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Who knows the answer to this question about negative rake scrapers?

Odie

Odie

Panning for Montana gold, with Betsy, the mule!
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For about 5+ years, I've been extensively using negative rake scrapers on my bowl interiors.....after years of thinking they were no different than standard scrapers using both flat to the tool rest. My thinking here, was the tip of the bur is no different in it's presentation, if the standard scraper is simply tipped slightly to match the presentation of the NR scraper.......but, I'm wrong about that, and I do not understand just why this is so. (Speaking strictly here about ground burs, and not manually raised burs, and both used at the centerline.)

In practical use, the standard scraper is noticeably more "grabby" than the NR scraper, used identically at the same angle and height, and flat to the tool rest. This advantage that NR scrapers have, allows for a little more manageable control.....and, is very useful under certain conditions.

I know the NR scrapers have this advantage over standard scrapers under certain conditions......but, I am unable to conclude why this is so.

Here's the question: Why do NR scrapers have an advantage? It just doesn't seem like they should, but there must be a reason that is comprehendible from a physics perspective.

-o-
 
Positive rake scrapers are cutting on the edge with very little metal support for the bur or the chip path. Negative rake scrapers are also cutting on the edge, but the bur has an angle which gives bur some metal support and deflection of the chip path to additional metal support.
 
My opinion is that the burr only lasts for seconds in the cut and with the negative rake scraper, you then have the same angle you should have with a regular scraper if you had the end of the handle higher than the cutting edge. So many bring the scraper in horizontally, or handle low, and that is grabby.
 
John Snow said:
IIRC, Richard Raffan made a comment in one of his videos that he thought old-style scrapers worked as well as negative-rake scrapers. I'm also in that camp, and yours, Odie. I think it's an issue of skill and experience in positioning the scraper, the tool rest, and cutting edge movement.
Click to expand...

Hmmm.....Undoubtedly, tool handling and skill are an acquired proficiency, and I know that it's a continuing paradigm that keeps improving with time and experience......however.....I have run tests with standard scrapers vs NR scrapers on the same piece of wood at the same time and placement....nothing different, except the style of scraper. For me, the NR scraper simply cuts differently......and as I also said, it defies any logical explanation on my part. In this, I can't say the NR cuts any cleaner, but the one thing that is significant for me, is the NR scraper is more under control, and less "grabby".

OTOH.....the standard scraper is capable of removing wood at a faster pace than the NR scraper....

-o-
 
Oldie - perhaps rather than using identical setups for the scraper angle, use identical angles of the cutting edge to compare. This would require the standard scraper ‘s handle be elevated to present the same angle of the cutting edge to the work…
 
A video I need to do..... I have been saying that for years. Okay, first, a standard scraper will not cut like a NRS no matter what you do with it. One turner commented that you get a 'trailing' cut if you really raise the handle on a standard scraper. Think of using a standard card scraper. You burnish a burr on it, and start cutting by rubbing the bevel and slowly angle the card until the cutting edge engages. If you keep tilting the card into the cut, you end up doing more of a rake than a slicing cut.

As for the NRSs, I prefer a burnished burr to cut with. Some like the upside down burr, saying it is sharper. This is done by sharpening the bottom bevel with your scraper up side down. I did try that a few times and couldn't really tell any difference.

The original NRS was a skew chisel. Allen Batty told a story about turning billiard balls out of elephant ivory, which was a couple of years process. My first NRSs were skew chisels. I did chat a bit with Tom Wirsing, hope I spelled that correct.... He said 2 things, the NRS is a high maintenance tool, and if you have to push at all, it is dull. For sure, the skew chisel types go dull really quickly, like in a couple of seconds. I am in the school that believes that unless there is a certain amount of metal under the burr, it will not last very long. So, now days, my standard NRSs are 65/25 for the 2 bevels. I have heard that the combined angles should not exceed 90 degrees. Don't know about that. I am thinking it is more of a suggestion. I will use the grinder burr, and then burnish it down and back up for an extended life. I did find that with the skew chisel ones, they didn't burnish very well. I could hear the edge fracturing as I burnished a burr. I may have been pushing too hard... Que in here the old song by the Seeds, "Pushing too hard:....

For sure, the NRSs are not made for heavy stock removal. They are for very fine cuts, mostly to smooth off those tiny ripples that we can leave with a gouge. They excel in finish cuts on end grain, to the point where if you sand with 400 grit, you are roughing it up. For side grain, the results can vary a lot, mostly depending on the wood. Softer woods do not do well with a NRS, but harder woods do tend to finish cut better.

