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#1
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Shelf strength & thickness for 4ft adjustable bookcase span
I am building a built-in bookcase with a 48" shelf span and want to
make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. - The sides and back of the bookcase are 3/4" cabinet-grade birch plywood. - Plan is to create 48"x12"x1" shelves by gluing together two sheets of 1/2" birch plywood. - Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes. I would like to avoid adding shelf-pins along the back because I don't want visible holes. Also, I would prefer not to split the span since I like the open look and don't want to have 2 narrow 2ft spans. Questions: 1. Is this construction strong enough to support a 4 ft span loaded with textbooks? (I have had 3 foot span bookcases constructed from 3/4" material and they seemed to hold) 2. Does anything else need to be done to stiffen the span? e.g., Thicker shelves? (I could use 3/4" to replace one or both of the 1/2" pieces but then the shelves get heavier and bulkier looking Torsion box construction? (I could sandwich a 1/2" layer between two 1/4" layers but this is added work and added opportunity to mess up Shelf pins along middle of back wall? (I really would like to avoid the visible line of holes down the middle) 3. Will the 1/4" shelf pins with sleeves be strong enough to hold the load? (e.g., should I use stronger or more shelf pins?) Also, do the sleeves do much to strengthen the holes or am I just as well off without them? |
#2
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blueman wrote:
I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. - The sides and back of the bookcase are 3/4" cabinet-grade birch plywood. - Plan is to create 48"x12"x1" shelves by gluing together two sheets of 1/2" birch plywood. - Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes. .... I personally think it will be marginal w/ ply. I'd make every attempt to find solid lumber to make the shelves from. It would take some clearance away, but you could use an edge on the shelves to gain some extra stiffness if you could live w/o having close margins between tops of books and next shelf. Overall, for a span over 3-ft, I'd really try to put in a vertical center support if I could. The 1/4" end pins are probably marginal and w/o the inserts I definitely think they will tend to round if a 4-ft shelf is loaded to the maximum possible. There are library systems that are more sturdy. Seems like Woodworker Supply had some??? Or was it Woodworkers' Hardware in MN??? I can't recall just now.... If it were me, I'd split it I think w/ single carcase, center support. |
#3
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Why don't you mock up a shelf with some scrap, load it up, and test various
designs? "blueman" wrote in message ... I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. - The sides and back of the bookcase are 3/4" cabinet-grade birch plywood. - Plan is to create 48"x12"x1" shelves by gluing together two sheets of 1/2" birch plywood. - Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes. I would like to avoid adding shelf-pins along the back because I don't want visible holes. Also, I would prefer not to split the span since I like the open look and don't want to have 2 narrow 2ft spans. Questions: 1. Is this construction strong enough to support a 4 ft span loaded with textbooks? (I have had 3 foot span bookcases constructed from 3/4" material and they seemed to hold) 2. Does anything else need to be done to stiffen the span? e.g., Thicker shelves? (I could use 3/4" to replace one or both of the 1/2" pieces but then the shelves get heavier and bulkier looking Torsion box construction? (I could sandwich a 1/2" layer between two 1/4" layers but this is added work and added opportunity to mess up Shelf pins along middle of back wall? (I really would like to avoid the visible line of holes down the middle) 3. Will the 1/4" shelf pins with sleeves be strong enough to hold the load? (e.g., should I use stronger or more shelf pins?) Also, do the sleeves do much to strengthen the holes or am I just as well off without them? |
#4
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See http://www.woodbin.com/calcs/sagulator.htm
Gut tells me that two 1/2" sheets of ply glued together would be much weaker than 1" thick ply. Mark |
#5
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You can offset the shelf sag and use 3/4 ply for the shelves if you wrap
them with a verticle grain face. Make a "frame" for the shelf out of 3/4" x 1 1/4" hardwood and rabbet in for the 3/4" plywood shelf. The shelves will appear more "massive" but will actually be lighter and more ridgid. Even if you stick to your 1/2" laminated idea any sort of verticle grain facing is going to add to rigidity. Knothead |
#6
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IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves
1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. -- Rumpty Radial Arm Saw Forum: http://forums.delphiforums.com/woodbutcher/start - - - - - - - - - - - - - - - - - - - - - "blueman" wrote in message ... I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. - The sides and back of the bookcase are 3/4" cabinet-grade birch plywood. - Plan is to create 48"x12"x1" shelves by gluing together two sheets of 1/2" birch plywood. - Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes. I would like to avoid adding shelf-pins along the back because I don't want visible holes. Also, I would prefer not to split the span since I like the open look and don't want to have 2 narrow 2ft spans. Questions: 1. Is this construction strong enough to support a 4 ft span loaded with textbooks? (I have had 3 foot span bookcases constructed from 3/4" material and they seemed to hold) 2. Does anything else need to be done to stiffen the span? e.g., Thicker shelves? (I could use 3/4" to replace one or both of the 1/2" pieces but then the shelves get heavier and bulkier looking Torsion box construction? (I could sandwich a 1/2" layer between two 1/4" layers but this is added work and added opportunity to mess up Shelf pins along middle of back wall? (I really would like to avoid the visible line of holes down the middle) 3. Will the 1/4" shelf pins with sleeves be strong enough to hold the load? (e.g., should I use stronger or more shelf pins?) Also, do the sleeves do much to strengthen the holes or am I just as well off without them? |
#7
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[snip]
- Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes. [snip] Are the shelved attached to the case? If not, you could finish both sides and flip the shelves occasionally. What bends one way will bend the other - that sort of thing. I do this with closet top shelves of 1" pine that sag over time. When flipped they straighten out and eventually sag the other way. Josie |
#8
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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 |
#9
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"Rumpty" writes:
IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. -- Rumpty I like this idea... Curious though why you pick 1/8" wide by 1/2" deep rather than the other way -- i.e. rather than a wider narrow strip. Also, where is a good place to get steel strips like that (I have seen steel rod at the borgs but not strip) |
#10
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"Mark Wells" writes:
See http://www.woodbin.com/calcs/sagulator.htm Gut tells me that two 1/2" sheets of ply glued together would be much weaker than 1" thick ply. Mark Excuse my ignorance, but why would it be weaker? |
#11
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Duane Bozarth writes:
blueman wrote: I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. - The sides and back of the bookcase are 3/4" cabinet-grade birch plywood. - Plan is to create 48"x12"x1" shelves by gluing together two sheets of 1/2" birch plywood. - Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes. ... I personally think it will be marginal w/ ply. I'd make every attempt to find solid lumber to make the shelves from. What type of solid wood (without knotw) would you recommend that I could buy in 12" width, 1" thickness, 4 (or 8) ft length without breaking the bank? I don't have a jointer so I would prefer not to have glue up a panel using narrower stock. |
#12
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blueman wrote:
..... What type of solid wood (without knotw) would you recommend that I could buy in 12" width, 1" thickness, 4 (or 8) ft length without breaking the bank? I don't have a jointer so I would prefer not to have glue up a panel using narrower stock. I'd assumed since you were building a sizable project you had stuff w/ which to do it... |
#13
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blueman wrote:
"Rumpty" writes: IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. -- Rumpty I like this idea... Curious though why you pick 1/8" wide by 1/2" deep rather than the other way -- i.e. rather than a wider narrow strip. 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. Also, where is a good place to get steel strips like that (I have seen steel rod at the borgs but not strip) 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).... |
#14
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alexy wrote:
Duane Bozarth wrote: blueman wrote: "Rumpty" writes: IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. 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. Actually, using epoxy to seat the steel should eliminate this concern, shouldn't it? 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.... |
#15
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Duane Bozarth wrote:
blueman wrote: "Rumpty" writes: IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. 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. Actually, using epoxy to seat the steel should eliminate this concern, shouldn't it? -- Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently. |
#16
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Mark Wells wrote: See http://www.woodbin.com/calcs/sagulator.htm Gut tells me that two 1/2" sheets of ply glued together would be much weaker than 1" thick ply. Mark I've read that a sheet of 3/4 ply is stronger than two 1/2 sheets glued together. I can't prove it (lost the references), but it makes sense. To the original poster. If it was me and I was dead set against having a center support and it had to be 48" long.. I would use 3/4 ply, and put a 1.5" strip of hardwood on the front. If you attach the shelves to your 3/4 back, that will help too. Another option is to just make the bookcase 32" wide. 3/4 ply (particularly with a 1.5" edge) would definitely handle that span. |
