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Glue_ Grain_ and Joints


  • pg 1
									          Reprinted with permission.
          American Association of Woodturners

       Glue, Grain, and Joints

It Together
                                          By Jim Rodgers

J   oinery is one of the great
    challenges in woodworking,
    whether you’re building cabinets
or a segmented vessel. Although
                                          4. Improving the quality of the
                                          fastened surfaces.
                                          5. Selecting the best glue types for
                                          the application.
                                                                                   relative density of the species being
                                                                                   joined so that the turning task is
                                                                                   more manageable. This is important
                                                                                   because moving our tool from soft to
cabinetmakers can employ a wide             Our solutions lie in:                  hard wood while turning may cause
variety of worked joints as well as       • Minimizing the differential in         tool control and tear-out issues.
hardware, we woodturners have             movement between wood surfaces
more limited choices. As a result,        being joined.                            How wood moves
turners need to pay careful attention     • Increasing the joined surface area     There are a number of possible varia-
to all of the details to achieve joints   to improve the joint strength.           tions of grain alignment between
that are tight and long-lasting.          • Using adhesives suitable to the        joined pieces. The strengths of those
                                          specific task.                           you’re likely to use are listed in the
5 key considerations                      • Wise species selection.                sidebar “Wood Orientation and Joint
Material movement and joint                                                        Strength,” opposite, prepared with the
strength are the two interrelated         Special challenges for                   technical assistance of Dr. Roman
concerns. The more the wood               segmented turners                        Rabiej, a scientist formerly at Frank-
moves, the greater the problem; if        In segmented turning, woodturners        lin International (manufacturer of
neighboring pieces move in different      usually deal with more different         Titebond glues) and now a
directions, it becomes even more of       species of wood in a single project      professor at Western Michigan
a problem.                                than other turners do. In designing      University. But before diving into the
  Here are five key challenges:           a project, then, we must consider the    three most common configurations,
1. Joining woods that move at             relative rates of movement of the ad-    let’s briefly review wood movement.
different rates.                          jacent species. If each wood species        Wood moves in three dimensions:
2. Using joinery to aid fastening.        is moving at approximately the same      radial, longitudinal and axial (some-
3. Joining woods that move in             rate, the joint will be less stressed.   times called tangential). The rates
different directions.                        We also need to consider the          of movement differ dramatically in

                                          36                                               American Woodturner Fall 2008
            each dimension and from species                                          Radial grain to axial grain            • Always seal all exposed surfaces
            to species. Typically, longitudinal                                      While the two presentations of         to reduce the movement due to
            movement (along the grain) is near                                       the wood are moving at different       moisture gain or loss.
            zero, but axial movement can be as                                       rates, the relative movement to each   • Work with species that have
            much as 8 percent and radial move-                                       other is all that is important. The    minimal movements.
              Metal      Brass
            ment about half of that.                                                 concern here is that even though       • Assure that wood grain is aligned

                                                                                     no end grain is involved, the move-    in the same plane before cutting the
                                                                                     ment between the two faces can         segments for a ring.
                                                                                     still be substantial. Large glue-ups   • Use glue with elasticity (creep)
                                                                                     (even single species) may fail over    such as polyvinyl acetate (PVA)
                Pine           Walnut                     Cherry

                                                                                     time due to continued flexing if the   glues. More about that later.
            n g e n ti al )

                                                                                     species’ axial and radial movements
                                                                                                                            End grain to radial or

                MDF                 MDO                Hardboard
                                                                                     differ greatly. Solutions include:
                                                                                     • Be sure that the surfaces to be      axial grain
                  ( Ta

