Discussion in 'Newbie' started by Charles Hill, Apr 9, 2013.
How thick should I make my green bowl sides. For the best result
The rule of thumb for the thickness of green wood that you plan to rough turn and then air dry before finishing is approximately one-tenth of the bowl's roughed out diameter. Don't go measuring this with a micrometer -- eyeballing it is close enough. See the figure below.
Here are some other guidelines:
Round over the top edge of the bowl. This helps to reduce stress concentrations that can occur if there is a sharp corner.
Coat the bowl with Anchorseal or something similar or otherwise do something to slow down the rate of drying. Fast drying is more apt to lead to cracks.
Some woods warp more than others so take that into consideration and make the roughed turning thicker. It is disappointing if you have a nice bowl that warps too badly to be able to get it round again after drying.
Bowls can have all sorts of shapes (spherical, paraboloid, calabash, etc) and all of these shapes have an effect of the way that the wood will behave or misbehave while drying. The only way to know for certain is by experience.
There is a high likelihood that a bowl that has an abrupt change in bottom curvature such as fairly flat bottom and steep sides is a very good candidate for cracking during drying. So it is advisable to shoot for creating a well behaved curvature that is fairly continuous sort of the way that is shown in the figure . This is probably one of the more difficult parts of bowl turning for a beginner who is doing well just making a bowl that holds together while turning.
Bill hit the important Parts.
I did a demo on turning green wood about a year ago and am doing it again in a few months for another club.
I start with a 10 minute slideshow which is the handout, then pass around 3 bowls and urn a owl for drying.
There are diagrams of how wood Ives and a shrinkage chart of some poplar species.
I think his link will show you're slides.
https://www.dropbox.com/s/5rl6v4aa8rpc0fl/WORKING WITH green wood.pptx
I call the following the five habits of successful greenwood turners
Start with crack free wood
Work fairly quickly
Even wall thickness
control the drying
The deck is sort of stacked against beginners. If you are learning on you own it is hard o get nice curves, even walls, let alone work fast enough to keep the wood from dying on the lathe.
The smoother the surface on the roughed out bowl the easier it is to return
Shape and grain more important than thickness. The 10% business is very popular. It'll guarantee you plenty of wood to do a reshape if you like. If you want to dry the wood faster, go thinner. No reason why a regular grained piece can't be cut to 7% or even a bit less and have enough to be made circular. Not much left over for restyling, though. It'll cure in a bit over half the time of a 1". What you don't want is a broad section of face grain, as in a squared-across bottom. The less slope on the sides, the thinner you can go if you're anxious, and the safer to dry in any thickness.
It's mechanical stress that exploits the surface checks or cracks which form from uneven moisture loss. And there will be uneven moisture loss, which is why the business of "equal thickness" doesn't apply. When the difference in rate of loss is a factor of ten or twelve, depending on grain orientation, a quarter inch difference in thickness is not significant. In the right place, even a half! To reduce the effect of mechanical stress - squeeze across the grain and droop in the walls - we like to keep the rate of surface loss low, and fiber expanded for as long as we can without growing mildew. Coating or containing are the common ways to do it. It's actually control of the relative humidity that you're trying to accomplish.
http://i35.photobucket.com/albums/d160/GoodOnesGone/LogEnd.jpg Is a good teacher. Since the earlywood is less dense, it shrinks proportionately more than the harder, denser latewood. The greater the span of earlywood, the greater the total contraction, and the greater the stress on any check on the end grain. Simple mechanics says if you tie a string (earlywood) across a span (two latewood rings) and then try to shrink the string, the span must shorten or the string must break. The same mental string will explain what makes your bowl oval, as well.
Not sure why a smooth surface on a rough bowl would help anything, but good tool technique will produce one automatically, even when you're hurrying hauling trash out of the way. Turns out the easiest and fastest way produces the best surface, with wet wood easier to slice than dry in most cases.
the business of "equal thickness" doesn't apply.
You're saying even/equal wall thickness isn't applicable?
The ten percent guideline is there for people learning like yourself. It is a very good guideline to start with until you gain some experience with the woods you're dealing with. A lot of folks do not vary from that guideline at all and are very successful with drying. Cutting the walls thinner as MM suggests will work but is best used after you have gained experience which can take a few years so be patient.
Your wall thicknesses can very a bit but try and keep an even wall thickness. If nothing else this is great practice while roughing to achieve an even wall thickness and gain the confidence to get the final thickness when finish turning. If you're going to be a little thinner anywhere the bottom of the bowl is a better location. It has less movement therefore will not diminish your ability to finish turn the bowl. Some have suggested this helps make the bowl a little more elastic thus less cracking but I can't say that anyone has proven this theory.
