Thompson Scraper grind

[Keith Harrell]

I purchased a Thompson 1" /14 scraper yesterday and have been looking at the best grind to put on it. I want to be able to use it on finishing the inside of bowls.
I think a full round tip with a 20 degree grind but wondered about the double bevel or a grind on the top and bottom to limit catches I see on some scrapers. It looks like a very small grind on the top of the scraper.
Thanks

[Greg Just]

for my finishing cuts with a scraper, I use a burnisher to reduce the size of the burr on the scraper, then hold the scraper at a 45 degree angle and take really light cuts

[Harvey Ghesser]

Why not grind a half circle? Still good for inside of bowl and outside too. I have the same gouge and unless I don't know something (still new at this) I'm very happy with it. Keeps an edge forever!:)

[Dave Rudy]

That small grind on the top turns the scraper into a negative rake scraper.

Negative rake scrapers have several advantages over traditional grinds. They are not self-feeding, so the chance of a catch is virtually eliminated, so long as the scraper is level or pointing down. I love these scrapers, although they are not the best for all kinds of woods and all grains.

There are basically four kinds of sharpening scrapers -- conventional scraper with burr and without burr (burr honed off after grinding), and negative rake scraper with burr and without burr.

When to use what depends on hardness of stock and which grain you are addressing.

As far as shape goes, may I suggest that Raffan's latest video on turning wood (ca 2007)demonstrates multiple kinds of scrapers in multiple applications.

As for me, I decided to grind the same Thompson tool into a square-ended scraper with slightly radiused corners. Works great outside and inside.

[Reed Gray]

I prefer a swept back design on mine, so more of a quarter round, then swept back on the left side. The nose can be used on the inside of the bowl with the tool on its edge at about a 45 degree angle. Note, do not use a scraping cut, with the tool flat on the tool rest. Also, the shear cut on the inside for me is a pull cut where you pull from the center towards the top of the bowl. Very light cut, for removing small tool marks, not for stock removal. Especially, near the rim which will make the edges of the bowl start to vibrate, then explode. With the swept back part, you can do shear cuts on the outside of the bowl as well.

As to how to sharpen, I prefer a blunt angle, in the 70 degrees range (never measured it). I prefer the burr straight from the grinder. The negative rake scraper has a burr that is so delicate that if you look at it hard, it goes blunt. A burnished one is more trouble to make, but does better with problem woods that are more trouble to get a good clean cut on. A honed burr is like the burr on a negative rake scraper. A honed on both top and bevel is fine, but I prefer the burrs.

robo hippy

[Keith Harrell]

I received the scraper today it is flat across as advertised but does have a 20 degree slope on it already. I tried using it on the outside of a bowl as it came out of the box and boy is it a nice scraper for the outside as is. I'm thinking about just taking one corner off so I can use it on the inside and the outside. I can really see the advance of a good scraper now.

[Richard Madison]

Keith, when you say 20 degree slope, are you referring to the relief angle? For wood turning tools the usual practice is to refer to the included angle at the cutting edge. Thus your scraper is probably ground at 70 degrees.

[Keith Harrell]

You are correct it is a 70 degree angle.

[Bob Hampton]

talking about scrappers has got me all a twitter:)
I have some older scrappers that i hardly use anymore and am thinking serious about trying my hand at this negative rake thing
Can someone clue me in on how to grind one correctly?
thanks
Bob

[Keith Harrell]

I have been using the scraper and will have to say it turned a whole new avenue for me and turning. I watched one of Raffan videos and it did help. It was the latest one which was 2008.
Questions
If I use the scraper at a 45 degree as a sheer scraper does can it be done with a quarter round tip? I have mine with the left side corner ground slightly and the rest square. I'm trying not to grind off more until I know what is best. I have a old spindle scraper I can convert if needed for a second one with a different grind.

