negative rake scraper

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

  1. Dave Fritz

    Dave Fritz

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    Generally speaking would you want a thick heavy scraper or a thinner, lighter one to grind as a negative rake scraper.
     
  2. robo hippy

    robo hippy

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    I use smaller ones. This tool is for 'whisper' cuts, so very light, and not a heavy roughing tool. The 'more mass = less vibration/more stability' thing does not apply. 1 is a repurposed skew, another a Raffen spear point tool which is the same as the skew 1 inch by 1/4 inch, and the last is a small scraper, 3/4 by 1/4 inch. Some people have heavy ones, but I don't know why...

    robo hippy
     
  3. Dave Fritz

    Dave Fritz

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    Thanks Reed. Always appreciate your posts.
     
  4. john lucas

    john lucas

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    Well I have both. I repurposed a thick scraper and a thin one. I don't notice much difference. I use both for the whisper shavings. The biggest dissadvantage of the thick scraper is that it takes too long to grind. The thin one just takes a second.
     
  5. Dave Fritz

    Dave Fritz

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    Thank you John. I reground an old skew and am using that. So far it's working well.
     
  6. robo hippy

    robo hippy

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    One of my NRSs was changed from 35 to 25 degree bevels. I took it to the 40 grit belt and took it down. The belt bevel is over 1/2 inch wide, the grinder bevel is about 1/8 inch wide. I do relieve the heels on all of my standard scrapers as well. On most grinding situations, it doesn't do much, but going from 180 to 600 or 1000 grit wheels, it can save time, even if the platforms are set to an 'almost' perfect height for angles from one grinder to the other...

    robo hippy
     
  7. Jamie Straw

    Jamie Straw

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    From the experience I've had so far, it really depends. I have two, a 1/2" and 3/4" that are ground specifically for box making -- the bevels are asymmetrical. They are both 1/4" thick, and they feel just right on the tool rest. I have used a 1" that was beveled all the way around the tip, and is often used with either side laid long-ways on the tool rest. Not sure how thick it was, but certainly no less that 1/4". NRS's are mostly a finishing tool, not a hogging out tool (except for Reed?:D)

    I can attest that one can make an NRS that's tooooo light -- a 1/4" round-nose that I ground with symmetrical bevels (22.5 degrees both sides). It's about 5/32" ;thick, and requires a much steadier hand than I have. Might have to "weight it up.":p
     
  8. Jamie Straw

    Jamie Straw

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    Wouldn't that mostly apply to the first grinding?
     
  9. Bill Boehme

    Bill Boehme Administrator Staff Member

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    It would apply every time that you sharpen it because the bevel is much longer on a thick tool so more metal to remove. If the bevel is twice as long then twice as much metal to remove. I have some miniature tools that are about ⅛" thick. I repurposed the skew to be a tiny NRS. As you said, we're only taking away whispers of wood (except for Reed of course) so no need for a big slab of steel.
     
  10. john lucas

    john lucas

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    Bill nailed it. I grind one side, use that until the burr is gone then flip the tool and grind the other side to create a new burr. That's how Stewart does it and it works wonderfully. The thick metal doesn't really take much longer once you have actually established the grind but the thin tool raises raises that burr with just a touch of the wheel. Repurposing an old skew is an excellent way to get a good negative rake scraper.
     
  11. Dave Fritz

    Dave Fritz

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    John, obviously then your NRS has a symmetrical grind?
     
  12. Jamie Straw

    Jamie Straw

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    When sharpening the NRS during turning, which has to be done frequently, it's only to re-raise the burr. Swipe-swipe, takes perhaps 5- 10 seconds on a platform that's ready and waiting. The two turners I've taken workshops from often just leave the grinder running.:cool:
    The top bevel needs grinding only periodically, the bottom frequently during turnig.
     
  13. robo hippy

    robo hippy

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    Eric Lofstrom uses a carbide tipped rod to burnish a burr on his NRSs. This burr does out last anything from the grinder, and can be used for more heavy cuts. The burrs from CBN wheels way out last the ones from standard grinding wheels, but still go dull fairly quickly.

    robo hippy
     
  14. Mark Hepburn

    Mark Hepburn

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    I hope I'm not hijacking this thread but I'm going to little off topic based on what you mentioned a moment ago Jamie.

