Scrapers: Part 3
Shear scraping. With this method, the scraper is not laid flat but is rotated to an angle of 45º or more to the tool rest. This brings the cutting edge more in line with the approaching wood so that the wood encounters the cutting edge at an acute angle instead of 90º, producing a shearing cut. The method is known as shear scraping. It is widely used because it offers several significant advantages.
The primary advantage is that the shearing cut typically leaves a much better surface. On the other hand, the 90º angle of flat scraping is the worst possible orientation in regard to producing tearout and torn grain.
Considerable latitude exists for the type of burr placed on the cutting edge. If
you choose to do so, you can go straight from the grinder to the workpiece, and it
will work just fine. With round-
An additional benefit is that as long as you are careful to follow one simple rule, you can forget about getting catches (explained below). You have to pull the cutting edge against the wood to get it to cut, and there is no tendency for it to self feed. You have complete control. Pull harder, and you get a deeper cut. Ease up, and you will get a lighter cut.
One new turner I know was terrified of letting the tool contact the wood without it being flat on the rest. The concern was that the wood would grab the tool and slam it down hard. Rest easy; this won’t happen as long as (1) the active part of the cutting edge is toward the lower part of the tool; and (2) you follow the rule given below for avoiding catches.
Remember, shear scraping can be done with the bottom wing of a gouge, but a dedicated scraper brings more steel to the task.
Avoiding catches while shear scraping. The wood exerts a force on the cutting edge that causes the tool to try to skate backwards and back away from the cut. This is good because it lends stability to the whole process. Further, because one section of the tool is anchored on the tool rest, the skating action causes the tool to pivot about the anchor point. As a result, the end of the tool, when it skates, travels in the arc of a circle.
Here’s the way to avoid catches:
Always position the tool at such an angle to the cut so that when it skates, the cutting edge swings away from the wood into thin air.
In many cases, this position will occur naturally; you don’t even have to give it a thought. But, it’s easy to wander into risky territory, as shown in a drawing a bit farther down in the section on shaping the bottom of a bowl. Keep your guard up.
Longevity of the burr. It’s generally accepted that a burr applied to a scraper is good for only about 15 seconds of actual cutting. It goes away quickly. This is in line with what I observe for burrs formed by grinding. However, quite the opposite is true for burrs formed by burnishing. The difference is easy to understand.
In Part 1, a burr produced by grinding is described as being not straight, continuous, nor uniform in size and shape. Further, it consists of highly disturbed metal that is not strongly attached to the metal underneath. It is easily removed with the point of a needle.
On the other hand, a burnished burr is straight, nearly continuous and uniform, and consists of metal that is largely undisturbed. It is strongly attached because it arises from deforming the native metal. You cannot remove it with the point of a needle. (The drawing at right is from Part 1.)
The fact that a burnished burr is nearly continuous contributes to its ability to withstand the cutting action of shear scraping. The part of the burr downstream from where the cut is actually being made supports the part that’s doing the cutting. This is because the forces exerted on the burr are directed primarily along the length of the “ridge,” a consequence of the shearing cut.
Here is a real-
The technique used was shear scraping using a scraper with a burnished burr. One burr did the entire job of truing and shaping the inside, which took at least 20 minutes to do. When viewed under a magnifier after the job was completed, the burr was still there. The finished bowl is about 8” in diameter with a wall thickness that varies between 1/16 and 3/32”.
Shear scraping with the bevel rubbing? Even though you don’t hear much about this technique, it will work as long as the burr is in line with the bevel. Grinding and honing is the best way to produce such a burr.
The advantage of rubbing the bevel is that the tool is easier to guide. It handles much like a gouge. Further, slight irregularities in the rate at which you advance the cut have practically no effect. The photo at right is a comparison of the surface left by shear scraping with the bevel rubbing and the surface left by a skew.
The nominal orientation of the tool that will result in a bevel-
Applications for Shear Scraping
This section briefly describes several applications for shear scraping. Unless stated
otherwise, a round-
Making a straight cut on a spindle. This a good practice cut and it may be used to check out the characteristics of a burr, but a skew is the preferred tool from the standpoint of surface quality. On the other hand, shear scraping is much safer if you aren’t up to speed with a skew, and if you rub the bevel, the surface will be almost as good. To gain experience, try different types of burrs – straight from the grinder, ground and honed, and burnished.
Creating a concave surface on a spindle. Normally, this is done with a spindle gouge, but it can be done, and with good results, by shear scraping. Cutting uphill or downhill makes a difference. You will usually get a better surface by cutting downhill.
Keep in mind that a scraper is usually not the most efficient tool for removing large quantities of wood. A spindle roughing gouge or a regular spindle or bowl gouge is a better choice for rough shaping. You can then switch to shear scraping to refine the surface.
When you’re working within a concave area, be careful to follow the rule for avoiding catches while shear scraping. That is, position the tool so that any tendency for it skate backwards causes it to pivot away from the wood.
Removing high spots or tool marks from the outer profile of a bowl or other vessel is a task for which shear scraping is ideal. Light, skimming cuts are the norm. Another option is to use the tool flat on the rest, as described in the previous article.
Working under an in-
Shaping the bottom of a bowl. Almost always I use shear scraping to form the final contour at the bottom of a bowl. This avoids having the rim get in the way of a gouge, and the resulting surface will be as good as can be produced in this application.
I usually work from the center toward larger diameters because of the better visibility. In some cases this gives rise to a cut that is slightly uphill, but it usually does not present a problem.
Be careful about anchoring the tool on the rest in line with the axis of the lathe
and then pivoting the cutting edge to the left. This can cause you to violate the
Smoothing the wall of a bowl is just a continuation of working at the bottom. I almost always work from the bottom toward the rim. However, I seldom go up the wall very far if it is fairly thin because of the problems with vibration and flexing described below.
Caution. On the inside of a bowl, a curved scraper will frequently match the contour of the surface so that an inch or more of the cutting edge could possibly engage the wood. This is a hazardous situation because the force required for such a wide cut is very large. At a minimum, the tool will be hard to control, and at the worst, the piece could be pulled off the lathe. My rule of thumb is to let no more than about 3/8” of the cutting edge make contact with the wood at one time, and this is a lot.
Do not fall into the trap of thinking that you can transfer the curve of the scraper to the surface of the bowl by simply easing the scraper up against the wood. You will be in for an ugly surprise if you do. I learned this the hard way when I was trying to find my way through the shavings a few short years ago.
Working near the rim of a thin-
The difficulty arises from the flexibility of the rim and the fact that turning tools exert a force on the wood. You are likely to get vibration and chatter, or worse, the tool may grab and destroy the piece.
The best option is to use a shear-
Make sure the tool is sharp and use a very light cut. Present the tool with a rotation well beyond 45º to the rest and with the handle swung well to the left. Don’t use pressure against the surface as a crutch for guiding the tool. Visualize the process as guiding the tool through the air at zero distance from the surface.
This is risky, especially with bone-
Flattening the bottom of a platter or plate. This amounts to a straightforward application of shear scraping, but the challenging part is determining where the scraping should be done. Using a short straightedge may be helpful initially, but it will take you only so far. Once you get it close to flat, rub your hand over the surface. You can feel the high spots better than you can see them. If the surface texture tends to confuse the issue, put a thin handkerchief or a paper towel between your hand and the surface.
Creating a chamfer (bevel) at an edge. It’s a good practice to always clean up corners and sharp edges on a turning, for safety reasons. A shearing cut makes quick work of this. Simply touch the tool to the corner for about 2 seconds, and it will be done.