Sheer scraping at a 45 degree with burr
1. Should it be square for the outside and rounded for the inside?
Removing wood using it flat on the rest
1. Is square best or round it for the outside of the bowl? I assume rounded is best for the inside.

I have looked at a number of you tube videos and still have some confusion when to use one for cleanup and when to use one for removing wood.
Thanks again for the help. I feel I'm getting closer.

[Reed Gray]

You can not use a square ended scraper for a shear cut on the inside of a bowl. Square peg in a round hole thing. The only thing I use a square scraper for is for making the recess for a lid on boxes. With the scraper flat on the tool rest, you get a scraping cut. You can get clean cuts this way if you are cutting into end grain, like on boxes or hollow forms, but not on flat grain like on bowls. This cut will 'pull' at the fibers more than a shear cut where the tool is on its edge, making a shear cut. You can use a round nose, or an inside bowl grind scraper (quarter round with sweep back on the left side if the tool is flat on the tool rest) on both the inside and outside of the bowl. On the inside of the bowl, you can only use the nose, but on the outside, you can start with the nose, and as you pull towards the rim, drop the handle, and move up to the wing, getting freshly sharpened burr on the wood through the whole cut. Do not try to use a flat scraping cut on the inside of the bowl near the rim. The bowl walls will start to vibrate, you will hear strange screeching and howling noises, and the bowl walls will start to vibrate to the point where you can get a big catch and your bowl will explode. Very gently, with the scraper at a 45 degree angle, make light cuts near the rim. You can use your left hand as a steady rest (make sure to round over the rims of the bowl because they are sharp and can cut) if you have turned the bowl all the way down, or you can turn down the inside in stages, as in turn down an inch or two, finish shear cut that part, then go down another inch or so, and finish cut that part and blend into where you left off the time before. These shear cuts are for touch up work only as in cleaning up the little ripples that you leave behind from your gouges. You have to take several passes over an area to get the marks out. The shear cuts are not for stock removal and shaping.

robo hippy

[Keith Harrell]

I think I'm getting it now. Thanks
One final question
What speed do you suggest for sheer scraping and just cleanup scraper work.

[Reed Gray]

Speed is whatever one you are comfortable with. If I have a 10 inch bowl on, I am turning at 1800 rpm. That is comfortable for me, but I had to work my way up to that. Of course, I never turn a bowl that fast if it has ANY defects in it like bark inclusions, knots, or cracks.

robo hippy

[Dave Rudy]

talking about scrappers has got me all a twitter:)
trying my hand at this negative rake thing
Can someone clue me in on how to grind one correctly?
thanks
Bob

Bob,

Negative rake simply means ground on two opposing surfaces. Think skew chisel, one of the ideal negative rake scrapers. If you take a conventional scraper and grind even a slight bit on the top, it becomes negative rake.

Because of the geometry of the tool, negative rake scrapers do not self-feed as conventional scrapers do.

Negative rakes are great on super-hard materials (like plastics, ivory, and dense exotics). They do not work nearly as well as conventional scrapers on bowl bottoms (long grain).

As with conventional scrapers, they can be used with and without a burr.

If you want a kick, grind one edge (the top) of your skew a little harder than the other until you can feel a definite burr. You can put the tool directly into the side of a bowl -- JUST MAKE SURE it is level or pointed downhill -- never point the skew above horizontal.

You will find an incredibly smooth finish on hard or medium-hard woods and a surprisingly gentle tool.

Negative rake scrapers have been around for centuries -- originally used by ivory turners. The name negative rake was coined by Stu Batty fairly recently. If you do a search on the AAW online index, I think you will find an article within the last couple of years.

HTH

Dave

[Richard Madison]

Am wondering how a "negative rake" scraper is any different in practice from simply presenting a conventional scraper in a slightly nose down attitude. I know how a negative rake scraper is ground.

