Sharpen back of bowl scraper?

[dickhob]

I've been turning some bone dry cherry bowls that I turned green and set aside about 2 years ago. I'm getting an ungoddly amount of tearout and have been reading here for solutions. While my tool technique is not expert, I think I can pretty well hold my own as an intermediate/advanced turner.

The best result so far has come from (I think) Al Hockenberry when he say to spritz the tearout to swell the fibers and then cut with a sharp tool. Works pretty good, but it is still there to an unacceptable extent. I'm finding that scraping the wet fibers leaves a better surface. (any thoughts on the tearout issues would be greatly appreciated, but to get to the point of this post)

I'm use 3/8 thick inside bowl scrapers. While doing the required frequent sharpening to restore the burr, it occurred to me that the backside of the scraper would be a good surface for a flat scraper edge, allowing the tool to do double duty on both the inside and outside of the bowl.

I don't do any wild curvey surfaces, but stick to more "classic" shapes. It seems using the back of a bowl scraper for the outside of a bowl should work well.

You know there's a question in this--- ready? Is there any real problems in sharpening the back of a round nose scraper and using it in this fashion?

Dick

 

[Odie]

I've been turning some bone dry cherry bowls that I turned green and set aside about 2 years ago. I'm getting an ungoddly amount of tearout and have been reading here for solutions. While my tool technique is not expert, I think I can pretty well hold my own as an intermediate/advanced turner.

The best result so far has come from (I think) Al Hockenberry when he say to spritz the tearout to swell the fibers and then cut with a sharp tool. Works pretty good, but it is still there to an unacceptable extent. I'm finding that scraping the wet fibers leaves a better surface. (any thoughts on the tearout issues would be greatly appreciated, but to get to the point of this post)

I'm use 3/8 thick inside bowl scrapers. While doing the required frequent sharpening to restore the burr, it occurred to me that the backside of the scraper would be a good surface for a flat scraper edge, allowing the tool to do double duty on both the inside and outside of the bowl.

I don't do any wild curvey surfaces, but stick to more "classic" shapes. It seems using the back of a bowl scraper for the outside of a bowl should work well.

You know there's a question in this--- ready? Is there any real problems in sharpening the back of a round nose scraper and using it in this fashion?

Dick

Howdy Dick......

Cherry is among the more difficult of woods to get a clean cut.

If I'm understanding your terminology and questions correctly, I'd say you could sharpen both sides of a scraper, but there will be some problems with it for anything but using it flat on the tool rest. If you intend to shear scrap, the opposite side will not slide as well on the tool rest if it has also been sharpened.

edit: On second thought, it could be done if you spend some effort rounding off and polishing the edge on the bottom of the scraper. This will allow it to slide on the tool rest much more easily. Sorry.......my mistake for not thinking this completely through before responding........

ooc

 

[Odie]

Dick.......

If you will allow a suggestion which includes a totally new approach, I might be of help.

I used to use scrapers on the interior of bowls, but have found that I can get cleaner cuts using gouges exclusively. Depending on the interior curvature of your bowl, and if we're discussing the very bottom, or side of the interior, the gouge can be shaped accordingly. (There is not a "one size fits all" solution.)

If it's the very bottom of the bowl, the gouge will/may require a profile shape that doesn't seem to be in common usage here in America. To my recollection, I first became aware of these shapes from studying the techniques of English and German turners of early last century. Some of these turners were using gouges with very blunted edges sharpened close to 60,70,80 degrees. The blunted deep flute gouge works very well here, and is what I'd try as a last resort......

.......My first attempts would be with a standard ground gouge. (done with V-arm on Wolverine, not Ellsworth grind) This grind can be used for the entire bottom of the bowl, from side to very bottom. To use this grind with the best possible cutting action, one must learn a technique of twisting the gouge shaft as it progresses from the interior's side to the very bottom.

