Popular Mechanics (South Africa)
WOODWORKING: SHARPEN YOUR TOOLS, PART 2
Following on from our introduction to tool sharpening in our previous issue, we now explain primary and secondary bevels, and how to achieve them.
WHETHER it’s on YouTube or elsewhere, the internet is packed full of experts such as Deadly Sharp, Scary Sharp and Magic Sharp and their toolsharpening systems. The truth is that most of them work well because their basic principles remain the same.
I choose to stay far away from sharpening on belt sanders and bench grinders, simply because the inherent danger of overheating the blade and changing the temper of an expensive edge tool is all too real. I prefer to use the ‘cool method’ instead of the hot one, which gives me infinitely more control over the entire process.
STEP 1: GETTING THE BACK MIRROR-FLAT
The first process is to get the back of the blade absolutely flat. Never assume that if the back of the blade looks flat to the naked eye, it is. And don’t take this stage lightly. Pour yourself a cup of good coffee, put on some gentle music and get comfortable, and then lap the back of the blade slowly, methodically and with even pressure.
Perform this part of the process properly on a tool and you will only need to do it once. Without this step, your only guarantee is that you’ll never be able to achieve a perfect arris (refer to our sharpening article in the March issue) and the tool will never perform as intended.
On a bad blade, I recommend starting with 180-grit wet- and dry paper with a little water to create a slurry, then working through the grits – 220, 240, 320, and 400 up to at least 800 – before moving over to your first whetstone.
Next, work through your whetstones, namely 1 000-, 3 000-, 6 000- and 8 000 grit. As you carry out this process, there’s a good (and repeatable) test that you should do using a black Artline 70 permanent marker or a spray can of engineers’ blue. Cover the surface you wish to work on with the ink. Give it a few seconds to dry and then stroke the tool once or twice lightly across the abrasive medium.
Now take a look at the surface. If the ink has completely disappeared, you are good to move on to the next grit level. If not, you need to work the surface on your current grit (repeating the ink test every time you think you’re almost ready to change to a finer grit). After some practise, you’ll be able to dispense with your fibre-tip pen or engineer’s blue, as you’ll be accustomed to seeing the requirements of your blade with the naked eye.
When you have progressively worked the back of the blade through all the grits, up to 8 000, and it is mirror-flat, it’s time to change over to working on the bevel side. At all times during the process, keep checking that your whetstones are properly flat, and re-flatten as necessary.
STEP 2: CREATING THE PRIMARY BEVEL
The primary bevel is the initial angle – it’s not used to cut at all. We’ll get into creating the secondary (cutting) bevel later on.
The table outlined below shows my recommendations for primary (and secondary) bevels:
As you can see from the table, 25° is the most common primary bevel angle generally used on tools. Importantly, these angles are what I use and recommend to my students, and they have served me well for many years. But they shouldn’t be regarded as a rule; rather treat them as a guide. Experiment with bevel angles on low-angle bevel-up hand planes when dealing with a particularly difficult grain. Bear in mind that if you choose to produce a more acute primary bevel, you run the risk of the edge becoming brittle, as there is too little material at the arris to take the pressure of the work that the tool is intended to perform. The only bevel I create that’s less than 25° is on my paring and inlay chisels. These are only used with hand pressure and a slicing action, and never with a mallet. Because of this, I can bevel them more acutely at around 20°. On paring chisels, I don’t employ a secondary bevel at all, because I need the most acute arris that I can practically achieve.
To sharpen a blade at a specific angle, whether it’s your primary or secondary bevel, you need a guide of some form to keep the blade absolutely square at your required angle. It must also be able to exactly repeat that angle over and over again. Having tried a number of sharpening and honing guides over the years, I keep coming back to the Veritas Tools device from Canada. It’s extremely well made, absolutely accurate, and offers repeatability of all of the required angles on its set-up gauge, and it allows you to set and repeat any angle you wish. This is a great tool that can be used with all plane blades, bench chisels and mortise chisels with equal ease. It’s also by far the best tool to learn whetstone sharpening with easy, repeatable accuracy.
When it is mirror-flat, it’s time to change over to working on the bevel side.
STEP 3: CREATING THE SECONDARY BEVEL
Having achieved truly flat backs on all of your blades, up to 8 000 grit, and worked primary bevels on to your tools, you’re ready to start creating your arris via the secondary bevel.
The reason for creating a secondary bevel is so you don’t have to grind away the entire primary bevel to create a fresh arris each time you need to sharpen a tool. Instead, you simply re-grind a tiny secondary bevel and don’t touch the primary. This saves a lot of time, and because the secondary bevel is so small, you’re wearing your stones very little in the process.
A secondary bevel is simply a smaller bevel that meets the back of the blade, forming your arris. It’s worth noting that this secondary bevel only needs to be a millimetre or two wide. The bigger the secondary bevel, the longer it will take to work it through the grits to achieve your perfect arris.
Some woodworkers recommend putting a third bevel on to the chisel or plane blade. This is more correctly called a micro bevel. To recap: your primary bevel would be at 25°, the secondary bevel would then be at 27° to 30°, and the micro bevel would be somewhere around 33°. The micro bevel is very small and requires far less effort to hone back into keen condition.
When it comes to mortise chisels,
I do things quite a bit differently. Because they’re struck hard with a mallet, and then levered a bit forwards and backwards to open up the cut, thereby making it easier to break the waste and extract the chisel, I don’t leave the sharp ‘meeting point’ between the primary bevel and the front of the chisel blade. Instead, I blend the primary bevel and the front of the blade into each other by creating a curved area.
This easing of the fulcrum between the two surfaces helps dramatically when levering out the waste from the mortise. It also helps to reduce compression fractures on the ends of the mortises. Interestingly, the Veritas Tools PM-V11 mortise chisels arrive already ground with a curved front fulcrum. If you manage to lay your hands on an 18th- or early 19th century mortise chisel, you’ll see that this is exactly the profile that was historically employed.
Properly sharp edge tools are a joy to use. When you’ve gone through the effort to get all your tools operating at maximum efficiency, it just makes sense to keep them in that condition.