negative rake scraper

Discussion in 'Newbie' started by Dave Fritz, Nov 17, 2016.

  1. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Sorry, I should have said whatever floats your pirogue. :D

    Mon ami, you no got a pirogue? Dem gators, how you gonna catch? Should I start a "Go Fund Me" to help you through this rough spot in the swamp?
     
  2. RichColvin

    RichColvin

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    From a safety standpoint, I very much disagree. Even though I work in my shop alone, I follow certain safety rules religiously. It takes only one slip up to cause a long lasting injury.
     
  3. Mark Hepburn

    Mark Hepburn

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    No, mes ami, I'm on high ground me. Dis here land is tree feet above de sea level. I can see dem gator comin and choot em. :cool:
     
  4. Fred Belknap

    Fred Belknap

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    I have mine at 75 degrees with the top bevel the lowest position on the Roborest which I think is 25 degrees. I tried the 70 degrees but went back to 75 as it seemed to work a little better. I do mostly NE bowls.
     
  5. Jamie Straw

    Jamie Straw

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    Oh, methinks it's not a noticeable difference. With a CBN wheel, and the upside-down approach for re-establishing the burr, it takes no time at all to swipe the tool. It takes me longer to turn, take two steps to the grinder, and turn it on than it does to swipe the tool once or, occasionally, twice. Perhaps I'll shoot an 8-second video.:D The whole bevel is on the wheel, tool pressed against the platform, index finger pressing it down and serving as a pivot point. Taking off a very small amount of metal.
     
  6. Jamie Straw

    Jamie Straw

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    I find myself more concerned about the stress of the grinder powering up with a CBN wheel than I do about it running for a period of time (not an hour, I'm too ADD for that). I actually spin the CBN wheel by hand before I throw the switch for start-up to give it a little boost. You're grinder has a leg up, because it's labelled. Mine is a no-name from Woodcraft.:p Back in the day, they didn't reveal who made their grinders.
     
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  7. Jamie Straw

    Jamie Straw

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    Thanks for that post, Bill, didn't quote the whole thing but I found it very educational. Electrical stuff is an uphill climb for this kid, but I keep trying.
     
  8. Jamie Straw

    Jamie Straw

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    That's very interesting, Fred. The only so-called rule of thumb about NRS's seems to be that the total of the 2 angles should be no more than 90 degrees. Sounds like that may not be a bullet-proof "rule."
     
  9. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Thanks for not pointing out some grammatical mistakes. I knew what I wanted to write, but my fingers were lazy.
     
  10. Dave Fritz

    Dave Fritz

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    I've heard the combined angles should not exceed 90 degrees as well. Does anyone know why? Is it more aggressive, less aggressive?
     
  11. robo hippy

    robo hippy

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    Coming up to full speed time with the D Way steel wheels is about 3 seconds with both my Baldor and my 1 hp Rikon. The Rikon appears to have an electronic brake. Can't remember how long it took with the old blue no name grinder from Woodcraft, but much longer. I never ran the heavy wheels on it other than to see what happened, and sold it years ago.

    I have no clue about why a more acute angle is 'supposed' to work better. Don't think I have ever heard it explained either. I haven't played with them enough to notice any real difference, and both seem to work fine.

    robo hippy
     
  12. Mark Hepburn

    Mark Hepburn

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    That's about 1/5 the time of my grinder. I knew when I bought the 1/2 hp units that I was going cheap but it's a low priority as it provides similar utility for the next year or two and I can move up in quality over time. As long as I don't burn these up, which I won't because, as a hobbyist, I don't get anywhere near the max duty cycle of these. Still, I do plan on going with a better grinder. I like the one that Leo posted - a really old monster, military surplus unit that will outlast me.
     
  13. Bill Boehme

    Bill Boehme Administrator Staff Member

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    If the angle is more acute then it might have a larger bur. When I first started using a NRS about a decade ago, I ground it with an included angle that was greater than 90 degrees. The bur was smaller and probably didn't last as long, but it left a nice smooth finish. The advantage of the blunt nose is that it couldn't damage the wood if pushed too hard. The disadvantage is the short lived bur.
     
  14. Jamie Straw

    Jamie Straw

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    Ahhhh, but one should not "push" an NRS. Boy, was that one point that Tom Wirsing emphasized during our workshop.
     
  15. Jamie Straw

    Jamie Straw

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    With 2 D-way steel CBNs on a Jet grinder, one reason folks leave the running is how long it takes them to stop spinning.:D They will often still be rolling along when it's time to touch up the burr, seems silly to shut 'em down. Wish I had that problem, but a Jet grinder is way down the road for moi.
     
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  16. Bill Boehme

    Bill Boehme Administrator Staff Member

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    And you're not supposed to do a lot of things that people do anyway. And, yes, I've heard the aphorism about the futility of trying to make things "idiot proof".

    But, also consider that a finer bur takes finer shavings ... although there's probably a fine line somewhere between getting a smooth finish and parsing molecules. :rolleyes:
     
  17. Jamie Straw

    Jamie Straw

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    Wow, that is way beyond splitting hairs. Love it!!
     
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  18. john lucas

    john lucas

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    I will have to go out and measure my negative rake scrapers. I use the Robo rest to sharpen them but can't remember whether I have it set at 35 or 40 degree setting for those tools. I'll try to remember tomorrow when I go out.
     
  19. john lucas

    john lucas

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    Just measured my negative rake scrapers. 68 degrees. I use the Robo rest at 40 degrees so I don't know exactly why it comes out to 68. Anyway, the actual angle doesn't matter as long as it's less than 90. More than 90 raises a lousy burr if at all. Anything less than 90 raises a good burr on all scrapers whether they are negative rake or not. By flipping the tool for each grind to raise the burr you remove less metal. Ideally when you raise a burr you grind or hone off the original burr and then grind or use a diamond hone to raise a new bur. Flipping the tool eliminates this step because it grinds off the old burr and forms a new on one the opposite side at the same time.
     
  20. Bill Boehme

    Bill Boehme Administrator Staff Member

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    If your scraper is meeting the grinding wheel above or below the grinding wheel centerline then you also need to take that into account when determining the angle of the grind.

    I don't have a Robo rest, but I seem to recall that it has an angle scale. The scale would need to be relative to where it meets the grinding wheel in order to give an accurate indication of the grind angle.
     

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