Now, another option. Saw a demo by Nick Stagg, out of the Salem, OR area. He was turning a lamp base out of hard/sugar maple. His finish cut was with a scraper. He sharpened it, then honed the burr totally off. It left an almost glass smooth surface. For sure that would not work with softer woods. I did try it at home, and it did work. Hmmmmm....

robo hippy
 
Dennis J Gooding said:
A few years ago I posted an analysis of this subject in the Tips and Tutorials Forum.

Here is the link

Conventional Scraper/NRS Are They Equivalent?

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...
www.aawforum.org www.aawforum.org














.
Click to expand...
 
With a negative rake scraper all the forces go straight down onto the tool rest. This puts less force into the wood. With a flat scraper tilted down to mimic a NR the wood is trying g to pull it down and into itself. In my experiments trying it both ways you have a more controlled and cleaner cut with the NR.
 
The one thought that I've had with NRS's is that maybe it's because the top bevel acts as a "limiter" (for lack of a better term) to prevent you from taking too deep of a cut, similar to a raker on a saw chain or blade. On a standard scraper you don't have the top bevel to limit your cut so it can easily bury itself, similar to if you were to remove all the rakers on a chainsaw.

Does this seem logical or am I just babbling? It's just one of those deep thoughts I've had.:rolleyes:
 
Like others I've wondered the same thing about the dynamics of why they work. I'll take a look at your article Dennis. All I know is, they work for me and I use them a lot to refine curves and surfaces and have them in a variety of sizes and shapes. I don't have any "regular" scrapers and in countless hours of using a NR scraper in all sorts of positions I've never had one catch on me. I even have a very long one that I use to clean up the first 5-6" of hollow form openings. I saw the Richard R video where he comments that it works the same as a flat scraper tilted downward. Haven't tried it since I don't own one.
 
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For me, I have several positive rake scrapers and one I think negative. I often lift the handle to bring the scraper to the negative angle, always on the internally, rarely on the outside. I must qualify that my internal scrapers are for want of a better description 'paddle scrapers' that perhaps lean toward this method
 
I have a small NRS I made based on a suggestion I read somewhere back in 2011 to cut a bit of a backbevel on the topside of a round nose scraper. Mine isn't shaped quite right because didn't totally grasp the concept, and I had forgotten about it until I unpacked my lathe earlier this year and have been amazed at what the thing can do without wrecking a nice bowl from a horrible catch. So even my mongrel NRS is a fantastic little tool. I'm going to eventually grind a proper NRS profile on it.

I think the reason a NRS scraper doesn't function the same as a "standard scraper held at an angle" is because the NRS is being supported a lot more by the tool rest and your hand. In other words, held more rigid.
I've used scrapers many times held at an angle because I feared a huge catch but never could control it enough to get smooth cuts.
 
Odie

My guess as to the difference from a physics perspective is that with a standard scraper held at an angle, lowering the handle gives tremendous leverage against the wood. Plus, the rotation of the wood is pulling the tool down against that leverage. So it’s much easier to have a light touch with the nrs.
 
hughie said:
For me, I have several positive rake scrapers and one I think negative. I often lift the handle to bring the scraper to the negative angle, always on the internally, rarely on the outside. I must qualify that my internal scrapers are for want of a better description 'paddle scrapers' that perhaps lean toward this method
Click to expand...

This is the key for regular scrapers. Raise the tool rest and lift the handle so that the scraper contacts the wood at a downward angle.
 
Dean Center said:
Thanks for asking this, Odie. I've had the same thought process and turning experience you have had.
Click to expand...

You bet, Dean.....We're having some interesting responses here.

It's not a thread about whether NR scrapers work, or not.....we already know they do.

it's the "why?" that I'd like to sort out....so that my mind can embrace it.


-o-
 
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Jeff Jackson said:
The one thought that I've had with NRS's is that maybe it's because the top bevel acts as a "limiter" (for lack of a better term) to prevent you from taking too deep of a cut, similar to a raker on a saw chain or blade. On a standard scraper you don't have the top bevel to limit your cut so it can easily bury itself, similar to if you were to remove all the rakers on a chainsaw.

Does this seem logical or am I just babbling? It's just one of those deep thoughts I've had.:rolleyes:
Click to expand...

