Replies

1. 
   mlbfreestyle | Dec 06, 2003 09:50pm | #1

   I don't have the specific values on hand, but I do know the principle.  Wood will move a certain percentage of width with any given change in humidity.  If the total width of the piece of wood is not that great, then the total change in dimension will also be relatively small.  You might want to do a web search on "shrinkulator".  There you will find a calculator that will estimate wood shrinkage based on species and change in MC.

   I suppose from there it is an enginnering judgment call as to how much change in dimension any peice can handle.  My guess is that by the nature of the box you are talking about, that there would be little room for dimensional change; thus splitting would occur.

2. 
   PlaneWood | Dec 06, 2003 11:31pm | #2

   I used to know a guy that made high dollar pistol cases.  He made the sides out of one piece, with the grain running around the sides.  The tops and bottoms were soaked in PEG before they were applied.  PEG stops the expansion/contraction.  The tops and bottoms were generally about 1/4" thick and 12"+/- wide.

   There are 3 different expansion rates.  Up and down the board (i.e. - up and down the tree).  This rate is the smallest and is almost negligible.  Then next is perpendicular to the center line (heart) of the tree.  I think this is called Longitudinal.  Or, perpendicular to the grain when looking at the end of a board.  The 3rd is Tangential, or between the growth rings (i.e. around the tree). 

   Longitudinal expansion/contraction is less than Tangential.  Thats why quarter sawn lumber is more stable.

   Also a board that has a mix of grain orientations will have a tendancy to warp or bow because of the unequal stresses.

   I have an expansion rate table around here some where.

   PlaneWood by Mike_in_Katy (maker of fine sawdust!)
   PlaneWood

3. 
   aksophie | Dec 07, 2003 12:39am | #3

   No, but it may split if you had made it in an area of high humidity and then move it to one of low humidity.

4. 
   YesMaam27577 | Dec 07, 2003 03:41am | #4

   It may crack, but it depends on a lot of things.

   Are all four sides going to have horizontal grain orientation? All four vertical? Or will there be a mix. If a mix, then cracks may eventually show.

   Next comes how you will attach the bottom of the box. If the bottom floats in a groove, then no cracks. If you glue the bottom to all four sides, then the bottom may crack, and/or two of the sides may also.

   As noted in another reply, wood expands/shrinks across its grain -- boards will get wider, but not longer, and not appreciably thicker. (In this case, 'across the grain' means tangiential to the growth rings.)

   So, if you glue two pieces with parallel grain, then no problems. If you glue them "cross grain", then one will try to shrink/expand, and the other will try to prevent it -- not a good thing. The board that "wins" in one direction will crack in the other -- and vice versa.

   If you're really worried, and really numbers-oriented, go to http://www.woodbin.com/calcs/shrinkulator.htm

   You can use those numbers to determine actual shrinkage and expansion, in terms of real dimensions.

   Vast projects should not be founded on half vast ideas.
5. 
   JohnWW | Dec 07, 2003 04:31am | #5

   Mike,

   Your questions certainly aren't stupid, and more people should ask them before they start making furniture.

   Since, out of necessity, some of the sides on a six sided box must be cross grain to the others, there will be cross grain stress on two of the boards.  Whether or not the stress would be enough to crack the boards or the joints would depend on a number of secondary details, but the stress will definitely be there.  

   When the humidity goes higher, the corner joints will be stressed by the cross grain sides trying to expand.  If the humidity drops, the cross grain panels will possibly crack as they shrink. 

   Finishes will slow the movement of moisture in and out of the wood, but they are powerless to prevent it from happening.  An unfinished side might dry out in a week, while a finished panel might take up to a month to lose the same amount of moisture and develop the same amount of stress. 

   Being that moisture swings in most environments go on for months, dry winters and damp summers are typical here in New England, eventually finished wood will gain or lose just as much moisture as a unfinished wood and will develop the same stresses if the furniture isn't designed to accomodate the movement.

   John W.

   
   

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   Edited 12/6/2003 8:34:37 PM ET by JohnW
   

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   Edited 12/7/2003 11:19:35 AM ET by JohnW

   
   

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   Edited 12/7/2003 11:28:16 AM ET by JohnW

1. 
   Strongbow | Dec 07, 2003 03:01pm | #7

   Now I am struggling to get my head round this moisture thing.

   Why dont we see lots of complaints from people buying furniture that have developed splits?

   for example..a piece of work is made in a commercial workshop, with the humidity at say average for the area. This is shipped to an area with a very different humidity, or goes to a store which has vastly different humidity than the manufacturers shop.

   It stands there for 12 months then I come along and buy the article, and take it home to stand in my home with the heating turned up all year round.......but no cracks appear, or warpage........how do they do that??? and if it was a problem there would be lots of returned furniture.

   and does it ever worry anyone out there who is making small production runs for craft fairs etc  that the stuff they sell might be sat in someones home twisted and split????

   So is the relivance of taking into account shrinkage realy necessary when building stuff?????....

   Mike

1. 
   JohnWW | Dec 07, 2003 06:07pm | #8

   Mike,

   The reason that the majority of furniture doesn't crack or warp is that the better cabinetmakers chose materials and techniques that reduce movement and/or allow the wood to move freely to accommodate expansion and contraction. 

   Wood never "just cracks" after it has been made into a piece of furniture, the failure is always due to poor design or technique.  Although there are many ways to accommodate the natural movement of wood, there is nothing mystical about it and no luck is involved in making a piece that doesn't crack.  To me one of the hallmarks of an accomplished furniture maker is their skill at dealing with wood movement.

   In the case of your hypothetical box, four of the sides should be assembled end grain to end grain at the corners which wouldn't create any stress because all four sides would be moving in unison.  The top and the bottom of the box could either be floating panels that sit in grooves so they would have freedom to change in size, or the the top and bottom could be plywood which doesn't expand and contract appreciably.

   Hope this helps, John W.

   Edited 12/7/2003 11:29:20 AM ET by JohnW

1. 
   Strongbow | Dec 08, 2003 01:10am | #9

   Thanks for the help....

   I will remember the advice..and now I can sleep well at night.

   On second thoughts I better not discuss this with the wife because when the bed collapses I can just tell her it was a design fault not a humidity problem.............

   mind you she will just say it was due to my weight problem....

   Miike

2. 
   JohnWW | Dec 15, 2003 03:44am | #10

   About a week ago, I answered some questions you had about wood movement, I just ran into an excellent magazine article on how cabinetmakers deal with movement by using proper design.  The article is in Fine Woodworking, issue #165, October 2003.

   John W.

6. 
   robkelly1 | Dec 07, 2003 04:58am | #6

   I just sold a 6x6x6 mahogany box with a ship's compass in it , built in about 1910, and the top had shrunk 1/8", enough to buckle the brass edging.  The wood was about 3/8" thick. It was from Germany. It lived in a heated house for the last 30 years which may have had something to do with the shrinkage.