The bottom bevel shouldn't matter much. I haven't tried one ground like that but you could probably just burnish a bevel on your existing grind and it should work fine. In fact it may work better than what I use. I sometimes use spindle gouges upside down for fine scraping in coves - similar in cross section to your scraper grind.
scraper_gouge3.jpg
All three of these are ground the same way. The differences are one is ground from a skew chisel and one is wider than the others.
scrapers_neg_rake.jpg
The bottom bevel does not have to match the top bevel. The wood doesn't care. However, it does let me use the tool identically when flipped over and a new burr applied. I make other negative rake scrapers with a large and large angle bevel on the bottom and a small but sharper angle bevel on the top. Here are some very small ones made this way - they give a glass-smooth surface on end grain in good wood:
scrapers_small_thompson.jpg
There are several ways I know and have used to make a burr.
- One way is to grinding on the bottom bevel last. This will deform and curl up a ragged burr which will cut very well for a short time. The size of the burr depends somewhat on the grit and pressure used in that final grind. You can easily feel the size of the burr with your finger. Many people use this method and replenish the burr often, sometimes after just one or two scrapes. If you look at the burr under a microscope you might see why - the burr looks like a jagged mountain range and it's easy to imagine the peaks wearing off quickly.
- Another way is to use a ceramic or diamond hone and hone the bottom bevel/edge, usually not flat on the bevel but at an angle. This will raise a burr the same way but it will be much the same as the grinder burr except smaller. This is a good way to raise small burr for a very fine and delicate scrape. It also doesn't last long.
- The third way is to use a burnisher. You hold the burnisher so it is not flat against the bottom bevel but forms an angle with the the bottom bevel. Apply some pressure and slide the burnisher along the entire scraping edge. The amount of angle, the diameter of the burnishing rod, and the amount of pressure determine the size an shape of the burr. The metal is actually deformed at the edge and curled upwards. If you look at it under the microscope the burnished burr might look like a smooth, polished cutting edge instead a ragged row of teeth. A burnished burr can be very gentle or quite aggressive. It can even be curled too much and won't even cut without holding the tool at a downward angle instead of horizontal as you would otherwise.
It is my experience that a burnished burr can last a lot longer than a burr from the grinder. I will typically grind the scraper, use a diamond or ceramic hone flat on the bevel to remove any grinder burr, then polish the edge by stropping on leather or some polishing compound spread on a piece of mdf or wood. After the edge is polished I raise the burr with the burnisher. When that burr wears away, I may re-hone, polish, and burnish several times before going back to the grinder.
Burnishing the cutting edge is the way hand and cabinet scrapers have been prepared for ages. I use the carbide burnisher on scrapers such as these, prepared and sharpened the traditional way used by cabinet and instrument makers:
scrapers_.jpg
Besides the hand burnisher I showed, Veritas also makes a scraper burnisher you fasten to the workbench. I used to use this on large scrapers. With this it is easy to put too much burr on large scrapers and make them way too aggressive.
burnisher-veritas.jpg
As mentioned, you can burnish with a carbide rod, a HSS steel rod (even the shaft of a tool) or almost any piece of steel in a pinch. The smooth end of an old HSS steel drill bit is perfect, especially if it is hardened all the way to the end (some are not). It helps if the burnisher is the same hardness or harder than the burnished tool so I prefer to use carbide. You need less pressure with a smaller diameter rod but a larger rod works fine - I have a larger carbide burnisher and used it for everything for years.
(sorry, in a big rush this morning - no time to proofread this, hope it's not too bad)
JKJ