                                                                                     glued are as flat as possible, thus    This glue joint has a significant
                                                     Treated                         increasing the bonding surface area.   problem because the side-grain
                                                                                                                            timber is moving in two planes at
                                                                                                                            different rates. Additionally, the end
                                                                                                                            grain adsorbs the glue into the long
                                                                                                                            cell structure of the timber,
                       Wood Orientation and Joint Strength                                                                  reducing the bonding strength.
                                                                                                                               Cabinetmakers address this issue
                                                   A      N
                                                                                                                            by choosing joints that present
                                                                                                     1 Radial to radial
                                                   B      O                                                                 stronger grain orientation and
                                                   C      P                                                                 sometimes even add mechani-
                                                   D      Q                                                                 cal strength as well. For example,
   Cherry                                                                                                                   box joints increase the side-grain
                                                   E      R
                              2 Radial to axial                    3 Cross radial to axial      4 Cross radial to radial    surface area, reducing reliance
                                                   F                                                                        on the end-grain bond. Although
                                                   G      T                                                                 dovetail joints do the same thing,
                                                   H      U                                                                 this joinery introduces mechanical
                                                          V                                                                 strength. Panel doors, tabletops,
                                                                                                                            and other tongue-and-groove
                                                   J      W
                                                                                                                            connections eliminate the glue in
                                                   K                                                                        troublesome areas and allow the
                              5 End to axial       L               6 End to radial             7 End to end                 larger wood surface to “float.”
                                                          Z                                                                    Neck rings, disc feet, and other
                       Wood Orientation M                                                      Glue Joint Strength
                                                                                                                            cross-grain segmented design ele-
                       1 Radial grain to radial grain (flat sawn to flat sawn)                            100%              ments create situations similar to
                       2 Radial grain to axial grain (flat sawn to flat sawn)                             65–70%            those faced by cabinetmakers, but
                       3 Cross radial grain to axial grain                                                50%               turners have fewer possible solu-
   O                   4 Cross radial grain to radial grain                                               50%               tions. Here are a few suggestions:
   P                   5 End grain to axial grain                                                         15–20%            • Preseal the end grain with thinned
                                                                                                                            glue (1:1 mixed with water). Allow
   Q                   6 End grain to radial grain                                                        15%
                                                                                                                            the glue to dry and then reglue the
   R                   7 End grain to end grain (sized joint, see text for details)                       30%
                                                                                                                            joint as usual. The initial sizing coat
   S                   Dr. Roman Rabiej, professor of Engineering and Applied Sciences at Western Michigan                  will close the open cell structure,
                       University, provided technical information for this chart on the relative strength of various        thus increasing the glue surface
                       orientations of glued wood joints. Dr. Rabiej is a recognized expert on wood technology.             area. The joint will be a glue-to-glue
   U                                                                                                                                                            Filename: G
                                                                                                                            joint (joining glue rather than wood).
                                                                                                                                                                R LeMoine
   V                                                                                 Filename: Grain blocks
                                                                                     R LeMoine                                                              9-16
   W                                                                                 9-16
   X        woodturner.org                                                                                                                                    37
Analysis of a simple                                                                       Is your glue
ring-segmented vessel                                                                      good to go?
                                                                                           To assist in building the strongest
All segments are arranged with horizontal running grain. This means that each ring
                                                                                           joints in vessels, our glue must also
was glued up end grain to end grain. Each ring was brick laid to add strength by gluing
                                                                                           be in prime condition. Glue degrades
side grain to side grain. All joints were overlapped to add more strength.
                                                                                           over time, and poor storage conditions
                                                                                           shorten that period even further.
                                                                                               According to Franklin International,
                                                      Veneer added here was made           Titebond guarantees its polyvinyl
                                                      from a single sheet of veneer,       acetate (PVA) glues for a one-year shelf
                                                      cut into a circle and glued          life because the company does not
                                                      between the rings. At only two       know and cannot control the conditions
                                                      side locations is the grain of the   under which the glue is stored.
                                                      veneer running parallel with the
                                                      grain of the adjoining rings.

                                                      The veneer acts with the
                                                      adjacent woods and not as a

                                                                                                                                  Photo: John Hetherington
                                                      separate wood element.

                                                      Spacers in the feature ring were
                                                      too wide (1⁄ 8") to act as veneers      However, Titebond II can enjoy
                                                      and were cut so that their grain     a shelf life of up to four years when
                                                      runs parallel with the stabilized/   stored under optimal conditions
                                                      dyed box elder burl feature          (moderate temperature and dim
                                                      wood.                                lighting). Other Titebond products have
                                                                                           shelf lives of two to three years.
                                                                                              Titebond products can stand up
                                                      The vessel was finished both         to six freeze/thaw cycles with some
                                                      inside and out to further reduce     decrease of strength with each cycle.
                                                      uneven expansion/                    After being frozen, the glue may need
                                                      contraction possibilities.           to be stirred to homogenize it. If the
                                                                                           glue is stringy, however, discard it.
                                                                                           Similar degrading problems occur with
                         The solid wood foot attached with a veneer spacer layer           extreme heat cycles. If your shop area
                         has both parallel-grain and cross-grain attachments to the        isn’t climate controlled, store your glue
                         first ring. Good surface preparation, gluing, and its small       in a living area of your home.
                         size (under 4") minimizes problems.                                  To determine the manufacturing date
                                                                                           of your Titebond product, check the
                                                                                           date code stamped on each bottle. The
                                                                                           date code can be found in the batch
This vessel is 10×8". The base is black                                                    stamp on each bottle indicating the
acacia with a body of redheart. The fea-                                                   year (number) followed by the month
ture ring is built from stabilized and dyed                                                (alpha code) of manufacture. The
box elder burl and yellowheart banded                                                      additional digits encode other batch
on either side with a narrow ebony ring.                                                   information.
The neck is ebony.                                                                                —Jim Rodgers

