Sharpening is the most difficult part of learning to turn. It is especially hard to sort out on your own because you just don’t know where to start and you do not have a properly sharpened tool with which to compare your efforts. The same skills and motions required to actually turn are necessary to sharpen (albeit in reverse or mirror image). But wait a minute, you may not have learned to turn yet. Adding insult to injury, you can’t learn to turn unless a sharp tool is placed in your hands. Which came first: the chicken or the egg, turning or sharpening?
I have come to the conclusion that it is not difficult for anyone to become proficient at wood turning if a sharp tool is placed in his or her hands. The concept of how a turning tool cuts is easy to grasp, and the execution only takes a bit of practice. The reason more people are not proficient turners is that they don’t start out with a sharp tool.
By sharp, I mean really sharp, sharp enough to shave hair off your arm. But a sharp turning tool entails much more than this because I have had numerous students bring tools that shaved hair but did not turn wood. In addition to being sharp, the tool must also be of the correct shape and have the correct grind angles on the bevels. This is a tall order, since most turning tools are not delivered sharp or with the correct grind angles. Most are not even the right shape. This is why I have included illustrations of each turning tool in chapter 6. You can compare your tool with the appropriate illustration to better see whether you have the correct grind angles.
When I first discovered wood turning, I had to learn to sharpen freehand, which added time and difficulty to the learning curve. Today there are a number of good, reasonably priced jigs that take 80 percent of the freehand skill out of the sharpening equation. I teach jigs in all of my turning classes, and every student can sharpen his or her own tools within half a day. When I had to teach freehand, I was lucky if two out of eight students got the hang of sharpening in five days. While I can still sharpen freehand, I now elect to use the jig because I get perfect symmetrical grinds (the same angles on both sides of a tool). I recommend that you start with a jig as well.
Sharpening can be divided into three distinct operations: establishing the correct shape and bevel angles, grinding when the tools become dull, and honing. The establishment of the correct shape and bevel angles is really a heavy grinding operation; after the correct shape is obtained, only light grinding is necessary to maintain the tool. Honing makes the ground edge really sharp. I’ll look at the equipment needed for each of these operations in detail.
Bench grinders for home and light-industrial use come in 6-in., 7 -in., 8-in., and 10-in. sizes. The smaller three sizes are generally built on a 3,450-rpm motor frame, while the 10-in. one is constructed on a 1,725-rpm frame. Many think the larger machine emits less heat because of the slower speed, but there is really not much difference. A 6-in. grinding wheel at 3,450 rpm has a surface speed of 5,416 ft. per minute, while a 10-in. wheel running at 1,725 rpm goes 4,5 16 ft. per minute. Several companies offer 6-in. grinders built on a 1,725-rpm motor, which gives a surface speed of 2,710 ft. per minute.
Although many tout the virtues of low-speed grinding, there is actually a loss of efficiency at surface speeds much below 4,000 ft. per minute. Because things happen more slowly, though, you’ll have more control, which is why beginners are more comfortable with these machines. If you are going to use a jig to sharpen your tools, a normal speed, 8-in. grinder works best. A low-speed grinder would only waste time because sharpening takes longer.
Because it is difficult to rock a tool up off of a flat table when freehand grinding, many turners like to fabricate a rest for grinding gouges that emulates the tool rest on their lathes (see the illustration above). This idea makes sense because the movements of freehand sharpening are somewhat similar to those of actual turning. For an experienced grinder, this is the best rest for sharpening gouges freehand, but with it you have no way to judge that the bevel angle is correct. Fortunately, the process is actually self-regulating. When the angle becomes too acute, the tool gets catchy and the edge wears too quickly. When the angle becomes too obtuse, the tool does not cut cleanly and beads become more difficult to roll.
For gouges, a rest that emulates the tool rest on your lathe works best because it allows you to roll the tool easily to create the complex shape required of a bowl or combination gouge. You must learn to turn on the grinder, set the tool on the rest, slide it forward into the wheel, and immediately start moving it laterally. This is easy to do with roughing gouges, parting tools, and scrapers but difficult with spindle, combination, and bowl gouges.
