The Technique of FURNITURE MAKING

OTHER EDGE TOOLS Chisels

It is hardly necessary to have complete sets of mortise-, firmer- and bevelled edged chisels for furniture-making, as the bevelled-edged will do everything necessary. Nor are complete sets of any one type essential, for there will always be three or four favourites to which the hand auto­matically turns. A first selection could be 1/8 in (3 mm), 1/4 in (6 mm), 3/8 in (9 mm), 1/2 in (12.5 mm) and /4 in (19 mm) bevelled edge (61:lA) with the addition of a 1/16 in (1.5 mm) firmer type (61:lB) and perhaps a 1/2in (12.5 mm) mortise-chisel (61:lC), with the later addition of a long paring-chisel (61:4B), and a butt - or sash-pocket chisel (61:4c) for fitting work away from the workshop.

Various types of chisel handles are shown in 61:3. Figure 61:3A shows the universally popular box or ash carving handle; 61:3B the traditional round ash; 61:3C the oval splitproof plastic handle, either in polypropylene or the transparent cellulose acetate butyrate which is tougher but more expensive, and 61:3D the typical steel-hooped mortise handle. It is, of course, false economy to buy cheap sets of chisels, for the keenness of the edge depends entirely on the grade of steel, which varies from maker to maker, and a really good chisel should last a lifetime. A recent development is the introduction of high-speed tipped firmer and

Grinding wheels

Most edge tools are now dry ground on high­speed grinders instead of the much slower cutting water-lubricated grindstone. The latter is always the better method as there is never any danger of withdrawing the temper of the cutting edge; nevertheless the high-speed grinder will give good results if care be taken. The wheels for these grinders, which can be hand - or power - driven, are either carborundum (silicon carbide) or Aloxite (crystallized fused alumina), and there is little to choose between them except that carborundum is harder and Aloxite tougher. (Generally speaking any grit - for-grit carborundum stones cut faster and wear more rapidly than the denser, tougher Aloxite stones, therefore carborundum is normally used where the stone is applied to the tool, and Aloxite where the tool is applied to the stone.) They are available in a variety of shapes and sizes from 1 in (25 mm) diameter by 1/4 in (6 mm) thickness upwards, and in a range of grits, 6 to 10 very coarse, 12 to 24 coarse, 30 to 60 medium, 70 to 220 fine and 240 to 700 very fine. They are also letter graded which denotes the strength of the bond and not the hardness of the grit which does not vary, from A soft to Z hard, with the main grades F to T. A fair average for the carbon steel used in wood­cutting tools would be about 60 grit F grade, always remembering that the softer the grit the cooler the cut. Therefore hard steels require soft wheels and soft steels hard wheels, while narrow-faced wheels cut cooler as do slow - running wheels, for less is taken off at each revolution and therefore less heat generated. Glazed wheels generate heat rapidly, and all wheels should be regularly dressed with a proper wheel dresser. To prevent burning of the tool edge, which not only draws the temper but also causes minute cracks in the bevel, the steel
should be frequently quenched in cold water, with the side feed across the face of the wheel as rapidly as possible so that only a fractional amount is removed at each pass. Providing all these precautions are taken there is little danger, but careless grinding can soon ruin a good tool, and a blued edge must be avoided at all costs. The above remarks do not apply to high-speed tipped edges.