Geoff's Woodwork   

 

for Students of Woodwork                                            


Carcases.

Timber when it’s moisture content changes, shrinks or swells far more across the grain than along the grain. Because of this the carcase must be constructed to allow this movement. Any jointing of members that fail to take this into account will split in the event of movement. All carcase forms and table tops, etc. must therefore take this into account.

There are three main forms of carcase construction.

Framed

Stool (table)

Box.  plus:

Drawers of various types

In the majority of cases you will find that the construction you are undertaking will take the form of one these three.  (these drawings are intended to show the grain direction to eliminate shrinkage problems only and not jointing details)  Refer to the paper on M & T's for details

The frame overcomes the worse effects of timber shrinkage and provides strength in all directions. The panel which is normally fitted loose is allowed to shrink or swell without affecting the frame.  This overcome the major problem in years gone by of obtaining stable wide boards.

The stool (or table) usually consists of four posts or legs at each corner with rails joined with a mortise and tenon joint. Because the grain runs along length and height there is no shrinkage problems. Furniture made in this form are; table, chairs, and some cabinets.

Box construction is used for solid wood carcases as opposed to panelled frames. Shrinkage does not occur because the whole the whole carcase moves equally. But, any runners or rails fixed longitudal across the grain must not be glued into place along the length.  Use elongated holes and screws, etc. to allow the contra movement of the different grain directions.  The middle or one or other of the ends may be fixed allowing the slotted end(s) to move as required.   Furniture of this form includes chests of drawers, sideboards and cupboards, 

Drawers may be flush to the front of the cabinet frame in which case the dovetails are 'lapped'.  On cheaper constructions and perhaps for kitchen cabinets the drawer front may be 'laid on'.  In this case a common or 'through' dovetail is used on the front and a 'false' front screwed on.  Drawer bottoms should be inserted in grooves worked into the sides or planted on mouldings.  The mouldings are usually quadrant shaped and are glued and pinned to the drawer sides.  Alternatively a flush slip may be used which gives a neater appearance.  The bottom should be inserted loose with the grain direction left to right and fixed into place with elongated countersunk screws into the bottom of the back rail. On wider drawers a muntin is dovetailed into the front rail to support the load.   Other types of drawers include mechanical mechanisms and runner/housings especially in kitchen and knock down type constructions.    A nice feature in quality drawers is the 'cock bead'.  Not only is this decorative I have used it to hide some defects and other 'sins' during the lock fitting operations.

Grain Direction.   It must be emphasised in all the above forms, any solid timber board fitted across it, such as a top or other cross member at 90 degrees to the grain, should be allowed to move otherwise it is likely to split. The front or middle however may be anchored and the outer edges allowed to float.

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all rights reserved ~ 18th August 2003 ~ addendum 29th November 2003 ~ 3rd January 2004