I think what Al is suggesting with a smooth surface is that when you go to finish turn the bowl the first few cuts will go a little better for you. When you combine the rough surface and warp of the bowl it can make for a bumpy cut for the less experienced. Having said that don't get to concerned with a smooth surface for drying sake be more conscious of form, thickness then surface.
As a final note, make sure you take the time to practice while rough turning. It is the best time to practice different cuts, forms, looking at the piece to study grain, holding methods, using a caliper to check thickness, how to make a good tenon or recess, turning with the opposite hand rather than the dominate hand, your stance and movement, slower speeds rather than crank it up, cutting while standing out of the line of fire and etc. etc. Most importantly have fun.
Shape and equal thickness are related.
A hemispherical bowl is more forgiving with unequal walls.
I usually rough my bowls with slightly less wall thickness in the bottom 1/3 of the bowl.
A bowl with sharp angles vertical walls flat bottom it very difficult to dry without cracking.
Wall thickness is critical
Hollow forms and once turned bowls even walls and curves are more important.
I have turned quite a few 3" solid balls from live oak.
They dry without cracking because the old can move in all directions and the wood has an interlocking grain.
They end up looking ellipsoid but don't crack
Everyone is assuming he's talking about twice turned bowls. I do a fair amount of those but most of my green wood bowls are turned thin to completion and just let them warp. Most of these are natural edge style so the warp isn't as noticeable although if they are highly figrued they get lumpy of course.
I turn them to 3/8" thick or less. Woods that have a tendency to crack I turn 1/4" or even less for small, say under 6" pieces. Many are from limb stock so they may be 4 to 6" wide and 8 to 12" long.
I can get buy with 1/2" thick if I put them in a paper sack for the first week or so to limit the moisture loss.
Smooth surface makes it easier to return the dry bowl.
If a roughed bowl has bump, ridges, tearout, it will be harder to reshape round and will require more wood removal.
When I return a bowl I try to turn the out of round to the one or two lines on the outside of the bowl that are in round.
This yields the largest bowl.
Rarely do I need to reshape this curve.
Yes I am. I'll go one further and say that the business of a thinner bottom doesn't apply. Since there's no consistency in dry rate, there's no need to try to be consistent in thickness. What you want to avoid is a long continuous area of wood to contract and exploit drying checks in endgrain. The broad bottom, steep slope syndrome. When you hollow you interrupt these broad areas until you get to the bottom of the bowl. There the continuous portion will have the greatest absolute shrink. http://i35.photobucket.com/albums/d160/GoodOnesGone/Picture-Package-14.jpg Shows the mortise on the bottom of a bowl. Shrink from green to ~10% has caused the 1/8" difference you see. 1/8 over 2 should come out to 7/8 over 14, but it doesn't, as you can see. You can also see that the walls are 3/4 or so at the top, and the bottom is as thick as the entire piece. None of that stuff is important to survival.
Steeper sides squeeze proportionately more. http://i35.photobucket.com/albums/d160/GoodOnesGone/Eleven-Across.jpg
Shoulders droop depending on how close the rings are and what the slant is. http://i35.photobucket.com/albums/d160/GoodOnesGone/Edge-Droop.jpg
NB I haven't ever had any problem leaving the edges sharp while drying. But it makes sense to round them over in case you should brush against them after they dry.
Require more wood removal? Something happening other than the normal shrink because you didn't make things slick? The difference along versus across grain will be the same regardless.
http://i35.photobucket.com/albums/d160/GoodOnesGone/BroadOutProgressive.jpg The short (end) grain needs to be cut to get to circular.
Inside http://i35.photobucket.com/albums/d160/GoodOnesGone/BroadInStaging.jpg The long grain needs to be cut.
The wood shrinks the same,
If you are returning a rough surface you sort of have to go through the roughing process again
Any ridge will have the same undulating warp of the rim.
The final surface of the rough turned bowl is just a light pass with the bevel riding push cut.
Something you could sand with 120. Nothing special.
My beginning bowl students usually get this by their 2nd or 3rd bowl.
If you are returning a clean surface and you line up the rim of the dried blank well you can use Finnish cuts to turn down to two thin lines of the dried surface.
95 % Of the time this will be real close to the final curve and the rim has the smaller diameter of the dried blank.
It just simple, if you leave a rough surface you will be turning away more of the outside to get a finished surface.
With your post in the middle method it probably doesn't matter as you are stuck with what the post allows.
I would mostly agree in that close counts, but I haven't found a cracked bowl yet that I would attribute to the wall thickness variations vs a surface or latent defect.
Figure you leave the tenon on, that adds extra depth (thickness)
The 10% for twice turned bowls is a guide line, and not an exact measure. With some woods it is more than you need, and with other woods, you need way more, or have to boil them to be able to twice turn, and Madrone is one of those woods. With a twice turned bowl, you can leave the bottom a bit thinner than the walls as it moved some what differently than the sides and end grain. Having a smooth surface does contribute to more even drying stresses, but what you don't want is big differences, which can create more stress in some areas, and stress is relieved by cracking. Rounding over the rims really helps as well.
