What Makes a Scraper a Negative Rake Scraper?

I'm a bit confused and am hoping to get some clarification. It has been said that merely tilting the handle upwards on a conventional scraper does not turn it into a negative rake scraper. Furthermore, the contention is that a negative rake scraper requires a small top bevel. It would seem to me that a properly tilted scraper would create the same angle of presentation to the blank that would be achieved with a top bevel. IOW, how does the wood know? All it sees is that angle. It has no idea how many bevels are present.

Case in point. Below is an image of a scraper that Crown is marketing as having a negative rake. Obviously, there is no top bevel. What's the difference between this and tilting a conventional scraper?

Attachment 389071

Glen,

My opinion is the top angle is (almost) everything, whether ground on the tool or held at an angle. Also important is the included angle - some say it should be less than 90-deg. I like the angles maybe 50-60 degreees for side grain work and usually larger angles for end grain. Under the right circumstances there will be no difference in the cut between a conventional scraper tilted downward and a NRS held horizontal.

But in practice there is a HUGE difference in how controllable and easy to use the tool is in different situations. Miscalculate a bit and it can get exciting in a hurry. A negative rake scraper is almost always held horizontal which makes it easier to control, consistent in the edge presentation, and nearly impossible to catch.

If a conventionally ground scraper (flat on the top, bevel say at 80-deg) is tilted down 20 degrees, the edge is presented to the wood the same as an NRS tool with the top bevel of 20-deg and an included angle 80-deg. However, the NRS more controllable and easier to use since it is held flat on the rest and the top angle is always the same. If tilting the conventional scraper down, I would have to be careful to keep it at the same angle and keep the contact with the wood consistent and stable. This is actually not too hard IF the scraper nose is flat or gently curved and the piece is flat or gently curved, such as the bottom of a platter where the tool shaft is always nearly perpendicular to the tool rest when viewed from the top.

This all falls apart when dealing with profiles that are more curved and when scraping work that is angled or significantly curved. It would be theoretically possible to swing the conventional scraper to engage the wood consistently, but difficult. For example, if you swung a round nosed scraper around to smooth the inside wall of a bowl while holding the tool angled downward, as the shaft changes from perpendicular to the tool rest the angle of the edge on the work would change, requiring a lot of effort to keep it where needed.

Compare that with a NRS ground with a round nose. Since it is held horizontal it can be swung in either direction and the edge will still stay flat and stay at the same height on the work (most likely on or close to the centerline) even when not perpendicular to the tool rest. This makes it much easier to control and no fancy gyrations are needed to keep the edge in the right place.

It gets even more interesting with a scraper with significantly curved sides. For example, these are my favorite negative rake scrapers for face work, inside and out. I ground them from Thompson steel somewhat based on scrapers I've seen some well-known turners use but with my own modifications. (so they are better! Ha!) The side curve goes to a nearly straight tip which give me a better surface on relatively flat areas like the bottoms and wings of pieces when "turning air".

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When swinging the scraper left or right to scrape inside or outside curves, the edge stays exactly at the centerline (or where I want it) requiring no care other than keeping the tool horizontal. The angle of the shaft to the tool rest doesn't make any difference. I can almost use this tool while daydreaming or half asleep. :)

Note, the photo may make it look like the tool is angled up but it is actually horizontal.
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I often use small negative rake scrapers with various profiles when turning end grain, mostly on smaller work. With hard, fine-grained wood the surface is almost unbelievably glass smooth. These are some that I've ground from Thompson steel, some from round stock, some from flat, and one made from a shallow detail gouge. I keep a variety of sizes and shapes. The round stock allows me to easily tilt the edge for sort of a shear scrape, nice in some situations.

Attachment 389076

BTW, I use all of these scrapers with no handles. I find handles are not needed for smoothing since there are no forces involved and no chance of a catch. I do use handles on scrapers used for hollowing and when extended far over the tool rest such as when working deep in a lidded box.

Also, just as with conventional scrapers, the burr on the edge is extremely important. I usually burnish a burr with a carbide rod but also on occasion use a honed burr or ground burr. The type and size of burr make a huge difference in how the scraper works in various situations and with different woods. The burnished burr definitely lasts longer.

The Crown scraper you showed is obviously designed to be held horizontal, making it functionally the same as a NRS when used straight in. In this case the flat top is identical to a top bevel and ground side of the teardrop cutter is the bottom bevel. The round shaft would let you rotate to tilt the cutter when smoothing the inside of the walls of a bowl, but keep in mind the angle of the cutter to the edge will change significantly with the tilt. I use a similar Sorby cutter the same way.

I hope this is not too confusing. It's one of those things that might take 10 minutes to describe and 10 seconds to demonstrate! (I'll probably read it later and get confused myself. :)

JKJ

Quote:

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Originally Posted by Glen Blanchard

I'm a bit confused and am hoping to get some clarification. It has been said that merely tilting the handle upwards on a conventional scraper does not turn it into a negative rake scraper. Furthermore, the contention is that a negative rake scraper requires a small top bevel. It would seem to me that a properly tilted scraper would create the same angle of presentation to the blank that would be achieved with a top bevel. IOW, how does the wood know? All it sees is that angle. It has no idea how many bevels are present.

Case in point. Below is an image of a scraper that Crown is marketing as having a negative rake. Obviously, there is no top bevel. What's the difference between this and tilting a conventional scraper?

Attachment 389071

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John - Thanks for your thoughtful reply. I agree that tilting a conventional scraper into a negative rake position could be a bit dangerous. My post was primarily about understanding the difference between doing that and using a true negative rake scraper with a top bevel. I have never understood why some turners state that you cannot obtain a negative rake by tilting the handle up on a conventional scraper. If I understand you correctly, doing so does indeed transform a conventional scraper into a negative rake scraper but does so at the risk of introducing a level of danger into the operation.