I could probably demonstrate these things to someone who would be interested in knowing just how these techniques and grinds work, fairly easily, and dependent on the comprehension level of that individual.......but the limitations of putting it all into script, and making it all understandable is an obstacle that is too much to overcome......but, I've given you the basics from which you could arrive at some understanding of these things.......with a little practical "hands on" experimentation.....

Anyway, good luck, and Al's spritz suggestion is a definite improvement over scraping very difficult to cut bone dry wood.......especially Cherry.

ooc

 

[john lucas]

I find cherry a pretty good wood to turn without tearout. Much easier than say Box Elder. The trick if you call it that is to use a sharp tool, possibly raise the speed of the lathe and take a light cut with as little forward pressure as possible.
I use the sharpest grind I can that will let me run down the inside of the bowl and still rub the bevel. If it's a steep sided bowl this my be something like 60, 70 or 80 degrees as Odie mentioned.
Try to rotate the tool so that it is cutting in a slicing motion as the wood crosses the tip of the cutter.
I find the Hunter tools excellent for this problem, at least when used in a bevel rubbing cut. The bevel angle is 82 degrees so it will go into a very steep sided bowl and still rub the bevel. However the cutter top surface is recessed so the actual cutting angle of the tip is about 30 degrees or so. It cuts extremely clean.
Here is my video showing how I use the tool.
https://www.youtube.com/watch?v=nfp2kvhH6Mo

 

[Bill Boehme]

Tell us how you are using the scraper -- are you using it flat or shear scraping? Personally, I don't think that a scraper is the tool to take care of end grain tear out -- it is liable to make it worse. My main use of the scraper for the inside of a bowl is to smooth out any small waviness in the curve followed by shear scraping to get a smoother surface.

I have not turned very much cherry, but I haven't found it to be difficult to work. Perhaps your gouges are not sharp enough or you are applying heavy pressure or the wood is punky.

I didn't follow your question about sharpening the back of the scraper.

 

[dickhob]

Clarifications

Wow, thanks for the quick responses. Some clarifications.

The cherry harvested from a healthy freshly cut tree that the owner had cut to “make space†in his back yard. It was all destined for the firewood pile- he gave me a shot of if first. I took all I could handle- about 24 logs, all 20plus inches in diameter. I chain sawed in half and used a McNaughton system to core some blanks, anchor sealed, and dry in my basement for 18 months. This stuff is bone dry.

As for scraping- I have pretty much been using a negative rake grind of around 70 deg with the scraper parallel to the lathe bed. I’ve done some shear scraping also but not much.

My tools are sharp- I’ve got a D-Way CBN wheel and a wolverine system. All gouges are sharpened at 50deg- set by using raptor gages. I sharpen often (sorry Odie, I don’t do honing well except for skews). I’m pretty much a fanatic about sharp edges.

My profiles are pretty close to Ellsworth, but I also have some no wing grinds at around 70deg to get the inside wall/bottom curve and to flatten the bottom- pretty sure this is what Odie and John are talking about. I can get a fairly clean bottom with these.

I’m working up to justifying the expense of something like a Hunter tool. If they truly do an outstanding job on end grain, my rationalization will go into high gear. (that’s what happened with the d-way CBN wheel). Have a lot more research to do on this.

I’m turning at 800-1000 rpm. If I go faster, the top 2in or so goes too out of round and the pucker factor go way up- I have had my share of exciting moments. I like to turn at higher rpms, but not on something this size. I have slowed things down and try to keep the cut in proportion to the work- that means I’m taking much lighter cuts than I used to. I have a little mouse constantly reminding me to rub the bevel, and take lighter and slower cuts. My tooling has dramatically improved in the last 2 years.

My thought on the sharpening back of the scraper was/is why waste a perfectly good cutting edge? My intent for use of this edge would be shear scraping the outside of the bowl. I’m looking for the downside of this, but so far there doesn’t seem to be much.