#17
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Just to clarify on my other post.. when I say to add a 1.5 inch of
hardwood to the front of the 3/4" shelf, I mean that the top 3/4" of the edging will be fastened to the shelf, and the bottom 3/4" of the edging will hang down. This helps resist sagging. After you glue the hardwood to the ply, you can put a 1/4" roundover on the on the hardwood edging. I do this because I think it looks nice. |
#18
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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 |
#19
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blueman wrote: I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. - The sides and back of the bookcase are 3/4" cabinet-grade birch plywood. - Plan is to create 48"x12"x1" shelves by gluing together two sheets of 1/2" birch plywood. - Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes. I would like to avoid adding shelf-pins along the back because I don't want visible holes. Also, I would prefer not to split the span since I like the open look and don't want to have 2 narrow 2ft spans. Questions: 1. Is this construction strong enough to support a 4 ft span loaded with textbooks? (I have had 3 foot span bookcases constructed from 3/4" material and they seemed to hold) Probably strong enough, but probably not stiff enough. I.e., the shelves probably will bend (objectionably, I'd guess) but not break. 2. Does anything else need to be done to stiffen the span? I like Rumpty's idea of the imbedded steel stiffener. But other possibilities might be worth mentioning. Thicker shelves? (I could use 3/4" to replace one or both of the 1/2" pieces but then the shelves get heavier and bulkier looking Torsion box construction? (I could sandwich a 1/2" layer between two 1/4" layers but this is added work and added opportunity to mess up 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). Shelf pins along middle of back wall? (I really would like to avoid the visible line of holes down the middle) 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. -- Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently. |
#20
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alexy wrote:
.... 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. 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... |
#21
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blueman wrote: I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. - The sides and back of the bookcase are 3/4" cabinet-grade birch plywood. 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. - Plan is to create 48"x12"x1" shelves by gluing together two sheets of 1/2" birch plywood. 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. - Shelves will be held in-place by a pair of 1/4" shelf pins at each end. I plan to use sleeves to reinforce the shelf-pin holes. I would like to avoid adding shelf-pins along the back because I don't want visible holes. Also, I would prefer not to split the span since I like the open look and don't want to have 2 narrow 2ft spans. 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. Questions: 1. Is this construction strong enough to support a 4 ft span loaded with textbooks? (I have had 3 foot span bookcases constructed from 3/4" material and they seemed to hold) some testing is in order. 2. Does anything else need to be done to stiffen the span? e.g., Thicker shelves? (I could use 3/4" to replace one or both of the 1/2" pieces but then the shelves get heavier and bulkier looking Torsion box construction? (I could sandwich a 1/2" layer between two 1/4" layers but this is added work and added opportunity to mess up Shelf pins along middle of back wall? (I really would like to avoid the visible line of holes down the middle) 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. 3. Will the 1/4" shelf pins with sleeves be strong enough to hold the load? (e.g., should I use stronger or more shelf pins?) 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. Also, do the sleeves do much to strengthen the holes or am I just as well off without them? the sleeves increase the load area, and if you glue them in they help resist shifting. |
#22
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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. |
#23
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"bf" wrote in message oups.com... Just to clarify on my other post.. when I say to add a 1.5 inch of hardwood to the front of the 3/4" shelf, I mean that the top 3/4" of the edging will be fastened to the shelf, and the bottom 3/4" of the edging will hang down. This helps resist sagging. After you glue the hardwood to the ply, you can put a 1/4" roundover on the on the hardwood edging. I do this because I think it looks nice. I like the suggestion to add a 1x2 (3/4x1-1/2) piece on edge to the shelf front and I think it's a great way to go because: 1. It covers the plwood edge of the shelf. 2. It adds quite a bit of stiffening. The part I don't like is that it stiffens the shelf in an asymmetrical way, therefore as the shelf is loaded, you may encounter twist, etc. It seems to me that if the books can clear a 1x2 on the shelf front (assuming you don't insert them horizontally and rotate), they can clear a 1x2 on the shelf back as well. The second 1x2 will increase the stiffness even more and the "C channel-like" shape will be loaded more uniformly. Opinions??? Bill Leonhardt |
