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Conventional Scraper/NRS Are They Equivalent?

Dennis J Gooding

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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 actually NRS’s in disguise. For this discussion, I will define a NRS to be a flat scraper in which the upper edge has been ground off at an angle such that when the scraper is presented in a horizontal orientation, the scraping face is angled down by a desired amount (negative rake). Also, I will ignore burrs, because they can be raised equally well on either type of scraper.

When is a conventional flat scraper, tilted downward to produce the same rake angle as a NRS, equivalent to that NRS? The short answer is “when the point of scraping is broadside to the tool rest at the point where the scraper rests (and tilts)” and the tool rest height of the conventional scraper has been adjusted to scrape at the same height on the wood”. This is a simple matter of geometry. In this situation, the scraping edge is presented to the rotating wood in exactly the same way in either case. Furthermore given this situation, if the tool rest is parallel to the wood being cut, one could simply slide the scraper along the rest, maintaining the same tilt angle, and the results again would be the same as that of a NRS. This situation might arise for example when scraping the inside or outside of a bowl using an appropriately curved tool rest.

Now suppose that the tool rest is not parallel to the wood being cut. A common example would be scraping a curved turning using a straight tool rest. In this case, having adjusted the rest height correctly for a particular point along the rest, we would find that if we were to slide the conventional scraper to scrape a different region of the wood, we would need to move the scraper in or out and readjust the rest position and height to maintain the same scraping conditions. If instead of sliding the conventional scraper, we were to swing it flat on the rest, to either side in order to scrape an adjacent area, the scraper would rotate slightly, yielding a shearing presentation to the wood, and each point of contact with the wood would occur at a slightly different rake angle and different scraping height. In contrast, in either case, the NRS, if held level, would require only moving the scraper in or out to follow the contour of the wood. Clearly, in these situations a tilted conventional scraper is not equivalent to a NRS. While it might achieve the same satisfactory end result in its own way, the conventional scraper would require frequent tool rest adjustments in order to mimic the NRS at each step of the way.
 
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The main thing you forgot to mention is shear scraping and that is not what you are doing the way most turners are using the double bevel scraper. The flat top burred scrapers should only be used up on edge in a shear scraping position. The thought of trying to duplicate the action of the NRS with a flat top scraper I think is ridiculous and like you said would be extremely difficult and potentially dangerous.
 

john lucas

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I played with flat.vs negative rake a.lot for a while trying to understand the difference. What I learned was the flat scraper tilted down wants to be pulled onto the work whereas the negative rake scraper does not. This was most noticeable when I used them on a thin natural edge bowl. The flat scraper would create chatter and the negative rake would cut clean. The rest of your article is dead on and quite interesting.
 

Bill Boehme

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What I learned was the flat scraper tilted down wants to be pulled onto the work whereas the negative rake scraper does not.

This is one of the points that Stuart Batty makes in his instruction manual for his negative rake scraper. When you're trying to use a delicate finishing touch this can be a big difference in controlling the tool.
 

Dave Landers

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the flat scraper tilted down wants to be pulled onto the work whereas the negative rake scraper does not
Exactly.
The initial thought is to look at the angle of the tool vs the wood, which is about the same for both. So what's the difference?

The difference is tool rest support. A flat scraper held handle-up is going to want to slide down the tool rest as the wood pulls it down. The NRS, held level, has alot more support perpendicular to the cut.
 
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I need a video to help me understand this.... For bowl turning, on the outside, with a scraper, you want to be below the center line. On the inside you want to be above that center line. The burr on a standard scraper, fresh from the grinder lasts a lot longer than the burr on a NRS.

You can do peeling cuts on spindles with a scraper. Peeling cuts on bowls, no matter if you are using gouges, scrapers, or a SRG (spindle roughing gouge) are suicidal....

robo hippy
 
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@Dennis J Gooding, you are dead on. In addition to your comments and those that follow I'd add that it is a great deal easier to hold a tool in a horizontal position than nose down. And tools that are held horizontally are amenable to mounting in an articulated arm hollowing rig.
 
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A large number of wood turning accessories and tools have been adapted from the metal machining industry, if you ever have a problem with processing wood on a lathe you can usually reference the methods utilized in the metal machining world to adapt it to wood turning.
 