Thanks again.

This was some thing that puzzled me for a long time, but raising the handle on a standard scraper does not turn it into a NRS. The 'why' was finally explained to me by some one over on Woodturner's Resource. He called it a 'trailing' cut. So, if you have played with the traditional card scrapers, you burnish a bevel, then to get it to cut, you rub the bevel, and then tilt the card towards the cut, which is usually at about 5 degrees off of vertical., so not really a bevel rubbing cut, but close. If you tip the card to 20 or 30 degrees, you get the trailing cut, which is more like dragging a rake through the wood than slicing. That tool from Crown would be difficult to get a catch with, but would not cut very cleanly, except in end grain. That type of tear drop cutter could be ground into a NRS fairly easily. Inside of a hollow form, I would want a burnished burr, about 45 to 60 on the bottom, and 20 to 30 on the top.The skew chisel type of NRS does not take a burnished burr very well.

robo hippy

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Originally Posted by Reed Gray

This was some thing that puzzled me for a long time, but raising the handle on a standard scraper does not turn it into a NRS. The 'why' was finally explained to me by some one over on Woodturner's Resource. ...
The skew chisel type of NRS does not take a burnished burr very well.

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This may be a matter of definition. As for the trailing cut explanation from this Woodturner's Resource guy, I think something is missing.

What do you mean by a "skew chisel type of NRS"? All of the scrapers and tools I've used as scrapers, including skews, could be ticketed with a carbide rod. (The small diameter rods you have are the best and easier to control than the larger ones I used to use - they've been in such demand I'm about out!) The angle and pressure are the big variables, both difficult to quantify.

There are good diagrams and definitions of rake in the machining literature. Rake is simply the angle of the cutting face relative to the direction of the cut.
(From https://en.wikipedia.org/wiki/Rake_angle, as good a reference as any)
Attachment 389113
POSITIVE RAKE:-A tool has a positive rake when the face of the cutting tool slopes away from the cutting edge at inner side.
NEGATIVE RAKE:- A tool has a negative rake angle when the face of the cutting tool slopes away from the cutting edge at outer side.
This says nothing about how the tool is supported or the type of material being machined. We can argue that machining wood is different but it's basically the same except for one big difference and one difficult to measure and quantify - the burr. For definitions, diagrams, and outstanding photos of cutting tools in action Darlow's "Fundamentals of Woodturning" is good.

Given two pieces of steel, both presented to a moving wood surface with the same horizontal pressure, the same negative rake angle relative to the wood, both with the same included angle, AND both with identical burrs (relative to the wood surface, of course), they will produce the same shaving. How would the wood know if that edge is being held by a handle angled down or a handle held horizontally? Besides obvious differences in the geometry of the support by the tool rest which will certainly play an increasing part as the force into the wood is increased, the difference I see is in the ease of handling to keep the edge consistent as the scraper is moved. The actual size and angle of the burr, of course, is a big variable difficult to keep consistent.

I tried this experiment with two scrapers. One big difference was angling the tool downward increased the overhang - the NRS allowed the tool rest to be closer to the work with all the benefits of that.

I also use a variety of scrapers quite a bit hand held, in the air, without tool rest support. The rake angle there is what I choose at the moment. In fact, varying the angle can make a big difference in the material removed and the smoothness of the surface, depending on several factors, the most important being the burr or lack of burr. Held in the hand, either with the wood spinning or stationary, is more like using a card scraper. I rely a lot on hand scraping for smoothing. (One huge advantage is the ability to scrape with the grain on every spot on the surface.)

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This NRS discussion reminds me of one about 15 years ago concerning the height of the tool rest. Some seemed to insist on the existence of intrinsic parameters relating the cut to the position of the tool rest relative to the center line of the work, defined as perpendicular to the line through the center of the earth. I maintained that the wood didn't care one bit, as long as all other forces were kept the same. Besides the extremely small effect of gravity on the tool itself, the reason one tool rest position works better than others is related to the person - how he stands, leans, bends, moves, feels and sees. Some positions sure would be awkward and make it difficult or impossible to control the tool, especially when moving.

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BTW, a fun thing to try is cut with a skew chisel with the lathe in reverse with the skew supported UNDER the rest. Rather than devise an inverted tool rest, use one with a fairly long round rod. After a few seconds of confusion I could make sloppy planing cuts, albeit like turning blindfolded since it was hard to see the wood. I told John L once that would be a fun thing to do in a skew demo - have two turners turning the same piece at the same time, one on the back side turning inverted! For those looking for something a little different, Graeme Priddle told about another fun thing, cut with two skews at the same time, one in each hand. I tried that also and while surprisingly easy (I was already used to turning spindles one-handed), I'd have to practice a bunch before I tried it in front of an audience! Maybe by my next spindle demo in a few months...

JKJ

By skew chisel type of NRS, the first ones were skew chisels, same angle grind on both sides and you didn't hone the burr off. At least as old as Allen Batty. His story about making billiard balls out of elephant ivory was very interesting. It was a couple of years process and they used a straight sided jam chuck and take off a very tiny bit at a time and then let the ball rest for a bit, then repeat as needed...

robo hippy

Greame Priddle used to start his demo of turning a large spindle blank by starting the lathe upside down and putting the bowl gouge upside down underneath the tool rest. He would just be happily turning away and the crowd would be laughing and he would say "what, don't you all turn this way" Everybody knows things run backwards on the other side of the equator.