Liutaio Mottola Stringed Instrument Design



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Sharpening and Burnishing a Card Scraper

Scrapers are among the simplest and most useful woodworking tools. Like most folks that use them, I wouldn't consider being without them. Even with a good sized arsenal of power tools at my disposal I find myself turning to simple scrapers often. In a lot of cases they are quicker and more effective than sanding, even power sanding. And there are certain lutherie applications where hand scrapers are the traditional and still preferred tool for final wood smoothing.

At first blush the process of sharpening and burnishing a hand scraper for use seems daunting. In reality this process is both quick and easy as I hope the following instructions will show. It is probably a good idea to read through the instructions once before trying them, just so you know where all of this is leading.

Last updated: Sunday, June 01, 2014



>Scrapers for woodworking applications can be had ready made in many sizes and shapes. For general use in preparing wood for finishing (or for quickly removing lacquer, shellac or other finishes) a rectangular cabinet scraper is a must have tool. For lutherie applications a selection of shaped card scrapers is also very useful. Such collections can be purchased from the luthier suppliers. A lot of folks (including myself) augment these collections with custom shaped scrapers cut from sheet metal and shaped on the vertical belt or disk sander. Both new and used scrapers must be sharpened and burnished for use. With use, the edge of a scraper will dull. It can often be quickly re-burnished into usable shape. But eventually the thing will need to be re-sharpened and burnished. The steps below describe the entire process of sharpening and burnishing. Lots of folks have the idea that this is difficult to do, but it is easy to learn and quick and easy to do.

The edges of a well used hand scraper will look something like this from the side (magnified many times):

Notice how the scraping edges (on the right, top and bottom) are both rounded over and bent a bit away from the broad sides of the scraper. To get a well used scraper back into shape (or to get a brand new scraper into shape) it must be first sharpened and then burnished. The first step is to flatten the broad surfaces of the scraper. You can do this by rubbing the scraper around in circles on any flat abrasive surface of about 320 – 400 grit. I do all scraper sharpening on the horizontal belt sander with a 320 grit belt. Leave the sander turned off! Manually turn the belt so the seam isn’t showing, too. Although I have owned a number of stones and other sharpening gizmos at various time, they all had problems in that I ended up spending more time keeping them flat than I did actually using them. Eventually I ended up just using the belt sander with 320 and 600 grit belts and a piece of 1/2 inch thick glass with 400, 600, and 1000 grit sandpaper affixed to it with spray adhesive for all my sharpening in the shop. But I digress. Again, to flatten the broad surfaces of the scraper, rub it around on the belt sander like this:

Do both sides. Note that I’m doing a small oval scraper in the pictures. If the scraper has been used before, it is pretty easy to tell when the broad surfaces are flat by testing with your finger to see if there is any edge left. Now the edges will look something like this:

The broad surfaces are flat but the thin sides (shown on the right) are a little messed up. These need to be flattened and squared to the broad surfaces. To do this, cut a scrap of wood with a good flat side on the table saw, to make another side that is perpendicular with the first. You probably have such a scrap just lying around already. Put that on the belt sander and use it as a fence to keep the thin edges perpendicular to the belt as you rub the scraper across the belt.

I try to work across the belt instead of along the belt’s length, to keep the belt from moving. Rubbing the thin side(s) flat is pretty easy with a rectangular scraper, but it takes a bit more effort to do with a curved scraper. I kind of drag it across the belt and turn it at the same time, keeping the broad surface in contact with the wood “fence.”. The thin side is really thin so it only takes a few passes to do the work. When it’s done the edges of the scraper will be sharp and square.

Now the sharpening is all done and it is time for the burnishing. You’ll need a scraper burnisher for this. The burnisher is just a hardened steel rod with a handle. Such tools are cheap and readily available. First, put a few drops of oil on a rag, and rub the oil all over the burnisher rod.

You can burnish at the edge of your bench but I like to do it at the edge of a scrap board since my bench is not flat right to the edge. You can see the small oval scraper near the edge of the scrap board in the picture above. The first step in burnishing is drawing out the edge. To prepare for drawing out the edge, hold the burnisher by the handle in one hand and by the end of the rod in the other. Standing up and leaning over the bench, place the side of the burnisher flat on the scraper. Now lift the handle ever so slightly, about 15 degrees. Pressing down fairly hard, slide the burnisher along the edge of the scraper slowly, keeping the burnisher perpendicular to the edge, like this:

Slide the burnisher over the entire edge to be drawn out. For a rectangular scraper you can do only one side if you like, but for the oval scraper shown I’ll go completely around the scraper, taking a stroke, turning the scraper, taking another, etc. until I’ve gone completely around. What you’re actually doing here is compressing the metal at the edge of the scraper so as to draw the edge out toward the thin side. It may be hard to believe you can do this, but your pressing down is focused on a very small spot and so you can deform the edge of the metal scraper quite readily. Here’s a drawing of the burnisher approaching the edge of the scraper:

And here’s one that shows how the metal at the edge is deformed by the drawing out process:

See how the edge has been drawn out toward the thin side on the upper right? This is very exaggerated in the drawing, but this is what you’d see, more or less, if you examined the edge with a magnifier after drawing out.

Once the edge is drawn out, it must be turned (bent) so that it is pointing toward the broad side of the scraper. Turning the edge is also done with the burnisher. Clamp the scraper in the wood vise so that the side that has the edge you just drew out it facing up. When doing a curved scraper like the little oval one I’ll turn the edge on part of the scraper, then reposition the scraper in the vise to do the rest of the edge. The edge is turned by pushing on it with the burnisher. Here’s a picture of the burnisher approaching the edge. In the picture the orientation is the same as in the other pictures, not the way it would be when held in the vise.

Here’s how the operation looks in real life:

I'm pushing the burnisher forward with both hands, keeping good pressure on the edge. One thing that should be clear from the drawing above is that the burnisher is held at a slight angle away from the drawn out edge. When turning the edge (also called turning the hook) you want to turn the edge so it points toward the broad side of the scraper. It you don’t start out with a slight angle on the burnisher there is a possibility that you’ll just mash the drawn out edge over toward the thin side of the scraper. So, starting with that small angle, slide the burnisher over the edge while pressing down, so that the burnisher remains perpendicular to the scraper. Do the entire edge this way, pressing down firmly. Then go over the edge again, this time with the burnisher held flat horizontal (i.e. no small angling). You can take a third pass if you like with a slight angle toward the edge (approximately 15 degrees), if you want to make the hook tighter to the broad side. Once done, the edge will look like this:

Again, the picture is exaggerated, but you get the idea. A well sharpened and burnished scraper works a lot like a small plane. When you scrap, you leave tiny ribbons of wood.

As stated at the top, when a scraper edge dulls two things happen. The edge itself actually gets less sharp, but also the turned hook gets pushed out of shape and thus less hooked. You can often get a worn scraper back into shape by simply turning the hook again. You may be able to get away with this simple touch up a few times, too, but eventually you’ll have to go through the entire sharpening and burnishing process again.