Wondering about scraping action

[rsser]

A local respected turner, teacher and retailer once advised that a scraper fresh off a grind wheel was only good for medium density woods. Hard or softwoods should have the burr taken off.

Assuming he's right, why is this do you think?

 

[MichaelMouse]

Sounds like another one that should have "in his hands" appended. Softwoods, and hardwoods with great density disparity between early and late wood are tough to scrape with anything. As soon as you apply pressure, you start crumbling not only your shaving but the surface it comes from.

If you're a flat guy you recognize the steeper angle of a "York Pitch" for harder, complicated figure. Principle's the same in scraping, break the shaving into small segments so you don't dig out. Longer is not necessarily better with scrapers. I'm not a big fan of scraping, because I do cutting angles pretty well, and they give even the uncoordinated a bevel to guide on. Keeps me from overfeeding and digging in. If the access angle permits, you just use the tools with blunter bevels to break the chip. Note that you can scrape with a toothed blade fresh off the grinder, or a smooth(er) blade with the burr faced off with or without a secondary edge from a burnisher, just as you can with the toothed or smooth side of a sawblade. Toothed is for rougher leveling work, of course. The grinding burr is a few orders of magnitude finer. It's just high-angle cutting, with or without a burr, though you are using the tool in two dimensions, normally, so you either dip or skew the nose, rarely both. I find that forged carbon is preferable to harder HSS alloy in burr consistency, but others might not agree.

You're doing the turning. Experiment, and include other steels in the experiment as well as grind/none/turned edges. Betting you'll remove faster with the coarse, less with the smooth, and find that going no burr gives you a more consistent cutting edge than anything you can turn, unless you use one of those jigs.

 

[robo hippy]

I use scrapers a lot on my bowls. I always have a burr on, and need to experiment with the burr off some. I have found that harder woods will cut cleaner with a scraper than the softer woods. This is both with a scraping cut (scraper held flat) and a shear cut (scraper held at 45 degrees). This also seems to be true with gouges, although some times a shear scraper on very hard/dense woods seems to give a cleaner cut. I tried once with the burr off, and didn't notice any real difference. I haven't noticed any real difference with a burnished burr, or one from the grinder. When burnishing, I use a triangle burnisher, not a bench mounted one. The triangle burnisher will turn a burr up, where with a round burnisher, I can't raise much of a burr. I have heard some say that the burr off the grinding wheel isn't as good or as consistant as one that is burnished. I haven't noticed that. When raising a burr on the grinder, I do push the scraper into the grinding wheel with some pressure. I think this is better than just kissing the wheel.
robo hippy

 

[Malcolm Smith]

Sounds like another one that should have "in his hands" appended. Softwoods, and hardwoods with great density disparity between early and late wood are tough to scrape with anything. As soon as you apply pressure, you start crumbling not only your shaving but the surface it comes from.

If you're a flat guy you recognize the steeper angle of a "York Pitch" for harder, complicated figure. Principle's the same in scraping, break the shaving into small segments so you don't dig out. Longer is not necessarily better with scrapers. I'm not a big fan of scraping, because I do cutting angles pretty well, and they give even the uncoordinated a bevel to guide on. Keeps me from overfeeding and digging in. If the access angle permits, you just use the tools with blunter bevels to break the chip. Note that you can scrape with a toothed blade fresh off the grinder, or a smooth(er) blade with the burr faced off with or without a secondary edge from a burnisher, just as you can with the toothed or smooth side of a sawblade. Toothed is for rougher leveling work, of course. The grinding burr is a few orders of magnitude finer. It's just high-angle cutting, with or without a burr, though you are using the tool in two dimensions, normally, so you either dip or skew the nose, rarely both. I find that forged carbon is preferable to harder HSS alloy in burr consistency, but others might not agree.

You're doing the turning. Experiment, and include other steels in the experiment as well as grind/none/turned edges. Betting you'll remove faster with the coarse, less with the smooth, and find that going no burr gives you a more consistent cutting edge than anything you can turn, unless you use one of those jigs.

Most pertinent is the last paragraph. Different woods react differently to different cutting methods and different types of edges. There really isn't any way to avoid a lot of experimentation with different tools and sharpening methods and the type of wood one is turning ultimately dictates what works best. One point to add - A burr left by grinding applied to hard wood can have a very short life. That may explain the original comment. I like to work with a very clean, very sharp polished edge produced on a Tormek with no burr. I turn very hard tropical woods working mostly with Glaser tools which are made of very hard steel.

If you are using a high speed grinder, dipping the HS tools in water to cool them can put a lot of small cracks in the edge which will then tend to break down faster.

Malcolm Smith.

 

[rsser]

Thanks for the insights folks. Yes, it is testing that I'm interested in, in part inspired by Brent Beech's work with flatware tools and in part by a test of spindle turning tool sharpening by Farrance some years ago.

I'll be playing around with different 'sharpening' methods and checking the results with a photomicroscope.

The task is of course to identify and where possible control the variables apart from the one of interest, hence my question about the interaction of burr with wood density.

So here, burr, no burr or burnished edge.

Grain orientation should provide plenty to puzzle over. Eg. to take another principle from flatware MM, would a low bevel angle perform better on the end-grain of a bowl than a higher one.

Frankly, I know little about the science of cutting. Will have to do some reading.

[Added: yes robo hippy, how hard you press the tool to a (dry grind) wheel would vary the burr I'd expect, not to mention the grit of the wheel and maybe the bond strength. The coarse the wheel presumably the coarser the burr.

Malcolm, yes; my take out from the Farrance study comparing dry grinding with wet grinding/honing in spindle turning was that the latter improved the tool performance as no coarse burr was left but that possibly after several minutes the burr on the dry ground tool was worn off at which point tool performance improved.]