I can't say if it's babbling, Jeff....but, it does have a sense of logical deduction to it.

Thanks for your thoughts.

-o-
 
Mike Novak said:
Odie

My guess as to the difference from a physics perspective is that with a standard scraper held at an angle, lowering the handle gives tremendous leverage against the wood. Plus, the rotation of the wood is pulling the tool down against that leverage. So it’s much easier to have a light touch with the nrs.
Click to expand...

Thanks Mike......To my thinking, your explanation makes the most sense. There are others who have posted to this thread and hinted at the exact same concept you've described, but your post explains it, using the right words that tweak my thinking process in such a way that makes it clearer to my own scrambled brain! :)

-o-
 
The difference is the contact with the tool rest. When a standard scraper is tipped forward to mimic a negative rake, the bottom surface is already pointed downward and more easily slides toward the work making it more ”grabby”. In contrast, the tool rest contact of a negative rake scraper is typically horizontal and as such not biased in a “downhill” attitude, making it less apt to move forward when the burr edge loads up.

Tim
 
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John Snow said:
IIRC, Richard Raffan made a comment in one of his videos that he thought old-style scrapers worked as well as negative-rake scrapers. I'm also in that camp, and yours, Odie. I think it's an issue of skill and experience in positioning the scraper, the tool rest, and cutting edge movement.
Click to expand...
Ironically, it was on a Raffan scraper that I first ground a negative rake. I bought a Raffan box scraper and it often "grabbed" in use. I ground a narrow, 15 degree negative rake on the top, keeping the steep angle beneath. I haven't had a catch since! To sharpen, I just do a light pass on the steep face. Eventually I have to re-grind the top. All my curved scrapers are now negative rake.
 
...and if you look at page 23 of SB's Negative Scraper Manual, the illustrations are spot-on with my comments above; directly pointed to Odie's original question.

Tim
 
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Stuart did coin the phrase 'negative rake scraper'. The tool was in use before Stuart was born. There are saw blades, I think for cutting laminate material, and they have a 'negative rake' set to the teeth, which means instead of pointing into the material you are cutting, they angle back away from the cut. No clue how long they have been around. Why does it work? Well, for sure it is less grabby than a standard scraper. Also, the burr is very fine/minimal, so there is less to dig in and grab the wood. The purpose of the negative set to the teeth is to prevent chip out. I would speculate that the NRS due to a combination of factors, works because it is only for finish cuts. If you consider the carbide NRSs, they have no burr, but they still cut. The scraper that has the burr honed and stropped off may work the same, but again, much of that can be because of the wood.

robo hippy
 
odie said:
Here's the question: Why do NR scrapers have an advantage? It just doesn't seem like they should, but there must be a reason that is comprehendible from a physics perspective.
Click to expand...
It's the rake angle of the "hook", formed by the grinder or burnished, as presented to the wood. The lower the rake angle is, the less aggressive the hook is. Ever tried a scraper with the blade tilted up? If you haven't, DON'T! Whether a flat or NRS, for a given edge prep process, the burr is more or less the same for the same included angle (a 70° bevel flat and 35/35 NRS have the same included angle). The angle of presentation of the hook to the wood can be the same, by tilting the flat blade to the same angle as the top bevel of the NRS. All of this discussion pertains to the blade flat on the tool rest, not shear scraping.

As mentioned by others, the tilted flat scraper will tend to slide into the wood due to the downward tilt, whereas the NRS is "neutral" if held flat. Held flat, the NRS is X° less rake angle (top bevel angle) and therefore less aggressive.

Another aspect of tilting a flat scraper is being able to maintain the desired position of the tool and tilt throughout a pass along the form shape. This can be difficult at best and impossible in a lot of situations. The NRS solves the positioning problem.

The physics perspective is the NRS hook is that it is much more easily presented to the wood at a less aggressive angle, making it more easily controlled.

Twisting the tool blade, and the hook, at an angle to the wood, shear scraping, does the same thing, ie makes the edge less aggressive by changing the angle of attack. That's my primary use of flat top scrapers.

From the physics perspective it's all about geometry:
< of the hook as presented to the wood (tilting and/or twisting the blade)
< the amount of edge (hook) in contact with the wood (ground shape of the tool)
 
Matt Meadows said:
I have a small NRS I made based on a suggestion I read somewhere back in 2011 to cut a bit of a backbevel on the topside of a round nose scraper. Mine isn't shaped quite right because didn't totally grasp the concept, and I had forgotten about it until I unpacked my lathe earlier this year and have been amazed at what the thing can do without wrecking a nice bowl from a horrible catch. So even my mongrel NRS is a fantastic little tool. I'm going to eventually grind a proper NRS profile on it.

