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[Scott Peterson]

Hey guys,

  I have been working on some Domino Cubes and have learned quite a lot about glue joints by asking around to try to strengthen some of the weaker joints in this particular puzzle.  I got most of the info below from John, Robert, and Tom Lensch.  I am merely summarizing so it is all in one place we can refer to later if needed.  Maybe this is all old news...

  The troublesome joints in the Domino Cube are the ones where one domino butts into the side of another - forming an end grain to side grain joint.  This is also a small glue joint, only 3/8" x 3/4" for the dominoes I have been using.  Taking into account the suggestions of the others, I have started producing acceptably strong joints.  If you have a similar problem, you might try some of the observations form below - set up somewhat like a checklist.

1.  Make sure the pieces fit snugly together without any gaps (even tiny gaps).  Sounds easy, but when I got rolling, there was a tiny internal gap that I didn't see - and this also happened to be an end-side grain joint.  We all know the PVA (polyvinylacetate - yellow glue) glues don't work well to fill a gap.

2.  Suggestion from Robert:  On the end grain especially and maybe also on the side grain, cut small incisions in a criss-cross pattern.  This allows the glue to get down in the end grain and better bond the joint.  Of course end grain on some woods can draw in the glue, so a second application may be needed right before you put the pieces together.  Tom suggested rubbing the pieces together, then taking them apart, applying glue to the end grain the second time, then putting them in position to set.
 
3.  If possible, leave a couple pieces unglued at the troublesome joints for the first round of glue-up.  This may allow the puzzle to come apart without force the first time - which may give an opportunity to make sure everything slides easily (adapted suggestion from Tom).  If there are pieces that fit too tightly, these could be sanded down a little (if you plan it right) before the final glue-up. 

  I tend to make cubes very tight fitting - a habit I formed from making Convolution and Involute puzzles where all of the joints are very visible on the outside (not obscured by cmamfers, etc).  I usually do this using a square corner jig to build the pile of pieces into a cube with glue in all the right places, then clamping it tight from all 3 directions.  This turned out to be my own nemisis on the Domino Cube - since the pieces fit so tight that I almost always break the weak joints the first time I try to take the puzzle apart.

4.  Suggestion from John:  After the final glue-up and all the joints survive the first disassembly, I have started to take a little bit of thin CA (cyanoacrylate - "super glue" - but use a good quality CA) and get it to seep down into the joint to reinforce the PVA glue joint.  If there is a tiny gap left, this may assist in strengthening the joint since the PVA will be weak with a gap.

5.  If all that still fails - clean all the glue off the joint and try a 2-part epoxy on the failed joint.  Try to expose clean wood fibers if possible, and maybe cut new incisions before applying the epoxy.

  I have read other techniques including the following

Microwaving
Making small sockets inside the joint and filling with thick CA or Gorilla glue

Gorilla glue foams during curing, which can glue the whole works together - so beware of that.

Any other ideas, please add!  I hope this might be useful to someone. 

Thanks to all for the help!

Scott

[Eric Fuller]

I dont know that this makes joints stronger, but Bart Buie taught me a neat trick for making a puzzle fit really well.  After it is done, wrap it in a bunch of rubber bands and nuke it in the microwave for 20 seconds.  Then pull it out and let cool.  The heat softens the glue making it pliable, and the rubber bands push anything out of alignment into alignment.

Word of caution: if you do it too long the resin and oils will literally boil out of the wood, and it isnt pretty.  Less is more.  Start out at 10 seconds and go up from there since some microwaves are stronger then others.

[Scott Peterson]

  Thanks for the addition with microwaving.  This does indeed work well - I used it for the hollow Pennyhedron puzzles I started out making.  I really practiced that one - I think I made about 80 of them before I decided I had enough.  I think this tip came to me originally from Tom Lensch or Stewart Coffin.  Those last Pennyhedrons came out so tight fitting it was REALLY difficult getting them apart the first time!

  If you want a little more pressure, you might also try an elastic cord (like a mini bungee cord, without hooked ends), which can be wrapped around and around - a little easier and faster than a bunch of rubber bands.  I even made a square corner jig for making cube puzzles (without any metal fasteners!) - which I tried for microwaving with the Domino cube puzzles.  Results were pretty good.

  Some glues cure with moisture (CA, Gorilla glue) - and in these cases, the microwave will likely strengthen the joint since the moisture and resins in the wood will be excited during nuking - which will help the glue cure and may draw the polymer chains (glue) into the wood more, early in the curing process.  I think the strongest point with microwaving is that thicker liquids become thinner when heated - which will allow better penetration into the wood fiber matrix.

  I just read yesterday that the PVA glues cure through evaporation of the solvent - which I think is mainly water.  Even this could be helped along by microwaving I suppose.

  Some of the engineered wood products (plywood, LVL - laminated veneer lumber, and glue-laminated beams) use a glue that is microwave cured in the factories.  I think they use higher end glues than are accessible to woodworkers (and more potent than what you would want to use in your shop) - so nuking glues does have a sound basis I believe.  Nevertheless, when I asked the question about microwaving to someone in technical support at Franklin (they make Titebond) - they had never heard of microwaving their glues.

Scott

[Eric Fuller]

Thanks for yur reply Scott...I forgot to mention that I got tired of the rubber bands really quick as well, and bought a large spool if 1/2" elastic on EBay for like $9.  Makes it a lot easier.

I may hit you up for advice on the Pennyhedron puzzles soon.  I've been wanting to make the Pennydoodle forever.

[Robert Yarger]

This is a portion of a recent e-mail response I gave to Scott.  You guys let me know what you think.

______

Scott,

   I noticed in the reponses going back and forth about gluing that special attention should be given to areas where glue pools because this indicates an area where the wood pieces do not touch flush.  I understand the concept, but was surprised because I never really had a problem with this.  I realize that the best cut you can possibly make with a table saw will still leave an microscopically uneven surface due to vibrations and the sort.  However, once that same piece goes through a sanding jig, the side has no choice to but to be perfectly level, (with the exception of pores in the wood).  Perhaps that is why I do not bother with expensive blades.  Everything I cut is roughly 1/100" oversized and goes through a sanding jig to produce the exact dimension in need, so a poor initial cut does not really matter for me in the end. It seems to me that with the time taken to glue a piece, take it apart to see where the side is not touching flushly, then addressing the situation, a person could have already sanded 3 or 4 parts with a perfectly even surface and fit using a sanding jig.  (By the way, all of my sanding jigs are designed for manual hand sanding, as opposed to mechanical means. - One stroke can take off 1/200" and sometimes that is all that you need and a machine set up will frequenly take too much.)