Stock forms

All designers need to know the stock sizes that metals and alloys are available in. If stock sizes are known, designs can be manufactured more economically to reduce waste. Metal is available as a stock form in sheet, rod, bar and tube, and it is sold by length, width, thickness and diameter.

The different stock forms of metal and their measurements including the diameter and length of rods, the wall thickness and length of tubes and the thickness, width and length of bars.

Steel rod is a solid round piece of metal, and the diameter and length are needed when ordering. Steel bar can come in many cross sections, such as square and rectangle, and the dimensions of the cross section and the length are needed when ordering.

Using steel as an example: the thickness of sheet steel is is also measured using the standard wire gauge (SWG) scale. Conversion tables allow the purchaser to understand SWG sizes in millimetres (mm), eg a 2 mm thick piece of sheet steel has a SWG size of 14. When buying steel tube, the wall thickness is measured using the SWG scale and the outer diameter and length are needed when ordering.

When buying sheet metal, the SWG size gives the thickness measurement, but the length and width measurements are also needed. Bulk buying metal, as with most items, can save money.

Example

1 m2 aluminium at 3 mm thick (SWG 11) = £29.00 per m2

Twice the thickness would cost:

1 m2 aluminium at 6 mm thick (SWG 4) = £44.00 per m2

The percentage increase in the cost for the thicker aluminium can be calculated:

Increase in cost = £44.00 - £30.00 = £14.00

This needs to be calculated as a percentage of the thicker aluminium:

(14 ÷ 44) × 100 = 32%

This shows that 100% more steel has been bought for just 32% of the cost.

Question

1 m2 steel at 2 mm thick (SWG 14) = £45.00 per m2

Twice the thickness would be:

1m2 steel at 4mm thick (SWG 8) = £57.00 per m2

What is the percentage increase in the cost for the thicker steel?

Increase in cost = £57.00 - £45.00 = £12.00

Calculate £12.00 as a percentage of the thicker steel:

(12 ÷ 57) × 100 = 21

So it is 21% more expensive.