 

[john lucas]

One other thing to throw in with the mix. I raise the burr several ways, straight off the grinder or I use diamond hones. I find the 3 different diamond hones produce different burrs than the grinder. Off the grinder it's a course burr that cuts aggressively. The medium hone produces a smaller burr and finer cutting action. The Ulta fine produces a very small short lived burr but does a fine job of removing just the tiniest of wood.

 

[rsser]

Thanks for that John.

Yes, lots of variables.

I think I'll start modestly with one scraper and one kind of wood, and vary the 'sharpening' procedure - looking closely at the edge and somehow measuring edge efficiency (and finish, though that poses its own challenges!).

In the longer term it would be very interesting to look at gouge performance in bowl hollowing - given that there you can identify three (or four?) different grain orientations, let alone all the other variables of tool presentation, bevel angle, sharpening procedure etc.

But getting back to scraping, should we really call it that if the cutting is done eg. by the burr or by a turned up edge? MM, how else do you think variation in burr/bevel angles comes into play?

 

[MichaelMouse]

Scraping is really cutting, sure. The term to remember is what they call "sharpness angle." http://home.earthlink.net/~wooddude78/Woodlib5_2.html It seems to apply here as elsewhere that getting a high angle handles the mix of end, face, and in-between grain presentations given with each turn of the work. Consider the orientation of the grain in curly wood. It is just that mix.

You'll want to avoid the negative rake situation as well. It's an easy one to get into by keeping the handle low on a burred edge, and it will make compression rings that will require 60 grit to remove or perhaps remove the tool from your hand if you catch.

The Hippy mentions why I like my forged gouges over scrapers. I can get both a skew angle and bevel support with them so easily. Trying to stand a "bowl" gouge up to cut on the wing to get the same presentation is possible on a convex surface, but impossible inside.

 

[john lucas]

Trying to stand a "bowl" gouge up to cut on the wing to get the same presentation is possible on a convex surface, but impossible inside.[/QUOTE]

MM. I'm sorry but I don't understand what your trying to say here. Could you explain.

 

[n7bsn]

MM. I'm sorry but I don't understand what your trying to say here. Could you explain.

John, I think George is talking about a "Pull Cut", which is when you have the flute towards the bowl and and the handle is towards 4 or 5 o'clock. You are actually cutting with the wing on almost a scraping action.

John, George's wording sometimes reads very oddly, he has stated it's because English is not his first language, sometimes I wonder if he doesn't have a random word generator in his computer, injecting extra words.

 

[MichaelMouse]

Here you go, John. As you can see, the edges are similar as the work would see them, but the "bowl" gouge has problems with support, clearance, and cannot be skewed so that the bottom of the curve leads the cut as you do with your "shear scraper." When the bottom leads the cut you can lean the tool so that the most vertical part of the gouge cuts end grain at close to 90 degrees.

Ralph is in error. Though fluent in two, I was raised with one language, and it is English. As a prodigious reader, I have an immense vocabulary. My active is larger than most natives' passive vocabulary.

 

[rsser]

MM can speak for himself, but on other posts he's provided links to his vids. Not pull cuts but IIRC using a forged spindle gouge on a bowl with the cutting edge approaching vertical.

 

[MichaelMouse]

Yep, forged, "continental" or "traditional" gouge shapes, and I prefer to stay out of trouble by pushing the gouge. That way any bark chunk or dismount might hit me in the knuckles, but nothing more. http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view&current=HollowTwo001.flv Push and pull. http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view&current=CherryPeelIn.flv Pushing with forged to show slo-mo technique. . http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view&current=Gouge101.flv

I don't call 'em spindle gouges because I don't do Newspeak.

 

[Malcolm Smith]

Certainly an interesting series of posts. A few points. Some of the time we are cutting and some of the time we are crushing. The latter produced surface can be deceptive and sometimes should be examined carefully. I have very good loups and a low powered microscope in my shop with which I examine tool edges and occasionally the wood surface that is being produced. I have found this to be very useful and strongly recommend it.

I turn mostly very hard dense woods and often find that when one tool doesn't produce the result I want, often another tool will. It's some times a mystery to me why there is a difference, but a cut rather than a crushed surface is certainly worth the trouble. With some woods the crushed, really, damaged surface can go quite deep below the surface. This leads to a great deal of sanding which itself is a process that damages the surface though not deeply into the surface being sanded. This is why a lot of fancy furniture in exotic woods is scraped rather than sanded and why the finishing process can be quite complicated and often was a carefull kept secret.

Malcolm Smith.

 

[rsser]

Glad to hear you confirm the danger of crushing Malcolm. I had always wondered whether my sanding was hiding this.

I take it from the puzzlement you express that the 'scope hasn't revealed any obvious links between tool edge treatment and visible wood finish?

This is something I plan to look at.

 

[Malcolm Smith]

Glad to hear you confirm the danger of crushing Malcolm. I had always wondered whether my sanding was hiding this.

I take it from the puzzlement you express that the 'scope hasn't revealed any obvious links between tool edge treatment and visible wood finish?

This is something I plan to look at.

I've written elsewhere what works well for me and to repeat, the sharpening I do is on a Tormek producing an extremely sharp polished edge with no burr on very hard Glaser tools. This probably seems like a lot of trouble but it is an edge that lasts a great deal longer than other type edges such as those produced on a regular grinder.

There are visible links between tool edge treatment and wood finish. What's sometimes hard to define is why a particular scraper produces a different better result than another scraper that has been sharpened in the same way. With a considerable difference between different hardwoods, there are a great many variables in all this.

Malcolm Smith.