#24
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Or even better yet...weigh a shelf full's worth of books. (ie, stack
textbooks until they're 48" high on the bathroom scale) That'll give you an accurate weight to plug into the sagulator. (http://www.woodbin.com/calcs/s agulator.htm ) |
#25
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adb wrote:
Or even better yet...weigh a shelf full's worth of books. (ie, stack textbooks until they're 48" high on the bathroom scale) That'll give you an accurate weight to plug into the sagulator. (http://www.woodbin.com/calcs/s agulator.htm ) But, as was discussed in a thread here not long ago, the "sagulator" is based on homogenous material and (to my surprise) the only plies that add significantly in ply are those w/ longitudinal grain--the others only add roughly 20% or so. So, one would have to look at the plywood construction in order to estimate the proper dimensions to plug in. Those would be only roughly 1/2 of the actual thickness so that the two-1/2" sheets would be equivalent to only roughly 3/4", maybe... I posted a link to the US Forestry Research site w/ a detailed paper -- a Google over the last week or so should find it. |
#26
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#27
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On Wed, 13 Jul 2005 15:49:08 -0400, Bill Leonhardt wrote:
"bf" wrote in message oups.com... Just to clarify on my other post.. when I say to add a 1.5 inch of hardwood to the front of the 3/4" shelf, I mean that the top 3/4" of the edging will be fastened to the shelf, and the bottom 3/4" of the edging will hang down. This helps resist sagging. After you glue the hardwood to the ply, you can put a 1/4" roundover on the on the hardwood edging. I do this because I think it looks nice. I like the suggestion to add a 1x2 (3/4x1-1/2) piece on edge to the shelf front and I think it's a great way to go because: 1. It covers the plwood edge of the shelf. 2. It adds quite a bit of stiffening. The part I don't like is that it stiffens the shelf in an asymmetrical way, therefore as the shelf is loaded, you may encounter twist, etc. It seems to me that if the books can clear a 1x2 on the shelf front (assuming you don't insert them horizontally and rotate), they can clear a 1x2 on the shelf back as well. The second 1x2 will increase the stiffness even more and the "C channel-like" shape will be loaded more uniformly. Opinions??? Bill Leonhardt Bill is right on the money here. SWMBO bought a particle board shelf with 40 inch wide shelves 5/8 inch thick. It was my job to make it work. I added a 1.5 inch front rail to all the shelves, then added a 1.5 inch stiffener both front and back on each shelf. With a 1.5 inch face frame all around the case painted up nice, the shelves all look good and you don't really notice the stiffeners. She loads this thing up with heavy notebooks and it deflects just a bit (maybe 1/4 inch). A good rail both front and back on each shelf and perhaps some kind of stiffener would work. There are also shelf pins that are inset into the shelf so they aren't seen. These could be used along the back. DGA |
#28
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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. -- Rumpty Radial Arm Saw Forum: http://forums.delphiforums.com/woodbutcher/start - - - - - - - - - - - - - - - - - - - - - "Duane Bozarth" wrote in message ... alexy wrote: Duane Bozarth wrote: blueman wrote: "Rumpty" writes: IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. 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. Actually, using epoxy to seat the steel should eliminate this concern, shouldn't it? 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.... |
#29
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Rumpty wrote:
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. .... It's the depth that's significant... |
#30
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Bill Leonhardt wrote: I like the suggestion to add a 1x2 (3/4x1-1/2) piece on edge to the shelf front and I think it's a great way to go because: 1. It covers the plwood edge of the shelf. 2. It adds quite a bit of stiffening. The part I don't like is that it stiffens the shelf in an asymmetrical way, therefore as the shelf is loaded, you may encounter twist, etc. It seems to me that if the books can clear a 1x2 on the shelf front (assuming you don't insert them horizontally and rotate), they can clear a 1x2 on the shelf back as well. The second 1x2 will increase the stiffness even more and the "C channel-like" shape will be loaded more uniformly. Opinions??? Bill Leonhardt Bill, since he was making the back out of 3/4 ply, I don't think a second stiffener on the back is necessary, if he attaches the shelves to the back. Of course, if he prefers adjustable shelves, I agree with your idea. |
#31
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bf wrote:
