Introduction to Metal Stock Forms

Imagine trying to build a bicycle if metal only came in giant, solid blocks. It would take forever to carve out the frame! To make life easier for designers and manufacturers, metals are sold in standard shapes and sizes called stock forms.

In this chapter, we will explore the different shapes metal comes in, how they are measured, and how to calculate exactly how much you need for a project. Mastering this helps you reduce waste, save money, and pick the right material for the job.


Common Metal Stock Forms

When you order metal from a supplier, you choose from a list of standard shapes. Here are the main ones you need to know for your GCSE:

1. Sheet and Plate

Sheet metal is thin and flat. It is usually used for things like car bodies, drinks cans, or heating ducts. Plate is similar but much thicker (usually thicker than \(6mm\)). Plate is used for heavy-duty structures like bridge sections or ship hulls.

2. Bar Stock

Metal bars are solid lengths of metal. They come in many "profiles" (the shape of the end), including:

  • Round bar: Used for shafts and axles.
  • Square bar: Often used for decorative ironwork or frames.
  • Hexagonal bar: Perfect for making nuts and bolts.
  • Flat bar: Used for brackets and braces.

3. Pipe and Tube

These look like bars from the outside, but they are hollow. Tube can be round, square, or rectangular. We use hollow sections because they are lightweight but still very strong. A key measurement here is the wall thickness (the thickness of the metal "ring").

4. Wire

Wire is a very thin, flexible rod of metal, often supplied in large coils. It is used for everything from electrical cables to fences and springs. Wire size is often measured by gauge—the higher the gauge number, the thinner the wire!

5. Extrusions

Extrusions are complex shapes made by pushing hot metal through a shaped hole (a die). Common extrusion shapes include I-beams, U-channels, T-bars, and C-sections. These shapes are designed to be incredibly strong under heavy loads while staying light.

6. Castings and Powder Metallurgy

Sometimes metal is supplied in a form that is already close to its final shape. Castings are made by pouring molten metal into a mould. Powder metallurgy involves pressing fine metal powder into a shaped die under high pressure and heat. These are used for complex parts like engine gears.

Quick Review: Stock forms are standard shapes like sheet, bar, tube, and wire. Using these saves time and money because the manufacturer has already done the hard work of shaping the raw metal!


Measuring Sizes: Gauge and Wall Thickness

In the workshop, "size" isn't just about length. You need to understand two specific terms often used in exam questions:

Wall Thickness: This applies to pipes and tubes. It is the distance between the outside surface and the inside surface.
\( \text{Inner Diameter} = \text{Outer Diameter} - (2 \times \text{Wall Thickness}) \)

Gauge: This is a traditional way of measuring the thickness of sheet metal or wire. Remember: In many gauge systems, a larger number means a thinner piece of metal. For example, \(22\)-gauge steel is much thinner than \(10\)-gauge steel!


Quantity Calculations

In your exam, you may be asked to calculate how much metal you need or how much it will cost. Don't worry—the maths is usually similar to what you do in Key Stage 3.

1. Calculating Area (for Sheet Metal)

If you need to cut a rectangular piece of sheet metal, use the formula:
\( \text{Area} = \text{length} \times \text{width} \)

Example: If you need a piece of aluminium \(200mm\) by \(150mm\):
\( \text{Area} = 200 \times 150 = 30,000mm^2 \)

2. Calculating Volume (for Castings or Weight)

To find the volume of a solid bar or plate:
\( \text{Volume} = \text{Area of the end (cross-section)} \times \text{length} \)

3. Minimising Waste (Nesting)

Metal is expensive! Manufacturers use tessellation or "nesting" to fit as many parts as possible onto a single sheet.
Percentage Waste Formula:
\( \text{Percentage Waste} = \frac{\text{Total Area} - \text{Area of Parts}}{\text{Total Area}} \times 100 \)

4. Costing

The price of metal changes daily. Suppliers often use the London Metal Exchange (LME) price as a baseline. You might be asked to calculate the cost of a specific length of metal if you know the price per metre.
\( \text{Total Cost} = \text{Quantity Needed} \times \text{Price per Unit} \)

Common Mistake to Avoid: Always check your units! If the question gives some measurements in \(cm\) and others in \(mm\), convert them all to the same unit before you start calculating.


Key Takeaways for Revision

  • Standardization: Metals are sold in stock forms (sheet, bar, tube, etc.) to make manufacturing more efficient.
  • Tube vs. Bar: Tubes are hollow and measured by outer diameter and wall thickness; bars are solid.
  • Gauge: A measurement for wire and sheet thickness. Remember that high gauge numbers usually mean thinner material.
  • Efficiency: Designers use nesting to arrange parts closely together on a sheet to reduce waste and lower costs.
  • Pricing: The London Metal Exchange (LME) is the authority for metal pricing.

Note: To see how these metals are actually shaped and joined, check out the chapter on "Metals: Specialist tools, shaping, fabrication and assembly".