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Thompson negative rake scraper

john lucas

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I picked up a scraper blank from Doug at the symposium this last week. I wanted to make a thicker negative rake scraper than the one I now have. Mine is 1/4" thick and Dougs is closer to 3/8". I wanted a curved edge for box interiors and my square ornaments. I have made it an included angle of 78 degrees. I might try a more acute angle later but this one seems to work and was cut on the same table setting as my other scraper which is why it ended up at that odd angle. His tools are quite hard. I didn't ask what they are hardened too but on my 120 grit aluminum oxide wheel it didn't raise much of a burr. My older no name HSS scraper raises a nice burr on the 120 wheel. On the 180 grit CGN it raises a nice small burr. I polished that off and tried using a steel to raise the burr. It raises a larger burr. Maybe later today or tomorrow I'll have time to cut on a practice piece and see which way I like it.
 

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I wanted a curved edge for box interiors and my square ornaments
i like Doug's tools, very good quality. a while back I visited Chappell Hill Woodturners during one of their tool making sessions. I also picked up
a 3/8 thick tool, shaped as a scrapper that good for bottoms and a 90 degree corner of boxes. its what ever you prefer.IMG_20170201_091743001.jpg IMG_20170201_091908684.jpg IMG_20170201_091924621.jpg
 

john lucas

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Pete That angle was chosen just because I had the Robo rest set for doing my old negative rake scraper. Anything more acute than 90 degrees works if your raising a burr. One of these days I need to do an experiment where I try a much more acute angle vs the more blunt 80 degrees or so. I would think that it all boils down to how good a burr you can raise and how long the burr lasts. I can't answer that yet. I'm grinding long bevels on both sides per Stewart Batty's technique. You wear off the burr, flip the tool over and grind to get a new burr. When that one wears off you flip the tool and grind to get another. With a short bevel on top you would hone off the old burr and then grind. Probably not a lot of difference in actual wear of the tool, just one short step difference in the grind. It does take a whole lot longer to take an existing scraper and turn it into a negative rake like Stewarts. A lot of grinding. It only takes a second to grind a short bevel on top of an older tool to achieve the same thing.
 

Bill Boehme

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I use 75 and 25 degrees on my negative rake scrapers. I know that don't compute but it works.

Sure it computes just fine. All that it means is that the included angle at the nose is 100° as shown in the sketch below.

NRS.jpg
I have a NRS with something pretty close to what you have. A large included angle makes the NRS essential catch proof, but the downside is that the bur wears off faster. I'm considering trying the symmetrical 45° bevels that everybody is talking about.
 
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Interesting. Hope this is not off-topic but I recall seeing a video where a turner used a burnisher to raise a burr on a tool like a scraper would have. Anyone know anything about this?
 

john lucas

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Yes I do it all the time. Using a burnisher or what some call a Steel is an excellent way to raise a burr. I have used diamond hones to raise a burr and on John Jordan's shear scraper he sells a ceramic stone to both polish off the old burr and raise the new burr using the same technique as using a steel. I find that various techniques will raise a different burr depending on the steel. Some straight off the grinder and some work better with a burnisher.
 
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I get a burr on the Thompson Scraper by flipping it over and grinding it upside down on my fine wheel.

One fellow here posted that he takes a stone to the flat scraper and hones the grinder burr off and then uses a scraper burnisher to get a fine burr. I don't know what kind of steel that poster is using for that, but I tried it on my thompson and didn't get much result.

I got me a 3/8" by 1" length of A-36 Steel from Speedy Metals and I'm going to weld a length of High Speed Steel tool bit to it and grind a curved negative rake on it to replicate the Glen Lucas tool.

Just as an aside I've used hand scrapers to get little bits of tear out that nothing else I had would touch
 

Bill Boehme

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Interesting. Hope this is not off-topic but I recall seeing a video where a turner used a burnisher to raise a burr on a tool like a scraper would have. Anyone know anything about this?

That's the way that I have been doing it for over a dozen years. Are you saying that you don't do it that way? :D

I prefer M2 HSS for scrapers because I can get a better bur if the steel isn't too hard. The high vanadium steel is very good, but I wish the scrapers were tempered to a lower hardness than the cutting tools.
 
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Bill, never heard of burnishing to raise a burr. But, I just now learned to tie my shoes. I use the Bunny Rabbit method. :p
 

hockenbery

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Quite a few people burnish a burr with a hard steel rod.
Most put the tool on the rest and role the burr with the rod.

veritas has had a small burnisher platform on the market for many years.
It has a pivot pin and burnishing pins.
 

Bill Boehme

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I have a burnishing tool that I bought probably thirty years ago. It was made for burnishing cabinet scrapers and has an oval cross section. It must be at least as hard as a file if not even harder and highly polished. I have tried to find them in the last few years, but apparently they quit making them many years ago.

