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Scrapers:  Part 1

Introduction  


More words probably have been expended in discussing and debating the use of scrapers than for any other tool except, perhaps, the skew. And with all the arguments presented, I’m not aware of a single documented case where someone left the discussion with an opinion different than what was held when it began.


My guess is that a lot of the varied opinions on scrapers arise because of an unusual feature of the tool:  the burr formed on the cutting edge plays a major role in determining how the tool will behave in a cut, but you cannot see the burr without some sort of magnification.  You can feel it, but you cannot see it.  Consequently, two scrapers that look identical may behave quite differently when presented to the wood in the same manner.  


And so, the discussion continues.  This is an introduction to scrapers.


Scraping vs. Cutting.  A scraper may be used in two different ways. One is pure scraping where wood is literally scraped away by forcing a keen edge against the wood. This method works, but it is the one most likely to leave a rough surface.  


The other method is to use the burr as a cutting edge.  This technique produces fine shavings and leaves a good surface. Frequently, a scraper used in a manner known as shear scraping will clean up and smooth a surface better than any other tool you can bring to the application.  


Scraper types.  Scrapers are available in many shapes and sizes to accommodate a variety of needs.  The most versatile is the round-nose scraper.  A more specialized tool is the square-end scraper.  Then there are scrapers shaped for specific applications like reaching under the rim of a bowl, for example.


Another consideration is that, like a gouge, a scraper must not be extended over the rest farther than what its design will tolerate. Typical scrapers are 1/4” thick and can be extended, comfortably, about 4” over the rest before vibration and chatter begin to be a problem.  However, this is totally dependent on the type and depth of the cut being made.  For a very light, shearing cut, a 1/4” scraper may be extended as much as 5” over the rest with no problems.


Heavier scrapers are available for longer extensions.  The heavy duty versions are 3/8” thick; the thickness of the extra heavy duty type is a full 1/2”.  These are useful for reaching to the bottom of a fairly deep bowl or similar vessel.


Applications.  Generally speaking, scrapers are used for taking very fine cuts in the process of smoothing a surface or taking just a wisp of material away from a tenon or recess to achieve a fit with a matching part.  They do not represent the best choice for removing large quantities of waste wood.


Inside a bowl, a scraper can be used to finish the side-to-bottom transition area and making the final cuts on the bottom when the geometry is such that a gouge cannot be used.  Also, a scraper can often be used to clean up the area underneath the rim of a bowl where the rim turns in significantly.  


A small round-nose scraper is good for shaping the bottom inside of a goblet.  A square-end scraper is typcially used to flatten the bottom of a box and shape the transition area where the bottom meets the side.


Sharpening a scraper.  There are two aspects to sharpening a scraper.  One is grinding the bevel to the desired angle.  The other is forming a burr that is suitable for the application.  Many turners combine these two steps into one in which they simply grind the bevel and take whatever develops in the way of a burr.  My preference, however, is to grind the bevel and then form a burr that’s tailor made for the task at hand.


Grinding the bevel is a simple matter for most scrapers.  It can be done by setting the tool rest of the grinder to the proper angle and placing the tool flat on the rest.

The bevel is then pivoted against the wheel.  


Three ways come to mind for forming a burr.  One is by grinding.  Another combines grinding and honing, and the third involves burnishing.  The burrs that result from each of these methods are different and each one has its advantages.  



The burr.  The grinding process produces a tiny ridge of metal at the top of the bevel.  This is the burr.  A diamond hone applied to the bevel will also produce a burr, but as you might expect, to a much smaller degree than a grinding wheel.  


I have access to a stereo microscope that I can use to get a pretty good look at the burrs produced in various ways.  Its maximum magnification is only 7x, but that is adequate.  


It was an eye-opener for me when I first saw the burr on one of my scrapers. The first thing that caught my eye was the rough, plowed-field look of the bevel.  The ridge of debris that I took to be the burr was not straight, not continuous, and not uniform in size and shape.  It looked like slag from a welder piled on the edge of the tool and was a chocolate brown instead of being shiny and metallic.  Further, the burr was not strongly attached to the metal underneath.  It could be removed easily with the point of a needle.


Grinding and honing.  Another approach is to grind the bevel, remove the burr that results, and then reform the burr with a diamond hone.  This gives a much smaller burr that is better defined.


