How to Sharpen Chisels and Plane Blades | Dubbeld How To
Introduction
I reckon one of the main reasons a lot of people give up on woodworking is because their experience is based on using tools that aren’t sharp.
The coolest thing in woodworking may be using a truly sharp tool – that feeling of empowerment you get when all of a sudden you’ve been given a supernatural power to steer the tool almost effortlessly through the wood is intoxicating… no dust, just shavings and the song of steel on wood.
On the opposite side, working with a dull tool robs the fun from woodworking and can be quite discouraging, especially for those new to the craft. The results are inevitably messy, frustrating, tiring and potentially dangerous; and who wants that?
I don’t particularly enjoy sharpening but I do enjoy using sharp tools so my aim for this topic on sharpening is to explain as best I can the methods I use to sharpen tools effectively and quickly.
– Joel Dubbeld
Contents:
- Why is it necessary to sharpen?
- Chisel and Plane Geometry
- Grinding Machines
- Sharpening Stones
- The Process of Sharpening: Chisels and Plane Irons
1. Why is it necessary to sharpen?
A cutting tool is essentially a hard wedge that is used to cut a softer material (in our case, wood).
The cutting edge on that wedge will wear, though as it is pushed through abrasive minerals contained within the wood. As the edge blunts, surface area is increased, generating more resistance and compressing wood fibres. The result is typically a torn, rough cut and a greater risk of injury from the increased force required to make cuts.
So consequently, sharpening is required to maintain the tool’s cutting efficiency. The frequency of this sharpening is dependent on the material being cut and the formulation of cutter being used. Edge retention and durability therefore become most important factors when choosing which type of tool and the qualities of the tool steel that is most suited to the task at hand.
When purchasing hand tools with the intent of using them often, it is important to be aware of the differences in tool steels. Do your research on reputable makers and get the best quality for what you can afford.
A note on tool steel:
Tool steel is most commonly used for woodworking blades because it generally has that balance of toughness and hardness, which give it the ability to retain a sharp, durable edge.
My simple understanding of the types of steel we use for tooling in woodworking tells me that tool steel is basically a combination of iron and carbon with other elements (alloys) added to increase its toughness and hardness. The difficulty is to make a blade both hard and tough; as you add hardness (to retain an edge longer), you make the blade less tough (more brittle less resistant to shock and impact). Carbon is the most important hardening element but the higher the carbon content the more the tool reduces in toughness.
After forging and shaping, tool steels are hardened and then tempered to achieve a more durable edge. During the hardening process, at high temperature (a point somewhere around red hot) the carbon dissolves and forms iron carbide (not tungsten carbide, which is a different material). If the steel is allowed to cool slowly from this point, it will become annealed (soft). However, if the steel is quenched or cooled quickly at this point, the carbide will ‘freeze’ and the steel will become hard.
Tempering, a heat treatment usually performed after hardening, reduces some of the hardness and increases the steels toughness. The level and temperature to which this is achieved, is dependent on the mix of alloys and the desired performance of the finished product. For example, a file is generally not tempered at all to retain maximum hardness but at a chisel would fracture quickly if it didn’t have some malleability and springiness (toughness) so it is tempered accordingly.
The hardness of steel is generally determined by measuring its resistance to deformation. You will notice in many tool descriptions the symbol ‘Rc’ with a number following. This relates to the hardness of the tool as tested on the Rockwell C Hardness Scale.
It is important to be aware of these degrees of hardness because different tools are required for different tasks.
Handsaws will most commonly rate softer at Rc38-42 than Chisels or Plane Irons at Rc58-62. A chisel at Rc63 will retain an edge a lot longer than one at Rc58 but will chip much more easily.
This scale is only one factor in the ability of a tool to hold a good edge. Just as important are the steel alloys being used and the tempering process itself.
Very hard tools are brittle and are generally more difficult to sharpen, often requiring specialised equipment. Softer tools are more flexible and will sharpen more quickly and easily.
Carbide (most commonly tungsten carbide in woodworking) although not steel, is a very hard compound made by heating one or more heavy metals and carbon at very high temperatures. It is most commonly used in power driven tooling such as circular saw blades, router bits and shaper cutters. It is generally not suitable to be used for hand tools, as it is too brittle to hold angles less than around 45 degrees. It will, however, remain sharp 10-20 times longer than steel tools used under the same conditions.
