The bevel-edge chisel is the most common type of chisel. As its name suggests, its upper edges are bevelled (have sloping sides) so that it can undercut or cut side¬ways into the corner of an acute angle. The paring chisel has a long thin blade which is usually bevel-edged. It can be used for cleaning out deep holes or long slots in wood where other chisels cannot reach. It should never be used as a lever, only for paring off wood. The blade of a firmer chisel is rectangular in cross-section but otherwise it is similar to the bevel-edged chisel. The mortise chisel has a thick, rectangular section blade and a strong handle. It is designed specifically for cutting mortise joints in which the chisel is subjected to hammering and bending.
In theory, a firmer chisel is supposed to be stronger than a bevel-edge chisel and a mortise chisel should be stronger still. In practice, the strength of a chisel depends on the design of the blade, par¬ticularly the shoulder and tang, and on the type and heat treatment of the steel used. To satisfy the British Standard for chisels, the blade must be hardened to within 25mm of the shoulder, and must also be able to pass certain bending tests. There are now one or two brands of bevel-edge chisel which are sufficiently well designed and so well made that they can quite easily be used for mortising without any likelihood of damage. So it seems that a good set of bevel-edge chisels is all you need.
New chisels are not usually sharpened. To get a cutting edge, first make sure that the back of the chisel is flat by rubbing it across an oilstone until an even polished surface is obtained. Then hone the edge using the technique des¬cribed for a plane iron. The sharpening angle of a chisel is very critical; a vari¬ation of only 3 degrees can make as much difference to the life of the blade as the total variation in blade quality of most of the brands on the market. For good quality chisels on softwood, the best grinding angle is around 30 degrees. On hardwoods, this figure should be in¬creased to around 35 degrees. Increasing the angle means that more force is re¬quired to use the chisel. On the other hand, decreasing the angle will increase wear. Even a good chisel will blunt rapidly with a honing angle less than about 27 degrees.
Chisels are used for a wide variety of jobs and, like screwdrivers, they tend to be used for some rather unorthodox tasks – often to their detriment. Their main intended uses are:
• removing wood when making joints such as housing, lap joints, mortises and dovetails
• paring, or removing thin slivers from the edge of a piece of wood, often across the grain.
For paring, you use a chisel with its bevel upwards. Always make sure that you keep both hands behind the cutting edge – one hand on the handle and the other over the top of the blade. And always secure your workpiece firmly. Make sure that if the chisel slips it will not cause damage to you or the wood.
When removing wood from a mortise, you generally drive a chisel into the wood with a mallet. When you do this, start the cut well on the waste side of the wood, with the flat side of the blade nearest the cutting line. If you start the cut on the line, the wedge shape of the blade tip will drive the blade past it when the chisel is driven in. Moreover, the tendency to use the edge of the wood as a fulcrum when levering out the waste wood will bruise the edge of the wood left and create an unsightly joint. When most of the waste wood has been removed, clean up to the line by paring away the remain¬ing wood. You can speed up making mortises by drilling out some wood first.
Tauqeer Ul Hassan –
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