Scraper Sharpening
Collated from newsgroup postings.
Whether you need a burr on the cutting edge of a scraper depends on how you sharpen the tool, how you are presenting it to the wood, and what type of cut you are trying to make.
Therefore, I invite you to make a simple test in three steps, using a scraper and a piece of wood in your lathe. This will provide an illustration of how a scraper cuts, answer any question regarding the burr, and you just may find that your scraper will cut better and have more uses than you thought possible.
First, take a 1" full-round or radiused scraper that you usually use, and grind the bevel to a 75deg included angle between the bevel and top of the tool. Don't be concerned with accuracy of the angle. Anything between 70deg and 75deg is close enough.
First test
Dress the top surface of the tool with a fine stone. The grit isn't important. I use either a fine grit "India" slip stone or a diamond file. This will remove any of the burr that was raised from the initial grinding of the bevel. The first time may take some work if the surface is rough.
Grind the bevel again at the same angle. Observe the glowing "ball of fire" where the steel touches the wheel, and rotate the scraper with one smooth pass that maintains the spark at the tip of the tool along the entire edge. You will now have as good a burr on the scraper as you can get, and its cutting edge will have the same texture as the grinding wheel.
Put a piece of a piece of cherry, walnut, or similar wood that is relatively dense and easily turned, and rough turn the outside to a shallow shallow bowl shape with a gouge. Torn grain is good, because we will use it later.
Now we are ready for the scraper. Use the scraper flat on the tool rest and aimed downhill with the conventional "trailing" cut that we have all been told is the correct way to do it. Observe the wood surface, the chip that is produced, and what we have done to the torn grain.
Then relocate the tool rest as close as possible to the wood surface, and at a height such that the scraper is on the centreline of the work when it is held flat on the tool rest and horizontal. Make a cut with the scraper in that position.
Then rotate the scraper upwards so the heal of the bevel is resting on the wood and the tip is above the wood surface. Rotate it downwards until you are riding on the bevel and a chip is produced. The chip will be unstable because you are cutting on the back side of the burr, and it may catch.
Then rotate the scraper to 45deg to the rotation of the wood, and make a "shear scraping" cut while holding the edge in the trailing position.
Second Test
Again dress the top of the scraper with the stone or diamond file. Then hold the stone to the cutting edge and at 90deg to the top surface. Draw the stone lightly, downward across the cutting edge, rotating it around the edge as the stroke is made. Only one or two passes are required across the edge. I would try to make no more than four strokes with the stone, two on each side of the centre. You now have a scraper with a "micro-burr" that is the same grit as the stone.
Make the same four different types of cut with the tool as you did with the heavy burr.
Third test
Again, dress the top of the tool with the stone. This time ride the stone on the tip and heal of the bevel as you draw it across the edge of the tool, and don't use too many strokes. The tool should be the same as the second test, except there should be no burr at all. We have sharpened it as we would a wood chisel. If there is a burr, remove it with the stone on the top of the tool, and again stone the bevel, only lighter this time.
Repeat the four types of cut with the tool.
Results
You should notice that the heavy burr from the first grind is best for removing a lot of wood, but there will be torn grain, maybe more than there was after the gouge, and the edge doesn't last very long.
The "micro-burr" should have been the best for a finishing cut with the tool held at 90deg to the surface. Note the fine ribbon of wood that can be removed. This is truly a finishing tool, and its edge should stay sharp longer than the heavy burr.
The sharp edge in the third test is as far away as we can get from any comparison of our scraper to a cabinet scraper. This tool should cut safely while riding the bevel of the tool, there should be minimal tear-out, and little danger of a catch. When the tool is used in a "shear scraping" mode at 45deg to the rotation of the wood, and while riding the bevel, the torn grain can be removed to give an almost finished surface.
It will take some practice to develop the various tool techniques that are used with each grind, but doing this will expand on the usefulness of your scraper.
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