All about Chisels
Wednesday 16 December 2009
John Bullar gives his journeyman a comprehensive course on chisels and how to use them
Furniture makers chisels can be broadly considered in two groups, those with bevel-edged shafts and those with square-edged shafts. Long, slender bevel-edged chisels are designed for paring intricate shapes and joints. The sides are thin and taper down towards the front, allowing the edge to be worked into corners from a range of angles.
Square-edged firmer chisels are primarily intended for squaring the sides of sockets while deep-sided mortice chisels can stand heavy blows chopping deep into hardwoods without buckling. The width of the chisel matches the width of the socket it produces.
Bench chisels, short, sturdy and bevel-edged on top, are for general-purpose work, designed for either paring with hand pressure or chopping with the force provided by a mallet. For example, to cut a rebate for a cabinet hinge you might chop the sides vertically then angle the chisel to pare a shallow pyramid shape between the sides. Finally you could pare the middle flat to the required depth, all done quickly with a single wide chisel.
A chisel works like a wedge, severing fibres then driving apart the chip from the solid wood. Sharpness is essential. The less force you need to apply, the less damage will be done to the surrounding wood.
Sharp chisels are safer because there is no need to force them, so they give you better control. Bevels are ground to 25 degrees or so for paring, while mortice chisels need 35 degrees or more for heavy chopping.
To hold an edge, the steel must be good quality, but even then there are trade-offs to be made. Steel hardness for Western chisels is typically Rockwell C 58 to 61. High carbon steel blades were perfected years ago, with a carbon content around 1% while methods of cooling are used to control the hardness.
Toughness in steel is the most important characteristic so the edge does not wear quickly. Steel needs to be hard enough to hold an edge for a long time but brittleness also comes with hardness. If a brittle blade is pounded into a dense wood the thin metal will fracture, leaving a jagged edge. Another consideration is that the steel must not be so hard that it takes forever to sharpen.
Alloy steels have other metals such as chromium, magnesium or vanadium added to harden them and increase wear resistance. Some alloy tools are cooled in liquid nitrogen (cryogenically cooled) to control hardness and grain size.
Traditionally, handles were always made from hardwoods and most good-quality ones still are, boxwood or hornbeam being generally considered best while ash and beech are quite serviceable. Fine-grained tropical hardwoods like cocobolo, rosewood or ebony are offered on expensive chisels for their attractive figuring but with no claim that they will perform any better.
Polypropylene handles are normally fitted to general-purpose chisels aimed at joinery and DIY markets. They tend to feel bulkier and less well balanced in the hand but if the steel holds a good edge, plastic-handled chisels are robust and serviceable for heavy chopping work.
The socket chisel has a conical end turned on the handle and this is inserted into a tapered socket forged into the top of the steel shaft. This compresses the wood to make a friction fit. In use, pounding the handle with a wooden mallet will seat it more firmly in the socket and it will not split the wood.
Turning a replacement handle is easy and the fit is not particularly critical. The steel socket is a heavy feature so the metalwork involved in shaping it makes good-quality socket chisels quite expensive.
Tang-fitted handles are finer than socket-fitted ones, allowing paring chisels to be made slimmer and lighter in weight. A long square-sectioned tapered metal tang is forged on top of the blade. This is driven into the bored centre of a handle, as far as a hilt which is forged around the top of the tang.
Chisels with tangs are less able to survive a heavy pounding which can drive the tang too far, splitting the handle. Firmer and mortice chisels have a ferrule around the lower end to prevent the handle splitting. There is also a leather washer around the tang to absorb sharpness from the shock transmitted between handle and blade, preserving the life of both the handle and the cutting edge.
With a metal ring or ferrule fitted to the top of the chisel handle you can use a metal mallet for light taps. This provides a sharper, less massive force with more control over the depth than a heavy wooden mallet.
Ferrules are seated squarely at the top of handles to prevent them splitting when tapped with a steel mallet. Ideally the wood should protrude a millimetre or two through the ring so the mallet does not strike the ring directly.
Paring with chisels
Several grips are used for paring depending on the cut being made but the blade is always guided by one hand while the handle is driven by the other. Right-handers prefer to use the right hand to power the cut and the left to guide it and vice versa for left-handers, but practising the opposite way round improves versatility.
Paring can be horizontal with the chisel handle gripped like a knife, or vertical with the handle gripped like a dagger. Basic skills of horizontal and vertical paring need practice which is best done on scrap with the wood firmly gripped. Accidental overshoot is more likely when paring than when chopping.
