Sharpening Carpentry Tools
In the scope of carpentry and joinery, sharp cutting tools play a large part in the tasks that we carry out. Chisels, hand planes, and hand saws are staple tools that allow us to craft and shape timber components, especially in the workshop and second fix operations. For the most part, new tools are going to be somewhat sharp straight out of their packaging, at least enough to slice the fibres of softer timber with relative ease. Through use though, these tools will dull and lose their edge, reducing their effectiveness significantly.
It's often stated that more accidents happen with blunt tools than sharp ones. For those that are unfamiliar with cutting tools of this nature, this idiom may seem counter intuitive. The excessive forces applied in order to make blunt tools cut leads to less control, certainly leading to a higher risk of injury. Pushing or pulling with force using blunt tools in an attempt to take shavings can cause them to slip or jump away from the timber. Based on this principle, it's important to maintain good edges on our tools, if not for the simple fact that they perform better and leave cleaner, smoother finished surfaces.
Additionally, many of us inherit or are gifted tools because of our association with the trade - at least that is my experience. Friends are family that are clearing out sheds and garages often pass on unwanted tools to a new loving home, though these rusted antiques typically require much love before they can be put to use again. Over time as the tools rust, the edges become dull and require restoration - this in combination with a previous lifetime worth of working abuse. In this section we will look at how cutting edges are created on our tools, in order for them to perform their tasks as intended. We will look at the terminology and practices associated with sharpening, and how tools can be maintained for the best performance and longevity.
Sharpening principles and terminology
The creation of a cutting edge is at the point in which two different planes meet. To form an effective cutting edge, the two planes of the blade come together at a consistent fine point. Tool blades are commonly made from tool steel and other hard metals, as these materials are capable of being brought to such a fine edge. For chisels and plane irons, the underside of the blade is flattened using an abrasive surface, all the way up to the cutting edge. A bevel is then established on the upper front edge, tapering from the top of the tool to the bottom, forming the cutting edge where the two planes intersect. Material is removed to create these bevels using abrasive sharpening equipment. These abrasives are available in a range of different grits - a widely used scale that measures the coarseness and aggressiveness of the abrasive. The lower the grit number, the more coarse the abrasive, and the more aggressively it will remove material. Once the desired results are achieved with a low grit, the process must be repeated using a higher grit. This removes the scratches and imperfections left by the lower grit and refines the cutting edge to a more polished state. This series of repetitions must be carried out in multiple stages until a very fine grit is reached, and the scratches and imperfections left in the cutting edges are minimal. For most carpentry applications, 1000 grit is high enough to create a razor-sharp edge, as beyond a certain grit - typically around 2000, the increased results are nominal. That being said, some people like to sharpen past 10,000 grit.
For saws and axes, hardened files are used to remove the material until the two cutting faces meet at a fine edge. Once the edge of any tool is established and the use of the abrasive materials are finished, it's typical for the edge to be polished using a leather strop. This stropping process, possibly including the use of fine abrasive compounds, helps to refine the edge and bring it to a mirror finish - capable of shaving hairs. The exact process for each cutting tool will be looked at in detail shortly.
Types of abrasives and sharpening equipment
As mentioned, the shaping and sharpening of tool edges are carried out with abrasive equipment. There are many varieties of abrasives on the market, each with their own advantages and disadvantages. Here we shall look at the most common styles.
Sandpaper
Sandpaper, also sometimes called glasspaper or emery paper, is an abrasive material that can be used for sanding timber products, as well as sharpening tools. The paper is sold in many different grits, either one a roll, in sheets, or shaped to fit a specific sanding tool. Cheap sandpaper is formed using a thin card as the backer for the abrasive. More expensive varieties can be purchased that are made using more durable paper products, as well as fabric or mesh. These products last longer and are well suited to use with electric sanding tools. The abrasive material is typically aluminium oxide or silicon carbide, adhered to the backing material. Sandpaper is sold in a wide range of grits, on a low to high numbered scale that relates to the coarseness of the paper.
To sharpen tools effectively using sandpaper, it must be taped or stuck to a perfectly flat surface, such as a glass lapping plate. The paper can be lubricated with water, though regular, non-water-resistant sandpaper will easily tear when sharpening.