I think the reason a NRS scraper doesn't function the same as a "standard scraper held at an angle" is because the NRS is being supported a lot more by the tool rest and your hand. In other words, held more rigid.
I've used scrapers many times held at an angle because I feared a huge catch but never could control it enough to get smooth cuts.
Click to expand...
maybe not. I’m not sure I understand how the nrs is more supported by the tool rest or my hand. I’ve made miniature negative rake scrapers and they cut just like my massive 1 inch carter & sons. Both give me smooth cuts inside a bowl, whether it’s 1 foot or 1 inch. I think the word “scraper” is problematic on the nrs. It cuts.
 
just IMHO, seems to me if you visualize at a microscopic level what's going on, it is kind of like the hook on a NRS cuts as opposed to peeling (but as the hook breaks away it's back to peeling) the difference being the angle the cutting edge is meeting the wood - you have a curved surface spinning which means it will be dragging at a bit of an angle over the hook of stationary scraper , so at a , say, 25 degree angle (similar to a low angle hand plane) - With a NRS you're presenting the hook at a more obtuse angle which may be more like a 50 to 60 degree angle - Flatwood workers know that you can sharpen a plane blade back bevel at a steeper angle than usual to get a much higher effective cutting angle to handle difficult swirly grain (such as on curly or birdseye type wood grain), reducing tearout.... So I would imagine the same principle applies with NRS Vs. Regular scrapers...
 
Brian Gustin said:
just IMHO, seems to me if you visualize at a microscopic level what's going on, it is kind of like the hook on a NRS cuts as opposed to peeling (but as the hook breaks away it's back to peeling) the difference being the angle the cutting edge is meeting the wood - you have a curved surface spinning which means it will be dragging at a bit of an angle over the hook of stationary scraper , so at a , say, 25 degree angle (similar to a low angle hand plane) - With a NRS you're presenting the hook at a more obtuse angle which may be more like a 50 to 60 degree angle - Flatwood workers know that you can sharpen a plane blade back bevel at a steeper angle than usual to get a much higher effective cutting angle to handle difficult swirly grain (such as on curly or birdseye type wood grain), reducing tearout.... So I would imagine the same principle applies with NRS Vs. Regular scrapers...
Click to expand...
Thanks Brian.....because of this thread, I'm using my NRS with a little more awareness.

Last night, I was noticing that when I was forming the interior shape of a maple burl bowl, there was a point in the transition area where it was tending towards being a bit more sensitive and grabby. ("grabby" may be a little too strong of a word, but without careful attention to what I was doing, a catch could have happened at that point.) I noticed that when I tipped up the angle to a shearing mode, just a little, the action was a little more controllable. Since we are speaking of burl figure here, the direction of the cut didn't make much difference, like it does with straight grained woods.....

Note, a curved tool rest allows for a more controlled cut with NRS, the point here is the more overhang, the higher tendency to lose control of the cut. It's not possible to have the perfect short overhang in all cases, unless your tool rest exactly matches the shape of the interior you're currently working on. I've noticed that some turners swear by their straight tool rests, but IMNSHO, a good curved tool rest is an advantage that is without equal when comparing to a straight tool rest.

-o-
 
NRSs are scrapers, which to means that the cutting edge is presented to the wood at 90 degrees to the rotation. If the NRS has a burnished burr, it can make a good sheer scraper too. I do prefer a shear scrape for my finish cut on bowls. There are some that say scrapers scrape but don't cut. For them, I say, if it isn't cutting, then why am I getting nice thin shavings? A scraping cut is different from a sheer cut, but it is still a cut.

robo hippy
 
Good post Rich. So basically he’s saying that a negative rake scraper is always making a trailing cut - unless you drop the handle massively.

Who can explain though, why a scraper with a burnished rather than a ground burr, cuts nicely with the tool parallel with the floor?
 
The burnished burr is far sharper and much more durable than the grinder burr. With harder woods, like sugar/hard maple, you can get almost glass smooth surfaces. I still prefer a NRS for finish cuts on my end grain boxes.... 400 grit roughs it up.

robo hippy
 
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