Thursday 23 April 2009
John Bullar learns what to look for in a backsaw and how to use it for best results
The backsaw has a metal rib along the top edge of the blade to stiffen it and to distribute extra weight evenly over its length. This reinforcement allows the blade to be made light and thin so that it will produce a fine slot or kerf in the wood.
The large handsaw used for the hard work of cutting stock to width and length is nowadays largely replaced by the circular saw bench, either in the workshop or at the suppliers. However, backsaws are still used by most makers when they need fine work.
Tenons & dovetailsTenon saws and dovetail saws are the most commonly used backsaws, used respectively for cutting large and small joints. The tenon saw has a wider blade, allowing it to cut up to 75mm deep for large joints. The length is typically 300-350mm, providing a long action for rapid cuts.
It is unusual to cut dovetails more than 30mm deep, so the dovetail saw blade can be made narrower. This brings the back closer to the cutting edge, improving stiffness and accuracy for fine work. Length is normally no more than 230mm for smaller, more deliberate, strokes.
HandlesWooden handles are warm and comfortable to the hand while easily manufactured in small volumes. Beech works and lasts well while walnut or maple woods are offered on upmarket saws.
Horns above and below shape the grip to fit the palm of the hand and are used as styling features to make the saw look elegant. Small variations in the width, depth, length and shape of the grip are very noticeable and it is worthwhile holding different saws to get their feel.
The handle front is slotted to accept the blade, with recessed bolts to grip it. The slot widens at the top to fit around the back. Grain is selected by the manufacturer to provide strength in the narrow sections above and below the grip.
Dovetail saws commonly have a pistol grip, open at the bottom, allowing a small handle to be gripped in a large hand with lightness and flexibility of hold.
'Gents saws', used for very small work, have a similar blade and back arrangement to small dovetails saws, but the handle is turned and secured to the back rather than the blade, with the result that the grip is around the axis of movement.
BacksThe backs on good-quality saws can be steel but are more commonly made of brass. Being denser and softer than steel it can be made in heavy gauges and easily shaped around the blade. Brass is also corrosion resistant and polishes up to look good. The standard back is made from a strip of sheet metal folded double and swaged onto the blade to form a friction fit. Some versions are machined from 1/4in-thick rectangular bar with a slot milled in one edge to fit the blade, but there is no difference in function.
Teeth and sharpeningEven if you do not plan to sharpen a saw, it is worth looking at the process so as to gain a better understanding of the shape of the teeth.
The blade must be firmly gripped along its length before sharpening. The ideal tool for this is a saw vice which allows the blade to be tilted in two dimensions. Wooden saw chops are simple to make or improvise from two battens gripped in a conventional vice.
The first stage of sharpening is to top or joint the teeth by running a straight flat file along the top edges. This ensures the tips of the sharpened teeth will end up level.
Tenon saws typically have 12 teeth per inch or a pitch of 2mm while dovetail saws may have 15 to 20 teeth per inch or a pitch of 1.5 to 1.3mm.
The saw teeth are divided by V-shaped notches or 'gullets' with an angle of 60 degrees between the front and back edge. Fine triangular Swiss files or needle files known as 'three-squares' are used; these cut with their corners as well as their faces, deepening the gullets and restoring the teeth. The files are produced in bastard cut, second cut and smooth cut version
but only the smooth cut is necessary for fine sharpening.
Cross- or rip-cutBacksaws are produced with teeth prepared either for crosscutting at right angles to the wood grain or for ripping along the direction of the grain. To some extent the two are interchangeable but they work more cleanly and quickly when used in the direction intended.
Crosscutting requires the teeth to slice through fibres, severing them cleanly on either side. Crosscut teeth have their front edges raked at an angle of 75 degrees to the direction of cut and bevelled at 60 degrees to alternate sides.
To achieve this they are sharpened with the file held at a compound angle to the blade, shaping alternate gullets on one side before turning the blade around in the vice for the other gullets.
Rip-saw teeth gouge out the fibres following the line of the blade. They are sharpened more simply by keeping the front of each tooth at right angles to the sides of the blade and perpendicular to the line of cut.
Kerf and setThe thickness of a backsaw blade is typically just 0.5mm. Any friction between the blade and the kerf it runs in can cause local heating, distorting the blade as well as making hard work of the cut. To prevent this from happening, the teeth are bent with a small offset to either side.
This 'set' makes the kerf slightly wider than the metal thickness, typically 0.6 to 0.7mm. The set in the teeth is shaped using a tool called a 'saw set' which presses the tooth against a miniature anvil at the correct angle.
The set also gives the blade freedom to tilt in the kerf which is a mixed blessing - while it enables small corrections to be made in the first few strokes, it also allows the blade to drift from side to side, producing a wiggly line.
Makers often need to reduce the set provided on new saws to give them a finer cut. This can be ground out by laying the blade face down on a whetstone or it can be pressed out by squeezing the teeth lightly between metal plates in a vice. This latter method has the advantage that it does not reduce the width of the teeth.
SawingThe backsaw handle is gripped with three fingers and a thumb, the index finger pointing along the back. The feet are positioned apart and well back from the bench, allowing the arm to swing in a piston motion with upper-arm, forearm and wrist all aligned with the saw back.
At the start of a saw cut, the blade needs to be positioned correct ly and at the necessary angle for the intended cut. The saw must be pitched slightly forward for the first stroke so it enters the far side of the wood.
This is where things often go wrong, the saw jumping from its starting position or tilting off line.
Taking most of the weight off the teeth while guiding the blade against finger and thumb of the free hand reduces the chances of jumping before the kerf is established. Locking the wrist and keeping a hawk-like attention on every stroke is the best way to avoid sawing at the wrong angle and drifting off-line.
As the kerf develops, the saw is held level and the weight of the back is allowed to press on the teeth without risk of jumping. The arm now provides purely horizontal force, the saw guiding itself in the kerf. Constant attention is still needed to keep the angle correct because once a kerf has drifted even a couple of strokes off-line it is practically impossible to guide it back on again.
Joint cuttingBacksaws are primarily used for cutting joints. On tenons the critical cuts are the shoulder lines running across the grain, and that is why crosscut teeth are preferred.
The critical saw cuts on a dovetail are the cheeks which run along the grain and these are best cut with rip-saw teeth.
Joints are marked out before cutting. Non-critical lines are marked with a pencil line 0.5 to 1mm thick which the saw blade can split down the middle.
When it comes to sawing the mating half of a joint, a knife line is used and the saw kerf is run against the waste side of it, so closely that the knife line forms the edge of the kerf.
For more complex joints, sawing at a compound angle is a three-dimensional activity and to judge the start of the kerf accurately you need to be able to see it with both eyes. Positioning your nose above the saw back gives you this view, although it takes a bit of getting used to because each eye is watching the opposite side of the blade.
Wood is secured low in the vice or tight against a bench hook to prevent vibration. After a slow start the saw can progress quickly down the line, slowing at the end to ensure it does not overshoot.
Pros & consHand-cut joints are a matter of taste - some makers like using them and some customers are prepared to pay for their effort. On other styles of furniture, exposed joints would look out of place so machine-cut joints are more cost effective.
Even so, the backsaw can reach into awkward places with no setup time, producing angles and clearances a machine cannot achieve, so every maker needs some proficiency in using the backsaw.
Backsaws are precision instruments. Like most tool steel, the blades are not corrosion resistant and need to be kept very dry or lightly waxed to prevent pitting. Carefully chosen, used and maintained, a good backsaw will last a lifetime or more.