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Thread: burnished negative rake scraper

  1. #1

    burnished negative rake scraper

    I have been using a negative rake scraper with good results with a burr straight off a 180# CBN wheel. Yesterday I was getting some tearout with it on the inside of an ash bowl, although another ash bowl cleaned up nicely. I honed off the burr with a 1200# diamond card and turned a fresh burr with a carbide burnisher- problem solved. I turned the burr down and back up a couple of times before I was done. Many thanks to John K. Jordan and Robo Hipppy for schooling me on this technique.

  2. #2
    Join Date
    Feb 2020
    Location
    Martinsville, VA
    Posts
    25
    I had similar results today with a hickory bowl. Mainly from Robo Hippy's info I made a carbide burnishing tool last week and today was the first time I used it to turn up a burr on a NR scraper vs. just using the burr off the 180 CBN wheel.

    I can't say for sure it was entirely the different burr that made the difference as I was also concentrating on taking very light passes with the scraper. In any case, I did successfully get a much better surface inside my bowl before sanding.

    Who says you can't teach an old dog new tricks?

  3. #3
    Join Date
    Sep 2013
    Location
    Wayland, MA
    Posts
    3,655
    The burr off the grinder is generally way too big as well as being ragged and not very strong. A burnished burr will almost always work better, and can readily be adjusted (by altering the pressure you use to draw it) for a heavier cut vs a fine finishing cut. It's also faster to refresh than returning to the grinder. What's not to like? It's a little bit harder on a NRS since you have a smaller bevel to rest your diamond hone on than with a regular flat top scraper, but gets easier with practice.

  4. #4
    I prefer the burnished burr. I don't have much luck with it on the skew chisel type NRSs though, I can hear the edge fracturing as I draw the burr, even with very light pressure. My skews are sharpened at about 25 degrees, and if you have a NRS that is sharpened at 40 or so, it may work better. It takes surprisingly little pressure to turn a burr, and you don't want to really crank on your burnisher as you raise the burr. You can turn a huge burr that way, which may not cut as well as a smaller burr.

    As for the grinder burrs, again, how you sharpen them can make a big difference. Very light pressure, and you get a smaller burr. Really heavy pressure, and you get a bigger burr. Then, there are those who prefer an upside/down sharpening to raise the burr. This is supposed to make a sharper burr. I haven't really played around with that much though.

    robo hippy

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