A few swirls of the hone placed flat on the top surface will remove the burr.  The burr can be regenerated with a stroke or two with the hone placed flat on the bevel, but in doing this, you must be careful not to round over the cutting edge.  A scraper sharpened in this manner is ideal for using flat on the rest, as explained a bit later.


A burr from burnishing.  Another method for producing a burr is based on the technique used by workers-in-the-flat to put a burr on a cabinet scraper.  They first file a square edge on the scraper and then, with it held in a vise, run a hardened steel burnishing tool along the edge to form the burr.


Burnishing the cutting edge of a woodturning scraper is similar.

After forming the bevel, the tool is pivoted against a hardened steel pin mounted vertically on a base. This is described in detail in the next article.  


A burnished burr is quite different from one produced by grinding.  It is straight, continuous, and uniform.  Further, because it results from deforming the native metal, the character of the metal in the burr is essentially the same as that underneath.  And it is very strong – impossible to remove with the point of a needle.


Grinding onto and off the edge.  A burr is formed in both cases, but the one produced with the wheel or hone going off the edge is much smaller.  It is marginally stronger, or so it seems in the limited testing I’ve done.  Some turners grind scrapers upside down in order to achieve the burr produced by grinding off-the-edge.  


Burr vs. bevel angle.  I did a quick test to see what the nature of the burr might be at bevel angles close to 90º, and the result was a bit surprising.  With an angle of 92º, no discernable burr was formed – none.  At 87º, a small burr was generated.  At 80º, the burr was indistinguishable from what I normally get on my scrapers, which are ground at 70º.  


I checked this on two different scrapers.  One is made from M2 high speed steel; the other is made from an old file just to test the nature of the burr.  I used a 120-grit wheel for grinding.  The result was the same: no burr for an angle greater than 90º.  


I think there may be a simple explanation for why no burr is produced at a bevel angle greater than 90º.  During grinding, the wheel advances into the steel.  As soon as the burr is produced, it is ground away by the advancing wheel.  For angles less than 90º, the wheel still advances but the burr manages to barely stay ahead, and it survives.


I suspect the angle that marks the divide between burr and no-burr depends upon the metal.  When I ground the file to an angle of about 87º, a keen edge was formed with no burr at all.  M2 high speed steel definitely produced a small burr at this angle.


To check out smaller bevel angles, I examined the burr at the cutting edge of a gouge where the angle between the flute and the bevel is in the range of 30 to 40º.  I found an extensive burr, but one that was very thin and feathery, hardly capable of supporting itself. The lightest touch to the wood took it away.  


Curl.  This term is sometimes used to describe a burr, as in  “The burr tends to curl back over the top face of the tool,”  or  “The burr seems to exhibit a significant curl.”  In my thinking, the geometry of the burr produced by grinding is not sufficiently well defined to take the term at face value.  I have not seen one that exhibited a smooth, curved surface that even remotely resembled a curl.   


However, aggressive grinding will produce a burr whose ragged, jagged peak is set back from the edge of the bevel, and in a cut, such a burr will  act as if  it curls back over the top face.  Even though the concept of a curl may not be totally accurate on the microscopic level, it does communicate the fact that the peak of the burr is not in line with the bevel.  It provides a model  that makes it easier to visualize the characteristics of a burr.  Therefore, I will use the term because it is rather descriptive.  


Terminology aside, the effect of the curl (or having the burr set back from the edge of the bevel) is real and easy to demonstrate.  Take two identical scrapers, ground with the same bevel angle, but on one, form a burr that’s in line with the bevel by grinding and honing. On the other, form a burr with a significant curl by aggressive grinding or over burnishing.  


Now test these two seemingly identical scrapers.  Place the tool flat on the rest with the handle horizontal and ease it up against a small spindle, but don’t push.  The scraper with the in-line burr will cut and raise a small shaving.  The other, the one with the curl, will not.  

However, if you push on the one with the curl, it will “take hold” and then cut very aggressively.  Or, instead of pushing, raise the handle so the burr is tilted into the wood.  Again, it will be very aggressive and grabby.  It may also exhibit a tendency to self feed, which scares me just a little.


Next up: more on burnishing and a discussion of a negative-rake scraper.


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