Overall, sandpaper is amongst the cheapest abrasives due to the low cost of production.
Diamond plates
Diamond plates are abrasive plates that are used for sharpening tools. They are flat metal plates that are faced with synthetic diamonds that act as the abrasive. As is typical of abrasives, they are available in a range of grits. Some varieties are double sided, featuring a different grit on each face of the plate. Their construction is robust and requires little to no maintenance over their lifetime. The hard diamonds last a very long time before the plates need to be replaced. They can be lubricated with either water, oil, or lapping fluid, and can be easily cleaned. Diamond plates are commonly used to flatten the softer style of water stones. Premium diamond plates are amongst the most durable and easy to use sharpening tools, though can be relatively expensive.
Water stones
Water stones are synthetic abrasive stones that are made from abrasive compounds pressed together with adhesive to form solid blocks. Aluminium oxide and silicon carbide are common abrasive materials that are used to form these stones. As is the case with diamond stones, some styles feature dual sided grits. The stones are lubricated with water and must be soaked prior to use to fully saturate the stone for the most effective use. The nature of the stones means they are relatively soft, and repeated sharpening can cause the faces to hollow and deform. As such, they must be regularly maintained and flattened with diamond stones. These stones are renowned for being among the most effective sharpening methods but require the most maintenance to use.
Oil stones
Oil stones are sharpening stones that have been used in traditional tool sharpening for millennia. Modern oil stones can be made of synthetic abrasives in the same manner as water stones, though traditional oil stones are cut from natural abrasive rock. As the name suggests, these stones are ideally lubricated with oil, though water can be used to float the shavings away.
Leather strops
A strop is a strip of leather that is used for honing the already sharpened edge of a cutting tool. The leather is naturally a fine abrasive and can be additionally “loaded” with a very fine abrasive buffing compound to aid in the honing process. The strop can be adhered to a solid flat surface in order to apply adequate pressure to the cutting edge of the tool. Strops are often used during end stages of the sharpening process to hone the edge, bringing the material to a mirror finish that is razor sharp.
Lapping fluid
Lapping fluid is a specialty lubricant that is intended for use with diamond sharpening stones. The fluid is designed to float away the particles during the sharpening process to prevent the abrasives from being clogged up. Whilst it does work as intended, water and oil do just as good a job, for next to no cost.
Buffing compound
Buffing compound is an abrasive paste/wax that is used in combination with leather strops of buffing wheels to carry out the final stages of a meticulous sharpening routine. Whilst not strictly necessary, the very fine abrasive, typically upwards of 10000 grit, can help to bring a mirror finish to the cutting edge. The compound most often comes in a wax bar form that can be rubbed on the leather or buffing wheel to “load” the honing material. There are many varieties of buffing compound, each with specific intended uses, and typically being colour coded. Examples of this include the all-purpose chromium oxide green, or the far finer red coloured “jewellers rouge”, used for polishing precious metals.
Honing guide
Honing guides are sharpening jigs that are used to aid in the sharpening process. When sharpening plane irons and chisels, specific angles are typically desired for the cutting bevels. Skilled sharpeners are able to maintain these angles freehand, though this can be tricky for beginners. A honing guide is a tool that holds the blade in place at a specific angle in relation to the face of the abrasive, steadying the blade during the sharpening process. This stabilisation also helps to keep the cutting edge in full contact with the sharpening equipment during the process.
How to sharpen
Here we will look at the exact procedure for sharpening the most common selection of sharpenable tools. When tools are bought from the manufacturer or from a previous home, they often require an initial setup to bring them into working order. This process is longer and more meticulous than intermittent maintenance, to ensure that the tool is performing correctly. The process can take a while if the cutting edges are damaged, or if the initial quality of the tool is poor. Once the initial setup is completed, the cutting edges require less intense, regular sharpening to maintain the edge. If the edges are damaged during use, another more rigorous tuning may be required.
