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Tearout, hole in cedar (Read 602 times)
 
Mark Putnam
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Tearout, hole in cedar
May 19th, 2016 at 7:55am
 
I'm turning a piece of cedar for the first time, and I have a couple of questions about dealing with imperfections and what I believe is called "tearout." See the picture below.

First, there is what I can only call a hole in the middle of the side that was not evident when I cut the blank and started turning. It did not come from one of my tools. It was just...there. Does anyone have a suggestion on how I can address this imperfection? It's the only such hole in the piece. It is deep enough so that if I tried to turn down to clear it out, the walls of the bowl would be too thin.

Also, you will note the various rough spots along the side of the bowl. I believe this is called "tearout?" I am using a 1/2" Sorby deep-fluted bowl gouge with a swept back grind. It is, in my opinion, very sharp (sharpened on a Tormek). After roughing the bowl to shape, I did several shear cut passes. But this tearout still appears. Any ideas on what might be causing this or how I can address it? One thing I did notice is that there was a lot of "chatter" when conducting my shear cuts, so that might have something to do with it.

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Chris Gunsolley
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Re: Tearout, hole in cedar
Reply #1 - May 19th, 2016 at 9:30am
 
Mark Putnam wrote on May 19th, 2016 at 7:55am:
I'm turning a piece of cedar for the first time, and I have a couple of questions about dealing with imperfections and what I believe is called "tearout." See the picture below.

First, there is what I can only call a hole in the middle of the side that was not evident when I cut the blank and started turning. It did not come from one of my tools. It was just...there. Does anyone have a suggestion on how I can address this imperfection? It's the only such hole in the piece. It is deep enough so that if I tried to turn down to clear it out, the walls of the bowl would be too thin.

Also, you will note the various rough spots along the side of the bowl. I believe this is called "tearout?" I am using a 1/2" Sorby deep-fluted bowl gouge with a swept back grind. It is, in my opinion, very sharp (sharpened on a Tormek). After roughing the bowl to shape, I did several shear cut passes. But this tearout still appears. Any ideas on what might be causing this or how I can address it? One thing I did notice is that there was a lot of "chatter" when conducting my shear cuts, so that might have something to do with it.

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Mark, I am a beginner, but have enquired on these exact topics with experienced turners in person before, so I'll go ahead and let you know what others have told me, and the experts in this forum can weigh in.

For the hole, if you fill it with clear epoxy or super glue, let it dry, then turn the bowl, it will still look good and perhaps be a feature (though a small one in this case). If you like the look of the colored resins (epoxy + dye) to make it an apparent feature, that's an option, too, and I think black would look decent. That's not really my thing, as I prefer as natural of an appearance as possible, but some people like it. You could also mix some fine sawdust from this exact bowl with epoxy, then fill it with that. Personally, I'd either do that or just fill it with clear to ensure the structural integrity of the bowl whilst retaining a natural look, and I wouldn't try to make a feature out of it because it's so small. Also, I'd be more concerned about that crack developing from the pith. If you fill the hole, I'd suggest filling that crack too then rough turning it ASAP after the glue completely dries. If you use clear on these, they won't be apparent and it will still look beautiful. 

As far as the tearout, if your tool is indeed sharp and you know it, that tearout can be caused by running the lathe at too slow of a speed. When I enquired on this, one experienced turner took out his pocket knife and tried to slice a piece of paper at a medium speed, and it just hit the paper and didn't cut it. At the medium speed, the blade may even start to cut the paper then lose its cutting ability part way through--akin to a tearout while turning the bowl on your lathe. He then snapped down on the paper as fast as he could and sliced it all the way through without a problem. That was the moment of my epiphany on the tearout, and he ended with "Once it is round, turn that thing up as fast as it will go!" Lesson: slow speed for roughing only; fast speed for the rest. He then insisted that many turners run their lathes too slow, thinking it's safer because it's slower, but the opposite is true.
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« Last Edit: May 19th, 2016 at 10:04am by Chris Gunsolley »  
 
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Mark Putnam
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Re: Tearout, hole in cedar
Reply #2 - May 19th, 2016 at 10:03am
 
My lathe is a Grizzly H8259 10x18 benchtop lathe. It is not "variable speed," but you can adjust the belts to achieve five different rpm settings: 826, 1205, 1713, 2422, 3337. I do all of my bowl turning on the lowest setting of 826 rpms. While that is technically the slowest setting, my understanding is that it is faster than what many bowl turners set their lathes to. I may be incorrect here.