[Dave Rudy]

I'm afraid I don't understand the geometry well enough to explain it. It is definitely a gentler, non-grabby tool. I think you would have to point a conventional scraper down so far it would be dangerous to acheive the non-self-feeding characteristic of the negative rake.

Best thing I can say is try one, especially in long grain and on dense materials. You will feel the difference.

Stu is scheduled to demo at our club tonight so I will ask him to explain it to me again and if I can understand it, I will post here tomorrow.

By the way, Cindy Drozda's new video Finial Star discusses negative rake scrapers and shows them in use if you're interested. She is an advocate of the tool.

[Richard Madison]

Thanks Dave. Looking forward to your explanation. Back when I used to use scrapers I had to use a nose down presentation to avoid the grabbiness. Nowadays do scraping and shear scraping with a bowl gouge.

[Reed Gray]

I believe the negative rake does the same thing as raising the handle when using a regular scraper. I do need to play around with one more, but most of what I turn responds better to a burr. The negative rake scraper does work better on harder woods (Mountain Mahogany, Bubinga, Cocobolo, etc), and most of what I turn is on the softer side (big leaf Maple, Walnut, Madrone, apple, cherry). It isn't as grabby or made for stock removal. I always hold the scraper pretty much level, and above center on the inside of a bowl, and below center on the outside of the bowl. For me, if you are going to make a scraping cut, that is what the scraper is made for, and does it better than a gouge.

robo hippy

[Richard Madison]

Thanks Reed. Have a feeling I just never really learned to use scrapers properly. Will dust one off and try it again. Maybe will learn something.

[Ryan Baker]

A regular scraper presented at [even a slightly] downward angle provides the same geometry as a negative rake scraper -- granted with a larger included angle to the edge. I frankly don't understand this "self-feeding" business with scrapers. I've never had a self-feeding experience with a scraper unless it was incorrectly presented at an upward angle. If the edge is presented upward, it can dig in and cause a catch. If the scraper edge is presented level or downward, and appropriately at or above center line so that it is clear of the curve, it can only be knocked down and away from the surface -- a dig-in isn't possible. (Using too much edge in contact with the wood at once is often the reason a scraper is "grabby".)

It seems to me that the typical negative rake scraper does two things: it effectively presents the edge at an even greater downward angle, and it provides a much sharper included angle to the cutting edge. This should result in a finer, less aggressive scrape. But I notice the same effect presenting a conventional scraper in a similar fashion.

Nothing against negative rake scrapers. I guess i'm just not yet convinced. My regular scrapers work just fine and require a lot less sharpening.

[Dave Rudy]

I had a chance to discuss with Stu Batty last night and he explained concisely to me why a conventional scraper will never perform as a negative rake no matter the angle. The geometry that makes negative rake scrapers work is the location of the burr relative to the opposite face of the tool. A burr on a conventional scraper is in the front of the tool. This produces self-feeding. (The physical self-feeding property of scrapers is a matter of physics. Even though some turners may have more or less problems, it is a physical fact that scrapers (as tools ground at certain angles) will tend to draw into the wood).

The burr on the negative rake is on top of the tool. The geometry will tend to pull the tool down, but not into the wood.

Thus it is not the angle of the tool to the wood, but to the opposite face which makes the difference.

Stu said that even a 5 degree grind on top of any scraper will turn it into a negative rake scraper.

If you're not convinced, try it and see if you feel the difference. I sure do.

By the way, if you're looking for a witty and technically unbelievable turner for a club demo, consider Stu. You won't be disappointed. He is concentrating on opening a new turning school in Boulder Colorado and on production of his new tool line, so he's not easy to book. But he is among the best of all the demonstrators out there.

[Richard Madison]

Dave, I am really missing something here. Could you draw us a simple picture showing the relative positions of the burr on the two different tools? Maybe has to do with how they are sharpened?

[Reed Gray]

Dave,
I was lucky enough to take a 3 day workshop with Stu and Allen Batty this summer. He really is a great instructor, very knowledgeable, but all of his techniques don't work for me.