Thanks guys, keep the thoughts coming
Dick

 

[robo hippy]

I can't understand what you mean by sharpening the back side of the scraper. If you want 2 cutting surfaces, the only way I can see that being done is to have the bevel look more like a V where the tops of the V are the cutting edges. That would be more trouble than it is worth.

If you are considering doing that with a negative rake scraper, I don't think it would work there either. You cut with the burr from grinding, honing, or burnishing. The burr is on one bevel face only. I guess you could hone till there is no burr, but then the tool would not cut well.

The cleanest cuts I have gotten, using any tool, is a bevel rubbing cut. I never use a scraper flat on the tool rest any where near the rim of a bowl on the inside after most of it has been hollowed. It sets up explosive oscillations. You can do shear cuts with the scraper, round nose preferred, and working on the lower 1/3 of the tool (just like a skew), and make gentle pull cuts towards the rim. A scraping cut (scraper flat on the tool rest) will always leave some tear out, with the exception of end grain cuts, which can cut fairly cleanly.

If you go to You Tube and type in robo hippy, I have a couple of clips up on how I use scrapers and gouges.

robo hippy

 

[dickhob]

Scraper Picture

Sorry for all the confusion. Here's a picture.

If you notice the scraper (front) edge on top, that is what I am calling a negative rake. It isn't even 1/16 wide, but it allows me to scrap with the blade flat on the toolrest and at least for me, minimizes catches. When the scraper face is sharpened, there IS a burr and it does cut very thin shavings.

All I'm suggesting that if I put an edge on the back (see arrows) of the scraper, I'll have a second edge which I should be able to used on the outside of bowls in a shear scraping cut. As I said, why waste a perfectly good edge?

Robo, I've seen your videos- several times, along with John's and others. They have all been helpful and I appreciate your sharing.

 

[Bill Boehme]

I dunno' -- that looks like a very bad idea holding a scraper that way where there is such a short moment arm to control the angle of approach.

If it was a good idea, turners would have been using it already.

 

[dickhob]

Bill,
I don't follow "short moment arm to control the angle of approach."

I agree that if it was a good idea turners would already be using it, which is why a asked the question to begin with.
Dick

 

[Bill Boehme]

Bill,
I don't follow "short moment arm to control the angle of approach."

I agree that if it was a good idea turners would already be using it, which is why a asked the question to begin with.
Dick

OK, sorry about going engineer on you. I'll rephrase. "Holding it sideways like that means that even the slightest wiggle of your hand is likely to give you one heck of a time keeping it from having a dig-in if you are using it flat on the tool rest". I might add that even shear scraping may prove to be "interesting". If you have nerves of steel, then give it a whirl. Maybe it isn't as bad as it appears to be.

 

[MichaelMouse]

I dunno' -- that looks like a very bad idea holding a scraper that way where there is such a short moment arm to control the angle of approach.

If it was a good idea, turners would have been using it already.

Sounds like an endorsement for tools which steady on the bevel to me. Not only a short angle, but probably a fulcrum approximating a knife edge. May be one of the reasons people are always writing in about reconditioning the top of their toolrests.

I only scrape under duress, when there's no other way to get there. You do have a couple of gouge choices you might want to try. Here's a look at what the wood sees.
http://i35.photobucket.com/albums/d160/GoodOnesGone/Gouge-Curves.jpg

With my preferred push cut and the gouge type on the left.

http://i35.photobucket.com/albums/d160/GoodOnesGone/08-Forged-Gouge.jpg

With a "bowl" gouge and a pull cut. This with my old toolrest, (pre 2007) where I rode the tool on both the shiny areas. Can't do that with the new, which does sacrifice some control over depth of cut.

http://i35.photobucket.com/albums/d160/GoodOnesGone/09-Bowl-Gouge.jpg

Lets the wood slice itself away from the piece as it slides down the edge. Very little lift up the ascending endgrain means almost no tearout, even on this birch, which can be a bear, especially compared to cherry.

Give 'em a try. Just remember outside stay above center, inside, below.

