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Thread: Negative Rake Scrapers

  1. #1

    Negative Rake Scrapers

    Not the first time, and probably not the last time I ask questions about this tool... Anyway, I am trying to organize myself to do a video on the NRS. As near as I can tell, other than the multiple variations on profiles, are the differences in 'included' angles being less than or greater than 90 degrees. I am wondering if we need specific names for those variations, and what they could be... Acute for less than 90, and obtuse for more than 90 degrees? Any other suggestions? Do we even need different names??? The acute type seem to be used more on bowl/side grain turnings, and the more obtuse seem to be used more on end grain turnings...

    robo hippy

  2. #2
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    I have no insight or input on this topic, however I'm anxious to see the video that you're considering. I enjoy all of your videos and find them to be very helpful and informative.

  3. #3
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    Quote Originally Posted by Reed Gray View Post
    Not the first time, and probably not the last time I ask questions about this tool... Anyway, I am trying to organize myself to do a video on the NRS. As near as I can tell, other than the multiple variations on profiles, are the differences in 'included' angles being less than or greater than 90 degrees. I am wondering if we need specific names for those variations, and what they could be... Acute for less than 90, and obtuse for more than 90 degrees? Any other suggestions? Do we even need different names??? The acute type seem to be used more on bowl/side grain turnings, and the more obtuse seem to be used more on end grain turnings...

    robo hippy
    Those names sound good to me at first thought.

    But to muddy the waters some more, I found a big difference between angles of "almost but still less than 90-deg" and angles much sharper. (I've never tried one more with an included angle greater than 90-deg.)

    All else equal (type of burr, etc) I found the sharper (acute) angles cut better but the less acute angles gave me a smoother surface with either end grain or side grain.

    And to avoid misunderstandings in your analysis it might be helpful to better define side grain and end grain. For example, when I turn a box or a pepper grinder with the grain running parallel to the lathe bed some people call the whole thing end grain turning. But on the outside of the box I'm cutting side grain and the inside bottom, lid, etc is end grain. The near-90-deg NRS can make a glass-like finish on the top of a lid, for example, better than a scraper with a 45-deg included angle. (I must disclose that the wood may also make a difference - my boxes are often from hard, fine-grained wood.) When smoothing the outside of a face-turned bowl we are usually alternately cutting across end grain and either with or across side grain. Here again (in my experience), the less acute NRS don't cut nearly as well but still give me a smoother cut. (They can cut so slowly I only use them for that occasionally.)

    And what about "extremely acute" angles? The wing of a gouge is sometimes referred to as a negative rake scraper, although if so it's one with an extremely acute angle. We all know how well they work. (BTW, as I mentioned earlier my newest mostest favorite sheer scraper for outside a bowl is a big spindle gouge.)

    Perhaps there should be more ranges, such as extreme acute, acute, near obtuse, and obtuse. But this doesn't seem useful since they are arbitrary. Assuming you can see enough difference during your experiments, would it be better to just define some ranges in degrees? For example, 10-25 deg, 25-50, 50-... Or to simply list the actual angles of the scrapers you are testing and leave the categorization to the reader. ??

    JKJ

  4. #4
    Perhaps it finds its origin in 40 years of cross examination, but I am always troubled by descriptive labels that are subjective by their very nature, e.g., obtuse and acute. I prefer exact numbers.

    It would seem the three main factors to performance would be the angle of introduction to the wood, the top bevel angle measured against the surface of the scraper and either the combined angle, or even more simply - theunderside angle measured against the scraper surface.

    Left click my name for homepage link.

  5. #5
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    I was taught "negative rake" by a retired machinist that had turned woodturner when he retired. He made some of his chisels out of tool bits, most were 1/2x1/4x3, but he also used 3/8x1/4x3 and 1/4 square tool bits. The tool bits were about 3" long and stuck in the end of steel rods that had wood handles. He sharpened freehand, but he had about 40 years of experience doing it. This was a guy that did not use calipers to measure, he measured with a micrometer.

  6. #6
    I guess I am trying to figure out how to refer to them when doing a presentation because there are so many variables. Most people do okay with 'right' and 'left', but stumble on north, south, east, and west. The surfaces on end grain seem to be pretty much the same no matter which NRS I use. Probably in part because the end grain cuts better with scrapers than side grain does...

    robo hippy

  7. Some Input

    Quote Originally Posted by Reed Gray View Post
    Not the first time, and probably not the last time I ask questions about this tool... Anyway, I am trying to organize myself to do a video on the NRS. As near as I can tell, other than the multiple variations on profiles, are the differences in 'included' angles being less than or greater than 90 degrees. I am wondering if we need specific names for those variations, and what they could be... Acute for less than 90, and obtuse for more than 90 degrees? Any other suggestions? Do we even need different names??? The acute type seem to be used more on bowl/side grain turnings, and the more obtuse seem to be used more on end grain turnings...

    robo hippy
    As a non-professional turner, but a rather rank intermediate level amateur, I would like to add my thoughts. All the terminology in the world will not communicate if people have no idea what angle you are referring to. I will try to post a diagram of what I mean. I would suggest that a picture would clarify for the viewer exactly which angle one is speaking of. I suggest you incorporate diagrams as you guide the viewer to your thoughts on the angles. I hope this presents some input from the non-professional point of view.
    Diagram.jpg

  8. #8
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    Quote Originally Posted by Lloyd Richardson View Post
    As a non-professional turner, but a rather rank intermediate level amateur, I would like to add my thoughts. All the terminology in the world will not communicate if people have no idea what angle you are referring to. I will try to post a diagram of what I mean. I would suggest that a picture would clarify for the viewer exactly which angle one is speaking of. I suggest you incorporate diagrams as you guide the viewer to your thoughts on the angles. I hope this presents some input from the non-professional point of view.
    Diagram.jpg
    Until Reed can see this and answer, I can say what I understand about an "included" angle. In geometry an included angle is the angle between two sides of a triangle.