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A large number of wood turning accessories and tools have been adapted from the metal machining industry, if you ever have a problem with processing wood on a lathe you can usually reference the methods utilized in the metal machining world to adapt it to wood turning.
Do metal cutting tools rely on a formed burr to cut?
 
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Doug,

Metal cutting tools rely more on the geometry of the grind on the cutting tool and the hardness of the cutting tool depending on the metal being machined.
 

Emiliano Achaval

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I’m sure Stuart is referring to only wood turning. It is the only context he has referenced it I am aware of, as well as the context of this discussion.
I agree with you Doug. The remark about metal woodturning was unnecessary.
 
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I've taken a diamond hone to my round nose scraper to remove the flat side bur, then a screwdriver shaft to add a bur too the bevel side... Then use it upside down for negative rake scraping.
 
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The reason a standard scraper does not work like a NRS, no matter what the handle angle is, was best described to me by Ed Webber, moderator over at Woodturner's Resource. He calls it a 'trailing' cut. Most of us are familiar with card scrapers, mostly used in flat work. You rub the bevel, and gently tip the cutting edge into the wood until it starts to cut. If you roll much beyond that, you get the trailing cut where the cutting edge is more like a rake than a slice.

Jeremy, I have never heard of that approach, now I will have to try it.

Standard scrapers cut when pushed into the wood. NRSs cut by just barely touching the surface of the wood. If you have to push at all, the edge is dull....

robo hippy
 

Bill Boehme

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I've taken a diamond hone to my round nose scraper to remove the flat side bur, then a screwdriver shaft to add a bur too the bevel side... Then use it upside down for negative rake scraping.

I need to try that. My gut feeling is that the burr would be weak because there's very little supporting metal.
 

Dennis J Gooding

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This thread has meandered quite a bit from the original issue of under what conditions is a conventional scraper equivalent to a “negative rake” scraper. I realize now that I should have mentioned the “self feeding” issue. Yes, the tilted conventional scraper is subject to a slight longitudinal force toward the cutting point. This force usually is very small compared to other forces that peg the scraper to the tool rest. For example, if the rake angle is 30 degrees, which I believe is a fairly commonly used value for an NRS, the self feeding force is half the cutting force acting on the scraper. Unless one is hogging out wood (not a usual task for an NRS), I believe that this force will be far less than the restraining force resulting from manual downward pressure of the scraper against the tool rest. I have never encountered self feeding with a conventional scraper in my work, and despite the fact that there are far more conventional scrapers in use than NRS, there seems to have been virtually no mention of it in the forums.

Since my original posting, it occurred to me that there is another way to mimic an NRS besides tilting the scraper downward. Again, this method is subject to the “self feeding force”. This method involves positioning the tool rest such that, with the scraper held level, the scraper is presented at a negative angle to the wood. For example, when scraping the inside of a vessel with a conventional scraper, one could raise the tool rest such that with the scraper horizontal, the scraper meets the wood at the desired negative rake. For example to produce a 30 degree rake, the rest would be adjusted to yield a contact height that is halfway between the axis and the inside top of the vessel at the point of contact. Similarly, when scraping the outside of a vessel, one could lower the tool rest to scrape below center an appropriate amount. The catch is that if the diameter of the vessel varies, the tool rest would have to be readjusted in order to maintain the same rake angle. On the other hand, the exact value of the rake angle seems not to be critical, so perhaps few or no adjustments of the rests would be required.
 
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A standard scraper will not cut like a NRS, no matter what you do with it. At least that has been my experience with them, and I have played around with it a lot. Closest I have seen to that is with some sugar/hard maple, and using a scraper that has the burr honed off. It can leave a very clean surface with the harder woods.

I have never understood the concept of 'self feeding' scrapers. As near as I can figure, I think that concept comes from those who use the big wide heavy scrapers. When you sweep across the bottom of a bowl, and then come into contact with the transition area of the bowl, they do seem to dig in. This 'feeling' is more from having as much as twice the cutting edge into contact with the wood because you are cutting the bottom of the bowl, and the transition area.

robo hippy
 

Dennis J Gooding

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One area where this won't work would be on the inside near the bottom. Also, it wouldn't work on most of a platter.

