Replies

1. 
   Sgian_Dubh | Jan 31, 2000 01:20am | #1

   *
   I don't think this all that unusual, and it's a tough one to solve. It also happens with mortice and tenons as well as other joints when they are finished flush. Mitred joints often open up a little too, so it seems you've dodged the bullet on that one so far.

   One possibility that occurs is that the two halves of the joint are moving out of unison because the end grain of each piece is different. For example, you may have a piece that is rift sawn mitred to a piece that is tangentially (flat) sawn. If the pieces are both tangentially sawn, then the concavities of the end grain might be reversed. You get the idea I'm sure. Even though the differential movement is small, it's enough to cause the effects you can both see and feel.
   *Part of the solution might be to pay careful attention to this detail at construction, perhaps using one length of timber to get all the foreedges or lipping out of, cutting and reassembling in the order it came apart, and ensuring that the same face always goes up. I hope I made sense there!
   *Next, if you don't already use them, a tight fitting biscuit at each corner might help, but I'm wary of recommending a glue like epoxy to go with it. The creep that PVA exhibits might actually be of benefit.
   *You might consider reducing the size of your walnut to less than an inch, but this will exclude the use of a biscuit, and also won't prevent wee steps forming between the solid timber amd veneered main body, which in all likelihood has happened on your existing examples.
   *Lastly, you might try and fool the eye and fingers altogether by introducing something like a wee chamfer on the top and bottom face of each mitre, so that if they do move a bo'hair, who's going to notice?

   I can't offer any absolute guarantee that any of these strategies will work, and the last two may be aesthetically unacceptable to you and your customer, but maybe something I've said might help. I'm sure there will be something else that I haven't thought of off the top of my head that might be of assistance.

1. 
   AD | Jul 01, 2000 07:07pm | #2

   *YOU MIGHT LOOK AT THE DIRECTION OF THE GRAIN OF YOYR BORDER. DO THE GROWTH RINGS PARALLEL THE TOP OF THE TABLE OR ARE THEY PERPENDICULAR? IF THEY PARALLEL IT THE EXPANSION DUE TO HUMIDITY COULD CAUSE THE RISE. WOOD IS MORE DIMENSIONALLY STABLE IN THE DIRECTION PERPENDICULAR TO THE GROWTH RINGS. ONE FINAL AREA IS THAT SHELLAC AND VARNISH ARE NOT GOOD SEALANTS WITH RESPECT TO WATER/HUMIDITY. HOPE THIS HELPS.

1. 
   Sergio_Piery_II | Sep 22, 2000 06:35am | #3

   *Sgain is right on the money, and so is AD. Another point that I may bring up is the MDF is essentially thick cardboard(with some stabilizers). It also will absorb moisture and can affect the piece as it does move as its moisture changes. If you look at the MDF edge-on, you will notice that the large surfaces are of different density than the core. At the long joints your border has more contact with the high density part of the MDF. At the miters, you have a proportionately larger contact with the softer core where you are more likely to encounter dimensional change do to moisture. The MDF also will be more likely to retain moisture at the core, and if you are not using a moisture proof adhesive, it could degrade the strength of the bond. You might consider sealing the edges of the MDF prior to assembly.Good luck, Sergio.

1. 
   Ray_Smith | Oct 30, 2000 08:08am | #4

   *Wood is going to move all the time there is noting you can do about it. But what I did notice is that you only finished one side of the trim. Try sealing the trim entirely. It might help control some of the swelling.

1. 
   Scott_Braun | Nov 09, 2000 04:08pm | #5

   *everything I've read here is right on, but here's something else to consider:since we can't control wood movement, we can try to live with it without saying, "there's nothing you can do." whenever I have a seam where I want parts to stay flush (especially if there is a clear top and bottom), I move my joinery (or add additional joinery) towards the edge I want flush. this way, any uneven movement will be towards the bottom edge. In this case, maybe using a 1/4" spline in your miter, but with an 1/8" shoulder on top and 3/8" on the bottom could be some help. what it means to you is that you are only asking 1/8" of wood to stay stable, rather than twice that or possibly much more (you don't mention what your joinery was inside the miters). between that and better alignment of grain, you should gain a great advantage. good luck...by the way, anyone reading this be advised that this is NOT something to take to an extreme and still expect a stable joint. If you make a mortise and tenon joint with a 1/4" shoulder on one side and a 2" shoulder on the other side, that isn't what we're after!