2. Chisel and Plane Geometry
Sharpness and acute angles are not necessarily the same thing. Sharpness in tools generally refers to the condition of a tools edge and not to its angle.
Let’s put it into perspective; take an axe for example. An axe used for splitting firewood is not sharpened the same as one used for chopping down a tree. Both need to be sharp but the edge geometry is different for them to be effective in their function.
The same goes for a chisel; one used for hogging out house frame mortises in Aussie hardwood is not sharpened in the same way as one used for carving fine detail in a soapy wood like Tarzali Silkwood.
In both cases, the more acute the angle of the edge, the less resistance it has but the more acute that edge becomes, the more fragile it also becomes. So, edge angles (or bevels) are determined by what the tool is being used for.
By polishing the 2 intersecting faces of this edge we can achieve a keener and more durable edge that slices more easily through the wood fibres, leaving a smooth, even surface.
Ideal tool angle is dependent on the tool and what it is being used for. Chisels pushed by hand can be sharpened to quite acute angles because they will experience less force than ones driven by mallets. For the same reasons, power driven tools will have higher or more obtuse angles.
Typical primary bevel angles for chisels
Looking from face on, chisels are generally ground square to their edges.
Plane blades can be ground differently depending on their use. Shoulder, Rebating and Jointing plane irons should ground square, whilst for surface work they are intentionally ground with a slight curve. The curve helps prevent the corners of the blade catching when planing flat surfaces and also reduces the force required to push the plane when making deep cuts when roughing.
Shapes of Bench Chisels and Plane Irons
3. Grinding Machines
The tool angle is generally accomplished by grinding; either by convex grinding, hollow grinding (concave) or flat grinding.
Types of Grinds
The high angle of convex ground tools, such as axes, makes the cutting edge very durable but requires a lot of force to cut. Carbide tips on router bits are sometimes convex ground to help support the brittle tungsten cutting edge.
Hollow ground (achieved by using the face of a vertical wheel grinder) and flat ground tools (achieved by hand with bench stones or by using powered grinders such as abrasive belt machines, horizontal wheel grinders or the edge of a vertical wheel grinder such as a Tormek diamond wheel) are easier to grind and therefore more common than convex ground tools.
The speed of power grinders makes this option the most popular. Of these, wheel grinders are the most common in the form of dry grinders and wet grinders.
Dry Grinders are relatively inexpensive to buy, making them a popular choice.
A typical grinder, intended for more general grinding, runs at approx. 3500rpm and most commonly comes with hard, slow cutting medium-fine and medium-coarse wheels. These wheels are not ideal for tool grinding because their hardness, combined with the high cutting speed, makes for high frictional heat, which can easily overheat and burn tool steel. Replacing these wheels with softer aluminium oxide 60-100grit white wheels will reduce the chance of overheating, as these wheels will cut more rapidly. However, they will wear more quickly. Be careful with the airborne dust from these grinders – inhalation of the combination of aluminium oxide and steel will not increase your life span. USE A DUST MASK and SAFETY GLASSES!
It is also important to quench a blade after every few runs across the grinding wheel to keep it from overheating.
There are more specialised dry grinders available, which run at lower speeds, but these are also more expensive.
Wet Grinders are an excellent choice for tool sharpening. Their grinding wheels pass through a bath of water, which acts as a coolant and keeps the wheel clean by removing grinding swarf. There is also no airborne dust, so no need to wear a mask.
These types of grinders generally run at slow speeds and remove material more slowly than dry grinders, however, once primary angles are ground, there is little speed loss if jigs are present to aide in repeatability.
Grinding wheels, as with all cutting tools, need regular maintenance to keep them sharp, clean and balanced.
Dressing is the process of cleaning the grinding surface by removing metal or foreign particles and dull abrasive grains. In this worn or blunt condition, the wheel becomes glazed and creates excessive heat through friction. Dressing also trues and flattens out a wheel, removing grooves and irregularities.
Various different types of wheel dressing tools are available to achieve this and all work on the same basic sharpening principle; using a harder material to wear away a softer one.