You are quite right Bill, the "raise the rest" technique does not work near the bottom of a bowl. However, you could revert to the first method, angling the scraper down, in that area.
 
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A standard scraper will not cut like a NRS, no matter what you do with it. At least that has been my experience with them, and I have played around with it a lot. Closest I have seen to that is with some sugar/hard maple, and using a scraper that has the burr honed off. It can leave a very clean surface with the harder woods.
The standard scraper with either a rolled burr or off the grinder will do better than a NRS on the inside of end grain goblet forms but it is never set flat on the tool rest the way the NRS is shown used. Think of the rounded end being 12 o'clock at the tip then tip the scraper to about 45 to 60 degrees in the bottom of the cup and the 10 to 11 o'clock position will come in contact for a negative rake shearing cut. The end grain at the center of the cup can be cleaned up with the double angle (NRS) scraper since end grain does not benefit from a shearing cut. The picture should illustrate what I am saying about presenting the bur at a negative rake angle.
DSC00865.JPG
 

Dennis J Gooding

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This is in response to Robo's and Don's comments.

If you re-read my post carefully you will find that my claim is simply that a conventional scraper and a negative-rake scraper with the same burr (or no burr) and both lying flat on the tool rest and cutting broadside to the tool rest, will cut the same if the conventional scraper is tilted down to the same angle as the top face of the negative-rake scraper. This is not a matter of opinion but simple geometry. I also pointed out that the two may not be equivalent if any of these conditions are not met. The conditions most commonly violated are lying flat on the rest and cutting broadside to the tool rest.

I am not anti negative-rake scraper. In fact, I use them myself where appropriate. My reason for publishing this is that over the past 20 years or so, I have seen repeated claims that the two designs are equivalent, with no indication of any necessary conditions.
 
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Don, your picture is a bit fuzzy, but what you are showing is a shear scrape, not a standard scrape. The difference being a scraping cut, to me, is when the cutting edge is 90 degrees to the rotation of the wood. For a scraper or NRS, this means the tool is laying flat on the tool rest while the wood spins. Both gouges and scrapers can be used this way. I frequently see the swept back gouges that are used in this type of scraping mode, and the turner is calling it a shear scrape, which it isn't.

The shear scrape, or sheer as Lyle Jamieson calls it (sheer face of a cliff), is a non bevel rubbing cut, which for some reason is why it is called a 'scraping' cut, which it is not, but no one has come up with a better name. This is a high angled shear/sheer/slicing cut. Totally different from any scraping cut.

Now, I will not agree with you Dennis on this one. Can't explain why, but the standard scraper, with a burr does not cut like a NRS, no matter how high you hold the handle. I am guessing that the burr is presented at a different angle, which is more blunt than what you get with a NRS. It doesn't seem to be related to burnished or grinder burr formation. No clue about a scraper with no burr as I have almost no experience with that tool. If you have used scrapers as much as I do, then your claim may have some merit, but from my experience, it just doesn't feel like they cut the same way, or yield the same results. The NRS is a much finer cut, and leaves a different surface when done, and yes, I have used both cuts to compare. Have you done this to compare the actual cuts and resulting surfaces, or is this speculation on your part? For sure, different woods will yield different surfaces, like madrone which works wonderfully with the NRS, all the way up the side of the bowl.

The NRS and standard scraper both perform far better on end grain cutting like boxes. They both perform fairly well for sweeping cuts across the bottom of the bowl. Their performance for up the side walls of bowls and through the transition area of a bowl is not nearly as good as the surface you get when sweeping across the bottom of a bowl. This is due to the up hill/down hill cuts in bowl grain orientation. Both are scrapers, and they pull at the fibers more when cutting than a shear cut does. The resulting surface on the side walls of a bowl with a NRS still seems to be better than the resulting surface I get when using a scraper with the handle high.

robo hippy
 
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, I will not agree with you Dennis on this one. Can't explain why, but the standard scraper, with a burr does not cut like a NRS, no matter how high you hold the handle. I am guessing that the burr is presented at a different angle, which is more blunt than what you get with a NRS. It doesn't seem to be related to burnished or grinder burr formation. No clue about a scraper with no burr as I have almost no experience with that tool. If you have used scrapers as much as I do, then your claim may have some merit, but from my experience, it just doesn't feel like they cut the same way, or yield the same results. The NRS is a much finer cut, and leaves a different surface when done, and yes, I have used both cuts to compare. Have you done this to compare the actual cuts and resulting surfaces, or is this speculation on your part? For sure, different woods will yield different surfaces, like madrone which works wonderfully with the NRS, all the way up the side of the bowl.

robo hippy

My question comes from curiosity, not to challenge your statement. Have you tested a flat and nrs with the same included angle and the burr created with the burnisher held at the same angle to the lower bevel? I suspect a bit of angle change, resulting in a bit different burr formation and angle, could be the reason they dont cut the same.

