More Woodturning Magazine
This Article was printed exclusively for More Woodturning Magazine subscriber:
Preventing Catches through Tool Controlby Lyle Jamieson
I often hear turners refer to catches as if they are normal: “Everyone gets a catch now and then” they say. I disagree! I frequently discuss tool control and the four different cuts I use: push cut, pull cut, scrape cut, and sheer scrape cut. If you use these cuts properly, catches will be a thing of the past. Here are the basics of each of the cuts and when I use them. Let me talk about these cuts in a bowl first.
The first is the push cut. The push cut with a bowl gouge is the work horse and we use it a majority of the time. Two basic rules apply: 1) always have bevel support; and 2) only use the tip of the tool grind, don’t use the wing. The push cut is used to "hog off", to shape, and to finish. It is used on spindle work and on the inside of bowls. The push cut can be aggressive and take a large amount of wood away with each pass, or it can be a small finishing cut leaving a great finish on the wood that needs little sanding. It can also be a shaping or a refining cut.
The flute is facing the direction of the cut and the handle is parallel to the floor or slightly down. None of the four cuts have the handle up with a bowl gouge. The flute is tilted or twisted on a 45-degree angle to the wood direction so the tip of the gouge is slicing on an angle and leaves a clean surface on the wood (see photo: Push cut 1).
The direction of all my cuts should always be going downhill to the grain, slicing supported fibers, inside of a bowl cut from rim to the bottom. This slicing action will always leave a better surface and takes less sanding than with scraping mode. The first and foremost rule for the push cut is the bevel is supporting the cut (see photo Push cut 2). The bevel is the directional finder and the controlling factor. The bevel prevents catches.
Push cut 2: This is a push cut starting on the inside of the rim of a bowl. Your view is looking right down the bevel. The bevel is against the side wall of the vessel. The sharp edge is peeling off a shaving at the tip. The cutting action is toward the waste wood being removed, not toward the side wall.
I have a procedure of lining up the bevel with the existing surface of the wood before I take EVERY cut. I think Bonnie Klein first coined the phrase "A-B-C’s": Anchor, Bevel, and then a Cut. My modified A-B-C’s goes like this: (A)nchor, “line up the” (B)evel, then make a (C)ut. This process does two things. First, it gets the bevel lined up with the existing surface. The existing surface will give you the direction the cut will be advancing. You have two choices at this point: go forward with the cutting action, or make an adjustment to the direction of the cut before you proceed with the cut by swinging the handle. Without lining up the bevel before each cut, the direction may or may not be going in the right direction. The result will mean an adjustment of swinging the handle to correct the direction AFTER the cut begins. Not good!! Best case scenario is there will be undulations or tool marks on the surface from the adjustment. Worst case is you get a catch when the bevel is not supported by starting a cut with the edge first without the bevel support. So, the second thing this process does is make sure the bevel is supported before the cut even gets started. A push cut without the bevel will get you a catch.
Okay, now how do we line up the bevel first? See photo (Push cut 3). Let’s use a cut on the inside of a bowl for example. With the lathe running, place the heel of the bevel of the bowl gouge gently against the wood at the inside of the bowl near the rim where the cut will start. The sharp edge is now facing out into air. With the bevel rubbing gently against the wood surface, start “wiggling” it back and forth very slightly. Pushing the bevel forward and backward in small movements will allow you to see when a shaving will be produced. Now, push/swing the handle slowly away from you, to the right, and keep the wiggle going. As the bevel now moves closer and closer to the wood surface it will make a little wisp of a shaving right at the very tip of the bowl gouge. Then, and only then, is the bevel perfectly lined up with the existing surface. Proceed with the cut or pull the tool back out of the bowl to the rim and start a new pass without swinging the handle and losing the direction just found.
Push cut 3: For a push cut shown on the inside of a bowl, I line up the bevel before I start a cut. I touch the bevel against the wood first and move the handle to get a little wisp of a shaving to appear at the very tip to line the bevel up with the existing surface before every pass.
