The Technique of FURNITURE MAKING

Silica carbide

The hardest and most expensive grit, produced by heating mixtures of coal and quartz in an electric furnace, and commonly known as 'carborundum. Scarcely less hard than the diamond, its sharp yet even crystalline formation is eminently suitable for wet or dry grinding, and polishing of very hard substances with a compact surface, i. e. cellulose, polyester, paint, glass, plastics, stone, non-ferrous metals, etc. It is, however, more brittle than aluminous oxide, and the latter is to be preferred for wood finishing.

Grading of papers (grain or grit sizes)

Hitherto, papers were marked with the grade only, and the principle is still employed for glasspapers which range from 'Flour', 00, to 'Strong 2"; but the grading of other papers has now been standardized to the actual mesh size per square inch for grits within the range 800, which is the finest obtainable, down to 12, which is the coarsest. To avoid confusion most papers are clearly marked with both grade and grit size, and the following table gives the various equivalents together with the approxi­mate comparable gradings in flint - and glasspaper.

Approximate comparable gradings in flint - and glasspaper

Both aluminous oxide and silica carbide papers are available in all grits up to 800, and garnet-paper up to 500 grit. The equivalent values for flint-papers are fairly correct, but glasspaper values are an approximation only, for it must be borne in mind that the harder the grit the sharper the crystalline facets and the quicker the cut; therefore there is no exact comparison.

Choice of types

The final quality of any piece of furniture depends in large degree on a correct choice of abrasives at all stages of manufacture. A key factor in performance is the way in which grit particles are arranged on the backing support. Densely packed grains are used to form close - coat surfaces. In theory these should give a greater speed of cut, due to there being more cutting surfaces, but close-coat papers tend to clog rapidly with dust and wood resins. This defect is overcome by spreading the grit more thinly. This type of abrasive paper is known as open-coat, and is generally less expensive and longer lasting than close-coat paper. Open-coat papers should always be specified for use when hand - or machine-sanding resinous woods, or fine sanding lacquered surfaces. Clogging of the abrasive surface with dust is also reduced by applying the grit in a pattern of small dots to the backing support. This is because dust and loose grit is able to escape into the clear spaces between abrasive patches. Stearate lubricated silicon-carbide papers are now preferred for fine sanding of polished work, since the dry lubricant is clean and convenient.

Backings

Backings or supports for grits can be either vulcanized fibre, which is ideal for disc and drum sanders where the loading is particularly severe; 'combination', comprising a heavy­weight paper laminated with an open-weave fabric, and used with floor and drum sanding machines; close-woven pure cotton fabrics in various weights and degrees of flexibility for machine sanding; and tough raw paper of high - tensile strength in a variety of weights for both hand - and machine-sanding. Cloth backs last longer but are considerably more expensive, and paper is usually suitable for most normal purposes, although a very flexible cloth backing specially prepared for hand sanding is better for pronounced contour-work. In principle, orbital pad sanders require a fairly tough paper backing usually known as 'cabinet-papers' (C or D weight), and these are also used for hand or 'touch' sanding with a cork block in preparatory levelling off using a fairly coarse grit; but for final finishing and delicate work a lighter weight (A) is used, commonly known as a 'finishing' paper. Glasspapers, flint-papers and corundum (aluminous oxide) are usually heavy C weight papers.

Sizes, etc.

Glass- and flint-papers are only available in the standard sheet of 11 in (280 mm) by 9 in (229 mm), which is torn into four equal pieces for use with a suitable cork block; but all other coatings are available in the standard sheet, in strips for orbital pad sanders, and in an immense variety of roll sizes from which discs, belts, sleeves and drum covers are made up to special order. Belts for portable belt sanders can be made up in the workshop if necessary, but the joint should be diagonal across the belt and the overlaps chamfered off to form a scarf joint, or the double thickness will bump over the rollers. A selection of abrasive papers for general workshop use is listed at the end of this chapter.

33 Scarf joint