Sharpening Woodturning Chisels
by Jon Siegel
This article first appeared in The Old Saw, the journal of the Guild of New Hampshire Woodworkers (www.gnhw.org) Novemeber 2005.
Part 2 - Sharpening
A sharp edge, no matter how painstakingly achieved, is only temporary.
In the first article of this series, I explained my plan to present this topic as two subjects – Tool Point Geometry, and Sharpening. In this, the second part, I will now discuss sharpening, which is defined as the routine maintenance of the perfect edge. This operation assumes that the correct geometry has previously been achieved, and we aim to keep it that way as we sharpen the chisel hundreds or even thousands of times.
My Search for a Better Way
I had reached a plateau in 1977. I already had 10 years of turning experience, and I wanted to quit cabinetmaking and millwork to concentrate strictly on turning. The one problem which I had not successfully solved, and which was creating a bottleneck in my work was sharpening. My turning was plagued by an endless cycle of disappointment over how short a time the tool remained sharp, frustration over the interruption of my working rhythm to sharpen tools, and the resulting syndrome of sharpening procrastination. I decided to try to develop a system of sharpening that was so quick, so convenient, and so accurate, that I would no longer suffer from this problem.
I began to realize that sharpening was not something to be made into a ritual at the beginning of the day, or the end of the day, or the start of a project, or any other particular time. Sharpening should be as much as possible a continuous process. But in order to achieve this, I had to find a way to sharpen in seconds instead of minutes. The solution I eventually developed was a belt and buff system that takes seven seconds to sharpen a chisel.
I sell sharpening machines through my company Big Tree Tools, LLC. If I can convince you that a belt and buff sharpening system is the best, then I might get your business, but that is not why I am writing this.
Belt & Buff Systems
The belt and buff system of sharpening was invented (or at least popularized) by Woodcraft Supply in the late 1960s through a tool they called the Mark II sharpening system. On these systems, the belt runs up, that is, away from the edge, and this established an important principle for belt sharpeners. To those who have only used grinding wheels for sharpening, where the wheel runs down, it may seem odd to have the belt traveling up, or away from the edge. The reason for this is that while a wheel is hard, a belt, being cloth is soft and the tool compresses the belt slightly. This can round the leading edge of the tool. That’s why it is important for the leading edge during the grinding process to be the heel, or bottom of the bevel, and not the cutting edge. In this way the action of the belt is somewhat like a grinder but also like a strop. Nonetheless there are many turners who use the belt in the down direction on the theory that if you use very light force, the compression of the belt is insignificant. This may work just as well, especially with a coarser grit, and more study needs to be done on this question.
One of the great advantages of a belt system is that the grit can be changed in seconds without altering your jig or set-up in any way. So a belt system can satisfy Sharpening Rule #4 and give you two systems in one.
The difference between the Mark II and a modern woodturner’s belt and buff system is that two critical improvements have been made:
1. Flat grind – The Mark II has a large contact wheel which gives
a very slight hollow grind, but it still is not flat. The best sharpening
systems use a flat platen and the grind is absolutely flat.
2. Incremental set-up of the vee block – The Mark II has a vee block on a sliding bar, and many types of woodturner’s sharpening jigs (such as Wolverine) still use this old form. The best type of vee block is incremental, meaning that the block has discrete positions. This allows the jig to be set-up in about two or three seconds, and automatically establishes exactly the same precise position each time. An incremental set-up allows repeatability which is much more accurate than a sliding system, as well as being faster.
What is Sharp? What is Dull?
To understand sharpness, we need to consider the difference between sharp and dull. When a tool is sharp, the two planes or surfaces of the edge (dihedral) meet to form a perfect line. As the tool is used, the edge wears down and becomes dull. This is a gradual but continuous process, and eventually the turner notices that the tool is not cutting as well as it did before.
The symptoms of dullness are:
1. More force is required to keep the tool stable.
2. A rougher surface is produced with more tear-out.
3. More dust and less shavings emerge from the tool.
4. More heat is produced, especially on dry wood.
A close examination of a dull tool reveals that the edge is rounded over.
The abrasive wear has rounded the edge of the tool. You can feel it. It’s
smooth. And you can see it clearly in strong light. The edge now looks
like a bright line because it has a width which it did not have when it
was sharp. As a tool dulls, the width of the edge gradually goes from zero
(or close to zero) to about 0.0005 inches when we notice that it’s
not cutting as well as it did when freshly sharpened. Depending on the
operation at hand, you may continue to use the chisel until the width of
the dull edge reaches about 0.001 inches, at which time you will need to
sharpen it. If you are not familiar with these small measurements, I should
point out that 0.001 inches is less than half the diameter of a human hair.