Chris, at what setting do you turn your bowls? And I'd be interested to hear others' opinions on this as well.
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Chris Gunsolley
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Re: Tearout, hole in cedar
Reply #3 - May 19th, 2016 at 10:09am
 
Mark Putnam wrote on May 19th, 2016 at 10:03am:
My lathe is a Grizzly H8259 10x18 benchtop lathe. It is not "variable speed," but you can adjust the belts to achieve five different rpm settings: 826, 1205, 1713, 2422, 3337. I do all of my bowl turning on the lowest setting of 826 rpms. While that is technically the slowest setting, my understanding is that it is faster than what many bowl turners set their lathes to. I may be incorrect here.

Chris, at what setting do you turn your bowls? And I'd be interested to hear others' opinions on this as well.


I'm sure 826 rpm's is way too slow, and that's causing your tearout. Crank it up and you'll be pleasantly surprised Wink

I actually have a speed adjustment system very similar to yours. My lathe is a Delta 1460 (built in 1938), and has the simple 5 speed step pulley speed adjustment system that you speak of. For roughing, I put it on the slowest speed, which is the smallest diameter at the motor pulley paired with the largest at the spindle pulley, and according to my calculations that should be spinning at 777 rpm. (It sounds like you're doing something similar, but full time.) I don't have to do much roughing, because I cut my blanks in perfect circles on my bandsaw. It's only getting rid of the corners. After roughing, I switch directly to the fastest speed, which is the largest pulley at the motor, paired with the smallest at the spindle. I don't actually know the rpm of that.

When I started, I had been misled by a salesman that was trying to convince me that I really need a slower spinning lathe (that costs thousands of dollars), and consequently went home and was using only the slowest speed that my lathe was capable of, just like you. I had the exact same tearout problem, it seemed no matter how precise I was with my tools, and it drove me nuts. Then the above story happened, I cranked 'er up and it was beautiful  Smiley
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« Last Edit: May 19th, 2016 at 10:15am by Chris Gunsolley »  
 
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Mark Putnam
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Re: Tearout, hole in cedar
Reply #4 - May 19th, 2016 at 10:14am
 
Ok, so this is where I'd like to know the difference between "personal preference" and "proven best practice." Smiley

I also cut my bowl blanks in to circles on a bandsaw first. But my understanding is that for roughing the blank in to an approximate bowl shape, it is most advisable to set your lathe to the lowest setting. Because you are making some dramatic cuts to the wood, which--while approximately a circle--is still not perfectly round.

Now, it makes sense to me that once the blank has been rounded fairly well on the lathe, to crank up the speed. But until then, while you still have possible imperfections in the wood, it's best to take it slow.

To your point, however, given that this piece of cedar is already in its bowl shape, I may try turning up the speed slightly to see how that goes.
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Chris Gunsolley
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Re: Tearout, hole in cedar
Reply #5 - May 19th, 2016 at 10:28am
 
Mark Putnam wrote on May 19th, 2016 at 10:14am:
Ok, so this is where I'd like to know the difference between "personal preference" and "proven best practice." Smiley

I also cut my bowl blanks in to circles on a bandsaw first. But my understanding is that for roughing the blank in to an approximate bowl shape, it is most advisable to set your lathe to the lowest setting. Because you are making some dramatic cuts to the wood, which--while approximately a circle--is still not perfectly round.

Now, it makes sense to me that once the blank has been rounded fairly well on the lathe, to crank up the speed. But until then, while you still have possible imperfections in the wood, it's best to take it slow.

To your point, however, given that this piece of cedar is already in its bowl shape, I may try turning up the speed slightly to see how that goes.


I agree, and that's why I do want the experts here to weigh in as well, letting us know whether they agree with what I've advised.

However, even I, in my limited experience can tell you that after you have your blank in a general bowl shape that you speak of, you are in a whole different world of beauty when turning with a higher speed compared to a lower one. It is safer, too, because your tool cuts better and is consequently less likely to catch. 