Drying is a whole different art, and will change for every different piece of wood you turn. Also, your local environment contributes a lot to this process as well. A lot of experimenting is necessary.
Boy I'll agree with Reed on that. Where you live has huge amounts to do with how the wood dries and that's probably why there are so many different techniques. What works for me might not work for you and vice/versa. If we all had ideal storage areas it might be different but most of us are forced to store the wood in less than ideal situations and consequently simply have to experiment and find what works best for us.
Yes indeed humidity is our friend!
I agree with Steve that defects in the wood are generally the catalyst for cracking, but there are some types of wood that just love to crack at the slightest provocation (madrone mentioned by Reed).
As you say, good tool technique results in a smooth surface regardless of its role in cracking. FWIW, let's consider a worst case scenario of what may happen as a result of really bad tool technique where a bowl gouge may be used more like a digging instrument and the resulting damaged surface that bears some resemblance to a furrowed field.
I believe that the gist of the above paragraph has to do with shrinkage vs. grain orientation and wood thickness. I agree, but I think that it overlooks the effect of surface discontinuities on internal stresses that are created during the shrinking process.
The internal structure of the wood plays a significant role in wood movement, but there are other external factors that can create "stress risers" AKA "stress concentrations". These terms refer to the concentration of high stresses acting on relatively small regions due to discontinuities in the shape of the surface.
Thinking about the "bad tool technique" mentioned above, these discontinuities include rough edges on corners and openings, cracks, grooves, transitions with sharp corners, surface irregularities and other such things. These stress concentrations occur because the strain (movement) is different on each side of a discontinuity. This sort of failure is self propagating because the resulting material yielding weakens it and increases the magnitude of the stress concentration. When combined with the right internal grain pattern the chance of a crack developing increases over the chance of a smooth surface developing a crack.
Stresses can be due to either internal or external forces or both. It doesn't matter if the material is homogeneous or not -- the difference is that the nature of stress induced cracking is more likely to be analytically predictable in homogeneous materials.
In the real world, will this make a difference? Maybe, but it is mostly a moot question because we use good tool techniques and take various measures to minimize the chance of cracking.
I haven't either, but then I haven't turned many bowls out of crack prone wood and besides that I seldom twice turn things.
At the all day demo at our club last Saturday, Mike Mahoney said that he turns a large number of bowls and since he started rounding the rims a few years ago, he said that he saw a significant enough reduction in cracked bowls to continue rounding the edges.
And I agree with you that rounding the edge of the rim is a good idea even if sharp edge cracking has not been a problem. It only took one close encounter of the bowl kind with my fingers to convince me that rounded rims are the way to go. More recently, I dropped a mesquite bowl with a knife edge rim. That edge didn't fare too well on the rough concrete street. Made me say naughty words.
You always do begin again unbalanced, because the average 1/4" diameter difference between the cross and long grain is going to be greater than anything beyond leaving the outside absolutely unturned. I bandsaw to better than that. I also re-turn stuff that has holes in it or wings on it with no additional problem that I can detect. You must have some OTHER kind of wood, or be in some kind of super hurry not to turn the full surface.
That said, there were times when I didn't turn to round when roughing. Had an old Sears drillpress with 7 1/2" from center of bit to unmovable tube, and a lathe which would swing 15 3/4 (400mm). I just cut a 1/8 - 1/4" max depth flat on one endgrain portion to push against the tube to bore center. Since normal shrink pulled sides in and down about that distance, I never missed it.
If you think it is easier to return rough surfaced dried bowls have at it.
I find returning smooth surfaced dry bowls easier than returning rough surfaced dried bowls.
It also allows removing less material.
So I make my last passes with a bevel riding cut that leaves a smooth surface.
You are most welcome to keep on turning rough surfaced bowls if that is what you prefer.
If you balance the rim the for returning the bowl is not unbalanced just out of round.
Two high spots equidistance from the head stock,, two low spots equidistance from the headstock.
Easy to do with the bowl Jamed on a an open chuck.
Your post in the bottom doesn't lend itself to balancing which is why you have unbalance returns.
I made this statement in the my first post and you seemed to have gotten all wrapped up in refuting it.
Again if you want to recommend leaving rough surfaces fine, I think it bad advice.
Rounding over the rims does make a difference. Dale Larson does it with long cylinders, I do it on all my bowls. Now, as to why it works, that I haven't really figured out yet. It could be in part because the sharp corner will dry faster because it is so fine, where if you round it over, it keeps things closer to 'even wall thickness', so the stresses will balance better. Not really sure, but it does make a measurable difference in drying success. The same goes for wrapping the rims of my once turned bowls with plastic stretch film.