I also have the Veritas burnisher that I bought about a dozen years ago and never use. In a recent discussion here somebody said the Veritas pins were tungsten carbide. I had assumed that they were steel, but I confirmed that they were indeed tungsten carbide with a simple test.

Steel is strongly magnetic and tungsten carbide is weakly magnetic. I have a stack of super magnets on my drill press (don't ask, it just seemed like the right place if I suddenly need a magnet). It turns out that the pivot pin is steel, but the two burnishing pins really are tungsten carbide.

As a side note, stacking a bunch of super-magnets together may not be such a bright idea. Removing one from the stack without injury or breaking the magnet is a challenge.

image.jpeg

Bill, never heard of burnishing to raise a burr. But, I just now learned to tie my shoes. I use the Bunny Rabbit method. :p

I only tie my shoes on Sunday. I even brush the shavings off.
 

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I also have the Veritas unit for raising burs......and I believe it is a carbide tapered post that is the working element. I'm assuming that any steel is capable of raising a bur, as long as it's harder than the steel where you wish to raise the bur......:confused:
 

Bill Boehme

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Can I ask an ignorant question? Alright, here goes...

For side-grain turning, can a negative rake scraper ever achieve a finish equal to a sharp gouge?

I think the answer might be "can Bill or Zach or Odie or Al use a NRS skillfully enough to achieve a surface equal to what he can get with a sharp gouge". There are situations where a NRS isn't needed and other times when a scraper is better than a gouge. Most things that I make get sanded so maybe it's not worth worrying about.
 

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Can I ask an ignorant question? Alright, here goes...

For side-grain turning, can a negative rake scraper ever achieve a finish equal to a sharp gouge?

I don't think I'm qualified to directly answer that question, because my experience with NRS's is very limited.......but, I can alter the premises slightly, and insert "shear scrape" in the place of NRS, and give my observations. For me, a shear scrape (with a sharp and properly raised bur) can usually, but not always, result in a better tooled surface, than the same surface directly resulting from the use of a sharp gouge. In this case, the word "usually" probably applies to more than 90% of the time.

To me, leaving the best possible tooled surface is extremely important for those times where I wish to have a an absolute minimum of sanding......surfaces which may include a small inside radius or corner, intersecting planes that result in crisp outside corners that visually please, and detail grooves that do not vary in depth and width.

There are other times when I just don't care about geometric integrity, and find that it's faster and easier to just sand the surface beginning with a coarser grit than would otherwise have been possible......and I can't be bothered with the time consuming goal of creating a surface with the absolute minimum of sanding requirements.

OK......now back to your regularly scheduled programming.....the discussion about NRS. :D

ko
 
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I find that NRS are a "sometime" answer. Sometimes they work great on a piece and sometimes not. They are just another tool to help get a good finish on a bowl. I do mostly natural edge and burls and sometimes with interrupted cuts a gouge just doesn't work as good as a NRS. On walnut bowls I get fuzzies on the the section of the bowl where it is cutting up hill with the grain. Most of the time I use the NRS to clean up the gouge cut on the wings of NE bowls and the transition on the bottom. One thing that I have found is that a NRS will work ok at a lower RPM than a gouge. Sometimes I need to keep the speed below 500rpm for out of round pieces. NRS are not the answer to all turning problems but "sometimes" they are the berries.
 
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Questions are never ignorant Zach, it's the stupid answers that are ignorant. None on this thread though. Another interesting thread with diversions---
Prompted by seeing the excellent finish on the White Oak bowl that Odie recently showed us I had a brief off-line conversation with him about what tools he uses. A ground burr on a scraper obviously works well for him.
We all figure out what works best for our own style by asking questions and experimenting with the various answers. I just watched a quick Richard Raffan video about avoiding "catches" that is currently on the main AAW site. Towards the end, he talks about scrapers and "the current fad" of NRS. I am not calling it a fad, but it's worth watching his take on it.
 