..... Bill, since he was making the back out of 3/4 ply, I don't think a second stiffener on the back is necessary, if he attaches the shelves to the back. Of course, if he prefers adjustable shelves, I agree with your idea. But, OP did specifical want adjustable shelves and was looking to avoid the holes in the back being obvious if he used pins there as well as the ends... |
#32
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blueman wrote:
"Rumpty" writes: IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. -- Rumpty I like this idea... Curious though why you pick 1/8" wide by 1/2" deep rather than the other way -- i.e. rather than a wider narrow strip. Also, where is a good place to get steel strips like that (I have seen steel rod at the borgs but not strip) 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? -- Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently. |
#33
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On Wed, 13 Jul 2005 00:02:47 GMT, blueman wrote:
I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. I would like to avoid adding shelf-pins along the back because I don't want visible holes. Also, I would prefer not to split the span since I like the open look and don't want to have 2 narrow 2ft spans. Questions: 1. Is this construction strong enough to support a 4 ft span loaded with textbooks? We built a library for an attorney who wanted his bookcases as large as possible with adjustable shelves. Our limit, because of sheet goods, was 4' wide with shelves basically 46 1/2". We told him that there was a possibility the shelves could sag but that we would try to make them as strong as we could. We ended up gluing 2 layers of 3/4" oak veneer core plywood together then put a 1 1/2" x 3/4" routed oak face on both the front and the back of the shelves. We told him that if the shelves did sag he could just turn them over. The bookcases are full of big ass law books, been there for ten years and he's never turned them over. Mike O. |
#34
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Duane Bozarth writes:
blueman wrote: .... What type of solid wood (without knotw) would you recommend that I could buy in 12" width, 1" thickness, 4 (or 8) ft length without breaking the bank? I don't have a jointer so I would prefer not to have glue up a panel using narrower stock. I'd assumed since you were building a sizable project you had stuff w/ which to do it... My wife already complains about my tool budget... |
#35
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"bridger" writes:
blueman wrote: I am building a built-in bookcase with a 48" shelf span and want to make sure that I am designing the shelving system properly to support the span with minimal deflection. The shelves need to hold heavy textbooks. - The sides and back of the bookcase are 3/4" cabinet-grade birch plywood. 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. 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. 2. Does anything else need to be done to stiffen the span? e.g., Thicker shelves? (I could use 3/4" to replace one or both of the 1/2" pieces but then the shelves get heavier and bulkier looking Torsion box construction? (I could sandwich a 1/2" layer between two 1/4" layers but this is added work and added opportunity to mess up Shelf pins along middle of back wall? (I really would like to avoid the visible line of holes down the middle) 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. 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") 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. Agreed, but I like the adjustability 3. Will the 1/4" shelf pins with sleeves be strong enough to hold the load? (e.g., should I use stronger or more shelf pins?) 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. Also, do the sleeves do much to strengthen the holes or am I just as well off without them? the sleeves increase the load area, and if you glue them in they help resist shifting. thanks |
#36
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Roland writes:
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 Well, I was using the term "textbook" pretty loosely plus suprisingly my textbooks do vary a fair bit in height |
#37
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"Rumpty" writes:
IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. 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 |
#38
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blueman wrote in :
"Rumpty" writes: IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. 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 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 |
#39
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Patriarch writes:
blueman wrote in : "Rumpty" writes: IF you want to use the two 1/2" plys, rout out matching 1/8" wide groves 1/4" deep in each ply sheet, epoxy in a 1/8' X 1/2" steel strip into the groves and glue up. This won't sag. 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 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? ;-) Absolutely -- how else do you save money by buying toys (I mean tools)? |
#40
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On Fri, 15 Jul 2005 05:30:38 GMT, blueman wrote:
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. 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") 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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