Nic Westermann demystifies sharpening carving knives and chisels
A few years back the (now ex) MP Ann Widdecombe appeared on a TV series having lost a large amount of weight. Sensing blood or money in the water, publishing houses circled trying to persuade her to write a book on her dieting secrets. In typical style she said that it would be pointless as the book would contain only two instructions: ‘Eat less, exercise more.’
Sharpening is the same. There is a massive amount of seemingly conflicting literature on the subject but the basic principles are very simple and can be distilled to just two, very simple points.
Firstly, you need to cut the bevels to the right basic shape, the form of these bevels being dependent on the tool. This is best done using a coarse grit, especially if reshaping of the bevel form is needed. Secondly, you need to refine the finish of these bevels to produce two smooth planes meeting, producing the final cutting edge.
That’s it; it really is that simple, but like dieting it can also be hard to accomplish these tasks. In the following articles in this series we will be looking at specific tools and applying these two rules to sharpen them efficiently; but first, let’s look at these two points a little closer.
Types of Bevel
The form – what shape should the bevel take?
There are many choices, but the most common symmetrical forms are shown in the drawings, Fig 1, below.
Understanding how and why a blade cuts also helps you decide what form is most appropriate, so here is a little bit of cutting theory:
- When a hand plane cuts, the blade is held at the correct angle by the sole gliding over the wood, so the bevel does not need to engage and clearance can be left. See drawing, Fig 2, below.
- A carving tool does not have the luxury of this built-in support. Knives especially derive support from the bevel engaging and guiding the cut, thus the bevel must match, or more accurately mirror the cut being taken. See drawings, Fig 3, below.
- So, we see that bevels can be flat or convex to match the cut. They can even be hollow ground (concave); however, these are rarely truly hollow, but allow sharpening to be achieved much more easily. I actually view this as a type of inbuilt jig. See drawing, Fig 4, below.
Making sharpening easier and more efficient can only be a good thing. Few people, if any, look forward to sharpening. As long as it is done well, our view is that it should be achieved as quickly and easily as possible. Carvers hone their skills by spending time carving, not sharpening.
Once we know the basic form of the bevel, the included angle needs to be looked at. There is no point spending ages achieving a hair-splitting edge ground to 20° on a chisel that will be driven into centuries-old oak burr. The included angle of the bevel needs to match the wood being cut and the force that will be used on it. A knife used to gently undercut primary feathers on a bird’s wing will be better served with a more acute bevel angle than a knife used for roughing out the shape even on the same piece of wood. Generally though, 20–30° will cover the vast majority of your carving needs.
There is also the issue of the quality of the steel the edge is made from. Even if the angles are technically correct, if the edge keeps rolling or chipping, the only option is to increase the edge angle. This can be done by putting a micro or secondary bevel on the edge in some circumstances.
Refining the Edge
Once we have these bevels set to the correct form the edge will, in close-up, look like a saw tooth and those tips will quickly blunt. A finer finish will give more TPI to our saw and will cut for longer.
To work down through the grits you only need to remove steel to the depth of the scratches lef t by the last grit – a tiny amount of steel as long as you are replicating the same angles. If you are cutting subtly new bevel angles with each progressively finer grit, it can take ages. This is why jigs to help maintain constant angles help so much. Without this, you will of ten end up with a bevel consisting of multiple facets and scratches left from the coarser grits that you were unable to remove with the finer finishing grits.
The jump down to finer grits can be quite dramatic and is largely dependent on the width of the bevel being cut. A narrow bevel will consist of a small cross-section of steel and this will allow for much larger jumps in grit. It is common to put a very narrow secondary bevel on a plane blade. This allows a jump from 1,000 grit to set the angle to an 8,000 grit water stone to put the final edge on, all achieved in a few seconds.
This is why hollow grinding, secondary bevels and micro bevels are so helpful and are of ten chosen when setting up the bevel form. Micro bevels and secondary bevels also have the effect of increasing the bevel angle. This increases the strength and longevity of an edge, at some cost to clean cutting. In many instances bevel support is lost so use this option with care. Then there is the issue of how far you take this process; how sharp does a tool really need to be?
Time Spent
Hair splitting sharp can be really short lived even if you don’t use the tool, the lightsabre pictured at the top of this article above actually blunted in the air – this blade would not perform the same feat when picked up a week later. The time taken is worth considering. Good carvers look like they have really sharp tools, but it is technique, not perfect edges that count. Spend more time carving rather than sharpening; this how carvers improve.
Jigs are of ten looked down upon, but when appropriate, they will dramatically speed up your sharpening sequence, especially when working down through the grits and are not cheating – it just a sensible use of your time and something we actively encourage.
If you are one still one of those people that stubbornly says they enjoy sharpening, I will ask you to consider this: You have just sharpened your favourite tool, it is razor sharp and ready to go, but just as you prepare for the first cut it slips from your hand, clatters to the floor and is blunted – is this a good thing? For me certainly not, as a parallel I enjoy carving but don’t enjoy going straight through the bottom of he spoon bowl, its not a chance to carve more, just wasted time, use yours wisely!
Hewn & Hone
This article is bought to you by the team at Hewn & Hone. The team comprises Nic Westermann, a blacksmith and creator of carving tools and sharpening accessories, Don Nalezyty, an IT specialist and respected Kolroser and carver, Alex Yerks, an internationally renowned green woodworking teacher and kuksa carver and Adrian Lloyd, a UK-based full time craftsman, teacher and toolmaker.
To learn more visit: www.hewnandhone.co.uk