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Scrapers,  Part 2

Burnishing, Applications, and Negative Rake

More about Sharpening Scrapers

Here’s a quick summary with a few additional comments, followed by more about the burnishing process.

To grind the bevel, position the tool rest of the grinder at the proper angle, lay the body of the scraper flat on the tool rest, and then pivot the scraper as necessary to bring all parts of the bevel into contact with the wheel.  For straight-edge scrapers, you may need a fixture or guide similar to what is used for sharpening a skew.  

To form the burr by grinding, use as light a touch as you can manage.  Stop frequently and check to see if you can feel the burr. When you can feel it, you are done.  The practice of watching for sparks to come across the top of the tool may have you go too far, leading to excessive curl and a ragged burr.

Grind and hone.  Grind the bevel with a 120-grit wheel.  Then use a hone on the top surface to remove the burr formed by the grinder. Create a new burr by honing with the hone flat on the bevel.  

The honing part of this procedure doesn’t take much.  Literally, one or two strokes are all that’s required to form the burr.  In fact, a greater effort will be needed to remove the burr created by grinding the bevel, and you want to be sure that burr is gone before beginning to regenerate the new, smaller burr.

Here’s the method that gives me the best control in forming the new burr. Place the scraper, top surface up, across the corner of a worktable.  Press the hone against the bevel with just enough pressure to be sure it’s seated flat on the bevel.  Then, while keeping the hone in constant contact with the bevel, swirl the hone in an elliptical motion while advancing the point of contact along the cutting edge.

Feeling the burr.  The burr from a grinder is easy to feel; it is prickly, and it gives the impression of some serious sharpness. That’s not what you’re looking for when forming a burr by honing. Instead, the best burr may be one that’s almost impossible to feel.  

One technique for detecting a burr is to drag the soft pad on the tip of your (less-calloused) ring finger across the edge, first along the top surface of the scraper and then up the bevel.  If a burr is present you should feel the difference, but the difference may not be dramatic.  

I find the practice of dragging the burr over a thumbnail to be unreliable because a keen edge will raise a shaving even when no burr is present.  And it messes up your nail polish.

Does such a small burr make a real difference in the way the tool cuts?  Yes. Even though a keen edge with no burr will remove wood and even produce shavings, the major difference is in the quality of the surface left by the cut.  The burr is far superior.  

Burnishing.  Grind the bevel as described above then use a hone applied to the top surface to remove the burr.  The tool will then be ready for burnishing.  However, if you want to go the extra mile, hone the bevel to remove the worst of the scratches left by the grinding wheel.  This will give a burr that is straighter, more nearly continuous, and therefore stronger.   

The burnishing rig I use is shop-made and consists of two hardened steel pins stuck into holes drilled in a piece of quality plywood.  The pins are 5/16” in diameter and are about 2” long.  One pin serves as the burnisher; the other serves as a pivot for levering the tool against the burnishing pin.  The plywood is screwed to the top of a sturdy table.

(See drawing in previous article.)

Hardened steel pins can be purchased at a hardware store.  An alternative is to buy a ready-to-use burnishing fixture.  (Veritas scraper burnisher for woodturners, available from Packard Woodworks and other sources.)

If burnishing is new to you, my suggestion is to creep up on it. Mount a piece of spindle stock on the lathe for making test cuts. Test the scraper after removing the burr but before burnishing, just to establish a reference point.  Place the tool flat on the rest, handle horizontal, then ease it up against the wood exactly at center height. See what happens.  You will probably raise a little dust but few shavings unless you press the tool against the wood.  Note the quality of the surface left by the cut.

With the body of the tool horizontal, use a pivoting motion to run the cutting edge across the burnishing pin while applying “a little” pressure.  Can you now feel a burr?  Test it to see if it cuts any better.  Repeat the burnishing with a bit more pressure, and do the cutting test again.  At some point, you should get thin shavings just by easing the tool up against the wood with only the slightest pressure pushing the tool against the surface.  

If it doesn’t seem to be working, chances are that the bevel is rounded right at the cutting edge. In this case, regrind the bevel and remove the burr, but don’t go the extra mile and hone the bevel. That’s probably where the rounding occurred.

It’s possible to over burnish (press too hard) and wind up with a burr that curls back from the edge of the bevel.  A symptom of this is having to either push the tool against the wood or raise the handle to get it to cut, which it will then do rather aggressively. The remedy is to remove the burr by honing the top surface and then burnish again, with less force this time.

A scraper with no burr.  This refers to a scraper that has had the burr removed by honing the top surface. The tool is placed flat on the rest, handle horizontal, and the cutting edge is simply pushed against the workpiece.  The result is scraping in its most basic form. This cut is not likely to produce a catch but the resulting surface is likely to be rather rough.  How rough, exactly, depends on the wood.   

When using a scraper with a burr, some tight-grained woods tend to grab the burr and pull the tool deeper into the cut.  This effect gives a cutting action that essentially cannot be controlled. The solution is to remove the burr.  The best (or worst) example of this that I have encountered is dogwood, but there are other woods that act the same.   

Smoothing a surface.  Tool marks left by a gouge can be removed and the surface refined with a scraper.  Better control of the tool is possible if you use it flat on the rest.  Set the tool rest so the cutting edge is at center height. It’s best to use only a minimal burr in this application, one produced by the grind-and-hone method.  

