How do Negative Rake Scrapers work?

[David Gilbert]

I have a variety of scrapers and have tried a variety of sharpening and burnishing methods. My observation is that the main difference between a regular scraper and a negative rake scraper is that the regular scraper will pull into the wood and can cause a catch. With negative rake scrapers it's very difficult to get them to catch. Does anyone have an explanation for this difference? I have seen Stuart Batty's explanation but to be honest, I don't understand it. Can anyone help?

Cheers,
David

[Reed Gray]

I don't know that I can explain it correctly or not, but I think it has to do with the 'included' angle, which means the combination of the 2 bevel angles. The standard scraper has one bevel face, and the NRS has 2. You can not get the same cut on a standard scraper by raising the handle way up in the air to simulate the top bevel angle on a NRS. Even with the CBN wheel burr, the NRS will not cut anything like a standard scraper.

The main problem with scrapers catching is that in soft wood it isn't a problem as much as it is with softer woods. Any one who uses them on the exotic tropical very dense woods says to hone off the burr because it will make the tool dig in. I do always angle my scrapers downwards. Perhaps the other cause for catches is that generally scrapers are pretty big as in 1 1/2 inch wide and 3/8 or so thick. If you stop to think about it, that is a lot of steel hanging off the tool rest, and if you engage that much at one time, you will stall your lathe, and can, if you are not prepared, have the tool yanked out of your hands. Most gouges will not have that much cutting edge that you can get into the wood at one time. I have figured that this is one reason why the carbide tipped tools are so popular with beginners. The smaller surfaces are easier for them to control.

robo hippy

[Fred Belknap]

David I use NRS (negative rake scraper) a lot, I think all my larger scrapers are NRS. I use a main angle of around 65-70° with a small 10-15° on the top. I only grind it back about 1/8", it doesn't seem to matter to much. It will work with the tool point down below the center line or above the center line. I do mostly natural edge bowls. About 90% of the turning I use bowl gouges but the bottom and finish cuts are almost always with NRS. I use a Lee Valley burnisher on the NRS for fine finish cuts with the scraper turned almost upside down, gives a very good presanded surface. It is real hard to get a catch, and like you I wonder why. My opinion is that the small negative rake keeps the tool from digging in and self feeding. I agree with Reed Grey that they don't produce large heavy shavings like a gouge but can remove a lot of large soft fluffy shavings. I have never seen anyone but myself use these, I just do a lot experimenting. This is just my opinion for what it is worth, I really like my scrapers.

[Thom Sturgill]

I have always heard that the angle between the top face of a scraper and the wood be scraped must be less than 90 degrees. If it is greater, the edge will dig in and self feed - a catch. If the angle is too great the bottom of the relief will keep the burr from touching and thus the scraper will not cut at all.

With a standard scraper this means the tool tip must be at or above the centerline when hollowing (below center line for outside work) if the tool is held flat on the tool rest. If the tool is angled to the surface you can work *slightly* below center, but that's unstable when scraping the side with the tool on its edge - on the bottom it is accomplished by angleing the end downward. In a transition area this becomes very complex.

The negative rake creates a smaller angle between the top face (and thus the burr) and the wood without being unstable. The tool is held flat on the rest.

The degree and nature of the burr determines the type of cut, roughing or finishing, or even if the edge will cut. I use also a Lee-Valley jig to roll the burr and have over-done it and had to hone the burr off and start over to get a workable burr. In my experience and according to Alan Lacer who I saw last weekend, a grinder burr is comparably like a tomato knife while a honed and rolled burr is more like a chef's knife when looked at under a microscope. The grinder burr breaks off quickly and does not last as long as a rolled burr.

[Jeffrey J Smith]

As I understand the concept, in simple terms...have you ever raised the handle of your scraper and noticed that it is less 'grabby' and seems to take a smaller, more delicate shaving? The negative rake scraper does exactly the same thing. By grinding an angle on the top surface of the scraper you're essentially creating the same angle with the surface without needing to raise the handle. It's easier to control since you're using it flat on the toolrest, won't grab, and achieves the same kind of wispy shavings.
That's how it was explained to me by 'someone' in the Batty booth at the San Jose symposium.

[Reed Gray]

Jeffrey, that makes my sphincters pucker.... I do generally have my scrapers level, or angled down a bit. You can do a bevel rubbing cut with one pointing up, which would mean that a scraper with a 70 degree bevel would have to be pointing up at 20 degrees above horizontal. If you have it pointed up and are rubbing the bevel, it is a controlable cut, but it makes me nervous, and I have only done it at very low speeds. The problem is that if you come off the bevel at all, it turns instantly into a scraping cut and you are pointing into the fan blade so to speak, and you can get a major catch. Similar to what happens with a skew if you come off the bevel. How much or how little they cut seems to be related to how much you are pushing. The self feeding is worse in very hard woods than it is in softer woods. Maybe it is just that I 'feel' it after turning so many bowls and using scrapers as my main roughing tool. I can get shavings of Madrone which you can see through, and they seem to float in the air.

Thom, I think I am going to have to come down to the Florida Symposium so we can play with scrapers for a bit. The burr from a CBN wheel lasts as long as any burr I have been able to burnish. I don't have one of the Veritas burnishing tools though, I hand burnish with a triangle tool.

robo hippy

[Jeffrey J Smith]

Reed - don't know if I was clear - but the idea is that raising the handle slightly above level lowers the scraper below the toolrest edge at the work's surface. Didn't mean to pucker you up...but it's the technique I learned for final izing the OUTSIDE of the a bowl.

[Reed Gray]

Oh, I misread it, you raised the handle, and not the blade..... If you have seen the video clip on why not to use a spindle roughing gouge on a bowl by Ian 'Robbo' Robertson of Australia, where he is turning a square piece of board on a Nova, and is using a SRG, he starts the cut like we are taught when starting to rough out a square spindle blank to round. The tool square to the wood, and bevel high. He raises that handle/lowers the cutting edge till the bevel is rubbing. He finally gets a big catch when he extends out and comes off the bevel......

robo hippy

[Sid Matheny]

You can also lower the rest a little and not raise the handle and get almost the same effect.

Sid