Hollow grinding for edge tools is nothing new (ever seen one of those old-timey hand-cranked bench grinders?), but it has recently become a bit of a trend in the greenwoodworking world, and for good reason. Let’s have a look at the terminology, available Jigs, as well as the practical advantages and disadvantages of hollow grinding your greenwoodworking tools.
There are three good options for greenwood edge tools – full hollow, flat-over-hollow, and full flat.
FULL HOLLOW – for a full hollow grind, (assuming you’re not a knifemaker) you should use a slow-speed water-cooled grinder such as a Tormek and fully grind the bevel until you’ve apexed the edge on both sides.
- Very fine edge – good cutting performance
- Fragile edge – there is very little material behind the edge, it will easily roll or chip in harder woods.
- Upkeep – as soon as you touch your bevel to a stone to maintain it, you’ve created a flat-over-hollow grind. To maintain a full hollow you would have to re-jig the blade on your grinding set-up every single time, which is impractical.
FLAT-OVER-HOLLOW – for my flat over hollow grinds, I use the Hewn&Hone sloyd jig to set accurate 25 degree hollow bevels on the Tormek, apexing the edge and forming a small burr on both sides. I remove the burr with a few strokes on a washita Arkansas stone, and then polish rails / flats holding the hollow bevel flat on a finishing stone. In my case I used a #8000 King superfinish stone. I then strop with hard backed Kangaroo full grain leather and Veritas green honing compound.
- Best of both worlds regarding durability and keenness.
- Very quick and easy to maintain the edge:
- A flat-over-hollow grind has very little metal at the edge which needs to be apexed and polished to sharpen the knife. You can sharpen your sloyd knife in just a few passes on a finishing stone instead of having to go through the grits.
- Because the bevel is hollow in between the rails, there is a noticeable positive feedback when you have the bevel flat on the stone or strop. This makes it simple even for inexperienced carvers to maintain the correct bevel angle and avoid microconvexing their edge. I recommend a firm, hard backed strop for this reason.
- Because the geometry makes it simple to hold the correct bevel angle, I find that most people will produce a better edge when sharpening a flat-over-hollow ground knife as compared to a scandi or full flat such as a factory mora. Technically there should be no difference in the edge, only in ease and time spent sharpening, but in real world terms, making it easier and quicker to sharpen the edge will result in most people doing a better job as well.
- Price – typically, only premium sloyd knives come hollow ground. This is because it takes specialised equipment and a lot of skill and practice to produce an even, continuous grind with correct bevel geometry. You may have heard people say it’s not that complicated to freehand a hollow grind on a slowspeed grinder, but the results aren’t comparable to the more exacting approach I’ve taken.
- This is why I decided to start hollow grinding inexpensive mora 106 blades and making them available for purchase – to make flat-over-hollow ground carving blades more obtainable for the average carver.
- Working life of the grind – as you sharpen your flat-over-hollow ground edge, the hollow portion will become smaller and eventually disappear, reducing the benefits of the grind. This problem is minimal assuming you strop often and don’t oversharpen your blade. If stropping doesn’t refresh the edge, touch it up on a finishing stone and take care to remove as little metal as needed – this will prolong the time until your edge needs to be re-hollowed.
- Durability – flat-over-hollow ground blades are slightly less durable than a full scandi or flat grind. This is because there is slightly less material behind the edge, as it has been hollowed out. The more you use your knife and sharpen it, the stronger the bevel will be as the rails become larger.
- I would recommend you keep a separate knife for doing rough work, and try not to lever or pry with the edge. If you take care, you won’t have any issues.
Julian Jones (@littlebear_sloyd)