    For turning tools with two ground faces it's the angle that "includes" both of the faces. For example each side of a skew might be sharpened at an angle 25-deg from the flat face of the skew. The included angle would then be 50 degrees. If the upper angle of your diagram were 30-deg (from the top flat) and the lower 60-deg (from the bottom flat), the included angle would be 90-deg.

    In geometry, an acute angle is an angle less than 90-deg. (Acute means "sharp" here) An obtuse angle is one greater than 90-deg (and in geometry, less than 180 degrees). An included angle exactly 90-deg is a "right angle", neither acute nor obtuse. The included angle of the tool in your drawing is obtuse since it is greater than 90 deg.

    But you bring up a great point - any video or document meant to educate turners on angles of tools, whether they be scrapers, negative rake scrapers, or other, should probably include a clear cross sectional diagram like yours. There is already a lot of confusion about how to measure tool angles - a diagram would let everyone be thinking the same thing. In fact, I wish I had time to make drawings showing these angles but I have to quit since the chickens are not far from my bedroom window - and they wake up at 5am!

    JKJ

  9. #9
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    Going to the machinist side https://en.wikipedia.org/wiki/Rake_angle

  10. #10
    Well, to add more to the mix, a standard scraper with a 70 degree bevel would have included angles of 70 and 0..... So, included angles would be 70...... I may end up just saying more sharp/pointy and more blunt. KISS theory...

    robo hippy

  11. #11
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    Reed: I wonder, too, if the individual angles change the use/effect of the tool if the included angle is the same. For example, does a nr scraper with a top angle of 30 and a bottom of 40 cut any differently than one with both top and bottom at 35? Both have an included angle of 70.

    So, if someone advises that a good included angle is 70 degrees, there could be many variations of top and bottom angles resulting in that included angle. I don't pretend to know the answer to my question, but maybe you could address that in your video.
    Grant
    Ottawa ON

  12. #12
    Grant, that is one of the major puzzles for me concerning negative rake scrapers. Opinions abound.... They range from 25/25 bevels which is in the range that Stuart Batty uses, to Sorby's 'hardwood bowl NRS' which is a 70/25 grind and appears to be used on the inside of a bowl in a bevel rubbing cut, which is actually kind of like a peeling cut used on spindles. There are claims back and forth about 'they just don't work if the included angles are greater than 90 degrees'. Thus far, as near as I can tell, the more pointy variations are used more on bowls, and the more blunt types are used more on end grain, but there is a lot of back and forth again. I was chatting with Doug Thompson about them, and he mentioned that Ashley Harwood was wanting a 40/20 NRS, which is what some British turner (name escapes me) was using, so now I have another variation to try out.

    Another factor is what type of edge/burr do you use. The standard has been the burr straight off the grinder. Easy to do if the grind is the same on both sides. More difficult if the angles are different, but that depends on what platform you use... With any scraper, I prefer to remove the old burr first, before raising a new one. Two ways to do this. Hone or grind the old one off, or a very light burnish to turn the old one down, and grind a new one on. The burr you get from a new 80 grit CBN wheel is a lot different than the burr you get from a 600 grit CBN wheel or even an old almost worn out 80 grit CBN wheel.

    To add to this, you can burnish a burr... Thus far, with my experiments with burnishing the burrs, that burr works better on the more blunt angles, especially for end grain like boxes and spindle work (yes, they can be used on spindles). You can burnish a burr on the more pointy ones, but you need to be careful. Main reason is that the acute angle is fragile, and it takes almost no pressure to raise the burr. If you have the burnisher at too steep of an angle to the bevel, you can actually hear the edge breaking as you burnish it. With the more blunt edges, you can burnish the burr up and down several times before needing to go back to the grinder. The burnished burr is far stronger than the grinder burrs, so needs less maintenance. So, lots of experimenting to do yet to try to figure it all out... This is a heaven/hell type thing for me, love the challenge, but keep finding more things I need to figure out...

    robo hippy

  13. #13
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    I've watched your previous videos on your use of scrapers of all sorts and I've seen how you do your burrs. So, I understand what you are saying in the context of NR scrapers and burrs.

    I'm just a hobbyist turner and one that hasn't done much turning in the last year at that. I find most of the discussions here on sharpening and now on NR scrapers to be adding to my overall confusion rather than lessening it. With any kind of luck, your video on NR scrapers will take some of my confusion away.

    When I first starting reading about NR scrapers, I couldn't really see a difference between a NR scraper and a skew, except for the burr, of course. I'm not sure even now if I see a difference between a 25/25 NR scraper and a similarly ground skew. If I take my skew, which is somewhere around 25/25 and burnish a burr on it, presto, I have a NR scraper, right? Hone the burr off, and it's back to being a skew again.

    Sometimes I think that many of these concepts have more to do with manufacturers selling more tools to confused turners like me than to any real forward movement in turning.
    Grant
    Ottawa ON

  14. #14
    The first ones I ever heard of were skew chisels without the burr. All of the ones that are more acute make good skew chisels, well if there is such a thing..... I can take the more blunt ones as well and hone off the burr and they do excellent bevel rubbing cuts. 70 degree bevel Big Ugly that is honed works very well on spindles. I guess main point about any cutting edge is that if it is sharp, it cuts. Same with burrs I guess...

    robo hippy

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