Sharp corners on roughed bowls......produce cracks?

[Odie]

(IIRC) At some time in the past, I remember someone suggesting to round off sharp corners on roughed bowls. The object was to eliminate a certain kind of cracking that emanated from that point.

I roughed out several bowls yesterday, and recalled that advice while in progress. It only takes a second to take the corner off......not a problem. However, as I contemplated this, it occurred to me that I don't believe I've ever seen a crack that looked like the source was a corner edge. Just about all the cracks I've ever gotten originated from a weak spot in the grain that couldn't take the warp.....like the example photo I'm providing. A few others were pre-existing. (There have been a few cracks that were caused by my carelessness......but, that's a whole different matter!)😱

On most of my bowls being prepared for seasoning, there are normally sharp corners at both sides of the rim, and the tenon.

I'm just wondering if anyone else has had examples of cracks that are caused by a sharp corner.......? How can one tell if the sharp corner is the problem, or could these things either be pre-existing cracks, or weakness in the wood grain?

The premises of corners=crack seems valid at first thought, but I just haven't seen this particular thing that I can recall. Could certain woods be more susceptible than others?

If there are example photos, I'm sure I'm not the only one who would like to check it out........

ooc

 

[john lucas]

Odie I used to lose a lot of bowls to cracking. The checks usually started in one of two areas. The lip or the tenon. Now I'm not really sure what helped the most, sealing the lip and tenon completely with Anchorseal or leaving them slightly rounded or maybe both. I'm guessing that sealing the endgrain of the vessels probably worked the most but since those areas were where I had the most problems I round them over slightly.

 

[Bill Boehme]

Odie, in objects made of solid homogenous materials such as metals, load stresses tend to be the greatest along very sharp inside edges. For that reason, fillets are used to ease these edges and distribute the stresses over a wider area. Wood, however, is a completely different animal. Since it consists of bundles of directional fibers, its ability to withstand various kinds of loads whether internal or external is very dependent on what's going on in the wood structure. Internal stresses are commonplace in wood and so are defects. I would agree that your intuition is telling you right. If a roughed out bowl had a very sharp thin rim perhaps that might be enough to encourage development of a crack that probably would happen anyway, but "sharp thin rim" and "roughed out" sounds like mutually exclusive conditions to me.

I would say that if you see a crack at the rim then it probably means that the crack stopped at the rim rather than starting there. 😀

I hardly ever intentionally rough out anything and let it dry before finishing it, so I don't have a wealth of experience to back up my opinion.

My motto is: Life's too short to sit around and watch crappy wood dry.

 

[hockenbery]

I always do a light scrape on the corners of bowl rims so I don't get cut.
Sharp rims on a rough turned bowl can cut quite badly even when the Bowl is dry.

Forms that have corners such as a bowl shaped with Vertical side walls and a flat bottom will frequently crack because the wood cannot bend around the corner.
A hemispherical bowl rarely cracks because the wood can move as the bowl dries.

I generally turn the bottoms of roughed out bowls a bit thinner than the side walls. This makes the bottom plus the tenon close to the sidewall thickness.
the bottom does not warp much so you always have enough wood in the dried bowl. If the wall of 14" rough out is an 1.5" thick at the rim I would have the wall thickness at the bottom a fat 1" to 1.25" thick. I run the calipers down the side wall right around to the tenon in the chuck. I want the tenon plus the bottom to be close to the rim in thickness.

Bill - One wood that is fun to watch dry is wet madrone. It moves so fast and interestingly.

Have fun
Al

 

[Ian Thorn]

Hi just to put my add my thoughts, going back to the metal concret will crack at a sharp internal corner and to stop this you place a rebar at 45deg to the corner , we cant do this with wood but you may be right about the sharp corners from what i have seen using wood for 45yrs. I have started puting my wet turned bowls i 50/50 dishwash and water this displaces the sap after about 2 weeks i take them out to dry on a rack out of the light untill dry, they dont seem to move as much or crack and I have saved some bowls that were air drying and starting to crack

 

[Michael Powers]

Al - I have a ton of Madrone (a couple of blow downs from last winter's ice storm). Do you have any techniques to stop this stuff from cracking? I turned a goblet from a branch a couple of months ago. It looked great for 2 days - then almost split in half overnight. My theory was to drill out the pith so it would have a place to contract but I haven't tested it yet. One other note on Madrone that I have discovered is this stuff needs to be weatherd - I turned few cups/goblets from a branch that I found on the ground. It had been laying there for quite some time and was completely dry and weathered. It turned dark brown with black streaks where as the fresh stuff turned pure white. I guess it depends on what you are looking for.. I know about spalting but these 2 pieces of wood don't look like they are from the same species.

