I picked up a scraper blank from Doug at the symposium this last week. I wanted to make a thicker negative rake scraper than the one I now have. Mine is 1/4" thick and Dougs is closer to 3/8". I have made it an included angle of 78 degrees. I might try a more acute angle later but this one seems to work and was cut on the same table setting as my other scraper which is why it ended up at that odd angle. His tools are quite hard. I didn't ask what they are hardened too but on my 120 grit aluminum oxide wheel it didn't raise much of a burr. On the 180 grit CGN it raises a nice small burr. I polished that off and tried using a steel to raise the burr. It raises a larger burr. Maybe later today or tomorrow I'll have time to cut on a practice piece and see which way I like it.
Doug does carry a 1 inch wide by 5/16 thick scraper which I really like, though I haven'g turned one into a NRS yet. Eric Lofstrom uses a carbide tipped tool (carbide bit, about 1/8 inch diameter in wood) to turn a burr on his NRS, and it seems to work well. Depending on how you burnish, there can be huge differences in the burr. I do like the burr from carbide better than the ones I get from the steel burnishing rods.
I use a 1/4" carbide rod for all my scraper burnishing, NRS, card scrapers, etc. One thing I eventually learned: it doesn't take much pressure. I used to use a lot more but I found a fairly light pass works very well. I don't use the burr from the grinder.
Why not use the burr from the grinder? I sharpen my NRS on a blue 80 grit Norton, and it seems to work as advertised. Included angle about 70°. 90° and more = no burr; Less than about 60° = very fragile burr.
What I find is some metals work great off the grinder and some don't. Probably has to do with the heat treating and maybe the toughness of the metal. For example the no name HSS tool I've been using for years will raise a large burr off the 120 grit wheel on my grinder. It's very small on the Thompson tool. I can raise a larger burr on both tools with a Steel.
On John Jordan's shear scraper I can raise a burr with the grinder but using his ceramic hone raises a burr that cuts better, not necessarily larger but apparently one that cuts better. I played with that last night on a bowl just to see.
Why not use the burr from the grinder? I sharpen my NRS on a blue 80 grit Norton, and it seems to work as advertised. Included angle about 70°. 90° and more = no burr; Less than about 60° = very fragile burr.
Depending on the metal and the grinder and the wood (and possibly some other factors) the burr from the grinder may last only for a few cuts before it's worn away. The books even say this and my experience agrees. A look with a microscope shows this burr is quickly gone. A burnished "burr", more of a raised cutting edge rolled up from deformed metal, lasts much longer for me, on the tools and types of woods I often use.
Magnification shows a ragged burr from the grinder with lots of tiny teeth which may break off easily. A burnished edge looks more like a smooth knife edge. Cabinet hand-held scraper edges are traditionally burnished. I'd say for a given tool, wood, and method of work just try both: grind, scrape, then hone and burnish.
That said I did recently get a set of excellent Stewart-Macdonald hand scrapers made for luthiers. These are much thicker than cabinet scrapers, made from hardened steel, and are sharpened by honing the flat sides then grinding the edge. I didn't feel a raised burr. When I get time I'll examine them with the microscope to see if they actually have a tiny burr or if they have more of a toothed edge. They do work very well on some flat and curved things I made but I haven't tried them yet on turnings.
I do not have the facility to look at the burrs through a microscope, so the only tests are feeling with a finger and actually cutting. On card scrapers, I do sharpen them the traditional way, honing away the old burr, filing flat and square and raise the burr with a hardened steel rod.
For turning, my regular scrapers are all Thompson with a 70-80° angle. They cut well and the burr lasts a long time. The same with the burr on Thompson gouges, which still have a burr on the wings after turning an outside of a bowl. You cannot feel the burr on the tip of the tool - not sensitivity enough to differentiate between the tip and the wings - so no way of feeling if the burr is gone near the tip. The wings, which really scrape retains the burr longer. Standard they are all ground on 180 grit CBN wheel with 60° tip angle. I do not know the real angle of the wings - cannot measure.
I was not convinced about the NRS initially, so all my NRS are older Sorby/Crown/Woodcraft scrapers re-ground. They are ground on the 80 grit Norton blue with a 70° incuded angle - and the burr does not last as long as on the Thompsons.
I did do some half-hearted burnishing test with a hardened steel rod, but did not really feel any difference. Clearly there is more work to be done.......
I'm new to NRS and debating on getting one or making one. I looked at Thomson's page and see where they'll grind their scrapers to NRS upon request. So, tell me about angles? Is the top angle the same as the bottom one? Your picture, John, looked that way.
I use two CBN wheels on a low speed grinder; what grit should I use to sharpen with, or is this the same old story of "Do what works for you?" My belt sander is at the other end of the work shop but I often use a 220 grit belt for scrapers and skew; works pretty well too.
Otherwise, I don't know what kind of sharpening equipment you guys are referring to with "Cards." I use playing cards to burn the valleys on my beading so that's probably not the right answer.
Jerry it doesn't matter if the angle on the top and bottom are the same. It seems to work better if the included angle is less than 90 degrees. Some use 20 on top and less than 70 on the bottom. I am trying the Stewart Batty version which has both angle the same. With Stewart's method once the burr wears off which happens very quickly, you simply flip the tool over and grind the other side to raise a new burr. On the 20/70 style you have to polish off the old burr and then raise a new one. Not a particularly big deal either way. It does take a lot longer to grind the same angle on both sides especially with a thick tool. Stewart's tool had a relatively thin cutter held in a thicker support shaft so he didn't have a large bevel to grind on. My Thompson tool is about 3/8" thick and took a little time on a course wheel to grind.
What some are calling a card is simply a diamond hone. Some come shaped like a business card, some come as paddles. These are often used to remove the old burr and sometimes to raise a new burr instead of using the grinder. Some people also use a "steel" or ceramic stone to raise the burr. I find that different tools may require different techniques to properly raise the burr so it's hard to say what grit wheel will work for your particular tool. You just gotta try it and see. On my Thompson gouge a 180 grit CBN seems to do a good job but I'm going to play more with raising the burr using a Steel which is a hardened shaft that you push across the blade to raise the burr.
Hope that answered your questions. If not ask more.
Since the cutting actually occurs on the burr, it seems to me that the exact angle isn't important anyway. However, the idea of having both angles the same and just flipping the tool over on the grinder seems like it's a great idea. Probably could loose track of which one should be up next.
I'm not fond of using a wheel on a scraper because it tends to leave a slight curve on the face of the bevel; that's why I mentioned the belt sander. But, again, since the bevel is never going to touch the wood anyway, what difference does it make.
I like the look and feel of the Thompson tools and may order one cut to a Negative Rake. I've got two of his already ground down on the left for bowl cutting and they work wonderfully.
Thanks for the help. I think I'll do this and let you know how it worked for me. What's another tool amongst the hundreds we all already have?
I've actually ridden the bevel on a NRS. I didn't have a particular reason to do it, I just wanted to see if it could be done. Oh, and I also grind the top and bottom to the same angle. It makes it easier. I have no idea what the inclusive angle is. I guess I should check.
Had a great conversation with Doug Thompson yesterday based on what we've talked about. He even went so far, John, as to bring up your picture so he could see what I was talking about. He remembered what you and he had talked about so it was easy for him to set something up for me.
He's quite the character, really nice to talk with and stands behind his products. Can't imagine one of his tools failing, but if it did, he'll refund it, fix it or replace it. And the scrapers of his I already have are as fine as you could ask for.
So, a new Negative Rake Scraper is on it's way. Now I'd best go buy the wife some roses for Valentines day. There's a balance in there somewhere.