Your cedar bowl is far past the point at which you should crank up the speed. Since you're cutting your blanks circular like me, you're actually just about at that point even when you mount the blank, as long as the circular walls of your blank are exactly perpendicular to the faceplate after it's mounted. Of course, this is what naturally happens as you shave off the wood on your lathe. Once nothing is obviously protruding, and you've gotten rid of the corners, which will happen quickly for you since you're taking that wise shortcut of cutting circular blanks, you need to be at high speed to maximize the cutting power of your tools.
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« Last Edit: May 19th, 2016 at 10:30am by Chris Gunsolley »  
 
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Mark Putnam
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Re: Tearout, hole in cedar
Reply #6 - May 19th, 2016 at 10:50am
 
So at what speed to you hollow your bowls?
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Mark Putnam
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Re: Tearout, hole in cedar
Reply #7 - May 19th, 2016 at 11:03am
 
Here is some information from Richard Raffan's book, specifically regarding turning speed for facework:

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Chris Gunsolley
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Re: Tearout, hole in cedar
Reply #8 - May 19th, 2016 at 11:05am
 
Mark Putnam wrote on May 19th, 2016 at 10:50am:
So at what speed to you hollow your bowls?


After it's mounted, I put the lathe on the slowest speed (777 rpm), true up the top (the part that will ultimately be hollowed) then make my first two cuts in the center. Then, I switch to the fastest speed and leave it there for the remainder of the hollowing.

I experienced that same terrible tearout problem in your pictures when hollowing, which required me to keep making the walls on my bowl thinner than I was going to initially, to get rid of the marks. It was intense, as I didn't want to lose my precious bowl and I almost ruined it. Again, the tearout stopped after I cranked it up to a higher speed and enabled my tools to do what they were designed to do.   

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Chris Gunsolley
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Re: Tearout, hole in cedar
Reply #9 - May 19th, 2016 at 11:06am
 
Mark Putnam wrote on May 19th, 2016 at 11:03am:
Here is some information from Richard Raffan's book, specifically regarding turning speed for facework:

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Note that these figures are for unbalanced blanks. Meaning, before they're round. You're cutting perfectly round blanks on your bandsaw, so these speeds don't actually apply to the method that you're using. These would apply if, for example, you just mounted a block of wood up there to round it off, bulging rough corners and all. You can take a thin shaving off your round blank at these speeds if you'd like, just to ensure that indeed it's perfectly round, but after it is and it's trued up, you should be spinning it far faster than this.

In any case, once you have that general bowl shape, I'd turn the lathe up as fast as it can go without producing apparent vibration. You're going to get vibration if your lathe is running too fast for your bowl size, and if you don't feel that vibration, you're fine. The only times more speed is not better is when you're not mounted correctly or you're dealing with unbalanced or unstable wood.  
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Don Stephan
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Re: Tearout, hole in cedar
Reply #10 - May 19th, 2016 at 6:57pm
 
Mark

First I'd strongly suggest you look for a turning group in your area and join.  This forum is wonderful, but there is no substitute for learning safe and recommended practices from a mentor alongside.  Barring or in addition to that, videos from recognized instructors, including but not limited to Richard Raffan, Mike Mahoney, Lyle Jameson, and others, as well as videos on this web site (which have been previewed) are excellent reference material.

I can't read the chart, and my copies of Raffan's books are not here, but I firmly believe I've read in many different sources there is always a maximum RPM that should not be exceeded for a given piece of wood.  When the lump of wood is unbalanced, a vibration will be felt at some speed, and the common recommendation is to slow down the lathe until the vibration goes away.  A lump of wood can be unbalanced because it is not round, because it is round but not centered on the lathe, or because some parts of the lump are lighter or heavier than others.

I want also to caution you about a maximum turning speed for any given lump.  My recollection is I've read from any number of sources that there is always a recommended maximum speed for any piece of wood.  For bowls, the chart I have considers both the diameter and height of the lump.
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Re: Tearout, hole in cedar
Reply #11 - May 19th, 2016 at 7:00pm
 
Mark

Regarding the lump of wood in your picture, the general recommendation is to stat at least 1" away from the pith when cutting a bowl blank, because the pith often leads to cracks.

Second, there appears to be a crack running all the way up the side of your blank.  If it is such a crack I think it is very unsafe to turn at any speed.
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