Chisels and plane irons
Regular bench chisels and plane irons feature a flat underside with a parallel or slightly tapered upper face. The cutting edge is created by establishing a bevel from the top face down to the bottom of the blade, at a specific angle depending on the criteria. The point at which these two flat planes meet creates the cutting edge. To establish a good cutting edge as part of the setup of a blade, the first step is to ensure that the back of the iron is actually flat. For all intents and purposes, plane irons and chisels are sharpened in the same fashion.
To begin we must flatten the back of an iron so that the last inch or so of the blade makes full contact with a flat sharpening stone. In order to achieve this, we work our way through the grits incrementally on an abrasive stone. Flattening the entire back face of the iron is redundant, as we only need the very edge to make full contact the stone. A quality iron should be relatively flat out of the box, though cheaper tools may be warped or distorted.
Having lubricated the low grit stone appropriately, we can begin to work the back face of the tool on the surface, ensuring even downwards pressure is applied. The face should be moved along the full length of the stone to ensure even wear of the abrasive - prolonging its effective life. After only a few passes we’ll be able to identify where the iron is contacting the abrasive by assessing the scratch patterns just created. The areas that are shiny with deep visible scratches are where the face of the tool is being abraded. For those unfamiliar with sharpening, a marker can be used to colour in the face of the tool to aid in the process. As the tool is worked the marker is erased, helping to more easily identify the points of abrasion. Ideally, we’re looking for either full contact with the stone along the first inch of the tool, or a slight hollow in the underside face. The hollow can be identified by a lack of scratches in the centre of the face, but full contact around the hollow. In both of these scenarios, the cutting edge is making full contact with the abrasive stone. In a less ideal scenario, the scratches will only appear along part of the cutting edge, or even worse still, an area within the last inch with no contact along the cutting edge. In these scenarios, the face is uneven or possesses an outwards belly. This means that all of the excess material must be ground away until the cutting edge is making full contact with the stone. This issue can be rectified but can take a while and a lot of labour to fix by hand. An electric sharpening tool can make the process quicker. Alternatively, if there is a lot of material to be removed, a low grit of sandpaper on a flat surface can be used to remove material more quickly.
The face must be worked until there is an even scratch pattern across the full cutting edge of the iron. As the front edge of the blade makes full contact with the stone, a burr will begin to form on the cutting bevel of the tool. This is excess metal that is being forced upwards away from the abrasive grinding of the underside face. This burr is sharp and can cut skin if touched incorrectly. We can safely feel if a burr has developed by dragging our thumb over the cutting edge. If a burr is present, we’ll be able to feel it. We can address this burr once the back is fully flattened.
Once the back is flattened appropriately, we can begin to work our way up the grits to bring the back face to a fine polish. The process of flattening is repeated at incrementally higher grits, working on each grit until the more coarse scratches from the previous round are removed by the more fine scratches of the higher grit. This process is typically repeated 3-4 times. Starting with an out of the box iron, the grit stages might look as follows: 80 grit sandpaper, 300 grit stone, 600 grit stone, 1000 grit stone. Depending on the intended application for the tool, the faces may be sharpened to a higher or lower grit than these recommendations, with more or less intermediate sharpening rounds.
Once a high grit is reached and the scratch pattern is very fine, we can move onto refining the bevel of the iron. Most cutting tools of this nature are going to feature a cutting bevel of around 25 degrees - this is a comfortable compromise of the features of tool steel. The bevel is measured in degrees between the flat surface of the tool and the cutting bevel. This applies to chisels, plane irons, and hatchet and axe blades also, though the latter two don't feature a singular flat surface. Regardless, the shallower the angle in degrees, the finer the cutting edge will be, in other words the sharper it is. You could sharpen at an incredibly acute angle, such as 10 degrees to produce a razor-sharp edge. The trouble is, the shallower the bevel, the less steel is behind the cutting edge to reinforce it. Very shallow bevels are weak, and can chip, snap, or even fold over when working with harder timbers. 25 degrees is a good compromise between the sharpness of the edge, and the reinforcing material present behind it. A paring chisel that is being used in soft wood with hand pressure can be sharpened to a finer point such as at 20 degrees, whereas a mortise chisel that is going to be beating into hardwood may be sharpened at a steeper more reinforced 30 degrees. Experienced woodworkers will experiment with the angle that suits their style of work. That being said, 25 degrees is a good baseline to start from.