Bill, didn't turning originate in 2008? Anything worthwhile, I guess.

 

[dickhob]

Bill,
Even I don't have the guts to use a scraper resting flat- unless I can get the tool rest very close (within 1/4in) AND I've used my neg rake on the top of the scraper AND the edge is freshly sharpened AND the bigger the piece, the slower the rpm.

I don't seem to get this accross- I'd use the back edge only in a shear scrap position and only on the outside of a gentle form.

I'm working on the grind a little at a time as I sharpen the "Normal" edge. I'll keep you posted as to how it works out.

MM,
If I'm not mistaken, don't you have a few videos on using these gouges? I've got your way on my things to try. I used to use gouges like you before I got introduced to the deep flute and fingernail/Ellsworth grind. That has kind of taken over my bowl turning for the past few years.

I have a few older gouges that would take your grind- maybe today is a good day to revisit your technique.

Again, thanks for all the input. It's got me rethinking everything again- more than usual.

Like I said, I'll keep you posted.
Dick

 

[robo hippy]

Dick,

That picture cleared things up. There are, with scrapers as well as gouges, numerous profiles that can be ground and used for different circumstances. Probably most basic are 'inside' which is the one you are showing, and it is swept back towards the left side. There is the 'outside' which is swept back to the right side. Note here: The inside and outside variations are so called because the profile will work best, when the bowl has been reversed (bottom towards the headstock), and you are finish cutting and/or roughing and give you better access to that part of the bowl. The third variation is a round nose scraper, which if you think about it would be kind of like putting the two halves (inside and outside) together. If you have only one scraper, I would suggest a round nose, more C shaped nose instead of ( shaped nose. It would be useful for both inside and outside the bowl, for heavy roughing cuts, and shear angle cuts.

To simplify, if you want to sharpen the back side on the scraper you have, you could do that, though your nose profile isn't the best for that, it will work. A round nose, or outside profile scraper would be more suited to that type of sharpening. I don't really use negative rake scrapers except on end grain turning.

robo hippy

 

[Bill Boehme]

Dick and M,

I am sure that both of you guys know this already, but maybe misunderstood what I said: Flat is neither level nor parallel to the ways -- it means flat on the rest and I assumed that it is common knowledge that it should be trailing down a bit.

I do not do much scraping in that fashion -- mostly shear scraping since I learned about it in 2004. I attended my first symposium the following year and in one of the programs, the demonstrator started shear scraping and most of the people in the room gasped and started yelling at the demonstrator that he shouldn't be using a scraper that way, lest he kill somebody in a horrible accident. Things have changed a lot in the past few years -- much more so than in many previous years when woodturning was stuck in a "only one way to do it" frame of mind.

MM,

I have turned on a foot powered Velocipede so I am going out on a limb and guessing that turning has been around for a bit longer than the past five years although for the most part, turners were just "scraping by" until not too many decades ago.

 

[MichaelMouse]

Keyed an old memory. Think it was Joaz Hill who espoused using a broad sweep forged gouge upside down and flat on the rest. That way he could reap the benefit of the round nose and the rise from supported edge to the point of greatest depth as his skew angle. Not sure if he sharpened as an in-cannel gouge or not. Would give you a burr if you wanted.

I tried it with the standard gouge configuration. Didn't know it would be called "negative rake" at the time, but found it was more forgiving than a standard scraper flat on the rest when working endgrain. Should be OK on the other, as well, though I'd just as soon cut.

Can't find the article in a quickie search, but perhaps someone else might ring him for a repeat.

 

[Jake_Debski]

Keyed an old memory. Think it was Joaz Hill who espoused using a broad sweep forged gouge upside down and flat on the rest. That way he could reap the benefit of the round nose and the rise from supported edge to the point of greatest depth as his skew angle. Not sure if he sharpened as an in-cannel gouge or not. Would give you a burr if you wanted.=[QUOTE

MM I also read that article I'm pretty sure it was an in-channel grind.

Jake