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Thompson negative rake scraper

Discussion in 'Woodturning Discussion Forum' started by john lucas, Feb 1, 2017.

  1. john lucas

    john lucas

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    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 wanted a curved edge for box interiors and my square ornaments. 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. My older no name HSS scraper raises a nice burr on the 120 wheel. 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.
     

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  2. Pete Blair

    Pete Blair

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    Thanks for posting this John. I have been trying some NRS but only have a short bevel on the top. Is there a reason you center the angles?
     
  3. charlie knighton

    charlie knighton

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    I wanted a curved edge for box interiors and my square ornaments
    i like Doug's tools, very good quality. a while back I visited Chappell Hill Woodturners during one of their tool making sessions. I also picked up
    a 3/8 thick tool, shaped as a scrapper that good for bottoms and a 90 degree corner of boxes. its what ever you prefer. IMG_20170201_091743001.jpg IMG_20170201_091908684.jpg IMG_20170201_091924621.jpg
     
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  4. john lucas

    john lucas

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    Pete That angle was chosen just because I had the Robo rest set for doing my old negative rake scraper. Anything more acute than 90 degrees works if your raising a burr. One of these days I need to do an experiment where I try a much more acute angle vs the more blunt 80 degrees or so. I would think that it all boils down to how good a burr you can raise and how long the burr lasts. I can't answer that yet. I'm grinding long bevels on both sides per Stewart Batty's technique. You wear off the burr, flip the tool over and grind to get a new burr. When that one wears off you flip the tool and grind to get another. With a short bevel on top you would hone off the old burr and then grind. Probably not a lot of difference in actual wear of the tool, just one short step difference in the grind. It does take a whole lot longer to take an existing scraper and turn it into a negative rake like Stewarts. A lot of grinding. It only takes a second to grind a short bevel on top of an older tool to achieve the same thing.
     
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  5. Fred Belknap

    Fred Belknap

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    I use 75 and 25 degrees on my negative rake scrapers. I know that don't compute but it works.
     
  6. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Sure it computes just fine. All that it means is that the included angle at the nose is 100° as shown in the sketch below.

    NRS.jpg
    I have a NRS with something pretty close to what you have. A large included angle makes the NRS essential catch proof, but the downside is that the bur wears off faster. I'm considering trying the symmetrical 45° bevels that everybody is talking about.
     
  7. John Torchick

    John Torchick

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    Interesting. Hope this is not off-topic but I recall seeing a video where a turner used a burnisher to raise a burr on a tool like a scraper would have. Anyone know anything about this?
     
  8. john lucas

    john lucas

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    Yes I do it all the time. Using a burnisher or what some call a Steel is an excellent way to raise a burr. I have used diamond hones to raise a burr and on John Jordan's shear scraper he sells a ceramic stone to both polish off the old burr and raise the new burr using the same technique as using a steel. I find that various techniques will raise a different burr depending on the steel. Some straight off the grinder and some work better with a burnisher.
     
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  9. Raul McCai

    Raul McCai

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    I get a burr on the Thompson Scraper by flipping it over and grinding it upside down on my fine wheel.

    One fellow here posted that he takes a stone to the flat scraper and hones the grinder burr off and then uses a scraper burnisher to get a fine burr. I don't know what kind of steel that poster is using for that, but I tried it on my thompson and didn't get much result.

    I got me a 3/8" by 1" length of A-36 Steel from Speedy Metals and I'm going to weld a length of High Speed Steel tool bit to it and grind a curved negative rake on it to replicate the Glen Lucas tool.

    Just as an aside I've used hand scrapers to get little bits of tear out that nothing else I had would touch
     
  10. Bill Boehme

    Bill Boehme Administrator Staff Member

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    That's the way that I have been doing it for over a dozen years. Are you saying that you don't do it that way? :D

    I prefer M2 HSS for scrapers because I can get a better bur if the steel isn't too hard. The high vanadium steel is very good, but I wish the scrapers were tempered to a lower hardness than the cutting tools.
     