Wheel Dressing Tools:
Atoma Diamond Sharpening Stone 600 grit
4. Sharpening Stones
Sharpening Stones are probably the most important items used in hand sharpening. The materials that make up these stones are either natural or synthetic and they come in a variety of different shapes, sizes and grits. Stones are mostly used with light oil or water, which keep the stones clean by carrying away the swarf. Excellent results can be achieved with all of the following sharpening stones:
Coated abrasives (sandpapers) are inexpensive and probably under-utilised for sharpening tools. They use aluminium oxide and silicon carbide abrasive in a whole range of grit levels from 40grit right through to 4000grit. The sheets are adhered to substrates such as float glass or machined granite and used as flat stones or wrapped around other shapes to make slipstones. Silicon carbide wet & dry papers can be used with water to carry away swarf. Most tools can be sharpened with these abrasives, except carbide.
Advantages:
- Inexpensive
Disadvantages:
- Wear quickly
Sandpaper Sheets (Wet & Dry)
Abrasive Film is a relatively new product to the sharpening market. Formulated for industrial use, it uses fast cutting microcrystalline diamond, silicon carbide or chromium oxide particles applied to a film backing. This self adhesive film is applied to substrates such as float glass for flat sharpening or to various shapes for profile sharpening.
Light oil such as camellia oil is best for carry away swarf.
Most tools can be sharpened using these abrasives, carbide requiring diamond film.
Grading:
- 15 microns for fast cutting
- 3 microns for honing
- 0.5 and 0.1 for polishing
Advantages:
- extremely fast cutting
- extremely keen edge produced
- conforms to any shape
- portability
Disadvantages:
- relatively expensive
- bubbles can potentially form between film and substrate
Arkansas Stones are a natural occurring novaculite (a type of flint) quarried in Arkansas, USA. They are graded as hard or soft, soft stones being more porous a faster cutting. Hard stones cut more slowly and give more polished, keener edges.
Oil is used as to carry away swarf. Most tools can be sharpened with Arkansas stones except carbide.
Grading:
- Washita (P400-600) white-purple, white-orange
- Soft Arkansas (P600-800) white, grey black, green, pink or combination
- Hard Arkansas (P800-1000) black, grey, white and red or combination
- True Hard Arkansas (P1200+) black, grey, white, red or combination
- Translucent Arkansas (P1200+) translucent grey, t yellow, t brown, t pink
- Black Hard Arkansas (P1200+) black, blue-black
Advantages:
- Long wearing
- Extremely keen edge produced on fine stones
Disadvantages:
- Cuts slowly
- Oil attracts dust
- Oil may contaminate wood surfaces
- Difficult to re-flatten after dishing
Synthetic Oilstones are man made stones and are available as 2 types: Aluminium Oxide (India) and Silicon Carbide (Crystolon), bound in resin and made by Norton. They are oil filled by the manufacturer. Most tools can be sharpened with Synthetic oilstones except carbide.
Grading:
- Coarse India (P100) Brown or Tan
- Coarse Crystolon (P100) Grey or Black
- Medium Crystolon (P180) Grey or Black
- Medium India (P240) Brown or Tan
- Fine India (P280) Brown or Tan
- Fine Crystolon (P280) Grey or Black
Advantages:
- Inexpensive
- Hard wearing
Disadvantages:
- Do not produce a particularly keen edge because of the limited grits available
- Oil attracts dust
- Oil may contaminate wood surfaces
Waterstones, Whetstones or Japanese Waterstones are either natural or man-made. Man-made stones are made up of aluminium oxide or silicon carbide particles suspended in a binder, such as clay or resin. Generally, coarser stones are quite porous and must be soaked in water prior to use. Fine stones and natural stones need no preparation and only require a sprinkling of water. Water flushes away the swarf and also the worn abrasive grains, exposing new sharp edges and resulting in a fast cutting sharpening process. However, because waterstones cut faster, they also wear faster and need to be flattened frequently to prevent dishing. This is easily done though, most commonly with a diamond plate such as an Atoma Diamond Sharpening Stone 600 grit or with a sheet of P240 wet & dry sandpaper adhered to a piece of granite or float glass. Most tools can be sharpened with waterstones (my current favourite is Shapton) except tungsten carbide. Nagura (meaning ‘correcting’) stones can be used to help flatten and form slurry on the surface of fine waterstones. The slurry helps relieve the suction that develops between the tool and the surface of the stone.
