Читать книгу Ellsworth on Woodturning - David Ellsworth - Страница 28

All of my hollowing tools are scraping tools rather than cutting tools. I work at great distances from the tool rest and through very small openings with relatively small-diameter tool shafts. If I used a cutting-type tip rather than a scraping tip, the cutter would be too efficient for the diameter of the shafts, and I wouldn’t be able to control the cut. In effect, the cutting tip would be too sharp. However, the principles described above relating to edge sharpness and mass still apply. In fact, they are more dramatic.

The tools I use today are more sophisticated than my earliest tools of the 1970s. The tips are now 10% cobalt high-speed steel instead of bent and sharpened bars of 0-1 drill rod. The shafts are round instead of square so I can rotate them on the tool rest. The bent shafts carry the mass of the shaft closer to the tip of the tool for greater support. I use a burr edge right off the grinder with an 80- to 100-grit wheel. Honing the edges with a diamond hone works, but I find the honed edge burnishes away quickly, leaving a smooth surface that simply won’t cut the wood.

There are six principles that explain how hollowing tools work. A basic grasp of these principles is necessary to know whether the tools are working properly, especially since you can’t see the cutting going on inside the form.

Two of my earliest hollowing tools: The larger is ½" square stock drill rod from 1976; the smaller is a hard-to-see tip of high-speed steel brazed to a ¼" square shaft drill rod with a lot of wear, from 1977.

My current hollowing tools have evolved to 10% cobalt high-speed steel tips set into rounded shafts that rotate easily on a tool rest when I need to maneuver the tips inside the pieces. The tips are glued into a hole in the end of the shaft with superglue.

Why use a parting tool to shape bases?

Viewing the photos at right, you can easily see how confined the region at the base of the piece is and why my large gouge won’t fit. By using the parting tool, I am able not only to shape this region, but also to shear the lower surface of the form with the top edge of the parting tool. There’s nothing special about this edge except that its angle when meeting the wood is elevated to approximately 45˚, which makes it ideal for making a brief shear cut in this narrow location. This edge comes right off the grinder, and its sharpness is limited to only a few cuts. But it does an excellent job.

Working in this confined area with a gouge is nearly impossible (left), but using a parting tool makes shaping the lower region of a form a breeze (below).

Principle #1: Cutting with the shaft horizontally positioned and the edge of the tip at the centerline of the work piece will produce the most efficient and controllable cuts. Cutting below the centerline causes the edge of the tool to drag on the wood, and cutting above this centerline can cause the tool to skip or pull into the wood, which invariably causes a catch.

Principle #2: The greater the amount of surface area of a tool’s edge that makes contact with the wood, the smoother the cut will be.

However, greater surface contact also means greater drag of the tool’s edge on the wood, often resulting in a catch. It is for this reason I use ¼"- and ³/₁₆"-wide tips on my straight and bent tools, respectively. In fact, I never go larger than a ³/₁₆"-wide tip on my bent tools, because even the ¼"-wide tip creates so much drag on the wood it can cause excessive vibration when cutting at depth in a hollow form.

Principle #3: Extending the tool tip further and further from the support of the tool rest makes it harder to control the tool when cutting the wood.

I’m sure this last statement sounds a bit obvious, especially since part of the challenge in doing hollow forms is to be able to cut at some distance from the tool rest. However, understanding this principle is critical. It relates to every aspect of the design of every cutting tool that has ever been used.

When turning wood, the deeper you try to work off the tool rest, the longer the tool needs to be. This means longer handles to create enough balance so the front end of the tool won’t drop down when cutting the wood. Similarly, when going deeper into an object, the shaft of the tool needs to also be proportionately larger in diameter.

Unfortunately, there is no specific formula for determining what exact diameter and length of tool will cut at various distances off the tool rest. And if there was a formula, all you’d have to do is change the density or dryness of the wood and the numbers would go out the window. This is where trial and error and good old-fashioned common sense become your best guides. And as usual, it’s always best if you err on the side of greater rather than lesser when it comes to support. If the tool shaft starts to flex or vibrate on the wood, you should use a larger diameter, longer shaft, smaller-diameter cutting tip, or all three.

Bevel angles affect the sharpness and edge durability of a tool. The left tip is very steep, making it sharp but weak. The middle tip isn’t sharp enough, and the bevel could rub against the surface of the wood. The right tip solves these problems by using a 65˚ edge. This tip is both sharp and durable enough to cut with efficiency and control.

Principle #4: The bevel angle on interior-scraper tips determines the efficiency of the cut.

The ideal angle for the bevel is approximately 30˚ back from vertical (right tip in the photograph at top). If the angle of the bevel is cut back too steeply, say, 45˚ (left tip), the edge will of course be sharper. But this is also a weaker edge that will tend to vibrate on the wood, particularly dense or dry woods. In effect, there isn’t enough mass behind the edge to support it when making the cut. Conversely, if the angle is cut too vertically, say, 85˚ (center tip), the edge may not be sharp enough to efficiently cut the wood. In most cases, this will cause you to push the tip harder than necessary against the wood to get the cut to start. Unfortunately, once the edge finally does engage the wood, it is quite likely it will rip uncontrollably into the fibers rather than cutting smoothly through them.

This principle will be magnified when you are working on forms with very thin walls where the surface can flex under the pressure of the edge pushing against it.

Principle #5: If you happen to be working at some length from the tool rest on a thin-walled vessel and you accidentally make too deep a cut, the bent tool will kick the tip down and away from this thin surface, thus preventing it from ripping through the wall. Conversely, because the tip of the straight tool is so well supported by being positioned directly in-line with the shaft, it will rip through the thin wall before you have a chance to retract the tip from the wood. I find it an interesting phenomenon that when using bent tools in thin-walled hollow forms, the tip that does the best job for interior-finishing cuts is also the one with the least support from the shaft that holds it.

Principle #6: Creating triangles throughout the body is the most efficient way to create good support for the tools. Triangles also help to maximize the energy to the cuts and minimize muscle fatigue. For more on how to set these triangles up, see Chapter 8, "The Body," page.

If you put the principles together, what you come up with is a basic understanding of the meaning of the term support. That is, a combination of an edge that’s supported by a certain mass in the tip, a tip that’s supported by a certain mass in the shaft of the tool, a tool that’s long enough for the turner to support the cut at long distances off the tool rest, and a body that is comfortably triangulated for balance in support of the tool. All of these factors need to be working together to provide efficient, safe, and vibration-free cuts.

The straight boring bar is used to initiate cutting the interior of a hollow form. Cuts are made from slightly left of center toward the center in a plunging movement. This will create a cavity of some depth that is then widened in order to make room to introduce the bent tool.

The straight boring bar is used to make the first interior cuts in a hollow form and do the bulk of interior cutting.

The bent tool is used to remove additional mass from the interior and to establish the finished surface at whatever wall thickness is desired.

The bent tool is used after a central opening has been bored with the straight boring bar. The bent tool will be used to cut and define the final thickness of the walls.

Hook and loop tools

Hook and loop tools have been used for centuries, primarily in Europe and Japan, and are used almost exclusively for end-grain turning in bowls, vases, and hollow forms. These are cutting tools as opposed to the scraping tools that I use, and they work off the fibers from a central hole drilled in the forms. The primary reasons I do not use these tools is because virtually all of my forms are cross grain, but also that the entrance holes I make in my pieces are too small to get the shafts of these hook tools inside.