Chisels for paring
Any chisel can be used for paring provided it is sharp enough. The paring chisel is thinner, more manoeuvrable and lighter in weight, but cannot take heavy pounding from a mallet. The flat back is used to locate the cutting edge and, as with the sole of a plane, if it is not flat the result will either be a hollow surface or no cut at all.
Surprisingly, long-handled chisels provide better positioning accuracy than short ones. This is because the guiding hand pivots the chisel near its edge, while leverage from the driving hand enables it to control fine movements.
When paring across a flat surface it is helpful if the chisel handle does not interfere with the plane of the back. Cranked paring chisels keep the handle and fingers well above the plane of the underside of the blade, ideal for cleaning across flat faces into corners or for smoothing the base of a housing.
Reaching into the tight corners of a lapped dovetail socket where the angle is less that 90 degrees is possible with a conventional narrow chisel. However, it is much more satisfactory to use a pair of skew chisels or, better still, a single fishtailed chisel.
Any sharp edges on the sides of a chisel should be rounded off as these could easily injure the hand that is steering it.
Chopping with chisels
Chopping with a mallet and chisel is the traditional way to cut mortices. A succession of shallow chops is made along the length of the socket to remove a layer of chippings.
Chopping a socket back against a line you can judge by eye, looking sideways-on at the chisel to keep it vertical. Alternatively press the back of the chisel against a square-sided block clamped against the line.
Chopping away too much waste in one go will force the chisel backwards over the line, making the socket too long and misshaping the shoulder. The solution is to chop away the bulk in front of the final line then chop or pare back against it.
A mallet will exert a strong force but often it is used to deliver light well-regulated taps. Wooden-headed mallets, normally made from dense lignum vitae, are best for driving wooden-handled chisels.
Chopping away too much waste in one go will force the chisel backwards over the line, making the socket too long and misshaping the shoulder.
Chisels for chopping
Firmer chisels have quite shallow, square sides while mortice chisels have deep, square sides. Mortice chisel blades are thicker than their width. The purpose of this is to avoid bending caused by continuous pounding, which exerts a larger force on the sharpened side, leaving the top unsupported, and also to locate, or register, the blade squarely in the mortice slot.
For this reason deeper chisels are often referred to as registered mortice chisels.
Chisels were traditionally the only tools for cutting a socket. Nowadays they are more commonly used to square up the edges of a socket after machining with a morticer, router or drill, not requiring the depth of a mortice chisel.
The main feature of Japanese chisels is that the blades are made from a lamination of very hard steel, up to Rockwell 64C, faced on a shaft of soft iron or mild steel. The two red-hot metals are pressure welded together in the hammer or roll forging process. The hard underside of the chisel will hold a fine edge while the soft upper reduces sharpening time and acts as a shock absorber to improve edge life.
Because the lower face is so hard, the toolmaker grinds a large hollow in it, removing the necessity to flatten any more than the edges of the underside. In time, the user will also need to hone this hard steel rim.
In paring and chopping wood the rim around the hollow can be used as a flat reference surface but extra care is needed to align it. Some Japanese chisels have a series of narrow hollow grindings on the underside so the intermediate flat areas improve alignment, especially with wider blades.
The bevel angle can be as low as 25 degrees, which is fine for chopping softwood or paring hardwood but likely to fracture on chopping dense hardwood. Reshaping the bevel to 30 degrees would be better for this but would interfere with the hollowed underside so is not practical.
The Japanese manufacturing process is complicated and good chisels are pricey. Some handmade Japanese chisels from master blacksmiths are very expensive.
Be cautious about buying cheap ones as it is not easy to spot poor materials or quality problems. Check the edge is perpendicular to the shaft, the transition weld has no visible voids and the grinding is well centred.
Moderately priced Japanese chisels last well and once you get used to their unfamiliar shape they give good results.
Japanese chisel blades may not fit a jig so they need to be honed by hand. The combination of soft and hard steel tends to choke up abrasives. They can only be effectively sharpened on Japanese water stones where the soft substrate rapidly wears away to reveal new abrasive particles.
While it is tempting to hollow-grind Japanese chisels, creating a secondary bevel at a wider angle, this will leave unsupported hard steel proud of the laminate likely to fracture so is not recommended.
The underside of a Japanese chisel should not often need flattening other than a wipe to strop away the wire edge. However, when the bevel reaches the hollow it will need flattening again so the hollow becomes smaller with use while the rim at each side becomes wider.