 

[john lucas]

I think the problem with sharp edges on wood is simply rapid drying. The thin edge loses moisture very fast and as we know, when wood dries it shrinks. If the wood next to it still wet and large something has to give. By rounding the edge slightly it evens out the drying.
I only do this on rough turned bowls because I'm going to clean up that edge anyway when I final turn it. For bowls I turn to completion I don't worry about the edge and cut it the way I want. I will however put that bowl in a paper sack or box for a day or so to keep the drying more even.

 

[Odie]

I think the problem with sharp edges on wood is simply rapid drying. The thin edge loses moisture very fast and as we know, when wood dries it shrinks. If the wood next to it still wet and large something has to give. By rounding the edge slightly it evens out the drying.

Good reasoning, John........this makes sense.

Maybe the reason I've never had a problem with sharp corners is all my bowls that require seasoning get completely anchorsealed immediately after coming off the lathe. It's also my rule to always finish roughing a bowl and anchorseal, once it's started.......so, from block form to roughed and anchorsealed bowl, there isn't much time for uneven drying to occur.

I may begin to take the sharpness off the rim and tenon, just as a hedge against the possibilities. If this were something that cost time and energy to do, I wouldn't bother with it......but, it's so quick and easy, that I'm having a hard time justifying not doing it.

ooc

 

[Steven Antonucci]

I forget which class it was, but when I was studying engineering at Stevens, one of my professors said something that always stuck with me. "Nature hates corners. They are stree concentrators." Funny how that always stuck with me, but the answer is yes.

I also agree with Al's reason, as I have also cut myself on a corner more than once. I either use a piece of sandpaper to give it a quick roundover or roll the edge with whatever tool I have in hand. Often, it is imperceptible to the eye, but it helps and keeps you from bleeding...

 

[Hugh]

Michael Powers,
I know this is off subject of the thread - sorry - not trying to steal the thread - just trying to answer Michael's question about Madrone wood.

Rough out what you want to turn - put into a large pot with water in it - bring to a boil - boil for 3-4 hours - let cool. Take the wood out and let dry like you do all the other rough outs. In the summer time when the weather dries out a bit ....you may need to seal the end grain to prevent cracking.
Hugh

 

[MichaelMouse]

(IIRC)


The premises of corners=crack seems valid at first thought, but I just haven't seen this particular thing that I can recall. Could certain woods be more susceptible than others?

Seems valid, doesn't it? But in thirty plus years of leaving 'em as is, can't say I've seen it. Which isn't to say it's not true, I better hasten to add, just that it isn't true for at least two of us. I think it's a bit like the "uniform thickness" rule, which I have consistently violated for the same period of time without losing stock. Too wide at the bottom (nearly vertical walls) will do it without intervention, though.

If you think in purely mechanical terms, there is only one very small part of the rim this might affect. I think what most people see at the very center of the heart is what happens when you have juvenile wood or true pith. Or neglected heart checks.

 

[robo hippy]

Well, I always round over my corners. Two reasons, one of course is to remove the razor saw edge that will slice you to the bone before you can say skew. The second is that it does help reduce cracking. I have tried to figure out why it works, and in my experience (thousands of green turned bowls) there is a much higher success rate for those that are rounded over. I was told that the sharp edge is much thinner, and will dry more quickly, and that builds up uneven drying stresses. I figure it has some thing to do with the "keep the walls an even thickness" idea. When you come to the rim, the thickness is uneven, and by rounding over the sharp edge, it does become more balanced.