Most tools are going to come with a bevel ground to 25 degrees, and so we can follow this existing angle when sharpening our cutting edge. Alternatively, if the chisel is in bad shape or not at the desired angle, a honing guide can be used to establish the desired angle. Using a honing guide is simple, as it effectively secures the blade at a specific angle in relation to the abrasive stone. A roller on the guide rolls across the surface of the to grind bevel at a consistent angle. Sharpening the bevel free hand is also possible, though requires a little bit of skill and experience to maintain the correct angle whilst sharpening.
Whether sharpening with a guide or by hand, the bevel is ground at the same incremental grit stages as the back face of the iron. Again, we will be able to identify the scratch patterns of the first few passes on the low grit stone or paper to assess if the cutting bevel is making correct contact with the abrasive. To establish a sharp cutting edge, we need to ensure that the bevel edge is also making full contact with the abrasive. Once the bevel is flattened, we can work our way up the grits in the same fashion as before. Once this is completed, we should be left with two flat and relatively polished faces, as well as a burr that has now developed on the flat underside of the iron.
If we’re establishing a micro bevel on the cutting bevel face, then we can do this now. A micro bevel is a secondary, slightly steeper bevel that is established using a few passes across the abrasive surface. This can aid in establishing the fine cutting edge and also makes future sharpening easier. Creating the micro bevel can create a burr on the back edge, and so we should do this now before addressing the existing burr on the back face.
To remove the burr on the back face of the iron, simply make a few passes of the back face on the high grit stone to grind it away, without removing enough material to create a new burr on the cutting bevel. Alternatively, this fine wispy burr can be removed by dragging the bevel through an offcut of soft timber.
Now that the cutting edge is established and the faces are well addressed, we can finish the chisel on a strop. We can load the strop with abrasive paste by rubbing it across the leather. The faces can then be pulled hard against the strop at their respective angles to abrade any remaining scratches. 30-40 hard pulls on each face are sufficient here and will bring the faces to a near mirror finish. This will polish the faces of the iron and increase the sharpness of the cutting edge. If all went well, the chisel will be perfectly sharp, enough to shave hair. As the tool is used, the fibres of the wood will slowly dull the cutting edge. Once this dulling effect is noticeable in the surface finish of the work or the feel of the tool, it's time to sharpen again. This sharpening is much less intensive than before. Around 10 seconds on each face of each grit is sufficient to resharpen an established but dull edge, with another 20-30 pulls on the strop. If the technique is correct, this light maintenance will have the iron in cutting order again.
Saws
Modern construction hand saws feature hardened teeth that are not able to be effectively sharpened. These saws are incredibly sharp out of the box and can be used effectively to cut across or to rip along the grain of softer construction related timber components. The teeth of these saws are configured in a “universal” style, being able to perform most cutting tasks suitably. As with any sharp tool, these saws dull over time and must be replaced relatively often to ensure maintained precision in our work. These modern construction saws are an adaptation of the more traditional style of panel saws. Panel saws are still favoured over disposable saws in workshop and joinery applications, due to the desirable variety of tooth configurations, as well as the ability to resharpen the non-hardened teeth. As these saws are intended to be maintained and resharpened intermittently, it's typical that the steel of the blade and handle ergonomics are higher quality than disposable saws.
Traditional panel saws are sold in and sharpened to a variety of different tooth configurations to suit a range of tasks. In broad strokes, the two main styles of tooth configuration are “rip cut” and “crosscut” styles. The “profile” of each tooth varies between the two styles, based on the criteria for cutting. The rip cut profile features a more upright tooth style, ranging anywhere from leaning a few degrees forward to a few degrees backwards. These relatively vertical teeth then taper backwards into a gulley that meets the next vertical tooth. The cutting faces of these teeth are sharp bevels, effectively forming many small chisel edges that shave away at the fibres during a rip cut. On smaller rip cut saws like gents or tenon saws, the rip cut pattern will feature little to no “set”. Set describes the amount by which, if at all, the teeth are purposely pushed outwards away from the blade of the saw. For larger panel saws, the set is increased to create a saw kerf that is wider than the thickness of the saw blade - to prevent it from binding in the cut. Typically speaking, the set of a rip cut saw is going to be less than the more aggressive set of a cross cut saw. The set of a saw is created through the use of a saw setting tool - a purpose-built pinching tool that bends the teeth evenly away from the blade of the saw.