Grading:
- 250 extra course (P180) colour varies
- 800 coarse (P400)
- 1000 medium coarse (P500)
- 2000 medium (P600)
- 5000 fine (P900)
- 6000 extra fine (P1000)
- 8000 ultra fine (P1200)
- 12000 ultra fine (P1200+)
Advantages:
- Fast cutting
- Large grit range
- Extremely keen edge produced
Disadvantages:
- Wear quickly but easily flattened
Shapton Japan Professional Ceramic 12000 grit whetstone (cream)
Ceramic Stones are made of aluminium oxide particles and baked at 1650°C under pressure in a ceramic bonding agent. They can be used to sharpen tungsten carbide. No water is necessary to carry away swarf but they tend to load quickly and require scrubbing with detergent and a fibreglass scourer under water.
Grading:
- Medium (P600) Grey
- Fine (P1000) White
- Ultra Fine (P1200) White
Advantages:
- Extremely long wearing
- Extremely keen edge produced
- Can be used to sharpen Carbide tools
Disadvantages:
- None to note
Diamond Stones, called plates are made of microcrystalline diamond crystals bonded to perforated or solid steel or aluminium flat plates. Because of this, the stones never dish out and remain flat. Diamond, being the hardest of any known material will sharpen all tools and can be used to flatten all other stones. Water is not a requirement but a few drops can be beneficial in removing the swarf.
i have found that you get what you pay for with diamond plates. High quality Japanese Atoma Diamond Sharpening Stones will not release their diamond as compared with cheap alternatives.
One thing to note though is the initial keenness of the diamond will become less aggressive over time as the sharp pints of the diamond crystal break away.
Grading:
- Coarse (P240) Silver Grey
- Medium (P320)
- Fine (P600)
- Extra Fine (P1200)
Advantages:
- Extremely long wearing
- Can be used to sharpen carbide tools
Disadvantages:
- None to note
Tormek DC-250 Diamond Wheel Coarse
Files are designed and used to sharpen many woodworking tools. Conventional hardened steel files as described in the note on tool steel are hardened to a higher degree than wood cutting tools, giving them the ability to cut the worn edges of the softer tool steel of tools like hand saws and drill bits. Files made for sharpening are generally single cut and they may also have safe edges or faces (without teeth). This becomes useful when sharpening cutting edges without changing the shape of the adjacent surfaces (e.g. spade bit). Diamond and ceramic files are used to sharpen harder materials including carbide
Strops are porous surfaces such as leather, paper or wood used with, or without, very fine abrasives to polish cutting edges. The motion used to stropping is to draw the blade away from the cutting edge and hold it at the exact bevel angle that was first established. Careful stropping will keep an edge tool cutting well for a long time but eventually stropping will create an obtuse cutting angle (dubbing) and the tool will need to be reground.
Kangaroo Tail Leather Strops are naturally abrasive and don not necessarily require additional abrasive compounds. They are regarded as the best of the leather strops.
Abrasives used in stopping include:
- Silicon carbide powders such as valve grinding compound
- Natural buffing compounds such as jewellers rouge, tripoli and emery
- Synthetic compounds such as chromium oxide
- Diamond paste
5. The Process of Sharpening
I believe the process of sharpening should be quick and in normal conditions be achievable in under 2 minutes but some preparation needs to be done to achieve this by Grinding the bevel angle and Honing the backs of chisels and plane irons dead flat.
The Grinding Process:
In the process of sharpening, we generally use the grinding process to establish the desired tool angle, or primary bevel. Use whatever your preferred type of coarse stone or powered grinder but remember to be very careful to not to overheat and burn (or blue) the edge. The tool must not be overheated during the grinding process. If the edge gets too hot and discolours, all of this metal must be ground away.
When using a dry grinder, take sure the grinding stone is dressed and kept clean, use a light touch and quench the tool in water after every few strokes across the grinding stone. It can be helpful to hold a finger on the back of the tool close to the grinding wheel to test the heat being generated and determine how often quenching is required.
If an edge has been burnt or a tool is dropped or chipped, the shape of the edge will need to be re-ground. I have found the easiest way to grind the correct shape is to first grind the cutting edge, perpendicular to its back and to the desired angle or shape and then grind the bevel until it meets that edge.
