Sharpening / Honing a Chisel Blade
Oilstones and slipstones
The first essential for sharpening is an oilstone. Oilstones are made in three grades of grit: coarse, medium and fine. Combination stones have sides of two different grades, usually medium and fine. Sizes are 152 x 50 x 25 mm (6 x 2 x 1 in) or 203 x 50 x 25 mm (8 x 2 x 1 in). The longer the stone the more effective each sharpening stroke will be.
Coarse stones are for removing large amounts of steel. They are not generally needed for home sharpening but are useful for removing a chipped cutting edge. Never use them on the flat backs of blades.
Medium grade stones cut more finely than coarse ones, but do not give a good cutting edge. This grade is usually used after grinding and before the fine stone.
Fine stones are used for giving the final cutting edge to a blade. Your stone should have at least one fine side.
Keep and use your oilstone in a homemadebox with a matching lid.
To make the box, mortise a slot in the wood the same width as the stone, 12 mm (½ in) deep, and the length of the stone plus 18 mm (¾ in). Cut two pieces of the hardest and closest end-grain wood you can find to pack out each end of the stone in the box, making them flush with the stone’s top. These prevent damage to blades which run off the stone and, by allowing the full length of the stone to be used, reduce chances of uneven wear. Drive fourinto the base of the box and cut them off so the ends just protrude; these stop the box slipping when in use on the bench. Make the slot in the lid slightly larger all round than the stone.
Slipstones are smaller and often finer than oilstones. They are used for sharpening less common tools, including gouges. Unlike oilstones, they are held in the hand and rubbed over the tool which is being sharpened. The four basic shapes are triangular, round, rectangular and the gouge, stone, which is tapered to give two curves of different radii.
Keep slipstones in their own box, away from workshop dust, or in paraffin
Never use an oilstone without lightly oiling it first; white spirit can be used if you run out of oil, provided the stone is treated gently. When sharpening, make use of the whole surface of the stone and do not run in one place all the time.
If a stone is worn hollow, regrind it flat with silver sand (as used in bird cages) and a sheet of old glass. Sprinkle the glass with sand and rub the stone over it with a rotary action. Keep the stone dampened with water as you rub. Continue rubbing until the stone is flat and clean all over.
If a stone is oil-clogged, drain it by putting it in a tray in a warm oven.
Honing chisels and plane blades
Chisels and plane blades have two angles forming their cutting edge, the ground angle of 25° and the honed angle of 30°.
The ground angle is formed on a grindstone and only needs occasional renewing. The honed angle is formed and maintained by rubbing on an oilstone to give a razor-sharp edge. To hone a chisel or plane blade, oil the stone then hold the blade at an angle of about 30° to the stone, rubbing it to and fro the length of the stone until a burr is built up all along the flat side of the cutting edge — you can feel this by running your thumb along it. Get the blade at the correct angle to the stone either by resting the ground angle on the stone then raising the back end up slightly, or by using a honing guide into which the blade fixes ready for rubbing.
When the burr has built up on the flat side, rub that from side to side on the stone until the burr turns back. The blade must be kept flat on the stone all the time you are doing this.
Continue rubbing each side of the edge on the stone in turn, using less and less pressure each time. The burr is thus being bent one way, then the other, getting thinner each time until it falls off, leaving a razor-sharp edge, which should cut a hanging sheet of paper.
Some chisels may not retain an edge for more than a few minutes’ work; better-quality chisels will last much longer. In any case, check that blades are sharp before using them, and hone if necessary.
When honing the angled side of. The cutting edge, keep the blade’s sides parallel with those of the stone. Plane blades which are wider than the oilstone are held at an angle across the stone, then worked to and fro in the same way as chisels.
Take care to keep the blade at a constant angle of 30° as you rub the honed angle to and fro along the stone — use two hands to hold the blade or buy a honing guide. When rubbing the flat side, keep the blade flat on the stone as you work.
How to hone gouges
The sharpening procedure of obtaining a burr on the cutting edge, then wearing it thinner and thinner until it drops off, applies to gouges as much as to any other chisel. The method for gouges only differs in the way the blade is rubbed and the type of stone used. Gouges, like flat chisels, have two angles at the cutting edge.
With firmer gouges, the burr is built up on the inside of the curved edge first, by honing the outer, angled face on a flat stone in a series of sideways, twisting actions which allows all the edge to come equally into contact with the stone.
When the burr is built up on the inside edge, it is reversed by honing with a slip-stone, which must be kept flat along the blade’s length. This two-fold process is repeated until the burr drops off.
The scribing gouge is sharpened similarly but in reverse sequence: the angled edge on the inside is honed with the slipstone at 30° to the blade, the burr being built up on the convex back of the blade and turned back by rubbing from side to side on a flat stone with a twisting action. The back of the blade must be held flat on the stone while this is being done.
When and how to regrind
When blades become gapped or curved at the cutting edge as a result of a lot of honing, they should be reground; that is, have the blade squared up and the ground angle of 25° restored.
Regrinding gouges should be left to a tool merchant as the process is tricky. Chisels and plane blades can be done at home, however, either by rubbing on a coarse oilstone (which is hard work) or by means of a grinding wheel.
Electrically powered grinders are better than hand-worked wheels, as the power model, among other advantages, leaves both hands free to hold the blade being reground.
To regrind a blade, first check that the cutting edge is at right angles to the sides, using a try-square. If it is not, or if the cutting edge is nicked or gashed, grind it back square by touching it on the wheel.
To restore the ground angle, rest the blade on a tool rest at the correct angle and touch it lightly against the wheel. Do not let the blade overheat — keep it cool by dipping it in water between grinding. If the cutting edge turns blue, it has lost temper and will be impossible to sharpen. If this happens, you must grind off the blued areas and start again
Move wide plane blades from side to side across the wheel as you regrind, so that the whole of the edge is properly ground. As a check that a blade is ground correctly, measure the length of the ground face; it should be 2-1/2 times the thickness of the blade.