john lucas

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Well it depends on the wood and how good you are with a cutting tool. Some woods will actually burnish with a cutting tool and leave a finish that looks like it's been sanded to 600 grit or higher. I often get that on spindles, sometimes on the outside of bowls, rarely on the inside (back to Odies question on interiors looking different for that answer). The problem is I don't always absolutely nail the shape. There may be the slightest change here or there or a tool mark where I hiccuped during the travel of the gouge. Sometimes you simply don't achieve the perfect cut over the entire surface so it looks uneven. That's where the shear scrapers come in to even things out so that sanding will improve the surface. On spindles I often have to sand not because the surface isn't cut cleanly but because certain areas don't have the same reflectivity. Sanding or shear scraping makes these areas reflect light the same.
OK that was the long answer to get around to negative rake scrapers. Shear scraping done correctly is actually cutting but usually leaves a surface slightly less dull than a pure well executed cut. Scraping which is where the wood comes across the tip at about 90 degrees with the tool held flat simply doesn't leave as good a surface as shear scraping and seldom if ever as good a surface as a good clean bevel rubbing cut. Negative rake scrapers do a fantastic job with a fresh burr and light touch. I just did a bunch of square ornaments with small bowls in all six sides. I used a Hunter carbide tool to do a bevel rubbing cut and switched back and forth between that and a negative rake scraper. I found 2 things. One, it took a lot of skill to get a bump free cut or perfect half sphere with the Hunter tool (or any cutting tool). When I did that was a surface that didn't need sanding and has sort of sheen to it. You really couldn't sand the inside of these because they had 4 holes that grabbed the sandpaper. I could take the negative rake scraper and get a surface that was extremely close and only the most careful inspection by someone who knows what to look for could see the difference. It didn't nave that same sheen, more of a highly sanded look.
Where I find the negative rake scraper comes into it's own is on thin pieces especially winged bowls, where any forward pressure on the wood causes chatter. I was experimenting with regular scrapers held at a high angle vs negative rake scrapers. My theory was that a high angle scraper duplicated the angle of the negative rake and there fore was the same. The problem is when tilting the scraper up you put just a tiny bit more force against the bowl and it was easy to create chatter or get thin the leading edge of a natural edge winged bowl. The negative rake scraper had less of a tendency to do this. I think it's because the forces push straight down onto the edge where the forces tended to want to pull the flat scraper held at an angle into the wood. Now this is pure speculation on my part because I don't really now how to test it but I could duplicate the problem when switching back and forth between the tools
Now I haven't had the luxury of playing with a lot of exotics and I've heard that negative rake scraping works extremely well on these. I have turned a lot of softer metals and sometimes the negative rake will leave a surface that is smoother and easier to sand than a cutting tool. I think it's because the cutting tool sometimes leaves a sort of spiral thread that must be sanded off and the negative rake scraper leaves a more even surface.
Don't know if that answered your question but will at least open it up to more discussion.
 

hockenbery

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Can I ask an ignorant question? Alright, here goes...

For side-grain turning, can a negative rake scraper ever achieve a finish equal to a sharp gouge?
You asked two questions. :) The forum exists for questions....

Sometimes I use a scraper on parts of a bowl or hollow form. But is usually get a sandable surface from the gouge. highly figurured woods will sometimes not cut well with a gouge and I will use a scraper to clean the surface. Every once in a while I need a scraper to fair a bump into the curve.

I seldom use a negative rake scraper and usually use shear scrape.

I usually shear scrape the outside of a bowl with the gouge.
On dense tropical woods a scraper is often leaves a better finish than a gouge I don't turn these often.

i also use a scraper on both sides of square edge bowl bowls.
 
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john lucas

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I meant to say that a negative rake scraper really shines when final shaping things with voids. On those square Christmas ornaments you had 4 voids. It took every bit of skill I had to cut a clean shape using cutting tools because the bevel wants to drop into the voids and those areas cut like soft butter compared to the solid wood. With a negative rake scraper it was easy to skate across those voids and get a good shape. I already mentioned the oval natural edge bowls. you have the same problem of tool control where your hitting the wood twice per revolution and a lot of air inbetween. The negative rake scraper makes this air wood transition easier, at least in my hands.
 

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Prompted by seeing the excellent finish on the White Oak bowl that Odie recently showed us I had a brief off-line conversation with him about what tools he uses. A ground burr on a scraper obviously works well for him.

Hi Tom......To clarify here, I don't use a ground scraper bur for anything but initial cuts. It's for final cuts, prior to sanding is where I would prefer a raised bur.

ko
 
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As a side note, stacking a bunch of super-magnets together may not be such a bright idea. Removing one from the stack without injury or breaking the magnet is a challenge.


Bill-I recently purchased some glass eye glass holders from Penn st. You place the magnet on the inside of shirt, and the decorative" button" thingy that you loop the bows of the eyeglasses thru (saves having a "rope" tying them to your neck!!) on the front side of the shirt. When gluing the buttons on, I removed the magnets and placed in a plastic bag so that any magnetization didn't glom the individual items together. I took them out of bag to reassemble , and 3 of the 10 magnets that were stuck together strongly had broken. PSI is replacing
 

john lucas

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I just got some strong rare earth magnets in the mail and was going to show my graphic artist how strong they were. I started to attach them to the metal door frame. It pinched my finger between the magnet and door frame and pinched a huge blood blister on one finger. I have a lot more respect now.
 

hockenbery

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I just got some strong rare earth magnets in the mail and was going to show my graphic artist how strong they were. I started to attach them to the metal door frame. It pinched my finger between the magnet and door frame and pinched a huge blood blister on one finger. I have a lot more respect now.
When I started reading I was envisioning the door frame being dislodged...:)
Finger will regenerate
 
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Okay, a few things here...

