General Safety Disclaimer:
Safety in the workshop is YOUR RESPONSIBILITY alone; I make every reasonable effort to advocate and illustrate safe woodworking practices wherever possible, but there are always exceptions. In order to adequately illustrate a procedure, some photographic license may be required to get a clean photograph, such as removing certain guards on machinery or adopting a normally unsafe stance while using a tool. Also, techniques that are safe for any particular tool or individual may not be for others, since the design of tools and machinery varies and the skill level and experience of each woodworker varies. Thus, your safety in the woodshop is your own responsibility and no one else's. You should always be familiar with all procedures and safety information listed in the owner's manual for each tool you use, and you should always use personal protective equipment, such as safety glasses. If you are ever in doubt about the safest way to accomplish anything in your shop, stop and consult a professional such as a woodworking instructor at a local high school or community college.
Having started woodworking in the carving field, I eventually realized that woodturning is really woodcarving in disguise. Same tools, same cutting techniques, different power source. Based upon that, there is no limit to the number of tools you can use or the ways they can be sharpened, any more than there is for woodcarving tools. Nevertheless, here are my own preferences and thoughts for some common tools:
Spindle Roughing Gouges:
45° is most common, but I've also got one sharpened to 65° for a less aggressive cut on bigger, badder chunks of wood.
There are two basic grinds here: regular and fingernail. A regular grind is a simple grind, the same as is used on a roughing gouge. A "slight fingernail" would be similar to a "regular" grind, but with the side wings ground a bit differently. An aggressive fingernail would have the side wings of the tool ground back severely.I use a variety of grinds on different bowl gouges for different cutting jobs, varying from a slight fingernail to a moderate "Ellsworth" grind for the inside, and a more aggressive fingernail grind that works best for shaping the outside of the bowl. The angle at the tip of my tools varies around 50°-70°, and the angle on the wings ranges from 45° to 15°-20° depending on the severity of the fingernail grind.
These are sharpened with a slight fingernail grind to 35° to 55° at the tip of the tool.
These are usually double-beveled, so I grind mine to a total angle of 45°-60°
Typically sharpened at 60° to 70°
IMHO, an oval skew is the only way to go here. Since the skew is never properly used flat to the tool rest, the generously rounded corners of an oval skew ride much more smoothly on the tool rest.The skew angle varies, but 30° to 45° is the norm. Mine is ground to a radiused edge that is angled at 40° to the tool's axis near the "short" point and curves around to about 80° at the "long" point.The blunter the angle the skew is sharpened to, the easier it will be to handle (less prone to digging in and skating off), but the more it will tend to crush the wood. I've seen people learning to use the skew with skews ground to a nearly 70° edge, but a 40° edge cuts much better on end grain and softer woods. The most suitable angles range from 35° to 50°.
You will have noted by now that I never give any single "best" angle to sharpen a tool to. That's because that angle depends on what woods you turn, the quality of steel from which your tools are made, and your own woodturning style. When I first started turning, I bought a piece 'O crap set of eight turning tools from Harbor Freight for about $11 on sale. I learned to sharpen these tools (and boy, did I do a lot of sharpening!), and I experimented with the edge shape until I had arrived at a shape that worked well for me. One I had some idea of what tools I wanted and how to grind them, I upgraded to high quality HSS tools (some Crown, some Sorby). This ended up saving a lot of HSS from being ground into oblivion as I learned the correct sharpening techniques and what edge shapes I preferred.
Learning to use the skew chisel is difficult, but well worthwhile. It saves a lot of time becasue it can do a lot of cuts well, and once you have mastered the skew, fingernail ground gouges automatically look "familiar," like two skews joined at the hip. Believe me, if you want crisp details on your spindles or you just don't enjoy sanding, the skew is the tool for you.
I've actually heard two opposing schools of thought on sharpening turning tools.
I know at least one full-time professional woodturner who sharpens his tools frequently using the Oneway Wolverine grinding system (IMHO, the best system just for turning tools). He doesn't hone or polish the edges, just uses them straight from the 120 grit wheel. His reasoning is that he'll need to sand a bit anyway, and the slight gain in edge longevity from honing does not offset the time spent doing it. Also, since the lathe's motor supplies all the torque he needs, cutting pressure is not a factor either. Since time is money, he uses the method that lets him maximize his productivity for the types of items he makes.
