  #### Glossary # What is meant by mean diameter? When forming metal into a curved shape, metal fabricators must always allow for the centre measurement of the metal remaining constant. This is called the neutral axis. The outside surface stretches and the inside surface contracts.

The neutral axis is better known as the mean diameter (MD).

### Calculating the mean diameter

Mean diameter = inside diameter + one plate thickness (of the material used)

MD = ID + T

or

Mean diameter = outside diameter - one plate thickness

MD = OD -T

### Calculating mean circumference

The mean circumference (MC) is also the cutting length. Use the formula:

MC = MD x . ( = 3.142).

### Example

What length of 10mm mild steel plate is needed to form a cylinder having an outside diameter (OD) of 650mm?

Calculate the mean diameter

MD = OD - T

= 650 - 10

= 640

Calculate the mean circumference

MC = MD x =

= 640 x 3.142

= 2010.624mm

= 2011mm (rounded off to the nearest mm)

The following diagram shows the cutting length (mean circumference) of the material. Therefore a piece of 10mm mild steel plate, 2011mm long is required to roll up to an outside diameter (OD) of 650mm and inside diameter (ID) of 630mm.