  11. John Torchick

    John Torchick

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    Bill, never heard of burnishing to raise a burr. But, I just now learned to tie my shoes. I use the Bunny Rabbit method. :p
     
  12. hockenbery

    hockenbery AAW Advisor Staff Member

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    Quite a few people burnish a burr with a hard steel rod.
    Most put the tool on the rest and role the burr with the rod.

    veritas has had a small burnisher platform on the market for many years.
    It has a pivot pin and burnishing pins.
     
  13. Bill Boehme

    Bill Boehme Administrator Staff Member

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    I have a burnishing tool that I bought probably thirty years ago. It was made for burnishing cabinet scrapers and has an oval cross section. It must be at least as hard as a file if not even harder and highly polished. I have tried to find them in the last few years, but apparently they quit making them many years ago.

    I also have the Veritas burnisher that I bought about a dozen years ago and never use. In a recent discussion here somebody said the Veritas pins were tungsten carbide. I had assumed that they were steel, but I confirmed that they were indeed tungsten carbide with a simple test.

    Steel is strongly magnetic and tungsten carbide is weakly magnetic. I have a stack of super magnets on my drill press (don't ask, it just seemed like the right place if I suddenly need a magnet). It turns out that the pivot pin is steel, but the two burnishing pins really are tungsten carbide.

    As a side note, stacking a bunch of super-magnets together may not be such a bright idea. Removing one from the stack without injury or breaking the magnet is a challenge.

    image.jpeg

    I only tie my shoes on Sunday. I even brush the shavings off.
     
  14. odie

    odie

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    I also have the Veritas unit for raising burs......and I believe it is a carbide tapered post that is the working element. I'm assuming that any steel is capable of raising a bur, as long as it's harder than the steel where you wish to raise the bur......:confused:
     
  15. Zach LaPerriere

    Zach LaPerriere

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    Can I ask an ignorant question? Alright, here goes...

    For side-grain turning, can a negative rake scraper ever achieve a finish equal to a sharp gouge?
     
  16. Bill Boehme

    Bill Boehme Administrator Staff Member

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    I think the answer might be "can Bill or Zach or Odie or Al use a NRS skillfully enough to achieve a surface equal to what he can get with a sharp gouge". There are situations where a NRS isn't needed and other times when a scraper is better than a gouge. Most things that I make get sanded so maybe it's not worth worrying about.
     
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  17. odie

    odie

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    I don't think I'm qualified to directly answer that question, because my experience with NRS's is very limited.......but, I can alter the premises slightly, and insert "shear scrape" in the place of NRS, and give my observations. For me, a shear scrape (with a sharp and properly raised bur) can usually, but not always, result in a better tooled surface, than the same surface directly resulting from the use of a sharp gouge. In this case, the word "usually" probably applies to more than 90% of the time.

    To me, leaving the best possible tooled surface is extremely important for those times where I wish to have a an absolute minimum of sanding......surfaces which may include a small inside radius or corner, intersecting planes that result in crisp outside corners that visually please, and detail grooves that do not vary in depth and width.

    There are other times when I just don't care about geometric integrity, and find that it's faster and easier to just sand the surface beginning with a coarser grit than would otherwise have been possible......and I can't be bothered with the time consuming goal of creating a surface with the absolute minimum of sanding requirements.

    OK......now back to your regularly scheduled programming.....the discussion about NRS. :D

    ko
     
    Last edited: Feb 3, 2017
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  18. Fred Belknap

    Fred Belknap

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    I find that NRS are a "sometime" answer. Sometimes they work great on a piece and sometimes not. They are just another tool to help get a good finish on a bowl. I do mostly natural edge and burls and sometimes with interrupted cuts a gouge just doesn't work as good as a NRS. On walnut bowls I get fuzzies on the the section of the bowl where it is cutting up hill with the grain. Most of the time I use the NRS to clean up the gouge cut on the wings of NE bowls and the transition on the bottom. One thing that I have found is that a NRS will work ok at a lower RPM than a gouge. Sometimes I need to keep the speed below 500rpm for out of round pieces. NRS are not the answer to all turning problems but "sometimes" they are the berries.
     