GRINDERS ARE AN OLD SHOP workhorse, but there are a number of safety concerns connected with them. Let’s take a look at how to grind safely.
Make it a habit to view grinding activity through the glass or plastic viewing shield because it provides a second barrier of protection (in addition to your eyewear) should the wheel disintegrate. Traditionally, the safety shields on any grinder were metal frames with tempered safety glass in them-two layers of tempered safety glass with a sheet of plastic in between. This “sandwich” creates a nearly shatterproof, scratch-resistant surface.
Most grinders today are supplied with plastic viewing shields, which reduces manufacturing costs. If your grinder has plastic shields in a frame, consider replacing it with a glass/plastic sandwich. If you must make do with plastic, some antistatic spray from a computer shop helps to repel dust.
Good lighting at a grinder is a must! Although industrial grinders often have lighting built into the safety-shield frames, economy grinders leave lighting to the end user. You’ll need a simple gooseneck lamp for direct local lighting.
Any grinder should provide guarding that encloses the wheel except at the area just above the rest. An open, unguarded wheel should be avoided under all circumstances. Most grinders have a sliding shield called a spark arrestor at the top edge of the guard. In addition to reducing the number of sparks that land on your hands during grinding, the spark arrestor helps to contain shrapnel inside the guards should the wheel explode. You should slide the spark arrestors to within 1/16 in. of the wheel, and as the wheel is dressed, you should readjust them.
Safety glasses are obviously a must around any grinding equipment. If your glasses are not true safety glasses with side shields, then you’ll need goggles or a face shield.
A good practice with a new wheel, whether it is a replacement or one on a new grinder, is to tap it to see how it rings. Like a baseball bat, it will not ring true if it is cracked. When you mount a new wheel in the grinder, it is best to stand to one side, start the machine, and leave the room for a few minutes. If there is a defect in the wheel, it will typically disintegrate at or shortly after startup. Never start the grinder without all guards securely in place.
Grinding wheels Most grinders are delivered with silicon-carbide wheels (available in most hardware stores). Such wheels are good for grinding almost anything from steel to glass or even bathroom tiles, but they grind too hot for tool steel. A better choice for tool grinding is an aluminum-oxide wheel. Often called pink wheels, they are just as often blue or white. An aluminum-oxide wheel has a bonding medium that is softer than that of the silicon-carbide wheel, which helps create the right amount of friability. Friability means to crumble. “Why would anyone pay extra for grinding wheels that crumble?” you ask. Because it makes the wheel self-sharpening and self-cleaning. Old, worn particles break off, which in turn brings up fresh sharp ones.
The trick is to find the correct amount of friability so that the wheel is not excessively soft and will only shed particles as they become dull. An aluminum-oxide wheel’s code number will start with A (for aluminum oxide) followed by the grit (36, 80, 100, and so on), then a letter indicating the friability. The letters A through G indicate very soft wheels that crumble easily, while the letters H through K represent soft wheels in the range that is useful for sharpening woodworking tools. The letters P through S appear on hard wheels, and letters beyond that are very hard wheels.
In general, an H or I wheel has about the right friability for general woodturning-tool grinding, as well as for carbon-steel tools such as plane irons and bench chisels. For exclusive use on high-speed steel (HSS), many turners prefer a harder wheel in the J or even K range. These harder wheels are more aggressive and work well for initial shaping of HSS (I use an A46K wheel for initial shaping of new too ls). Without great care, however, harder wheels will burn carbon steel.
Aluminum-oxide wheels can be found at industrial-tool suppliers. Friability standards are a voluntary trade standard and can vary from manufacturer to manufacturer. Once you find a brand you like, stick with it.
The vibration levels of many bench grinders may make them difficult to use-especially if the grinder is lightweight. Dressing with a diamond dresser usually improves this situation, but it seldom eliminates vibration altogether. If the grinder is not clamped to a bench, it will walk away. The principal culprit here is the average grinding wheel, which is often out of balance. How out of balance it is depends on the wheel? Norton wheels, for instance, tend to be very good, while the Chinese wheels that came on my imported grinder are horrible. It pays to buy a good wheel.
