Thompson Scraper grind

[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.