thermal processes the metal has undergone.

General Properties of Metals

  • Physical properties
  • Chemical properties
  • Mechanical properties: Tenacity, elasticity, plasticity, fracture resistance.

Classification of Alloys

  • Ferrous alloys (a.férreas)
    • Heavy ferrous alloys (a. pesadas Ferrer)
    • Light alloys (Aluminum Base)
    • Ultralight alloys (based on Magnesium)

The most important ferrous alloys are steel (0.1% and 1.7% C) and cast iron (1.7% and 5% C).

Non-ferrous alloys (a. no férreas)

Steel Manufacturing Process

  1. Ore is crushed.
  2. Charged with iron ore, coke, limestone, and hot-air cast iron.
  3. Undergoes a refining process: elimination of impurities in the liquid metal.
  4. Steel is obtained. Liquid steel will be made by conventional casting or continuous casting, thus obtaining semi-finished steel.
  5. Becoming just the semi-rolling or hot forging.

Converters

  • Air Refining: Thomas and Bessemer procedures. The Bessemer is distinguished from Thomas by its acidic coating and is used for pig iron low in phosphorus and rich in silicon.
  • Refining by Oxygen Injection: Pure oxygen is injected instead of air, blowing on the surface of the bath through a lance on cooled metal in a refining hearth.
  • Refining by electric oven.

Allotropic States of Iron

  • Alpha iron: < 0.025% C, < 723 °C, magnetic.
  • Gamma iron: 2% C, up to 1130 °C, non-magnetic iron.
  • Delta iron: 0.1% C, up to 1492 °C, weakly magnetic.

Constituents of Steel

  • Ferrite: Soft, low strength, ductile, and magnetic.
  • Cementite: Hard and brittle, found in high-carbon steels.
  • Pearlite: Harder and more resistant than ferrite but softer than cementite, malleable.
  • Austenite
  • Ledeburite
  • Martensite

Hot Spots: Temperatures at which structural changes occur in the different allotropic states.

Classes of Iron

  • Cast iron
  • Wrought iron

Steels (Aceros)

Iron-carbon alloys containing less than 1.7% C, are bluish-gray, hard, and elastic. Large and bright with low C content; fine and tight with high C content.

Classification of Steels

  • Low carbon steels
  • Medium carbon steels
  • High carbon steels

Alloy Steels

  • Nickel
  • Chromium
  • Tungsten
  • Silicon
  • Manganese
  • Titanium
  • Vanadium

Metal Forming

Techniques used to give objects a form.

The most common operations are:

  • Flexion
  • Casting
  • Forging
  • Pressing
  • Extrusion
  • Drawing: Technical strength with a matrix metal into the desired shape for rolling.
  • Die Imprinting
  • Drawing: Deformation is achieved without cutting the sheet, used to make hollow parts.