Simply run the tool back and forth across the surface while keeping the handle level. You should produce very small shavings with very little pressure applied to the tool. Keep the width of the cut fairly narrow. A cut 1/8” wide is wide enough.

If you find you have to raise the handle to get the shavings, the indication is that the bevel turns in right at the cutting edge. This probably results from failing to keep the hone parallel to the bevel when forming the burr.

If you have trouble taking only a very light shaving, the burr is probably too prominent. Remove and then reform it, with a bit less honing this time.

A word of caution.  If you take a bit of a shortcut and use the burr straight from the grinder for the smoothing cut, the result might be somewhat unpredictable, and disappointing. The burr will probably be more pronounced than what is needed, and the tool may be grabby and hard to control.  

In the worst case, the burr may have a significant curl which will require you to either raise the handle or apply significant pressure to get it to cut, and once it begins to cut, it may be rather aggressive.  

Flat-on-the-rest scraping with the handle elevated (tip of the tool pointed downward), in my view, is not a good practice.  If the burr turns out to be overly aggressive, the tool can be trapped (or so it seems) between the tool rest and the workpiece and produce a very heavy cut if not an outright catch. Further, with the handle elevated, the tool tends to be pulled toward the cut. The effect is slight, but I have felt it and I didn’t like it very much.   

To the casual observer who might be unaware of the idiosyncrasies of the burr, tilting the tool forward would seem to place it in trail, which should reduce the cutting action and give a safer cut. But such is not the case.

These unpleasant characteristics are not caused by having the tool rest set at the wrong height, and, in turn, changing the height of the tool rest will not remedy the situation.  The problem is, in a nutshell, too much burr.  

Many turners use the burr straight from the grinder with the tool flat on the rest and have no problems at all. But these folks almost certainly have many years experience and have developed considerable finesse in forming the burr and applying the tool. For the rest of us, using a minimal burr is the safe way to go.

Negative-rake scraper.  The oft-quoted advantage of a negative-rake scraper is that it will give a clean cut in wood so twisted, hard, and gnarly that ordinary scrapers or turning tools cannot deal with it.  

A negative-rake scraper is not just an ordinary scraper presented with the cutting edge in trail.  The negative rake is embodied in the geometry of the bevels that form the cutting edge.  As long as the bottom bevel angle is less than the rake angle, the angle between the bevels at the cutting edge will be greater than 90º.  Therefore, grinding will not produce a burr.  (See Burr vs. bevel angle in the previous article.)  

The salient feature of a negative-rake scraper is that you can sharpen the tool on a grinder and not produce a burr.  The absence of the burr, in conjunction with the negative rake, gives it the ability to produce a clean surface on gnarly wood.

Will it cut without a burr?  Yes, but how well it cuts depends upon it having a keen edge.  It’s a bit out of the ordinary to think of an edge as being keen when the included angle is more than 90º, but that’s what it takes.  Further, considerable pressure is required to force the cutting edge against the wood.  

There’s room for compromise.  If the bevel angle is just a degree or two greater than the rake angle, the angle at the cutting edge will be less than 90º.  This means that grinding will produce a burr, but it will be a minimal burr.  This will make the tool cut easier, but if you go too far, you will sacrifice the characteristics of the negative rake.  The tool then reverts to being an ordinary scraper.

I tested two negative-rake scrapers on a bone dry piece of walnut. One was M2 high speed steel; the other was a test scraper made from an old file.  Both produced a surface smooth as glass, all the way around, end grain and all.  

However, the negative-rake ran into problems when I applied it to a sidegrain blank of kiln-dried pine, which was relatively dense and hard, for pine at least.  Considerable pressure had to be applied to get it to cut, and some tearout was evident.  On this piece, an ordinary scraper with a burnished burr used in the shear-scraping mode gave better results.  

Two things, I believe, come into play here.  First, negative-rake scrapers do best with hard woods, the harder the better.  Second, a shearing cut will almost always yield a better surface than one where the wood meets the cutting edge straight on, as it does with the negative-rake scraper.  It appears that the advantage inherent in a shearing cut won out over that of the negative rake.

I reground the scraper I made from the file to give an angle between the bevels of less than 90º, by just a degree or two. But even though the angle was less than 90º, grinding did not produce a burr, which I attribute to the harder metal of the file. However, the small reduction of the angle made a significant difference in the way the scraper cut, requiring much less pressure against the wood.  

I have yet to form any definite conclusions about negative rake scrapers.  In the limited testing I’ve done, a spindle gouge used in an extreme shearing mode gave equal or better results. (See Gouges, Part 7.)  However, I have not done any tests on a really hard and gnarly piece of wood, which is where a negative rake scraper is supposed to excel.  

Finally, here’s a note of caution, a disclaimer:  files are brittle and are apt to break if placed under stress.  Consequently, you should not use old files to make scrapers.  I did it for testing purposes in very limited and controlled situations, but the file I used was 0.23” thick, a much heavier bastard (file) than an ordinary mill file.  In the manner I used it, it was not about to break.  

Up next: shear scraping and how to create a burr that lasts more than 15 seconds.  

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