    I've often thought about just leaving the grinder running and if I didn't have a cheap grinder I might do that. I'm hesitant to just flip it on and let it rip for an hour at a time.

    I'm using a half horse Rikon end it does take some time to come up to speed so does anyone else live their grinder running?
     
  15. russ stanton

    russ stanton

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    John or Robo

    What angle do you sharpen your NRS?
     
  16. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Here's something to mull over. A typical 4-pole (in other words, 1725 RPM slow speed grinder) half-horse single phase AC induction motor running on 120 volts draws a full load current (FLC) in the range of 8 to 10 Amps. For typical turning tool grinding, it doesn't put out anywhere close to full load power and in fact isn't needing much more than no-load current. BTW, no load current consists of current needed to overcome losses such as copper loss, iron loss, magnetizing current, bearing friction, and windage friction loss. All of these are unrecoverable losses.

    For a grinder, you can add a slight windage loss for the grinding wheels. I mention all of the above because they add up to a significant power loss. A good engineering rule of thumb for a motor of this size is that the no load current is approximately a third of the FLC. Using this rule of thumb and a best case motor that draws a FLC of 8 Amps, we have a NLC (no load current) of 2.66 Amps for our half-horse motor. If we bother to calculate how much power is being lost while the grinder is idly running, but not actually being used it comes out to 320 watts! That's not exactly a trivial number if you are going to let the grinder run for long periods.

    Before you rush for the OFF button there's more to consider. When you first turn the grinder on there is a very large start-up current that can have an initial peak as great as six times the FLC. The initial peak magnitude of the current before the rotor begins to move is called the locked rotor current (LRC). Because the rotor isn't actually locked, the current will quickly decrease to nominal running value as the motor spins up to its running speed. All this typically takes place in less than a half second. However, I wanted to mention this because many people have said that the smaller grinders like the Rikon with CBN wheels installed take much longer to come up to speed. How much longer, I don't know because nobody has given a number. Is it two seconds or twenty seconds? If we're talking many seconds then maybe that needs to be considered as a reason to let the grinder continue running. If you're touching up a NRS every few seconds and it takes X number of seconds for the grinder to come up to speed, then there would be a reason to let the grinder run under those circumstances.

    One other consideration is that the start winding often uses lighter gauge wire than the run windings because of its normally short and infrequent duty cycle. When the grinder is used outside of the design duty cycle and with CBN wheels that presents a far greater than normal inertial load and we are left with more unanswerable questions. This also puts a greater load on the start capacitor. Imported motors often come with limited technical information, usually nothing beyond what's on the nameplate.

    So, now is everything is perfectly clear about turning the grinder off or letting it run?
     
  17. Mark Hepburn

    Mark Hepburn

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    Yes. Very clear. I just ordered two Tradesman grinders and a photovoltaic array.

    Seriously, it seems that with a cheap grinder like mine what becomes more important to consider is its duty cycle and ability to withstand frequent starts. Mine takes 14.83 to come up to speed. It has 2 D-Way 1.5" wheels. That can seem like a maddening wait.

    If I'm using my gouges I don't have to go back and forth because I have three with the same grind. But intuitively, it seems like it might be better for this grinder to let it run if I'm doing an operation that requires frequent trips back to the grinder.

    That is if I'm not considering energy consumption. Am I even close here?
     
  18. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Sounds good to me. There's no clear cut answer. After all of the analysis, it boils down to whatever floats your boat.
     
  19. Mark Hepburn

    Mark Hepburn

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    I'm in a real bind here Bill. I'm one of those rare people in south Louisiana who doesn't own a boat.

    That means I could buy more tools though
     
  20. robo hippy

    robo hippy

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    I have seen 2 variations on the NRS. On the ones that have the same grind on both sides, they seem to be between 20 to 30 degrees on each side. I think Glenn Lucas uses 33 degrees or some thing like that. I don't know that I can feel any difference with how they cut, but that could be because I don't use them that much. The other variation is a 70/20 grind, which for me is my standard scraper with a relieved top bevel. I use them on the inside of smaller hollow forms and end grain forms.

    robo hippy
     

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