Segmented Vessel Base - Tahoe Turner

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the BASE-icsEditor’s Note. The following article is a collaboration by officersof Segmented Woodturners, a chapter of the AAW. It is anattempt at establishing guidelines and recommendations inthis important aspect of segmented woodturning.The tendency for wood to reactto humidity changes and, as aresult, change dimension hasalways challenged segmentedwoodturners. This is especiallytrue in the vessel base area.Segmenters have been discussingand experimenting for many yearswith the best method to constructtheir vessel bases. While there aremany ways to establish a baseon a segmented vessel, there hasnever been a consensus on whatconstitutes the best practices. Themost common base constructionsare a solid disc of wood, a pieshaped segmented disc, a pieshaped segmented disc with acentered plug, and a “floating”base. So what’s the best? First, afew questions:1. What is the effect of the baseon the integrity of the piece?2. Will a solid base disc create a5. Is a “floating” base worth theconstruction trouble?Creating a vessel base that islong-lasting is a challenge, andthe answers to these questions arevaried, depending upon your pointof view and personal experiences.While there have been studiesconcerning glue types and woodmovement, there are no formalstudies investigating the bestway to construct a segmentedvessel base. On the other hand,there have been thousands ofsegmented woodturnings, andtheir history helps us to formopinions. Here are a few thingsthat we know: When a solid disc is used forthe base of a stacked ringconstruction, there will be onlytwo areas of contact where thegrain of the bottom segmentedring and the base are incross-grain glue joint sufficientalignment. The remaining areasto create stress at the baseof contact will be comprisedarea of a vessel over time?of some amount of cross-grain3. How large does the basediameter have to be to causea problem?4. What constitutes the best lookjoinery. Wood fibers expand andcontract mostly perpendicular tothe direction of the wood grain. The wood in cross-grain gluefor the project–a solid base, ajoints will expand and contract insegmented disc, or no foot atdifferent directions, causing stressall?in the glue joint. The level of stress createdapr 2010managingthesegmentedvessel baseImage 13

depends on several factors:- The wood species’ coefficientof expansion.- The differences between twodifferent woods’ coefficient ofexpansions.- The environment in which thework resides and its changingconditions (humidity effects). We know that end-grain to sidegrain joints are weaker than sidegrain to side-grain joints. And we know that end-grain toend-grain joints are considered aweak type of joinery.Just because wood moves doesnot mean that vessel bases will fail,but it forces us to carefully considerour design options.Let’s discuss the various optionsmost commonly being used.A Solid Disc Style of BaseThis probably has been used byall of us at some time and withgeneral success as long as the solidbase is relatively small in diameter.A discussion at an AAW event afew years ago tried to establisha maximum diameter that couldbe used safely without creatingtoo much stress. An arbitraryagreement, among those inattendance, settled on 4” to 6” indiameter, smaller being recognizedas better. Even small sizes areaffected by the species of thewood being used and its dryness.Using very dry stable woodthroughout the turning will definitelyimprove the stability of the vesselbase.Jim Rodgers says, “I’ve neverexperienced a separation in myImage 2small transitional vessels (image 1)that are generally less than 6” indiameter. I use stable, dry woodfor the base discs, and I make surethat the glue joints are as flat aspossible. I use Titebond PVA gluethat allows for some tiny amount ofmovement without letting go.”Another way to reduce movementin solid bases, which we stronglyrecommend, is to use quarter-sawnstock for the base disc. Quartersawn timbers (image 2) havebeen used in furniture constructionfor generations, and this grainorientation is well-established tobe more stable and less subject tomajor expansion and contractionissues. Malcolm Tibbetts says,“As much as I prefer the use ofa floating base in items such as4segmentedwoodturners.org

segmented salad bowls, a floatingthat the wall thickness of the basebase is not practical. A solid discis not much larger than 1/4”or thisusing quarter-sawn boards is themethod will have the same failureway to go.”rate as using a solid block of wood.Bill Smith used a calculatorat woodworkerssource.com/I’ve had good success with thetechnique.” (image 3)movement.php when providingthis statement, “Using quarter-sawnwood for the solid base reducesPie-Shaped Segmented Discwood movement by 1/4 to 1/2At the 1st Segmenting Symposiumdepending on wood species. Inin 2008, the Instant Galley critiquethe Northeast, from summer towas particularly harsh on pie-winter, a 6” diameter quarter-sawnshaped disc bases. For starters,solid sugar maple base will movethe segment-to-segment jointsabout .014”. The same base, butare end-grain to end-grain, thewith flat-sawn wood, will moveweakest type of joint. Perfectabout .031” or a little more thanalignment of the points also is verytwice. Less stable woods will havedifficult. Misaligned points spoil theeven greater differences.”visual effect. But that’s only partEven with quarter-sawn wood,there is still some movement toaddress. Creating relatively thinvessel walls that can slightly “flex”partially addresses the challenge,and turning the base disc witha slight horizontal “hourglass”profile (thin in the center) also willaccommodate some movement.Curt Theobald says, “I always turna slight depression on both the topand bottom of my base discs.”of the problem. While the woodmovement of the pie-shapedImage 4disc generally matches the grainmovement direction of the ringabove it, the wood is confinedand not allowed to move withoutseparating a seam. If the woodexpands, it expands perpendicularto the direction of the grain. It isblocked from moving inward soit has to move outward. When itmoves outward, the circumference(diameter X 3.14) becomes longer,Because wood moves andbut the segments do not becomebecause the base disc maylonger. Something has to “give.”move slightly differently from thevessel walls, Bill Smith devised thistechnique many years ago. HeImage 3says, “Instead of a flat thin disc, Isometimes use a base disc about1” thick. Then during the turning, Ihollow the inside, creating abouta 1/4” (or less) wall. This allows fora little ‘flex’ at the intersection ofbase and vessel wall. It’s importantapr 20105