                                                38                                                American Woodturner Fall 2008
                                          down with acetone, naphtha, or          4. Wipe down oily exotic woods
                                          other solvent to remove accumulated     before applying glue.
                                          extractives, such as terpenes and       5. Never align glue joints in adjacent
                                          oils, immediately before gluing.        layers. Always overlap to add more
                                          • Crosscuts should be made with         strength to the joint.
                                          a sharp finish-cut blade. A 10"- dia.
                                          blade should have 60 or more teeth.     Final thoughts
                                          • Wood should be held firmly            Here are some general consider-
• Consider increasing the glue area       while cutting to eliminate acciden-     ations that will also aid in reducing
by using a tongue-and-groove              tal movement and vibrations that        the number of potential joint failures
construction for a foot or neck           would reduce the cut surface quality.   in your next segmented project:
ring. In The Art of Segmented             • Burrs and surface tear-out should     • When using a solid hardwood as a
Woodturning, Malcolm Tibbetts             be sanded away to prevent capture       bowl foot, keep the size small so that
suggests a great bowl foot idea that      in the glue joint.                      it is less affected by movement. Less
is exactly like the cabinetmaker’s        • The face of the cut should also       than 5" in diameter is best.
floating panel. As shown above, a         be sanded to improve the surface        • Use burl woods that have totally
rabbet captures the “foot” between        quality, especially on soft (western    dried to ambient conditions. (This
the first and second base rings of        soft maples) or open-grain woods        will vary depending on your
a vessel. The base is cut slightly        (walnut, wenge) as well as brittle      climate.) Burls are stable and don’t
smaller than the rabbet and allowed       species (purpleheart).                  move much because of their inter-
to float.                                                                         locked grain structure.
• Avoid this construction.                Choosing appropriate glues              • When inserting a veneer between
                                          Some glues permit movement and          layers of a vessel, maintain clamping
End grain to end grain                    some don’t. Other adhesives expand      pressure until the joints are com-
These are the weakest possible            and fill while many don’t.              pletely dry. Veneers tend to absorb
joints. The amount of surface area           For example, when cyanoacrylate      moisture from glue and will swell
to be glued is minimal and the glue       (CA) glue is catalyzed, it becomes a    and crinkle, causing the joint to
is wicked away from the available         solid acrylic material incapable of     open if unclamped prematurely.
surface by capillary action.              moving. Thus, CA glue can’t expand      • Other than appearance, the move-
These are your options:                   or contract with the wood move-         ment in cross-grain veneers is not a
• Don’t do it.                            ment. Therefore it is not appropriate   concern. The thinness of the wood
• Overlap an end-grain joint with         for segmented turning.                  when bonded to a solid wood causes
another side-grain joint (brick-laid         Polyvinyl acetate (PVA) glues        it to move with the wood to which
joinery), thus providing more face-       (white glues) and modified PVAs         it is attached (not as a separate
grain strength.                           (Titebond I & II are two examples)      element).
• Size the joint first with a 1:1 solu-   creep and will allow for some rela-     • All turnings should be sealed
tion of glue and water; allow it to       tively slow movement in the joint.      inside and outside to minimize
dry and then glue. This will create a     Some epoxy glues are also formulat-     uneven movement due to uneven
joint where the sizing seals the end      ed to allow constant movement and       absorption of moisture.
grain and creates a glue-friendly         twisting (West System epoxy and
surface, although the bond is             System 3 are two examples).             Jim Rodgers (jlrodgers@aol.com) has
actually glue-to-glue.                                                            written several journal articles about seg-
                                                                                  mented turning. He is a former president
                                          5 strategies for segmented
                                                                                  of the Bay Area Woodturners and lives in
Joint quality                             woodturners                             Martinez, CA.
Wood that is being glued must be          Here are five key guidelines to help
flat and clean, free of inhibiting        ensure gluing success:
burrs and debris. A poorly prepared       1. Use PVA glues.
joint may fail when all else is fine.     2. Prepare joints that are clear of
• Oily species should be glued im-        obstructions.
mediately after cutting or wiped          3. Increase the glue area.

woodturner.org                                                                                                            39

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