Sharpening Advice for Woodworking Hand Tools
Aug 18 '09 (Updated Sep 02 '09)
The Bottom Line A dull tool in your home or shop is a dangerous tool. For a small investment in dollars and time you can keep all your tools sharp.
In a wood shop, or anywhere else for that matter, a dull tool is a dangerous tool to use. Not only will its use be frustrating, but it can cause accidents with lots of blood. In this article we will try to define what sharp means and how to know when sharpness is reached. We will also give some examples of sharpening. These examples will mostly be tools we use in the wood shop, but we will touch on some other tools as well.
What Does Being Sharp Mean?
In this article we are limiting definitions to edge tools. Most are made of various metals, but ceramic and plastic sometimes come into play. In any case, to be sharp, a tool must have two edges that are at an angle to each other so that when they are flattened they will come together to make an edge. A knife usually has two sloped edges that meet, for instance. Other tools like a wood chisel usually have one edge flat and the other edge slanted. When those two edges meet in a straight line so that the end of the edge is one molecule of the metal thick, the tool is sharp. We need to understand two terms, sharpening and honing. Sharpening is the action that gets the edge to the proper shape for the particular tool and very nearly to a thin enough edge. Honing is using fine abrasive to remove all the scratches from sharpening so that there are no corrugations at the edge and the edge is down to one molecule thick.
There are several ways to tell when a blade is sharp. One is to use a magnifying lens to look at the edge for scratches. Another is to shine a bright light at an angle along the edge and look for reflections. For a small tool you can roll the edge lightly over your thumbnail. It should catch rather than slide. The ultimate test is to shave some hair off your arm. Even a broad axe or garden hoe, properly sharpened, will pass all these tests. Now you know what sharp means and how to test for it, but how do you get that tool really sharp?
Sharpening Tools
In this section we will describe some of the many sharpening and honing aids that we have actually used and talk about different strokes for different tools. To begin, the angle at which the two edges meet is very important. In general, the tougher the use, the steeper the intersecting angle needs to be. Thus an axe has a very wide angle and a surgeon's scalpel has a very shallow angle. Shallow angles can obtain a very sharp edge, but the edge will dull more easily. Broader angles won't take as sharp an edge, but the edge will hold up to hard use better. Each different type of tool has an optimum edge angle and you should learn this first before beginning a sharpening project.
Grinding wheels
When it comes to getting a tool to a new edge angle or cleaning up the edges of an abused tool, you can't beat a grinding wheel. These usually consist of a motor with two shafts that can have grinding wheels of various grits installed. Many years ago the big stone wheel with a handle or foot pedals for turning was a standard part of most farms. Today the grinding wheels for a home shop are usually motor driven with 6" or 8" wheels from ¾" to1" wide installed on the shaft. Generally these come with motors that spin at either 3450 RPM, high speed grinders, or 1725 RPM, slow speed grinders. When grinding many metals, high speed steel for instance, heat is the enemy. Too much heat at those thin edges will take away the temper of the tool, rendering it useless. Some grinding systems have wet wheels - where water or sometimes oil is run over the wheel to carry away the heat of grinding. A slow speed wet wheel grinder is the best for sharpening, but with care and a handy pan of water to dunk the steel in regularly, a dry wheel will do the job. Since sharpening a tool is done on the curved edge of the grinding stone, you will actually make a concave instead of flat surface on the side of your tool. In many cases this is not a problem as it actually makes the honing process quicker. An 8" stone is better than a 6" stone, though, particularly for smaller blades. A note here about those grey grinding wheels that come with many grinders. They are all right for putting a point on a piece of rebar or when sharpening the blades of your lawnmower. For real sharpening tasks, however, throw them aside and get a couple of real grinding wheels in white or pink or blue. It is often nice to have two different grits on the grinder. We use 60 and 100 grit wheels.
Other tools for sharpening are metal files and coarse bench stones. They take longer than a powered grinder, but, if the edge isn't too far off, do a good job.
It takes a lot of practice to get good at holding a blade at the proper angle for sharpening. Most of us use jigs to do the job these days. Most of the grinders come with an adjustable platform, some good, some not so good. Aftermarket jigs for the grinding wheel are readily available in a dizzying variety. Some grinding systems with all sorts of jigs can be had for several hundred dollars. There are less expensive alternatives for the average shop. If you have a good saw you can make wooden blocks to set the various angles you use most often.
Honing
Once you have used the coarser abrasives; grinding stones, files, or what have you, even coarse sandpaper on a flat surface will work, you will have a blade with an edge that is at the proper angle and has a straight (or correctly curved) edge with no nicks, but you will not have a sharp tool. You will notice that the sides are a little dull looking. If you use a magnifying lens you will see a lot of little scratches. You will notice that the edge looks more like a serrated knife than a regular knife under magnification. Honing, sometimes also called polishing, is a process where we will use successively finer grits of abrasive to remove the coarse scratches and replace them with finer scratches. We will continue this process until the scratches are so fine that they scatter very little light and the surface appears polished. The goal is to get the scratch pattern to zero, but this level isn't really necessary. Getting the scratches to less than a micron is enough for even the finest edge.
Honing Tools
Back in the early days of honing almost all abrasives were from natural materials like stones and polishing compounds made of a slurry of ground abrasives. Many of these are still available today and can be used just as successfully. Of these, oil stones are probably the most well known, followed by hard Arkansas stone, and various polishing compounds. Another natural stone was first used in Japan and since spread worldwide. This is known as water stone.