After the tool is ground, it is very important that there is a burr evident, which can be felt on the back of the tool.
After the grinding process, which gives the tool its proper shape and primary bevel angle, to make a tool truly sharp it becomes necessary to hone the cutting edge.
Honing is the process of using progressively finer abrasives to make the cutting edge as smooth as possible. In this process, the abrasives cut small grooves into both the leading and trailing faces. At the cutting edge, where these 2 faces meet, as the grit becomes finer, the edge becomes smoother. On a microscopic level, the closer this edge comes to a single straight line, the keener the edge will be and the better the edge retantion.
Although not necessary, a microbevel can be beneficial.
A microbevel, or secondary bevel, is a tiny bevel at the point of the tool that reinforces the cutting edge, making it more durable. Because the microbevel is so small, it only slightly increases the force required to make a cut. Probably the greatest benefit though of creating a microbevel is that we don’t have to hone the entire face of the bevel. This reduces the amount of metal that needs to be removed and can speed up the sharpening process.
Recommended tool angle and microbevel (chisels and plane irons) for general purpose woodwork
Recommended kitchen knife microbevel
Most Chisels and Plane irons have a bevel and a flat back, which makes them some of the easiest tools to sharpen.
The tool, especially a bench chisel, must have a perfectly flat back. Once the back is properly flattened, honed and polished, it should not be necessary to hone it again for the life of the tool, except to remove burrs by rubbing it lightly against a fine stone.
Terms associated with sharpening chisels and other associated single bevel tools, including plane irons
The Honing Process:
The degree of sharpness that you can achieve is directly related to how smoothly both the bevel and the back (or in knives, both bevels) can be polished.
To flatten the back of a bench chisel or plane iron, start with a fairly coarse sharpening stone (P320), making sure the stone is completely flat. Dress the stone as necessary. If using a waterstone, this is probably every couple of minutes. If using Abrasive Paper, ensure it is well stuck to a flat surface or you will roll the edges.
Never use abrasive belt sanders on the back of a chisel or plane iron – it will roll the edges. The side of a Tormek Diamond Wheel is a very effective tool for back flattening , especially in a coarse grit, as it will remain flat and can speed up the flattening process quite a bit.
Press the tool against the stone, covering as much stone as possible and rub back and forth. Keep checking the scratch pattern and as the surface, especially near the cutting edge, becomes a consistent dull colour, move to a finer grit. Repeat this process until you achieve a consistent mirror finish on an extra fine stone (P1200+). Again, make sure each stone is dressed and completely flat.
Depending on the initial flatness of the back of the tool, this process can take some time, even with new high quality chisels or plane irons but this should only ever need to be done once.
Japanese tools are typically hollow ground. This makes it possible to quickly flatten the back nearest the cutting edge.
One of the most common mistakes is to start out with an abrasive that is too fine. Most of the flattening work should be done with the coarse stone. Each subsequent grit level will take out the deeper scratches of the previous abrasive.
To form the microbevel, rest the primary bevel at the rear of a fine stone (say 2000 grit) so that the cutting edge and heel is in full contact and doesn’t rock. Raise the tool (about 2-5° for a chisel or plane iron), lock arms and draw the tool back. Repeat this for a few strokes until the microbevel is an even colour with no gaps. If any nicks are visible at this stage, re-grind, then re-hone. Repeat this process with finer stones, say 5000 grit and 12000 grit to polish the bevel to a mirror finish. If using waterstones, ensure they are regularly lapped to keep them flat.
Alternatively, a honing guide can be set up and used. This can be helpful to prevent rocking the microbevel and forming a convex shape.
When honing is complete, the burr that forms and curls up toward the back of the blade should have fallen away, provided a very fine stone has been used.
Follow up the honing process by stropping to achieve a crazy sharp edge. Remember, draw strokes only, and only strop the bevel, not the back.
Test the sharpness by cutting paper or paring thin slices from the end grain of softwood (the softer the wood, the sharper the tool needs to be). Any traces of the burr or fine nicks in the blade will leave lines in the cut surface.
The best way to maintain an edge is to polish or strop it a little, more often, than to sharpen it a lot, less often.