Say the flat scraper has a 70 deg bevel and burnished at 80 deg. The nrs has a 60 deg lower and 10 deg upper, burnished at 70 deg from below. With the flat top held at -10 deg it should be the same cut. The steel and the wood don’t care. Correctly testing this with various wood grain angles/types could become pretty time consuming and tedious. Would need some jigs and tool fixturing to get the burnishing and cut angles correct. Probably best done on a metal lathe. In the end its really an academic discussion, since holding a flat scraper at negative angles is not really possible for all needed positions.
 
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A standard scraper will not cut like a NRS, no matter what you do with it. At least that has been my experience with them, and I have played around with it a lot. Closest I have seen to that is with some sugar/hard maple, and using a scraper that has the burr honed off. It can leave a very clean surface with the harder woods.

Don, your picture is a bit fuzzy, but what you are showing is a shear scrape, not a standard scrape. The difference being a scraping cut, to me, is when the cutting edge is 90 degrees to the rotation of the wood. For a scraper or NRS, this means the tool is laying flat on the tool rest while the wood spins. Both gouges and scrapers can be used this way. I frequently see the swept back gouges that are used in this type of scraping mode, and the turner is calling it a shear scrape, which it isn't.
I know it is a shear scrape as I explained further on in the same post I never said it was a standard scrape and I am not sure what a standard scrape is or even want to know. The term "shear scrape" as I first learned applied to wood turning back in 1990 I believe came from the burred cabinet card scraper, which works well due to the fact that it is presented at a negative rake angle to the wood and you move it in the direction of the grain. The card scraper can be used at 90 degrees to the grain if necessary by turning to an angle to the direction of travel at a shear angle just like shear scraping on wood turning. The burred cuter (scraper?) has the advantage of a very shallow cutting depth due to the the minute size of the burr and thus will bridge over small grooves and cut the ridges away until you have removed all of the tool marks. The swept back gouges do work however I don't think they are easier to use or do a better job then a single angle skew with the burr formed toward the bevel. The NRS does work on side grain bowls however if I remember right the demonstrater did limit it's usefulness to certain hard woods. Final note any scraper with the burr formed on the bevel side will work exactly like the NRS when laid flat on the tool rest bevel up.
 
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Well, as I said earlier in this thread, I think, what happens if you raise the handle very high on a standard scraper to try to get it to cut like a NRS, is the same thing that happens with a card scraper if you tilt it too far in the direction you are cutting, it tends to 'rake' more than slice and cut. With the card scraper, you tilt the card towards the direction you are cutting, just until it starts to cut, which is generally maybe 5 degrees or so. You can over burnish that burr like you can on a NRS. If you keep tilting to 30 degrees or so, you will not get as clean of a cut. This is what happens if you raise the handle up high. The standard scraping cut is the most efficient cut I have seen for heavy stock removal when turning bowls.

robo hippy
 
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Well, as I said earlier in this thread, I think, what happens if you raise the handle very high on a standard scraper to try to get it to cut like a NRS, is the same thing that happens with a card scraper if you tilt it too far in the direction you are cutting, it tends to 'rake' more than slice and cut. With the card scraper, you tilt the card towards the direction you are cutting, just until it starts to cut, which is generally maybe 5 degrees or so. You can over burnish that burr like you can on a NRS. If you keep tilting to 30 degrees or so, you will not get as clean of a cut. This is what happens if you raise the handle up high. The standard scraping cut is the most efficient cut I have seen for heavy stock removal when turning bowls.

robo hippy
Why has this been brought up again I don't think anyone has suggested that it would be a workable method, I know that I have never done it and never heard of it until this thread had begun.
 
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