Some of you sent me private messages or posted comments asking about the kind of woodturning I like to do. My first reply was a video with 20 minutes summary of how I turn a goblet (the blog entry containing the video is here: http://lumberjocks.com/jocks/alindobra/blog/2719 ). While the video is spectacular, it helps little in understanding how YOU can learn how to turn. The explanations in the video are scant and my elbow covers a lot of the action. This blog series is designed to expand the shallow explanations and add more observations/knowledge.
Throughout this blog series, I will provide my experience turning green wood. While I might make references to various sources (books, web resources, etc.), most of the thinks I will talk about are my experience. Since I have been turning for less than 2 years and seriously for only 1 year, you have to understand that the opinions I express are not definitive. I am still refining my methods and there are a lot of things I do not yet know or I know only partially. You should use this blog series as an invitation to experiment yourself and, through comments, to share your experience. If you find the series interesting, please drop an occasional comment—it will keep me motivated. Questions also help a lot (thanks TechGuy for your batch of comments) it figuring out what to talk about.
Motivation for turning green wood
1. The biggest nuisance I have to deal with in my shop (I’m sure most of you will agree) is dust. While most machinery come with decent dust collection nowadays, lathes are an exception. Most dust collection setups for a lathe are improvisations that will help a little but not reduce the dust to an acceptable level. While fancy face masks that filter the air are available on the market, I have no desire to work with an “astronaut helmet” on my head for several hours. This is motivation enough for me to turn green wood.
2. Green wood is free. Yes, free. In my town, they cut so many trees every day that they have trouble getting rid of them fast enough. Even more, you have to pay to dispose of the wood; when asked if I can take a piece of the tree they are cutting, I am offered the whole tree delivered to my house if I want. It never happened to me to be refused as much wood as I want when I asked. Usually, most people would pay a reasonable fee to get the “good stuff”. When turning, at least for the first 3-6 months, you want the wood to be absolutely free since you will be reticent to use it as practice if you payed significant money on it. Since in this case only lots of practice makes perfect you do not want to be distracted by the fact that you payed for the wood so you’d better get something out of it.
3. The resulting turnings look more natural. I usually like to keep the bark on my pieces, and definitely to keep the sapwood. That is jut not possible with dried wood (the bark never stays on on thick pieces of wood, even with care). In this day an age when we have CNC lathes, it is hard to make something that a machine cannot. Making thin bowls from green wood is still outside the realm of production woodturning. Not being told by friends and family that they saw a turning like yours at Walmart is priceless.
4. While turning green wood, the tool stays cooler. That means less trips to the grinder and less opportunities for burned fingers.
5. Turning green wood is spectacular. Larger shavings can be taken and they literally fly off the lathe. Most of my friends are utterly impressed at the site (and I get a wholly mess in my shop).
Grain orientation in green wood turning
When turning wood in general, the rotation axis is either perpendicular on the direction of the grain, called side grain turning, or along the grain, called end grain turning. A lot of things are different in these two types of turning, things that become serious considerations when a piece is designed:
1. Behavior of the cutting tool. On side grain turnings, in a single rotation the tool moves from a cut along the grain to a cross cut and back. This introduces a small non-uniformity in the behavior of the tool. For end grain turnings, the cut is more uniform but not necessarily better. Depending how the tool is presented to the wood, a cut against end grain might be attempted, cut that can result in a lot of frustration since the effort of holding the tool is significantly greater than a cut that does not go against end grain.
2. Rules and knowhow. There are a lot of good rules for side grain turning and a lot of good rules for spindle turning. The most important rule of all is to cut with the bevel when using a gouge. While technically the wood turns in the same way for spindle turning and end grain turning, spindle turning consists mostly in decorative elements rather then large concavities. While cutting with the bevel always works for side grain and spindle turning, it might be awful, at least on some woods, for end grain turning. The problem is that going with the bevel might mean going directly into the end grain and producing mostly dust. The ability to use various techniques depends on the wood characteristics; you effectively have to learn how to turn each type of wood (i.e. to experiment a little before starting on your best piece). What all this means is that end grain turning is a lot more experimental then side grain and spindle turning.
3. Wood movement After turning the wood green, you have to let it dry. In the drying process, the wood moves, which produces deformations of the turning. Side grain and end grain turnings deform fundamentally differently. All side grain pieces will shrink a lot across the grain at not at all along it . Since the rim of the turning contains both side and end grain, the piece will take an oval shape. For this reason, the side grain pieces are first rough turned, dried (2 weeks to 6 months depending on method) and then finish turned to get the round shape. End grain turnings have only end grain, so the turning will tend to deform to accommodate the larger shrinking tangentially with respect to radial shrinking. This shrinking process, especially if a full section of the tree has been used, can produce spectacular bending of the wood. Take a look at this project: http://lumberjocks.com/projects/3799 to see an example of interesting bending due to wood shrinkage. While the oval shape for side grain is undesirable, the bending of end grain is, in my opinion, desirable. This means that an end grain piece can be turned in one sitting. If the wall thickness is 1/8” or less, the bowl will dry in 1-2 days and can be readily finished. This means 2 days from start to finish rather than almost 6 months for side grain.
4. Containment of the pith. The second rule of woodturning, after cutting with the bevel, is not to include the pith. The reason for this is the fact that the piece will almost certainly develop at least a small crack at the pith. This is happening since the wood around the pith shrinks differently than the rest of the wood (shrinks more). In side grain turning it is customary to avoid the pith. This means that the diameter of the wood being turned has to be more than twice the depth of the bowl. To get most of the grain pattern and interesting bending of end grain turning, the pith needs to be included (and prominently displayed). This inevitably leads to complications with the drying. What I noticed is that, if the wall thickness is below 1/4” everywhere, the effect of the crack originating at the pith is minimized to the point that it is not worth even filling. If the pith is included in the end grain piece, the wall thickness better be less than 1/4” anyway otherwise the drying stress is not dissipated by deforming the piece but by large cracks. Turning the bowl thick, then drying it is not a viable option for end grain unless the whole piece is cut from a piece far away from the pith (i.e. from a very large diameter tree). Moreover, the wall thickness better be uniform otherwise cracks will develop as well. Getting uniform thickness for a piece requires a lot of practice but the result is a better quality piece.
The four points above essentially suggest that end grain green wood turning is a nuisance. The main advantages though are very little wood preparation, ability to finish the piece in a couple o days and interesting looking pieces (end grain turning is relatively rare). Since was always drawn to hard things in my life, end grain turning is by far my favorite. This blog series will mostly be about end grain turning but some of the observations and techniques should hold for side grain turning as well.
Since this blog entry is getting rather large, I will continue in a separate entry with stock preparation, mounting the piece on the lathe and tool sharpening in the next episode.
The best resource on this subject is the book of Michael O’Donnell “Turning Green Wood”. I strongly recommend you acquire this book if you want to turn green wood.
Thanks for looking,
-- -- Alin Dobra, Gainesville, Florida