Shelf strength & thickness for 4ft adjustable bookcase span

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blueman wrote:

He's putting it on edge--the strong dimension. For this to work well you will have to have the depth of the grooves at pretty close tolerances.

Most any hardware store will have 3-ft lengths, some may have longer. Look in your phone book for steel suppliers in your area--some will deal w/ individuals (for a price, but not any more than the exorbitant piece-price at most retail outlets)....
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Actually, using epoxy to seat the steel should eliminate this concern, shouldn't it?
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Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

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alexy wrote:

Perhaps, but I don't think I'd count on it for long term...I suppose you could fill the groove sufficiently, but my experience is that epoxy and smooth metal surfaces tend to separate w/ age--or maybe I'm just unlucky.... :)
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Usually a 1/8 grove and a 1/8 strip of steel have a pretty good fit....if you can find cold rolled strip (McMaster Carr) it's exactly 1/8, drop in your epoxy and go.
We use this method for making the tables on the Radial arm saw. You could also use a sheet of 3/4 ply and rout a 1/2 inch deep slot in the bottom and glue in your strip, but you'll see the slots on the under side of the shelf.
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Rumpty

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Rumpty wrote:

...
It's the depth that's significant...
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The HD near me has them, with rather unique pricing: 3' for $3.27 or 4' for $2.17! Both seem high to me, but for only a couple of pieces, so what?
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Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

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I think I will go with this idea since it seems to meet my criteria best though it does entail additional work (but at least I get to play with my new DEWALT DW618B3 router :)
My plan is to do the following. 1. Use two strips 1/8" x 1/2" with each strip aligned with the depth of the shelf pin I assume that "two is better than one" and that aligning with the plane of the shelf pins should help transfer the weight directly from the shelf pins to the steel supported part of the shelf (Is this correct?)
Also, I assume that two strips spread out that way is better than one larger central strip of the same total mass.
2. I plan to build a template for routing the groove out of a similar 4ft x 12" piece of plywood with slots routed out. Then I will use a 1/8" straight cutting bit (is this right? I am new to the world of routers)
3. One other thought I had. Would I be better off adding an additional 3rd row of shelf pins down the center. i.e. even thought they are 1/4" and have glued-in sleeves is the support itself marginal?
Please let me know whether I am barking up the right tree now. Thanks
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Seems to me that 1/8" is exactly the width of the kerf of a regular kerf table saw blade. Big table. Nice fence. Excellent depth control. Good chip collection.
Use your new router. You told you wife you needed it, right? ;-)
Patriarch
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Absolutely -- how else do you save money by buying toys (I mean tools)?
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<snip>

I NEVER tell her I'm saving money with a new tool. She would wonder which aliens took over my body.
I tell her that the new tool will do the job exactly as it should be done, and with considerably greater safety than the other methods.
She is far less fearful of running out of money than she is of having to be my primary caregiver. I'm a real PITA when I'm hurting. Or so I'm told.
I never notice it.
Patriarch
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Shelf design is a matter of how much deflection is acceptable (some deflection is inevitable). The previous suggestion of making a simple mockup is very sensible. If I had to guess, I would say that the two half inch plywood shelves will have deflection noticable from across the room......but that is only a guess. Dave
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Andy Rae's book, The Complete Illustrated Guide to Furniture & Cabinet Construction (from Taunton) has a good listing of span limits for various materials. I doube your 1 inch plywood sandwich will be enough. You might add hardwood edges that will not only cover the rough plywood but add some stiffening.
RVH
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Probably strong enough, but probably not stiff enough. I.e., the shelves probably will bend (objectionably, I'd guess) but not break.

I like Rumpty's idea of the imbedded steel stiffener. But other possibilities might be worth mentioning.

To add stiffness, You'd have to make it thicker, which adds to the visual "weight", even if not to the mass.
A lip on the back, maybe both above and below the shelf to allow for reversing as longer term sag occurs, would stiffen the shelf (though not as much as front and back lips).