It is usually one of the glue jointsrecord, while other styles havedisc is the solution. I’ve never hadtoward the outside of the ring.a better track record. Why takea stress defect appear in my stave-Conversely, if the wood shrinksunnecessary risks?constructed vessel bases when(image 4), it’s blocked from movingusing the floating disc technique.inward, so the center area has toIt is also a great solution for basesmove outward, and usually a seamnear the center opens up.The Floating BaseThis is a variation on the floatingpanels used in door construction.Pie-Shaped SegmentedDisc with a PlugA disc is trapped between ringsin a small groove and allowed toon items such as stave-constructedtravel mugs. The floating base ismy answer for most all large bases– both stave-constructed andstacked ring constructions.”move without exerting pressureThe use of a plug in the center ofon the surrounding materiala segmented ring is perhaps the(image 5). This eliminates anymost controversial of all our optionsstress resulting from movementThe introduction of veneer layers– opinions vary. Tibbetts says, “Inbecause the disc diameter is cutbetween a solid base disc and amy early days of segmenting, Islightly smaller than the diameter ofsegmented ring can be beneficial.constructed many bases this way,the recess into which it is housed.Using several layers of veneerbut as my turnings experiencedThis technique has proven to bebetween a solid disc and themultiple seasonal humidityan especially successful solutionfirst segmented ring actually canchanges, I began to realize itwhen constructing vessels withcreate a bit of a shock absorbersimply was not the answer. Toolarger than average bases. (Forbetween the two elements. Themany (not all) bases developeda detailed explanation, see themultiple layers, using PVA glue cantiny defects as a result of wood“Floating Base” article in thisabsorb a tiny bit of movement, thusmovement. Even a few basesnewsletter.)reducing the stress between theUsing Veneersless than 3” in diameter beganbase disc and the first main bodyto show stress.” Most segmentersring. As a wood, veneers tend tohave come to accept that a largeplatter, constructed in this way,likely will experience some seriousfailures. Because wood moves as apercentage of its total dimension,it’s logical to think that the risk offailure will diminish as the overalldimension is reduced, but evenwood in tiny dimensions will stillmove. So what’s the verdict? Ingeneral, we do not endorse theuse of a “plugged” base ring,especially in larger diameters. Forthose turners who have had goodexperiences with plugs, our “failureto endorse” may seem extreme.The bottom line is that thesetypes of bases have a poor track6Stave ConstructionIn stave construction, vertical grainstaves intersect with a horizontalbe quite weak, but using plenty ofglue to fully penetrate the veneerfibers will help strengthen thevessel base, presenting a problem.Simply gluing a horizontal discof wood onto theImage 5staves is asking fortrouble. A large,vertical grain plugsometimes works (insmall sizes), but stillthere is risk. This isperhaps why manysegmenters avoidstave-constructedvessels. Tibbettssays, “The floatingsegmentedwoodturners.org

joinery. The addition of a few thina “shock absorber” between thecontrasting layers can add visualbase and vessel side by installinginterest. Perhaps more importantly,multiple thin veneer layers.they can improve long-term vessel Always thoroughly seal the vesselstability in the area between thewith appropriate finish. Wood invessel wall and the base.a well-sealed vessel will still move,photos ofthe weekbut that movement can beslowed significantly with a goodRecommendationsbarrier of finish.The officers of the AAW chapterSegmented Woodturners—As segmenters, we love wood, butMalcolm Tibbetts, Curt Theobald,we are forever challenged by itsBill Smith, and Jim Rodgers—wouldmovement properties. Properlylike to suggest the followingaddressing those movementgeneral guidelines:tendencies will keep glue joints Keep solid bases to a smalldiameter. Keep solid bases short or thin –tight for a very long time. No onewants to hear, “What happenedLuna IBill Hrnjak, Nipomo CAhere?” (image 6) about 3/8” or less (less mass tomove). When using a solid disc of woodat the base, try to use quartersawn boards. Avoid using a “plugged”segmented ring as a base. When possible, use a “floatingbase” on larger projects. Try to use only dry, kiln dried,stable woods with low coefficientVase with DiamondsRichard Thomas, Huntingtown MDof expansion. Woods such asmesquite, Gabon ebony, andwenge work well. Try to create a thin wall thicknessat the junction between thevessel base disc and thestacked rings on the vessel sides.Remember thatImage 6wood moves as apercentage of itsdimension. Lesswood equals lessmovement. Consider creatingapr 20107

this important aspect of segmented woodturning. the BASE-ics managing the segmented vessel base. 4 segmentedwoodturners.org Image 2 depends on several factors: - The wood species’ coefficient of expansion. - The difference

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