The second cut, the pull cut is on the outside of a bowl where the head stock will be in the way and I cannot use the push cut. I follow three rules for pull cuts: Rule #1: The handle is way down, like at a 45-degree angle down and tucked into your thigh (see photo: Pull cut 1).
Pull cut 1: This is a view looking directly at the outside bottom of a bowl. Note the angle of the sharp edge is on a 45-degree angle to the wood movement with the handle way down. This will slice the fibers supported from bottom of the bowl to the rim. The shavings are small and only at the tip with the bevel support from the side of the grind.
Rule #2: Use the tip only--do not get a shaving going down on the wing of the bowl gouge. The shaving is small here. We are refining the shapes not hogging off big shavings at this stage.
Rule #3: Bevel support is on the side of the gouge (in contrast to the push cut where the bevel is supported at the tip of the gouge). See photo: Pull cut 2.
To start a pull cut, the flute is pointing almost straight up. Bevel touching the wood, A-B-C's again. Roll your wrist on the handle-hand clockwise until you see a wisp of a shaving. At this point, the bevel is lined up with the existing surface. Pull the bowl gouge forward and cut in the direction the flute is facing from the bottom of the bowl to the rim. As you make the curved line of the bowl, you roll your wrist so the bevel follows the shape you want to make on the bowl. This is one of the hardest cuts to master because we don’t use it very often. It takes small shaping cuts. The pull cut is not usually a finish cut and we do not hog off with it. The bevel support is a must here, too, to prevent catches.
The third cut is the scraping cut. We can scrape with many tools. The rule for scraping is that the cutting edge must touch the wood at less than a 90-degree angle to the surface of the wood. When we scrape with the bowl gouge, we twist the flute in and face it directly at the wood surface. A parabolic flute is necessary. A flute with a “U” shape flute will be flirting with a catch (see Scraping figure 1).
We use the broad brush of the middle of the wing and stay away from the tip and the corner of the wing. This is typically used to clean up the concave area in preparation for the glue block or to prepare for a faceplate. I don’t use the scraping cut much because the push and pull cuts leave a better surface on the wood with less torn out grain and less sanding.
In contrast, when we are doing the inside of hollow forms with the HSS cutter we are in scraping mode. We cut slightly above the center line to keep from violating the 90-degree rule inside hollow forms. The carbide cutter I have on my hollowing system uses the bevel-supported push cut and a negative rake scraping cut, both with the little 3/16-inch nanograin carbide cutter.
The last and fourth cut is the sheer scraping cut. See photos Sheer scrap cut 1 and 2. This is scraping on a steep angle. So, using the bowl gouge, we would scrape with the handle down, way way down, the steeper the angle the cleaner the cut. In a bowl, I use the sheer scrape to refine and finish only the outside of my bowls. The importance of grain orientation comes in here again. On a bowl, to go downhill with supported fibers, we must make the cut on the outside of a bowl from the base to the rim. I never scrape or sheer scrape on the inside of bowls.
Sheer scrape 1: This view is looking at the side bottom of the bowl with the gouge flute facing at the sidewall. Note the 45-degree angle the sharp edge is to the rotation of the wood. This slices very fine shavings downhill with supported fibers.
On the outside of a hollow form we need to have the angle of the cut facing downhill. Downhill on an end grain hollow form (or spindle mode) is from the largest diameter to the smaller diameter. The steep angle is peeling the fibers with little angel hair shavings downhill.
All this is hard to visualize from text. To see the cuts in action, see my Basic Bowls DVD, or check my YouTube clips to get a glimpse of the cuts as I make them. But the better and faster way is to seek out a mentor in your club and get some help from someone who has the skills. Or, to really kick it up a notch, come up to Michigan and take a class with me!
Click here to read about the author, Lyle Jamieson.