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Conventional Scraper/NRS Are They Equivalent?

Discussion in 'Tutorials and Tips' started by Dennis J Gooding, Mar 24, 2020.

  1. Dennis J Gooding

    Dennis J Gooding

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    There has been a recurring issue as to whether (or when) a conventional flat topped scraper, tilted downward to produce a negative rake, is equivalent to a negative rake scraper (NRS). Clearly conventional scrapers work and are far more common than NRS’s. The issue here is whether they are actually NRS’s in disguise. For this discussion, I will define a NRS to be a flat scraper in which the upper edge has been ground off at an angle such that when the scraper is presented in a horizontal orientation, the scraping face is angled down by a desired amount (negative rake). Also, I will ignore burrs, because they can be raised equally well on either type of scraper.

    When is a conventional flat scraper, tilted downward to produce the same rake angle as a NRS, equivalent to that NRS? The short answer is “when the point of scraping is broadside to the tool rest at the point where the scraper rests (and tilts)” and the tool rest height of the conventional scraper has been adjusted to scrape at the same height on the wood”. This is a simple matter of geometry. In this situation, the scraping edge is presented to the rotating wood in exactly the same way in either case. Furthermore given this situation, if the tool rest is parallel to the wood being cut, one could simply slide the scraper along the rest, maintaining the same tilt angle, and the results again would be the same as that of a NRS. This situation might arise for example when scraping the inside or outside of a bowl using an appropriately curved tool rest.

    Now suppose that the tool rest is not parallel to the wood being cut. A common example would be scraping a curved turning using a straight tool rest. In this case, having adjusted the rest height correctly for a particular point along the rest, we would find that if we were to slide the conventional scraper to scrape a different region of the wood, we would need to move the scraper in or out and readjust the rest position and height to maintain the same scraping conditions. If instead of sliding the conventional scraper, we were to swing it flat on the rest, to either side in order to scrape an adjacent area, the scraper would rotate slightly, yielding a shearing presentation to the wood, and each point of contact with the wood would occur at a slightly different rake angle and different scraping height. In contrast, in either case, the NRS, if held level, would require only moving the scraper in or out to follow the contour of the wood. Clearly, in these situations a tilted conventional scraper is not equivalent to a NRS. While it might achieve the same satisfactory end result in its own way, the conventional scraper would require frequent tool rest adjustments in order to mimic the NRS at each step of the way.
     
  2. Don Wattenhofer

    Don Wattenhofer

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    The main thing you forgot to mention is shear scraping and that is not what you are doing the way most turners are using the double bevel scraper. The flat top burred scrapers should only be used up on edge in a shear scraping position. The thought of trying to duplicate the action of the NRS with a flat top scraper I think is ridiculous and like you said would be extremely difficult and potentially dangerous.
     
  3. john lucas

    john lucas AAW Forum Expert

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    I played with flat.vs negative rake a.lot for a while trying to understand the difference. What I learned was the flat scraper tilted down wants to be pulled onto the work whereas the negative rake scraper does not. This was most noticeable when I used them on a thin natural edge bowl. The flat scraper would create chatter and the negative rake would cut clean. The rest of your article is dead on and quite interesting.
     
  4. Bill Boehme

    Bill Boehme Administrator Staff Member

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    This is one of the points that Stuart Batty makes in his instruction manual for his negative rake scraper. When you're trying to use a delicate finishing touch this can be a big difference in controlling the tool.
     
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  5. Dave Landers

    Dave Landers

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    Exactly.
    The initial thought is to look at the angle of the tool vs the wood, which is about the same for both. So what's the difference?

    The difference is tool rest support. A flat scraper held handle-up is going to want to slide down the tool rest as the wood pulls it down. The NRS, held level, has alot more support perpendicular to the cut.
     
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  6. robo hippy

    robo hippy

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    I need a video to help me understand this.... For bowl turning, on the outside, with a scraper, you want to be below the center line. On the inside you want to be above that center line. The burr on a standard scraper, fresh from the grinder lasts a lot longer than the burr on a NRS.

    You can do peeling cuts on spindles with a scraper. Peeling cuts on bowls, no matter if you are using gouges, scrapers, or a SRG (spindle roughing gouge) are suicidal....

    robo hippy
     
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  7. Mark Jundanian

    Mark Jundanian

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    @Dennis J Gooding, you are dead on. In addition to your comments and those that follow I'd add that it is a great deal easier to hold a tool in a horizontal position than nose down. And tools that are held horizontally are amenable to mounting in an articulated arm hollowing rig.
     
  8. Doug Freeman

    Doug Freeman

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    Stuart Batty says he created the NRS term and method ~25 years ago. Starting @ 1:32:00 of this video is his description, discussion, and use of the tool. It's the best explanation I've found.


    View: https://www.youtube.com/watch?v=N7BjRcSDurM
     
  9. Mike Johnson

    Mike Johnson

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    Negative Rake Tooling has been used in the metal machining industry for many decades.
     
  10. Doug Freeman

    Doug Freeman

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    I’m sure Stuart is referring to only wood turning. It is the only context he has referenced it I am aware of, as well as the context of this discussion.
     
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  11. Mike Johnson

    Mike Johnson

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    A large number of wood turning accessories and tools have been adapted from the metal machining industry, if you ever have a problem with processing wood on a lathe you can usually reference the methods utilized in the metal machining world to adapt it to wood turning.
     
  12. Doug Freeman

    Doug Freeman

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    Do metal cutting tools rely on a formed burr to cut?
     
  13. Mike Johnson

    Mike Johnson

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    Doug,

    Metal cutting tools rely more on the geometry of the grind on the cutting tool and the hardness of the cutting tool depending on the metal being machined.
     
  14. Doug Freeman

    Doug Freeman

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    Thanks. While there are some similarities there are important differences.
     
  15. Mike Johnson

    Mike Johnson

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    Doug,

    Plenty of books have been written on the topic.
     
  16. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    I agree with you Doug. The remark about metal woodturning was unnecessary.
     
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