This brings us to Sharpening Rule #1 – Remove the minimum amount of metal possible. If you follow the above logic, you will realize that you can restore the edge to perfect sharpness by grinding off 0.001 inches of metal. This means that you can sharpen a chisel 1000 times before 1 inch of blade has been consumed. If you grind off more than this, you are increasing the risk of overheating the tool, wasting expensive chisel, wasting grinding media, creating excessive harmful abrasive dust, and wasting time which unduly interrupts the rhythm of your work.
This leads to Sharpening Rule #2 – Sharpening a chisel involves three steps – set-up, grinding, and deburring. With a good sharpening system, this should take less than 10 seconds total time for all three steps. The elements of the system which allow this are simple: incremental set up, automatic precise repeatability allows you to remove the minimum amount of metal (typically 0.001 inches or less), and a buffing wheel for deburring on the same machine. Your sharpening station should be at most two or three steps away from your lathe. Sometimes I put mine right on the lathe bed if space allows. Then I can sharpen by simply taking one step; and I’m back turning wood in about 7 seconds because of course, I leave the lathe running.
Many people buff too much, and over-buffing will round the edge. Two or three seconds should be the maximum time for buffing. It is only necessary to wipe away the burr left by grinding. Usually on a gouge, this means buffing inside the flute, and hardly at all on the bevel side. On a skew chisel, alternating for one second on each side will achieve a perfect edge in a few seconds. A narrow 1/4˝ buffing wheel works best because it will compress into the flute of even the smallest gouge. The best buffing compound is emery because it is designed for ferrous metals.
Grinding a Flat Bevel
In part 1 of this series, I explained that wood turning chisels should not be hollow ground, but flat. Obviously you cannot achieve a flat grind using the periphery of a wheel. There are several ways to achieve a flat grind: My favorite way of course is a belt, but you can also use a sanding disc or a face wheel. However, it is not safe to use the side of a grinding wheel unless the wheel was designed for this purpose – i.e. a face wheel.
Single vs. Multi-Tiered System
Some turners have more than one sharpening routine. That is, sometimes they grind, sometimes they hone, sometimes they strop or buff, etc. Their plan is to restore the edge quickly by honing (with a slip stone, for example). Then, after they have done that a certain number of times, they have to go back and grind. This is what I call a multi-tiered system, and I don’t like it. The edge is not consistent, that is, it is not the same after honing as it is after grinding. Also each time the edge becomes dull, you must decide which process is necessary at that time. I prefer a single-tiered system, which means that you do the same routine every time you sharpen, and there are no decisions to make. The edge is consistent and performs exactly the same way every time.
When the scraper is taken right from the grinder, it has a small burr. You can feel it with your fingernail. Your nail will not slide off the edge, but will be caught by the burr. Some turners enhance the burr by burnishing the edge with hard steel. The burnishing process “turns the burr” by forming the metal at the edge into a thin projection. The formation of a burr by this process is very delicate business, and different types of steel chisels react differently to the burnisher. This is because turning a burr depends on the malleability of the metal, which in turn depends on the metallurgy and the heat treatment of the steel in the chisel. The old saw, “If a little is good, then a lot is better,” definitely does not apply to forming a burr on a scraper. Too much burr will make the chisel not cut at all or become unpredictable and “grabby”. Many people tell me that they struggled with scrapers for years until they discovered that they had too much burr.
Grinding methods for scrapers are very straightforward. A platform jig holds the tool at the correct angle against a grinding wheel. A medium fine wheel dressed with a diamond tool will give good results, and the tool may be used directly from the grinder. If you need a more aggressive tool for roughing, burnishing may help; but experiment cautiously. Never buff a scraper.
Because gouges are ground on the bevel side only, it is easy to overlook the surface quality of the flute itself. It is necessary for the surface of the flute to be polished to a very smooth condition, because the quality of the edge depends on it. If the flute is rough, you need to polish it with a round edge slip stone. A gouge with a rough surface in the flute can never be sharpened to a fine edge by grinding or honing the bevel alone.
Side Grind Gouges
If you want to create a side grind on your gouge, you need a special jig. Since you cannot use the handle to anchor the rotation of the tool, you must employ a jig which clamps onto the blade in some way and provides a pivot point which is off the axis of the chisel. In order to achieve repeatability, this jig must be attached at precisely the same position each time it is used.
Various side grind jigs are available, or you can make your own. Some of these jigs allow the angle of the leg to be adjusted, while others are fixed, and the angle can only be adjusted within a small degree by clamping the jig to the blade of the chisel in different positions.
Figure 6 shows the geometry of the side grind jig. Notice that the angle of the leg, and the location that the jig is clamped to the chisel combine to form an offset angle. Theoretically, this angle can be anything from 0 – 90 degrees.
I hope that through these articles you have begun to think about sharpening in a new way. Most turners find their enjoyment of the craft is greatly increased when sharpening procrastination has been eliminated.