METAL FORMS.
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.
Fig. 3.—Suggestion for circular form.