The Modern Cabinetmaker
From the Maintext
Scratch stock design features
Most of the corners of the scratch stock are slightly rounded to make it more comfortable to hold. Do not use excessive rounding; this will make the tool difficult to control. Only two screws are needed to hold the cutter in place and the two halves of the scratch stock in alignment. The M6 nuts are set in flush with the back of the scratch stock. They have to be filed or turned down in thickness so that 5 mm (0.2 in) of wood remains in the thickness of the bar. The positioning of the screw centres allows cutters of different widths to be located anywhere along the bar. The lower corners of the bar are chamfered for three reasons, a) it improves visibility of the cutter, b) it helps to prevent wood chips building up either side of the cutter during use, c) it allows the bar to be tilted over more before fouling on the workpiece.
From the Abrasives supplement
Abrasive products, irrespective of the type of abrasive material being utilized, can be divided into three broad categories. These are known as 'loose', 'coated' and 'bonded' abrasives. Loose abrasives consist of loose particles of grit which may be used dry or in a liquid suspension; a typical use is lapping or polishing metals. Coated abrasives consist of a layer of abrasive grains (either single grains or, in some products for grinding metals, aggregate grains made up from several individual grains) bonded to various kinds of flexible backing material. ('Sandpaper' used for sanding woods or other materials is one variety of coated abrasive but the latter term encompasses a great variety of related products). Bonded abrasives consist of abrasive particles bulked and bonded together in various media to make abrasive products which replace natural abrasive stone in tools such as grinding wheels and sharpening stones.
Despite the efficiency of modern abrasives it remains important to select appropriate abrasive tools for each job, having regard to the nature of the material to be ground. A mismatch between the material of the abrasive and that of the workpiece can result in reactions (the mechanisms of which are not yet fully understood) which lead to rapid dulling of the abrasive particles and hence inefficient grinding action and overheating. Instances of this kind are silicon carbide being used for grinding steel, and aluminium oxide being used for grinding brass. In addition, the working properties of abrasive materials vary according to their purity. Economic considerations should always be taken into account when choosing abrasive products for particular purposes.
From the Adhesives supplement
Some advantages to the use of adhesives
The chief advantage to the use of adhesives in joints, compared with other forms of fastening, is that loads are more evenly distributed throughout the joint i.e. stress concentrations in the joint under load are minimized. This makes for stronger and stiffer joints.
Consider two blocks of timber which are fixed together and loads are applied to them in opposite directions. If the blocks are joined by adhesives, shear stress will be developed in the whole glue line (see Glossary of terms). If mechanical fasteners (nails, bolts, etc.) alone are used, stresses will be concentrated around the points of contact between the fasteners and each block. These localized stresses will be of greater magnitude than those in the glue line of the glued joint and if too few fasteners, or fasteners of an inappropriate design are used the joint will fail prematurely as a result of trauma to the timber, the fasteners, or both. Notwithstanding this, it is essential that glued joints in wood be properly designed and manufactured.
The use of adhesives may also result in greater economy or convenience in the assembly of products, or savings of weight in the finished product.
From the Metals supplement
Metals are important to the woodworker. The blades and bodies of many woodworking tools are of metal, and metal fixings and fittings fulfil vital roles in many kinds of wooden structure. Some knowledge of the nature and properties of metals on the part of the woodworker is required if he is to use them successfully. This section gives only a brief glimpse of the complex science of metallurgy.
The crystalline nature of metals
All metals are crystalline when in the solid state. Crystals are made up from atoms packed together in (predominantly) precise geometrical arrangements. The atoms in pure metals are bound together by 'metallic bonds'; the electrons in the outer shell of each atom are shared with all other atoms and dispersed freely throughout the material (hence they are good conductors of electricity). Metallic bonds, though weaker than either covalent or electrovalent bonds, still require large forces to make them 'slip' in perfect crystals. The structure of crystals is seldom perfect however; at intervals throughout the atomic lattice incomplete layers of atoms will exist. This gives rise to fault lines, known as 'dislocations' which give rise to mechanical weakness.
From the Plastics supplement
There are over 20 000 formulations of plastics materials, and their properties vary widely. Since 1976, plastics have been more commonly used than any other material. Whilst not directly useful to fine furniture-making in large amounts, plastics can be indispensable for making fittings, tools, jigs etc.. This section deals briefly with some of the most important natural and synthetic polymers. It includes notes on their history, uses, the machining of plastics materials, and a glossary. Most libraries hold some information on plastics and plastics applications. For more detailed help contact the Plastics & Rubber Advisory Service, see http://www.relderton.co.uk/links.htm under Information agencies & databases. Information on plastics can also be found elsewhere on the Internet.
From the Wood supplement
There are more than 30 000 tree species but only a few hundred are exploited commercially; about 100 in the UK. Hardwoods and softwoods are differentiated by their physical structure as well as by more obvious differences in botanical form. The evolution of softwoods predates that of hardwoods and their timber has a simpler and more uniform structure than that of hardwoods. Classification of the two groups is based on the way in which seeds are borne on the trees. Hardwood trees bear fruit which contain seeds, and are included among the plants known as angiosperms (from the Greek aggeion vessel and sperma seed). In softwood trees the seeds are more exposed and are not contained in an ovary; they are among the plants known as gymnosperms (from the Greek gumnos naked). (Both groups are subdivisions of the seed bearing plants, the Spermatophytae).