Today polishing is generally accomplished with an assortment of natural and man-made abrasives. One of these is sandpaper. Another is diamond. All of these come in various sizes and shapes to suit specific edge configurations. Just as in the rough sharpening phase, maybe even more so, it is important to hold the edge at the proper angle to the abrasive. Again, it is easier to use some sort of guide to hold this angle. Holding a tool at the exact angle and not changing that angle during the sharpening process is a pretty difficult skill to acquire.
An example
To get a feel for the steps in sharpening let's take a bench chisel. These tools are flat on one side and have a bevel on the other to make the edge. The goal is to get the angle of the slope so that the edge is as sharp as possible with enough metal behind the edge to give that edge some strength. A typical angle is 25 degrees. Many of these chisels use high speed steel for the blade. ‘The first thing we know is that we can't heat the edge too high or we will lose the temper of the metal. Let's assume the angle needs adjusting. We will start with a slow-speed grinder and a can of cool water. We can use something to set the grinder tool rest so the sloped edge strikes the edge of the grinding wheel ( a 60 grit white wheel for this exercise) at the needed 25 degree angle. We first dip the edge in water then grind for a short time. We frequently dip it in the water again. We continue until we have an even scratch pattern right up to the edge. A good way to tell if the grinding is even is to cover the sloped edge with magic marker ink. When the angle is correct all or most of the ink will be removed. When we are at the edge we want, the sparks from grinding will roll over the edge of the tool. The rough grinding is done and we move on to the honing process. Remember that we have actually produced a concave surface on the sloped side.
We will use a coarse diamond stone lubricated with a little water for the next step. We could use coarse sandpaper fixed to a flat surface, a water stone with water as the lubricant or an oilstone with oil as the lubricant. We use a honing guide that rolls along the stone. It is set for 25 degrees. We will see that the edge of the slope and the back edge near the thick part are getting polished while the concave part is not touched. This helps reduce the time required to get that edge to its desired sharpness. We will move on to a medium diamond stone. Again, other equivalent abrasives could be used. We will see the edges we are abrading getting a little more shine. The scratch pattern is getting smaller. We move on to a fine stone. We also change the angle of the guide to 28 degrees. This is called making a micro-bevel. Now we are only abrading the thin edge of the slope.
At this point we move to the back of the blade. Remember that two sides coming together flatly is what is required to make a sharp edge. We will flatten the back side of the blade until we see the same level of polish back at least a short distance from the sharp edge. For some tools this is probably enough. We may need to knock off what is called a wire edge. This can be felt by flicking a thumbnail up over the edge on each side. For our chisel we are going to move on for an even more polished edge. We go to an 8000 grit water stone. We could use an extra-fine diamond stone or even some fine sandpaper, usually available from auto supply stores, fixed to a flat surface and lubricated with a little water. We repeat both slope and back edge polishing. Now we make a slurry using fine diamond powder and place it on a flat surface like plate glass or, in our case, a piece of MDF. We are still using the guide set for 28 degrees and polishing both surfaces. We go through three levels of diamond paste and, finally, take a couple of light strokes to remove any wire edge left. Now we wipe the blade clean with a dry rag and slide the edge down our arm. We watch hairs fall over as the blade cleanly cuts them with no pulling at all. We are ready to use the chisel.
We have various shapes of abrasive stones for different tools. There is a list below of some of the sharpening devices we use or have used if you want more information.
Summary
You can spend a few hundred dollars for a single grinder like the Tormek that, with its accessories, can take an edge from terrible all the way to quite useful. You can keep a simple mill file on hand for keeping your shovels and hoes sharp and even touching up your axe and lawn mower blades. You can go somewhere in between with several different abrasives, a grinder, and a couple of honing guides to keep everything in your home and garage shipshape. You can spend hours trying to get that perfect edge or learn when it is good enough. Good enough will vary by the intended use. We have found that our lathe chisels can often be used right off the grinder. We like our paring chisels hair shaving sharp. Our yard tools are usually good enough with just a file being used on them.
My current arsenal consists of a low speed grinder with a jig for sharpening mostly lathe chisels, a two-sided diamond bench stone for bench chisels and plane blades with a Veritas sharpening guide system, an 8000 grit water stone, diamond files for touching up carbide edges, a curved diamond sharpening device for gouges, small diamond stones for touchup of lathe chisels during use, and three grits of diamond compound for that really sharp edge I sometimes pine for. I also have a mill file for garden tools and a sharpening steel for my knives. The last things I would give up are my sharpening guides.
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Some Sharpening Aids I Have Known
Power Grinders and Sharpeners
Delta 6" Variable Speed Bench Grinder
Drill Doctor Drill Bit Sharpener
Delta Wet/Dry Sharpening Center
Plasplugs 4-in-1 Power Sharpener Kit
Woodcraft 8" Slow Speed Grinder
Sharpening Jigs
Veritas Sharpening System
Wolverine Grinding Jig
Wolverine Skew Grind Attachment
Grinding and Honing Abrasives
DMT Duo-Sharp Diamond Honing Stone
DMT Diamond Mini-Sharp Extra Coarse
Commando Diamond Grinding Wheel Dresser
DMT Miniature Diamond Whetstone Set
DMT Diamond Wave Sharpener
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