Replies

1. 
   JeffHeath | Jan 23, 2008 05:26am | #1

   George

   This should help.

   http://www.woodbin.com/calcs/sagulator.htm

   This site will allow you to calculate your sag based on species, span, etc....

   Jeff

2. 
   WoodShopGuy | Jan 23, 2008 07:44am | #2

   A common and cheap support enhancer I usually use is 1/8” by 1” steel flats vertically inserted into 1/2"+ by 1/8”+ grooves cut in each of the 3/4" sheets of ply along their lengths.

   If the final shelf length is 40” long, I would cut the flats to a length of 38” and then center. Usually two steel supports, each about an inch away from the front and rear edges are adequate. This allows an inch of clearance for your final cuts-to-length-and-width after glue-up, and biscuits or whatever you want to use to attach the shelf to the vertical members, edge banding and so on. You could easily add a third or fourth steel support toward the center of the shelf if you wanted.

   I usually use a pallet knife to trowel in three or four equally spaced dabs of slow-setting two-part epoxy in the grooves before using wood glue to lam the two shelf sheets together.

   Normally you would think a thermosetting glue like plastic resin (urea formaldehyde) would be necessary but the steel supports add so much strength that a thermoplastic type (yellow) glue is what I usually use cause it is sooo much easier to fool with -- a small roller to apply and a bag press or lots of clamps to ensure uniform gluing across the surface with a little squeeze out along the edges.

   The only downside is that these steel flats come in 20 foot lengths and are covered in surface rust if they are stored outside. So when you go to pick it up from the iron merchant, bring a set of bolt cutters or a hacksaw, some gloves, and some towels and such to protect your hands and vehicle. Even at today’s prices, you are probably looking at a ten dollar purchase, or less!

   Cheers!

   Bob

1. 
   saschafer | Jan 23, 2008 08:22am | #3

   You can get reasonably-sized pieces (8' lengths) of steel bar stock, angles, etc. from Lowe's and Home Depot.

   -Steve

    

2. 
   WillGeorge | Jan 23, 2008 03:57pm | #4

   I have used 1/2 inch steel 'L' (45 degree) in some very long shelves. My son-in-law is a Dr. of Education with a huge collection of child care and other books. It's hard to believe the weight of a shelf full.3/4 inch ply shelves with two steels emended (equal spaced across width) from underneath. I cut a groove with a trench/housing to recess the steel flush (not seen). Yellow glue. No sagging of the shelves at all.Note: I forgot the exact size of steel 'L' I used but gives the idea.(equal spaced across width).. equal centered/spaced?

   Edited 1/23/2008 7:59 am by WillGeorge

3. 
   Mackwood | Jan 23, 2008 07:24pm | #5

   George,

   I built a bookcase 48" wide with 3/4" plywood shelves.  I used the hardwood edge you referred to; 1 1/2" wide and 3/4" thick on all four edges of the shelves.  They have supported very heavy loads (set of encyclopedia for example) for over 20 years with no visible sag.  I would recommend this method.

   Regards,

   Mack

   "WISH IN ONE HAND, S--T IN THE OTHER AND SEE WHICH FILLS UP FIRST"

1. 
   Buzzsaw | Jan 23, 2008 08:47pm | #6

   I have another suggestion. I built a bookshelf not too long ago and, at the suggestion of another woodworker, created each shelf by sandwiching two pieces of plywood separated by strips of wood for spacers. I installed cleats on each side of the cabinet the same width as the spacers and there is just enough room on each side of the plywood sandwich where the cleats would fit into. Then a hardwood front is added to the sandwich and you have the most solid shelf you could ever conceive of. I have my stereo on the shelves and there is no give whatsoever. If you need more information I may be able to find a diagram what this fellow woodworker sent to me. There is a name for this type of shelf but the name escapes me right now.One question for you. What software did you use to create that diagram?By the way, here is the bookshelf that I built. The faces are just over 2 1/4" thick (3/4" for each plywood piece on each side and the spacers were also 3/4" thick).
   Regards,

   Buzzsaw

   "Don't ask yourself what the world needs. Ask yourself what makes you come alive, and go do that, because what the world needs is people who have come alive." Gil Bailie

   
1. 
   georgep | Jan 23, 2008 10:21pm | #7

   Sounds like you built a torsion box.

   The program is Google's Sketchup, the base software is available for free at http://www.sketchup.com/  .  It's a great tool that can do some really complex designs.  I only know enough to do the equivalent of stick figures.

   Lots of information and options to think about.  Thanks everyone.  These shelves will not carry a lot of weight, maybe, at the most 20 lbs. each.  Given your experience, if I put 3/4 x 3/4 cherry on the front of the shelf and top and 3/4 x 1/34 on the back (shelf stock is 3/4 hardwood ply), would that be sufficient?  I prefer to overbuild it but I'd like to keep the 3/4 shelf.

   One thing I have no idea about is how a 45 degree angle on each end of the shelf affects the saq.  Since the front edge is about 38" and the back about 54 inches I'm hoping that the actual span, for saq purposes, is somewher in between.

   George