Tongued and grooved joints

Where there is considerable loading of the surface, as in unsupported table flaps, etc., edge joints should be strengthened with tongues, which increase the gluing area. Tongues can be worked in the solid (134:1) or both edges grooved and a loose tongue inserted (134:2). These loose tongues were formerly cut from cross grain solid wood but plywood is now univerally used, matching the ply to the thickness of the cutter. The boards are not planed hollow, while the tongue itself, whether loose or worked in the solid, should be a hand-tight fit and no more, and slightly bare of the full depth of the groove to allow for surplus glue, allowing 3/16 in (5 mm) thick tongues entering about1/4 in (6 mm) for 3/4 in (19 mm) boards, and 1/4 in (6 mm) thick for 1 in (25 mm) boards. Thicker boards require double tongues (134:3).

If the grooves are worked with a spindle-shaper or router they can be stopped well clear of the ends and the tongues cut back accordingly (134:5), but if worked with grooving- or combination-plane or circular saw it is more difficult to stop the grooves and they can be allowed to run through, either showing the ends of the tongues—which is not objectionable if well done—or cutting back and filling in with carefully selected end-grain pieces (134:6). Twin grooves and loose tongues (134:2) save timber width; moreover, solid tongues (134:1) worked with matched cutters in a combination-plane are not wholly satisfactory, for the carefully planed edge has to be cut back to form the tongue, and the tightness of the joint may suffer. With machine-cutters, however, solid tongues are so easily formed they are standard practice in production-work and 135 shows the variations possible.

A good tongued and grooved joint will help to pull up a warped board, but should not be relied upon to correct a pronounced problem. Faults in tongued joints are shown in 134:4 where in (A) the tongue is too long, preventing the edges meeting; (B) too thick, forcing the grooves open; and (C) too short, weakening the joint. Typical applications are also shown on this page. Figure 134:7 is a tongued and grooved table-top; 134:8, 9 show two forms of applied lippings to plywood or laminboard, etc.; 134:10 is a light framework with ply panel suitable for small doors, provided the panel is also glued into the groove; 134:11 is a carcass cheek tongued into a post or leg in preference to a series of small tenons which have to be cut laboriously by hand; 134:12 is the corner of a tongued carcass, with (A) a stronger variation as








These joints should be machined, glued and assembled without a dry fit. The short grained tongues are very weak the long lap at X is unsupported, and (B) a mitred lip and tongue; 134:13 is a simple box structure; 134:14 is a shaped carcass plinth or cornice; and 134:15 is a method of working tongues on a bevelled edge where the block (A) gives right-angled seating for the plough-plane. Other applications are shown in succeeding chapters.

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