Madrone: "That stuff starts to split when you fire up the chainsaw. Heck, I thought it started to split when you take the chainsaw out of the truck." It has much higher water content than other woods, and warps more than just about any other wood I have seen. Thing with the way it warps, is that it is never predictable. I have cored sets of bowls, and each one seems to warp in different directions. For end grain pieces, do not leave the pith in. Suicide! With end grain pieces, if you leave a tenon on it, it has to be sealed, with a very thick coat of sealer, finish, or glue, and that does not always work either. I do not let it sit out in the weather. Bugs will burrow into it, 6 inches deep within a month or two. You can boil it. 1 hour per inch of thickness. Bring it to a boil in the water, let it come back to air temp in the water, then remove. Seal and dry like any other wood. I don't do that because I don't like the color changes. I turn green to final thickness of 1/4 to 3/8 inch thick, then let it dry and warp. Do round over the rims, and put several wraps of plastic stretch film around the rim, about 1 inch over the top and the rest over the outside. Stretch it out as you put it on. I had bad luck with the paper bag method, single, double and even triple bagged. The stretch film (used around boxes to keep them on a pallet, available at big box office supply places), put my drying success rate up to almost 100%. Another reason to think that protecting the rim is very important. I was using the DNA soakers method of wrapping the outside with paper and securing it to the rim, and leaving the inside open. I then started using the stretch film to secure the paper, then just the plastic. Start drying on the floor of the shop. After a few days, put up on a wire rack.

robo hippy

robo hippy

 

[hockenbery]

Al - I have a ton of Madrone (a couple of blow downs from last winter's ice storm). Do you have any techniques to stop this stuff from cracking? I turned a goblet from a branch a couple of months ago. It looked great for 2 days - then almost split in half overnight. My theory was to drill out the pith so it would have a place to contract but I haven't tested it yet. One other note on Madrone that I have discovered is this stuff needs to be weatherd - I turned few cups/goblets from a branch that I found on the ground. It had been laying there for quite some time and was completely dry and weathered. It turned dark brown with black streaks where as the fresh stuff turned pure white. I guess it depends on what you are looking for.. I know about spalting but these 2 pieces of wood don't look like they are from the same species.

Sent you a Pm
Al

 

[Gretch Flo]

sharp corners

"I also agree with Al's reason, as I have also cut myself on a corner more than once. I either use a piece of sandpaper to give it a quick roundover or roll the edge with whatever tool I have in hand. Often, it is imperceptible to the eye, but it helps and keeps you from bleeding..."

A couple of years ago I had a white mulberry bowl tith sharp edge fly off a not so tight expansion mode chuck and hit me above my glove. Didn't bleed much (because it was plugged), and when it bled a little I washed it and applied bandaid thinking no more of it. 1-2 months later there was a fester on one edge, and I extracted a 1 cm piece of wood and 2 days later another in the middle, slightly smaller (see the 2 red areas on arm) . Still today when pinching the "incision" vertically I feel a 1 cm+ rigid structure, hence more it left, but very quiescent. Would it have happened had I rounded it??? Gretch

 

[MichaelMouse]

A couple of years ago I had a white mulberry bowl tith sharp edge fly off a not so tight expansion mode chuck and hit me above my glove. Didn't bleed much (because it was plugged), and when it bled a little I washed it and applied bandaid thinking no more of it. 1-2 months later there was a fester on one edge, and I extracted a 1 cm piece of wood and 2 days later another in the middle, slightly smaller (see the 2 red areas on arm) . Still today when pinching the "incision" vertically I feel a 1 cm+ rigid structure, hence more it left, but very quiescent. Would it have happened had I rounded it??? Gretch

Certainly. A splinter is a splinter, regardless the conformation of the other end. I'm guessing you don't cut on the push as much as on the pull?

 

[Odie]

A couple of years ago I had a white mulberry bowl tith sharp edge fly off a not so tight expansion mode chuck and hit me above my glove. Didn't bleed much (because it was plugged), and when it bled a little I washed it and applied bandaid thinking no more of it. 1-2 months later there was a fester on one edge, and I extracted a 1 cm piece of wood and 2 days later another in the middle, slightly smaller (see the 2 red areas on arm) . Still today when pinching the "incision" vertically I feel a 1 cm+ rigid structure, hence more it left, but very quiescent. Would it have happened had I rounded it??? Gretch

Hello Gretch.......

The answer is: "probably not!"

I've pretty much decided to take a moment and round the corners, because it's so quick and easy...... John Lucas gave a credible theory on how the corner might induce a crack (even though I don't recall of this ever happening to me). The safety concern is a very good reason to do it......you and Al are quite correct about the dangers a sharp corner does present.

ooc