Replies

1. 
   davamoore | May 15, 2003 04:26pm | #1

   No, you don't have to allow for wood movement with manufactured products such as MDF and plywood.  Based on the description of your project, movement shouldn't be a problem.

   David

   "The world that was not made is not won by what is done" -- Mundaka Upanishad

2. 
   JohnWW | May 15, 2003 05:42pm | #2

   I'm going to disagree with the first answer and suggest that you err on the side of caution.

   Plywood because of it's cross grain construction shows very little change when it's moisture content changes.  MDF actually shows quite a bit of dimensional change when it gains or loses moisture, I know this from experience with carefully made precision jigs that shift with the weather. 

   If your top only has a single layer of veneer on each face, so that there is no cross grain, the top will expand and contract perpendicular to the grain of the veneer and the top should be treated as though it were solid wood and mounted to allow for it's expansion and contraction.

   John W.

   
   

   ---

   Edited 5/15/2003 2:25:23 PM ET by JohnW

1. 
   davamoore | May 15, 2003 06:08pm | #3

   John, since I don't use MDF that often, I will bow to your experience.  But I have to confess that, on the few pieces for which I've used MDF as a veneer substrate, instead of plywood, with edge treatments and finishing I haven't noticed any movement.  Of course, they're only a couple of years old, so time might tell.  Even though I know that no finish fully prevents expansion and contraction, only reduces it, do you leave your jigs unfinished?  If so, could that account for some of the noticeable movement?

   I also have a question regarding the possible expansion/contraction of the veneer.  Given the thinness of the veneer, is it possible that the expansion might be held in check somewhat depending on the type of glue that was used?

   For example, standard yellow glue is known for its creep, so if the veneer is going to move I would imagine that the glue would do little to prevent that process.  On the other hand, urea resin and some other glues are quite rigid once they've cured -- wouldn't they have the strength to prevent the wood movement, given the thinness of most veneers?  And if not, would you end up seeing fine cracks or creases in the veneer itself, rather than gaps at the edges or connection points?  In other words, which is stronger:  the veneer, the glue or the substrate?  (I'm not sure if the veneer used by the original poster is sawn by him or not, and I am guessing that thickness would have something to do with this.)

   Thanks in advance for your thoughts.  Always learning...

   David"The world that was not made is not won by what is done" -- Mundaka Upanishad

1. 
   JerseyJohn | May 20, 2003 05:18pm | #8

   Quote

   ""I also have a question regarding the possible expansion/contraction of the veneer.  Given the thinness of the veneer, is it possible that the expansion might be held in check somewhat depending on the type of glue that was used?""

   Absolutely.  There are three things that you can do to minimize and / or eliminate creep  / movement

   1. Use a urea adhesive.

   2. Pretreat the veneer with a veneer softening agent.  This removes the post press tension in the veneer by making the cells pliable.  Therefore, when pressed in a particular shape or a flat, the veneer doesn;t want to go back to it's natural shape.  Especially when using burl, crotch veneers and those with similar characteristics

   3. Keep the core and veneer moisture contents as close together as possible.

   Regards,John Ersing

   Veneer Systems

1. 
   davamoore | May 20, 2003 05:43pm | #9

   Hey John, thanks for the additional information.  So far, I'm following points #1 and #3.  Regarding the veneer softening agent you mention in #2, would that be the same type of formula recommended by Lincoln and others to flatten warped veneer before use?

   David"The world that was not made is not won by what is done" -- Mundaka Upanishad

2. 
   forestgirl | May 15, 2003 06:48pm | #4

   Quote from one of the MDF manufacturers:  "Expansion joints of 2mm per 1500mm [are] therefore recommended."  Seems like your jigs are moving excessively.  forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl   ;-)

1. 
   JohnWW | May 15, 2003 10:38pm | #5

   Did some searching, even water resistant MDF expands and contracts with changes in relative humidity, about one quarter of one percent when the humidity in the air goes from 50% to 80% relative humidity.  This would produce a change of about 1/16 inch across a 24 inch wide panel.

   In a climate with the wider humidity swings typical of New England, a change of 1/8 inch in 24 inches, or more, is possible between winter and summer humidities.  Non water resistant MDF will move even more.

   In their technical literature, Sierra Pine which makes Medex and Medite brand MDF panels recommends that panels sit in the shop several days to stabilize their moisture content before they are used. 

   A couple of years ago I spoke with a designer at a company that makes the laminated tops for router tables. I recall he said they were very careful to control the relative humidity in the plant to prevent problems with dimensional changes in the MDF cores of the table tops.

   My most recent experience with MDF responding to humidity changes was with a shop made router table.  The drop in panel, approximately 9 by 12 inches in size, fit nicely in its routed cutout in the summer when the table was made but was jammed tightly in the opening by December after the air in the shop dried out.  The insert had to be pried out and the opening recut before it would fit back in.  The shop, by the way, is climate controlled, air conditioned in the summer and humidified in the winter so the relative humidity change wasn't as great as it might have been. 

   From my own experience I know that MDF moves a bit with changes in relative humidity, not as much as solid wood, but I would design a piece of furniture to accommodate that movement just to be on the safe side.

   John W. 

1. 
   davamoore | May 15, 2003 11:25pm | #6

   John, thanks for taking the time to respond so thoroughly.  Thinking on what you just wrote, I should probably point out that I have used MDF as a veneer substrate for smaller pieces, only.  I've always used plywood as the substrate for larger pieces, e.g. larger tabletops.  Perhaps that's why I haven't noticed the effects of humidity on the MDF-based pieces.

   David"The world that was not made is not won by what is done" -- Mundaka Upanishad

2. 
   forestgirl | May 15, 2003 11:29pm | #7

   Thanks for the heads-up John.  I haven't done any veneering yet, but will certainly take that info seriously when the time comes!forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl   ;-)