The other school of thought (the one I generally subscribe to), hones and polishes the tool. I find that I do get a smoother surface on the wood and slightly longer edge life by using a polished cutting edge. By polished, I mean that after sharpening, I can shave hairs off my arm with my skew chisel. It does take longer to achieve this type of edge, but since I'm a hobbyist and I don't enjoy sanding, I prefer to use freakishly sharp tools. Also, the less you have to sand the turning, the less you'll knock the crispness off of your sharp edges and details, so if you're out to make objets d'art, go ahead and polish those tools.
As for how to know when to re-sharpen, that really is learned best by experience. When you find that the tool seems difficult to control or otherwise hard to use, try sharpening it. Often, that's all you need to do to make the tool behave again. During a turning project, I keep an extra-fine diamond honing plate in my pocket or on the shelf by my lathe. I'll use that to quickly touch up any edge that might need it before that edge gets too far gone and needs re-grinding.
The whole idea of sharpening is to produce two smooth, polished metal surfaces that meet at the appropriate angle for the particular tool being sharpened. Before discussing the proper techniques for sharpening, it is necessary to understand the geometry of a sharp blade and the relationships between the various angles involved. At right is a diagram of the cutting edge of a plane iron.
Most plane irons are set in the body of the plane at a 45° angle to the work surface. This produces a rake angle of 45°. A steeper (smaller) rake angle would cause the blade to act like a bulldozer, crushing the wood fibers and shoving them out of the way by brute force. This would tend to reduce tearout when going against the grain, but would leave badly crushed fibers on end grain and in soft woods. A shallower (larger) rake angle produces a wedge-like cutting action, smoothly wedging apart the wood fibers on end grain, but causing severe tearout when going against the grain.
The sharpness angle is the angle most often referred to when sharpening a blade. In this example it is about 35°. When sharpening, the blade would be held at 35° to the stone. If the blade were double beveled like a carving knife, the blade would be held at half the sharpness angle on each bevel, giving a symmetrical edge. A small sharpness angle allows a large rake angle and cuts cleanly and without much resistance, however, because there is little material behind the cutting edge, it dulls quickly. A large sharpness angle stays sharp longer, but requires more force to cut and may crush end grain on softer woods.
On a plane, the clearance angle allows the blade to cut without dragging or crushing the wood surface. As the diagram above indicates, a dull blade has both a negative rake angle and a negative clearance angle, giving a generally lousy cut.
To sharpen a chisel or plane iron, a honing guide is most helpful, and some means of gauging the angle between the blade and the stone. Fix the blade into the honing guide at the appropriate angle.
If you are using a waterstone or oilstone, check that its surface is flat using a straightedge. Apply the proper lubricant to the sharpening stone (if any is required). For general sharpening of blades that aren't in need of nick removal (touch-up sharpening) a 1000 grit Japanese, fine diamond, medium ceramic, or washita stone are appropriate.
If you are using a water- or oilstone, it is important to wear the stone surface evenly, working the ends of the stone as much as the middle. This reduces the time between flattening the stone, especially on waterstones. After sharpening for awhile (depending on the initial condition of the edge), a small flap or burr of metal will form right at the cutting edge. The formation of a burr indicates that the bevel has been ground all the way to the cutting edge. Once a burr is formed all the way across the edge, the back of the tool may be carefully lapped on the stone just enough to weaken the burr, then it is time to move on to the next finer stone.
Continue honing on the fine stone until roughly 30 seconds after all coarse scratches from the previous stone have disappeared, and lightly lap the back of the blade again. Finish by honing on a 6000 grit Japanese, ultra fine (white) ceramic, or hard black arkansas stone (diamond stones are not graded fine enough for final honing).
The final step to achieving a properly sharpened blade is to polish off the burr, leaving two flat, mirror polished surfaces. The traditional Japanese method involves alternately polishing the bevel and back of the blade on an 8000 grit stone, however this requires both time and great skill to do properly. The western method is to use a leather strop and fine polishing compound. The blade is drawn backwards along the leather, so as not to cut into it, until both sides appear scratch free and the burr falls away.
You should now have a cutting edge sharp enough to shave with, or at least cut cleanly across end grain. For future reference, dull spots on a cutting edge can be diagnosed in a couple of ways. One method involves looking at the edge straight on. A truly sharp tool cannot reflect light from its very cutting edge, so any light spots indicate that the edge has been blunted there. The other method involves holding the edge at right angles to your thumb nail, and drawing it lightly down the nail surface. A slight dragging indicates a sharp edge, as the point digs into the nail slightly. The blade should be tested by dragging it both directions, as a burr will also catch on the nail, but in one direction only.
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