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  19. Tom Albrecht

    Tom Albrecht

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    Questions are never ignorant Zach, it's the stupid answers that are ignorant. None on this thread though. Another interesting thread with diversions---
    Prompted by seeing the excellent finish on the White Oak bowl that Odie recently showed us I had a brief off-line conversation with him about what tools he uses. A ground burr on a scraper obviously works well for him.
    We all figure out what works best for our own style by asking questions and experimenting with the various answers. I just watched a quick Richard Raffan video about avoiding "catches" that is currently on the main AAW site. Towards the end, he talks about scrapers and "the current fad" of NRS. I am not calling it a fad, but it's worth watching his take on it.
     
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  20. john lucas

    john lucas

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    Well it depends on the wood and how good you are with a cutting tool. Some woods will actually burnish with a cutting tool and leave a finish that looks like it's been sanded to 600 grit or higher. I often get that on spindles, sometimes on the outside of bowls, rarely on the inside (back to Odies question on interiors looking different for that answer). The problem is I don't always absolutely nail the shape. There may be the slightest change here or there or a tool mark where I hiccuped during the travel of the gouge. Sometimes you simply don't achieve the perfect cut over the entire surface so it looks uneven. That's where the shear scrapers come in to even things out so that sanding will improve the surface. On spindles I often have to sand not because the surface isn't cut cleanly but because certain areas don't have the same reflectivity. Sanding or shear scraping makes these areas reflect light the same.
    OK that was the long answer to get around to negative rake scrapers. Shear scraping done correctly is actually cutting but usually leaves a surface slightly less dull than a pure well executed cut. Scraping which is where the wood comes across the tip at about 90 degrees with the tool held flat simply doesn't leave as good a surface as shear scraping and seldom if ever as good a surface as a good clean bevel rubbing cut. Negative rake scrapers do a fantastic job with a fresh burr and light touch. I just did a bunch of square ornaments with small bowls in all six sides. I used a Hunter carbide tool to do a bevel rubbing cut and switched back and forth between that and a negative rake scraper. I found 2 things. One, it took a lot of skill to get a bump free cut or perfect half sphere with the Hunter tool (or any cutting tool). When I did that was a surface that didn't need sanding and has sort of sheen to it. You really couldn't sand the inside of these because they had 4 holes that grabbed the sandpaper. I could take the negative rake scraper and get a surface that was extremely close and only the most careful inspection by someone who knows what to look for could see the difference. It didn't nave that same sheen, more of a highly sanded look.
    Where I find the negative rake scraper comes into it's own is on thin pieces especially winged bowls, where any forward pressure on the wood causes chatter. I was experimenting with regular scrapers held at a high angle vs negative rake scrapers. My theory was that a high angle scraper duplicated the angle of the negative rake and there fore was the same. The problem is when tilting the scraper up you put just a tiny bit more force against the bowl and it was easy to create chatter or get thin the leading edge of a natural edge winged bowl. The negative rake scraper had less of a tendency to do this. I think it's because the forces push straight down onto the edge where the forces tended to want to pull the flat scraper held at an angle into the wood. Now this is pure speculation on my part because I don't really now how to test it but I could duplicate the problem when switching back and forth between the tools
    Now I haven't had the luxury of playing with a lot of exotics and I've heard that negative rake scraping works extremely well on these. I have turned a lot of softer metals and sometimes the negative rake will leave a surface that is smoother and easier to sand than a cutting tool. I think it's because the cutting tool sometimes leaves a sort of spiral thread that must be sanded off and the negative rake scraper leaves a more even surface.
    Don't know if that answered your question but will at least open it up to more discussion.
     
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