You can improve your grinder’s performance by installing One way hubs, but you will have to buy new wheels with larger arbor holes to accept the hubs. The hubs have a series of threaded holes that accept set screws as counterweights. The manufacturer’s directions suggest balancing the wheel, then dressing it (with its fixture), then balancing it again. The result is that a Canadian faceted penny, which is supplied with the balancing weights, will stand on edge on top of the grinder. Grinding is a joy: The grinder just sits on the bench and purrs. With these advantages, a One way balancing system is worth consideration.
Most people make the mistake of grinding with too fine a wheel. A good roughing wheel is 46 to 60 grit, while the finishing wheel should be 80 to 120 grit. For dry grinding, you should never use finer than 150 grit. Remember that the grit you select for grinding wheels should never be as coarse as what you would typically select for hand-sanding, which is how most woodworkers typically judge grit. This is because each particle of abrasive has less penetration when moving at speed.
Dressing wheels A new grinding wheel, whether it comes with the grinder or is a replacement, is about as round as a freshly mounted turning billet. It is impossible to grind well on such a wheel because the work will just hop up and down. Therefore, some means of truing your grinding wheel is as important as the grinder itself. Truing can be broken down into two operations. The first is the actual truing, that is, making the wheel concentric with the grinding machine’s arbor and making the face of the wheel square (90 to the sides of the wheel). The second operation is dressing.
Dressing is done to give the wheel the desired texture and to bring up sharp new particles when necessary. Only about 1 0 percent of the particles in a given wheel actually cut-the rest just rub. As you grind, the 1 0 percent of the particles that are doing the work dull. The area between them fills up with metal, causing the particles effectively to lose their clearance. The wheel needs to be dressed to sharpen and clean it. Dressing must be done any time you can see steel in the surface of the wheel, when the surface looks glazed, or when the wheel has grooves worn in it. It has to be done more frequently with a hard wheel than with a soft one.
Several accessories to true and dress are available. The best method to true the wheel is by using a diamond dresser, preferably in combination with some sort of jig to ensure that the wheel is dressed square as well as round. A diamond dresser is no more than an industrial diamond brazed into the end of a piece of cold-rolled steel. By setting the shank on the grinding rest and sliding the diamond tip laterally across the grinding wheel, the wheel will become round. The diamond dresser should be presented with a fairly light touch at a 5 ° negative (downhill) angle to the face of the wheel in a scrape cut. The Oneway dressing attachment does this automatically and dresses the wheel round and square to boot (see the photo below). If you don’t have a jig, it is easy to tilt the table a bit downhill and use it to support a standard dresser.
Buying a diamond dresser is like buying a diamond ring-the size dictates the price. Fortunately, industrial diamonds are cheap compared with their aristocratic gem-quality cousins. A good diamond dresser will cost you between $ 1 5 and $60. Bigger dressers, called clusters, contain more than one diamond and are typically in the $60 price range. Clusters, however, are not good for truing a wheel with a jig because they leave a finish similar to that left by a star-wheel dresser. Diamond dressers are available from industrial-supply houses, so pick one up when you are buying your aluminum-oxide wheels.
A star-wheel dresser, which is available at any hardware store, is good to use in combination with a diamond dresser. Since a star-wheel dresser follows the contour of a wheel, it does not really make it round–only a diamond can do that. But a star-wheel dresser does give the wheel a much rougher texture than a diamond, so it is great for dressing your coarse wheel (after it is made round with a diamond) in preparation for the initial shaping of new tools. A star-dressed wheel is much more aggressive than a diamond-dressed wheel, removing metal fast with a minimum of heat. After about two dressings with a star wheel, the wheel will be out of round and you’ll need to use the diamond dresser again. The worst type of dresser is a silicon-carbide stick. These sticks are only marginally harder than the wheel, so they leave a glazed surface that tends to grind hot.