The Technique of FURNITURE MAKING

Part IV Advanced areas of furniture construction 21 Carcass construction

GENERAL NOTE

The construction of any large carcass must provide for possible distortion, either under the weight of its own members or by applied forces—pushing, pulling, lifting, upending, etc.—or by setting on an uneven foundation such as sloping floors. The effect of any such disortion may not be detectable visually, but nevertheless doors will bind, drawers jam and connecting joints may fail; therefore overall stiffness and squareness under all normal conditions are essential. Assuming an open­faced rectangular box with top, bottom, two sides and fixed back, any force applied, no matter from what direction, will endeavour to twist the faces of the box, including the open face, which will tend to take on a diamond shape (174:1) with the corner angles greater or less than true right angles. If, however, one side of the box is so anchored that it cannot twist then all the other sides will be prevented from twisting and the opening will remain square (174:2), while if the opening itself is so stiffened with cross-bracing that the diagonals are preserved then the sides also will be prevented from twisting (174:3). Obviously, this latter method is not practicable in open carcass-work, therefore other methods must be adopted.

METHODS OF STIFFENING CARCASSES

Openings can be stiffened by applied side pilasters (174:4) and central pilasters (174:5), but the top and bottom members must be stiff also and the connecting joints as strong as possible. Other methods include pilaster attached to a stiff apron (174:6)" L frame (174:7)TU frame(174:8)rinverted U(174:9); T frame"(174:10); I frame(174:ll) and H frame (174:12). Methods of increasing the stiffness of flat panels include increasing the thickness (174:13); sandwich panel (174:14); closed box (174:15); cross-ribs reinforcement (174:16); double cross-rib (174:17) and pyramid bracing (174:18). Carcasses can also be stiffened by arbitrary fixed cross-rails or shelves or divisions, which in effect either adopt an 'H' or T formation and break up the carcass into smaller boxes, with increased wood thickness relative to the opening. This method is often adopted in open bookcases with loose shelves, where one or more shelves are housed and glued into the sides to increase the stiffness, but such carcasses will still distort fractionally if stood on an uneven surface, no matter how sturdy the wood sections are, for increasing the wood dimensions also increases the overall weight, therefore some form of pilaster, stiff apron rail or box plinth is preferable. The principles of deflection and the appropriate methods of combating them are examined in detail in the Design Manual for Cabinet Furniture published by the Furniture Development Council.

In all carcass work it should be remembered that while a well-fitted carcass back, grooved, screwed or glued in position will help to achieve overall stiffness and squareness, it will not automatically ensure such rigidity.

MOBILITY

Large carcasses are often split vertically into two separate carcasses coupled together after erection, and the maker should always check well in advance that stairways, elevators and door openings are wide enough for the passage of large furniture. There is a growing practice to fit small free-running castors to heavy pieces, wardrobes, etc. and this is a sound move.

MOVEMENT

Solid wood and plywoods etc. must not be mixed indiscriminately in carcass-work. For instance, solid sides and plywood tops, bottoms and fixed shelves would have different shrinkage values; so also would plywood sides and solid tops, bottoms, etc. There is, however, no objection to using relatively narrow bearer rails in solid wood to plywood sides or vice versa, as the shrinkage will be negligible. In planning any carcass construction, therefore, a moment's thought as to the probable direction and effect of the normal shrinkage of each part will usually suggest a satisfactory compromise. Framed-up (panelled) carcass sides can be treated as plywood, for here again shrinkage values are usually too small to cause trouble.