I don't understand about the burr being in front of the tool. On top of, yes, but the burr does not extend out in front of the bevel, and is actually behind in a bit, especially if you burnish a bevel on the tool. A burnished burr will not cut with the scraper held level at center height. A burr from the grinder is pretty much the same thing, but it isn't pushed as far back as the burnished burr. The burr from a hone on the face is similar, but very delicate. I do use scrapers a lot. More so than Stuart, and maybe some time we should 'discuss' it. Interesting to hear different perspectives.

robo hippy

[Richard Madison]

Reed, The "burr in front" is the part I am having trouble visualizing also.

[Ryan Baker]

Reed, The "burr in front" is the part I am having trouble visualizing also.

Me too. At least the way I am grinding, I can't see the distinction. Maybe Stu does it differently. I'm not saying he's wrong -- I would really like to understand what I seem to be missing. Stu IS a good presenter.

[Dave Rudy]

I think I understand it -- I'm having a very difficult time articulating it.

Let me try it this way -- in a conventional scraper, the burr is always on a surface parallel to the surface on which the tool rests. That is, the tool has two parallel sides. One is where the burr is developed; the other is where the tool rides on the rest.

With a negative rake scraper, the burr is always on a surface oblique to the surface on which the tool rests -- never parallel.

So the burr is always on a different plane on one tool compared to the other. The "angle of attack" or presentation of the tool to the work, does not affect this geometry at all. It is why a conventional scraper cannot be a negative rake no matter the angle to the work (or the angle of the grind).

That's about the best I can do. Sorry. I wish Stu would join this conversation -- I'll send him an email and ask him, but he does not frequent any of the forums.

Drawing is not my strong suit. But I can turn, at least moderately well. And my experience confirms that a conventional scraper (especially with burr) is a self-feeding tool. You can minimize and overcome to some extent the self-feeding with good technique and tool control, but the tool is a self-feeder. That is why even the best turners typically get catches with conventional scrapers. (Look at almost any Raffan video).

And I know that negative rake scrapers do not self-feed and produce an exquisite, gouge-like finish on dense hardwoods, especially on end and mixed grain.

I appreciate (and share) the desire to understand the geometric reason(s) behind the tool performance. I am just not able to do a very good job of explaining it.

So I say -- try the grind and you will undoubtedly appreciate the tool.

[Ryan Baker]

I seem to be getting more confused instead of less. (Maybe i'm just being slow today.) I am getting the feeling that the Stu negative rake grind in question here is different that what I have in mind. From what I have seen of Stu's grind, it is what I expected -- but I haven't seen one up-close in person.

I seem to recall that Stu had an article about his negative rake scraper grind on his web site somewhere. I think I need to go dig that out and take another look.

[Dave Rudy]

I seem to be getting more confused instead of less. (Maybe i'm just being slow today.) I am getting the feeling that the Stu negative rake grind in question here is different that what I have in mind. From what I have seen of Stu's grind, it is what I expected -- but I haven't seen one up-close in person.

I seem to recall that Stu had an article about his negative rake scraper grind on his web site somewhere. I think I need to go dig that out and take another look.

A skew is a negative rake scraper without modification of any kind. If you raise a burr on one edge of the skew and make that the top, it is a negative rake scraper with a burr.

Make any scraper with parallel sides a negative rake by grinding a shallow angle into the top of the tool.

Stu did an article in American Woodturner (AAW) vol 21:1, pp. 24-27.

[Ryan Baker]

Yes. That was always my understanding of the negative rake scraper. And I did look up that article from Stu which also confirms the grind. But I maintain my original comment that the geometry of the cutting end is exactly the same (or at least could be if ground to the same included angle) between the scrapers. The only difference is the presentation angle of the tool -- where the neg rake may have a slight advantage in that it can be held horizontal, putting the forces directly into the rest, making it slightly easier to control.