One of the tool skills that many people struggle with is sharpening scrapers. Set up correctly, a scraper should be able to pull shavings just like a plane. If all you are getting is dust, something is wrong.
There are two ways to sharpen them. One method uses a honed face which is 90 degrees to the scraper and two cutting edges are pulled off, one from each of the two corners. This is a common method for thick rectangular furniture scrapers and there are commercially-made scrapers based on this concept for instrument makers that are made from stock that is several mm thick. I don’t use this type at all, for anything. I believe in furniture making it’s intended as a final finish scraper, is not particularly aggressive, and is more difficult to maintain.
The other type uses a 45 degree bevel around the edge. This version has only one cutting edge, can be adjusted for fine finishing or aggressive, and is very easy to sharpen and maintain. It is the only style that will work with the very thin and flexible scrapers that are used for jobs like scraping violin arching. I also use this edge on the rectangular cabinet scrapers that I use for scraping ribs.
Rectangular cabinet scrapers are easy to get in the form of cards about 3×5 inches from the common tool sources. For thinner scrapers I use several different thicknesses of machinist’s blue shim stock. The most versatile thickness is 0.15 inch thick, and initially I would try that and then go from there if you want to experiment with stiffer or more flexible scrapers.
The most useful shapes are outlined below. Though I have many more, those are my “desert island” shapes.
Large kidney shape with one flat edge, one rounded, and small-radius ends (two types). I use this for large areas of the top and back, inside and out. The flat side flexes to the outside arch, and putting one’s thumbs in any spot will center most of the cutting on that spot, blending outwards from there, or the whole scraper edge can be used for larger areas. The convex side is similarly used for the insides of the plates and also for finishing the bottom cove of fingerboards.
Small rectangular with four slightly-curved edges and four radiused corners. this is used for the final shaping of the edge around the purfling. Making all of the edges slightly different allows slightly different areas to be scraped with a scraper that fits better, and I also use the corners of the kidney scrapers for this job and blending the scoop around the edge into the main area of the arch.
Medium squarish with one side rounded and the other concave. The convex side is used for whatever, and the concave is sharpened to a slightly flatter curve than a fingerboard for blending and finishing the tops of boards. When carefully sharpened this can even make the final finish for the top of the board short of a final sanding with fine sandpaper.
The first step is cutting the steel. I use tin snips and cut a few mm away from the final edge because the snips deform the steel near the cut. The deformed area needs to be ground off when the 45 degree bevel is ground. I start by grinding at 90 degrees to make the final shape. Then use a straight edge to determine if the steel is really flat or if one side is hollow, the other convex. You want the flat surface to be slightly concave, the grinding edge on the convex side. This is so you can place the flat side flat on your stone and polish the steel up to the edge. If you try polishing the flat side and are not polishing at the edge but somewhere inside away from the edge, the solution is to force the scraper concave by pounding on the side you want to be concave, forcing it into a very slightly dished shape. Don’t do this near the edge! Put the scraper on a flat piece of wood and hammer it towards the center until it cups. You may still need to do local areas so that the whole edge will polish, but don’t make dents too close to the edge–you want to use this scraper for decades, smaller and smaller.
In all sharpening jobs I follow a policy of doing the fine honing on the smallest possible amount of steel and scrapers are no exception. For knives, gouges and chisels, and plane blades this means maintaining a hollow grind so that all of the fine honing touches only the tip and heel of the bevel, and I start with a very fine stone–8000 grit or higher is sufficient to quickly remove all the grinding scratches from such a tiny cutting surface. Bringing a large bevel fully to high polish is only a waste of time.
For scrapers I do something similar. The ground edge should in theory be 45 degrees, but I grind it on my hand grinder somewhat sharper, say around 35 degrees (non-specific). Then if I’m being tidy I hone on my finest stone to a precise 45 degrees. I do this by using a triangular block of rosewood as a guide.
The block is a right triangle–two 45 degree corners and one 90. It’s floated on my water stone by the water. Honing is done very gently to avoid deforming the edge or scratching the stone. When the edge is literally like a knife (frightening to touch) I turn a hook with the burnisher very gently! This is where most people mess up. Even though the scraper is hard steel only fingertip pressure is needed to turn a hook. Any more pressure than needed needlessly deforms the steel behind the hook impairing future sharpening and can make the edge ragged. The burnisher is held a few degrees morre than the bevel–say at a 60 degree angle.
If you don’t damage the edge then as the scraper dulls all that is needed is to flatten the hook by burnishing flat on the flat side, then return the hook. This works a few times and when the scraper no longer cuts the fine 45 degree honing is repeated. For the finest finishing cut on spruce freshly hone the edge and leave it unturned.
Grinding the concave fingerboard scraper is a special case. I do this by holding it inline with the stone rather than 90 degrees to it and allowing the stone to cut the curve with its shape. Laying the steel down at a 45 degree angle establishes the bevel. Honing is a special case. The lighter you press on the stone the finer the scratches, so when you have the final curve lighten up to the pressure of a feather and let the stone polish the final edge. If you do this well, you can put a final scratch-free finish on ebony!