Ask yourself (then answer realistically) how important the movability of shelves is. Most are never moved, once set up to the owner's liking. If that is the case, work with the side pins until you have the shelves spaced like you want them, then attach cleats to the back and sides under each shelf, and attach the shelves to the cleats.
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alexy wrote: ...

Actually, your idea suggests to me that if the shelves are to be heavily loaded, the visibility of support pin holes in the back may not be such a big issue, particularly if only the ones very close to the actual height are drilled out instead of all possible. Then, a ledger strip could be made which fit into those holes which would add quite a bit of rigidity...
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blueman wrote:

why 3/4" for the back? that will ad a lot of weight to the cabinet without any real benefit. the back is mostly there to resist racking forces (keeping it from becoming a parallelogram). 1/4" ply is more than sufficient for that, although it will feel a bit hollow at 48" wide. 1/2" backs will take care of that and make the case a lot easier to handle.

consider preloading the shelves when you glue them up. that is, glue them so that they have a crown or upward curve equal to the amount that they will sag once they are loaded.

4' is a pretty long span for plywood shelves. given that you are laminating 2 layers you will likely want to cover the edge anyway, so make the edges wider than the shelf thickness by another inch or so. it'll help avoid sag.

some testing is in order.

if you determine that a preloaded shelf laminated from 2 layers of 1/2" BB is insufficient, a torsion box shelf or center pins may be in order. if you go torsion box, consider 1/2" for the top layer, a 3/4" web and 1/4" for the bottom.
another approach would be to make the shelves fixed. then you could rabbet the shelves into the sides and pin it solidly to the back. you lose the adjustability, but you gain a lot of strength.

the pins are plenty strong. where you may have failure is the wood carcase that the pins are supported by. it's a lot of load on a 1/4" wide section of wood. once the load reaches the crush point of the wood fibers the hole elongates, the pin starts to shift, reducing the area it has in contact with the wood, increasing the point load and failure quickly follows.

the sleeves increase the load area, and if you glue them in they help resist shifting.
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Well, it is a built-in, so I don't care much about moving it (other than the first time). Plus, since cost of 3/4" is not very different from a 1/4" sheet, I thought that the added stiffness to the back and lack of hollowness would be worth it.

Assuming that you are talking about 3 sheets of plywood sandwiched as above, does a torsion box like that really work in the sense of being stronger than just two 3/4" sandwiched together (both have the same total width of 1.5")

Agreed, but I like the adjustability :)

thanks
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the solid shelf will be slightly stronger. the torsion box shelf I was picturing involves dimensional 1x as the interior webbing. the advantages of this approach are weight, cost, ease of assembling a prestressed panel.
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Yes, use the sagulator to check this out. It can tell you if 1" ply deflects more or less than various solid materials.
Other ideas.
1. You could use a stronger attachment at the ends. Have you ever seen the old cabinet technique where you have a notched piece of molding at the front and backs of the sides (kind of looks like dentil) and you have a loose piece of wood say 3/4x3/4x12 that spans across a selected set of notches and the shelf sits on that? Of course this assumes you have a face frame to hide the ends of the shelves.
2. You can add a beam under the shelf. Say a 1" wide piece of 4/4 hard maple turned on edge half way back. Pretty un-noticable but adds a huge amount of strength.
3. If you use solid material you can go thicker but make it "look" thinner by using an edge treatment. I like to use a panel raising bit (without a back cutter) or a table edge bit (like a thumbnail). If I use the pane bit I then round over the underside front edge too.
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another option is to rebate the back of the shelves into the backing of the bookcase, you will lose the adjustability but you say you will be using the bookcase for textbooks - a known height. rebate 1/8 of an inch 1nto say 1/2 inch ply backing? you should gain a lot of strength and stiffness
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Well, I was using the term "textbook" pretty loosely plus suprisingly my textbooks do vary a fair bit in height :)
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