Metal forms can be used to advantage when the work involved is to be
repeated many times. If it is known or if it is probable that the forms
may have to be altered, the relative costs of wood and metal forms should
be carefully determined.
Metal forms of various types and designs may be purchased. Although the
first cost may be high, yet their use may lower the total cost when the
work is such as to warrant it.
Circular forms may be built as shown in Figure 3. The sheathing is
generally of wood 4 to 6 inches wide, or sheet metal, and, if of wood,
is laid perpendicular to the battens. In forms of small diameter, sheet
metal sheathing is necessary if a smooth surface is desired, as the
4-inch boards can not be made to conform to a true circle. The radius
used for cutting the battens of the inner circle should be the thickness
of the sheathing less than the inside radius of the structure and the
same amount greater than the outer radius for the outside battens.