First, scrape vs. shear. A scrape is a cut where the cutting edge is at 90 degrees to the spin of the wood, and best example would be a scraper flat on the tool rest. A shear cut, bevel rubbing or not means the cutting edge is at an angle to the spin rather than at 90 degrees. Generally the higher the shear angle is, the cleaner the cut because the cutting edge is able to lift the fiber more gently as it cuts, so think of one of Doug Thompson's fluteless gouges at 70 degrees compared to most gouges which can only roll to 45 or so, unless you have a swept back grind and drop the handle really low, which you can do only on the outside of a bow. On end grain, like boxes, you can get a good finish cut with a scraping cut because of grain structure all going the same direction as long as you are cutting down hill. With side grain, a scraping cut will not give as clean of a cut as a shear cut. Twice each rotation, you are cutting against the grain, and a scraping cut with either gouge or scraper will pull more than a shear cut. Makes no difference if you have a big burr, or a tiny one, regular scraper, or NRS, all scraping cuts pull at the fiber. So, on bowls, scrapers are fine for sweeping across the bottom of the bowl, and a tiny way through the transition and a tiny bit up the side. Go too far, you not only start pulling out fiber, you can start the bowl to 'screeching and howling and then it blows up'. A scrape works across the bottom fairly well because you are cutting across the grain rather than going down through it. Softer woods will tear more than harder woods.

Now, burrs... I prefer scrapers for all of my shear scraping, and I always use a burr. More mass to them when compared to gouges, and I just like scrapers... Different ways to raise the burr: straight from the grinder, honed, and burnished. There are huge variations in each. I have noticed little difference in the metals, but huge differences in the wheels, especially when comparing CBN to standard wheels. These can also vary a lot depending on whether you just kiss the surface or push it into the wheel. As near as I can tell, with the CBN wheels, you get an almost honed burr (very strong and suitable for heavy roughing), rather than the wire edge more common from standard wheels (very weak). The finer the wheel, the finer the burr, with 180 and 80 grit wheels giving an edge suitable for heavy roughing and pretty good for most shear scraping. With the 600 and 1000 grit wheels, you get a very fine burr suitable for fine shear scraping and not good for heavy roughing.

When honing a burr on scrapers, usually this is using a diamond card, and pushing either up or parallel to the bevel. Most of the time this is a very fine card, and these burrs are more suited to fine finish cuts, and not good for heavy roughing cuts. For shear scraping, you want fine. For the McNaughton coring blade tips, you want a coarser, like 220 card for honing since it lasts longer.

Burnishing is another method to raise a burr. Grind, hone off the burr, or as much as you can, then use a burnishing tool (hard steel rod like for card scrapers, or carbide tipped, either hand held or the one from Lee Valley tools which bolts to the bench). They can be used on standard and negative rake scrapers. The advantage to this type of burr is that it is supposed to be stronger and sharper than the burr from grinders, and I have to experiment more with them. Mostly in the past, I couldn't tell any difference. I saw a demo once where the turner stated flat out that it was impossible to hand burnish HSS. Being skeptical, I went home and tried it, and with minimal effort, I could do it. He was really cranking on the Lee Valley burnishing bench mounted tool, and probably raised a tsunami type burr. I was using the standard hand held ones intended for card scrapers. Anyway, a very light burnishing, with the burnishing tool at maybe 10 degrees to the bevel (like 70 degree bevel, and burnishing tool at 80 degrees) you can produce a very fine burr. This burr seems to be pretty good for roughing, but not superior to the one from the 180 grit CBN wheel. More than anything to me, especially since I got the 600 and 1000 grit wheels, the extra work of burnishing just doesn't show any real improvement of the cutting edge to me. I may have to go back to HSS and experiment some more.

Burnishing the NRS... I saw Eric Lofstrom recently, and he burnishes a burr on his NRS. Hmm, really having to think about that. The NRS is still a new tool to me, but I made a burnishing tool to experiment with about an 1/8 inch carbide tipped rod in the end and uses that. He seems to get good results with it on standard use, and it does seem to hold up for roughing. I have tried it on both 30/30 bevel NRS, and 70/25 NRS, and get good results, but can't really tell if I get better results than with standard off the fine wheel burrs, but they do seem to last longer. Tom Wirsing, who is a master with the NRS does not agree that the burnished burr works better. He and Stuart Batty will be doing a demo at the Symposium with a 300X camera on burrs and edges, and that is on my agenda as well, especially to compare the different grit wheels, and new vs. old CBN wheels...

So, more testing needed...

robo hippy
 
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