The crosscut style of tooth set and profile varies from the rip cut style to achieve the desired outcome when cutting across the grain. Where the rip cut saw chisels away along the fibres during the cut, the crosscut saw must be configured to cleanly sever the fibres as it cuts across the grain. To achieve this, the tooth profile more closely resembles sharpened knife edges. The front edge of each tooth is less upright than a rip cut pattern, with a steeper backwards fall towards each gulley also. This brings each tooth to a more central triangular point. Each tooth is then given another bevel on its front face, alternating between teeth in relation to the set of the saw. This creates a sharp, front facing edge on each tooth that comes to a fine point. These teeth sever the fibres on the outside of the set on each side of the blade, creating a “V” shaped profile in the cut.
There are many variants of these styles, and alternative set patterns, though these are the most prominent and basic applications. Now that we understand the differences and characteristics of the two styles, here is a brief section on how to sharpen them.
To sharpen a rip cut saw - Use a triangular saw file to follow the gulley profile between each tooth. Saw files are sold in varying profiles, with the correct file matching the gulley profile of the saw. Depending on the size of the saw and the size of the teeth, 1-3 even passes should be made in each gulley, to sharpen the front face of each tooth up to the point. The file should be kept perpendicular in both planes to the main blade of the saw. With each gulley attended to, every tooth should be sharp again. If the saw is still not performing up to standard, another pass or two can be made on each tooth until the saw is cutting well.
To sharpen crosscut saws - If the teeth are dull but in otherwise good condition, they need only to be filed on the angle in relation to the bevel of the teeth. If the teeth are damaged or irregularly shaped, the profile of the teeth may need to be attended to first, in the same way the teeth of the rip cut saw are sharpened. Once the teeth are the correct side profile, we can begin to sharpen the bevel of each tooth. As the teeth alternate their bevel based on the set of the teeth, we will sharpen every other tooth in the same direction, before changing our angle of approach and addressing all the other teeth we missed from the other direction. To begin, the file should be placed in the first correct gulley and lined up to the appropriate angle for the bevel of the teeth. 1-2 even passes are sufficient for each gulley. In doing this, we are sharpening the front edge of one tooth, and the back edge of the tooth in front. We then skip a gulley and repeat the process. Once we reach the end of the saw, we can flip the saw or change our stance and file each remaining gulley in the opposite direction to complete the sharpening process.
If the saw is very old and damaged, with rounded teeth and irregular profiles, we can restore the saw with relative ease following these steps. Using a long flat file, we should flatten the tops of the teeth in one flat plane until the damaged and rounded areas are removed. We then use the file perpendicular to the blade to deepen each gulley and re-establish the points for each tooth. We can then check the set of each tooth, and re-file the tooth edges if the saw is a crosscut pattern.
Hatchets and axes
Axes, and more typically hatchets in the scope of site carpentry, are sharp tools that are used for the rough shaping of timber. These tools feature a sharp edge, with a large amount of steel behind the bevel to resist against the strong percussive forces associated with heavy chopping. Whilst a shallow bevel is more conducive to cutting timber, the rough use of a hatchet or axe would soon roll or damage such a fine edge. As such, the thick metal of the axe head tapers in a steeper angle towards the cutting edge, as much as 30 degrees in some instances, in a flat or curved fashion. To sharpen a hatchet or axe, material can be removed using a file to remove any damage and quickly establish the cutting edge. As a burr is developed from the use of a file on each side of the axe, stones of varying grits can be used to refine the edge. This process is more of less the same as sharpening a plane iron or chisel. Once the high grit stone is finished with, the edge can be honed with a strop.
Whilst this list of specific sharpenable tools is not exhaustive, understanding the principles behind these three common examples fill us with the knowledge to sharpen other less common tools.