Replies

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

   If you expect the top to expand in the future, the tip of the button shouldn't reach to the bottom of the groove.  At the same time, the button needs to extend deep enough into the groove to handle any shrinkage without pulling completely out of the groove. 

   How much the top will expand or contract at various times of the year will depend on its current moisture level versus the extremes that it will be likely to reach.  I always presume that a piece of freestanding furniture will eventually be exposed both to very dry conditions and very humid ones during its lifetime and I make my calculations accordingly, figuring on a swing of 10 to 12% in moisture content.  Never presume that a finish will reduce the range of the wood's movement, it's wishful thinking.

   There is shrinkage calculator on line, I'm sure somebody will post its address, or you can buy a simple and inexpensive calculator from Lee Valley that I find very useful.  The Lee Valley calculator has a very good set of instructions on the various factors that affect the calculations.

   By the way, the only dimension you need to worry about when making the calculation for shrinkage or expansion is the distance between the buttons, not the total width of the top.

   Hope this helps, John W.

2. 
   BobSmalser | Dec 08, 2003 12:01am | #2

   Buttons and Drawbore M/T Joints

   Snug but not too tight and affixed with just one screw so's they can skew a bit if needed.

   If mounted like these to accommodate seasonal crossgrain movement of the top, then they can bottom out int heir groove w/o problem.

   “When we build, let us think that we build forever. Let it not be for present delight nor for present use alone. Let it be such work as our descendants will thank us for; and let us think...that a time is to come when those (heirlooms) will be held sacred because our hands have touched them, and that men will say, as they look upon the labor and wrought substance of them, ‘See! This our father did for us.’ “ --John Ruskin.

   
   

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   Edited 12/7/2003 4:03:40 PM ET by Bob Smalser

1. 
   15Broad | Dec 08, 2003 03:21pm | #3

   Thanks, Gents.  At this stage of the game any information helps.  Distance between the buttons is a factor?  How so?  I realize a 5x7 top would be floppy if only secured in the middle, so enough pieces must be installed to hold it securely, but does a top made from QS cherry (with a relatively low rate of movement) require the same number of buttons as a top made from flat sawn oak.  I'd think the number of buttons would remain fairly constant (ie;one every six inches or so) but the depth and width of lip and groove would vary depending on the materials involved.  Let me know if I'm at least on the right track.  Thanks again for the help.  Bob - Truly dig the message on your response.  Let us not build for today...  

1. 
   JohnWW | Dec 08, 2003 08:35pm | #4

   I'm sorry that I wasn't clearer in the comment about the distance between the buttons.

   When you calculate for the amount a table top will expand and contract, one of the factors in the equation is the width of the wood, the wider the board, the more movement there will be for a given amount of moisture change.  Once you know how much a table top will move, you can then make the depth of the grooves for the buttons deep enough to accomodate the movement.  What you are actually calculating is how much two buttons on opposite sides of the table are going to be moving as the top expands and contracts. 

   The point to my original comment was that when you do the calculation for movement, only the width of the wood between any two buttons on opposite sides of the table is used in the formula.  For instance, a table top might be four feet wide, but the buttons, being inside of the apron, will only be around three feet apart.  In this case, when you calculate the movement, the number you use for width in the formula would be three feet, the distance between the buttons, not the four foot width of the full top.  This may seem obvious, but I have seen mistakes made often enough that I felt obligated to point it out. 

   John W.

   Edited 12/8/2003 12:50:31 PM ET by JohnW

   Edited 12/8/2003 12:51:56 PM ET by JohnW

1. 
   15Broad | Dec 09, 2003 03:27am | #5

   Thanks for clearing that up, John.  You say two buttons on each end, do they not go along the sides also?  The wood there must also be allowed to move, or are slotted free holes in the apron the way to go?  I have a book on this, but haven't gotten to the table construction part yet.  Still reading about effective design.  If I may ask one other question, is it generally correct to say that a coarse grained wood will have a higher rate (and likelyhood) of expansion than a finer grained wood?  I've just finished a chapter dealing with the subject and want to make sure I've got the right idea.  I realize there are exceptions to every rule and many variables, but if I've a fine grained piece and a coarse grained piece in my living room, made of lumber at equal MC, the coarse piece will grow and shrink more than the fine one over the year... I hope.  Thanking you in advance.

1. 
   JohnWW | Dec 09, 2003 04:37am | #6

   You can place as many buttons along each edge of the table as you want.  It is the distance across the table from the row of buttons on one side to the row of buttons on the other side that I was pointing out is the critical dimension for calculating the movement of the wood, I didn't mean to imply there were only a pair of buttons on each edge.

   Off hand, placing a button every six inches along the edges is about right, though some might consider it overkill, but I like sturdy design.  The screws holding the buttons don't have a lot of penetration into the table top so the more of them the better.  In general, buttons aren't glued into place because it makes disassembly or repair in the future harder.

   Buttons are just as effective along the the ends of the table as they are along the long edges since there is movement to be accommodated and stress to be dealt with there also.  On the ends, the buttons will be sliding lengthwise in the slot instead of in and out of the groove as the top expands and contracts.  The buttons can be seated deep, but not bottomed out in the groove since lengthwise expansion of the table is minor. 

   To keep the top centered on the base, it is held by a pair of fixed screws run into the top, one screw on each short end of the table.  The simplest way to achieve this is to glue a block for the screw to the inside of the apron at the midpoint on each end apron and then run a screw through the block into the underside of the table top, making sure the top is centered before making the connection. 

   As for the coarse grain/fine grain question I don't know that I've ever seen it discussed, perhaps someone else knows the answer.

   John W. 

2. 
   15Broad | Dec 09, 2003 05:32am | #7

   Thanks again, John.  Still have about 8 million questions, but I'll read some more first to see if I can pare the list down a bit.  Stay safe.