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Woodwork 

Winter 2013-2014

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Floating Shelves

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Floating Shelves

Torsion-box construction creates sag-proof shelves that "defy" gravity.

By Tim Johnson


Some time ago, an Ace Hardware ad in American Woodworker sparked a surprising number of inquiries from readers. They all wanted to know how to build the cool-looking shelves that appeared in the background. We liked the shelves, too. Their contemporary design and invisible mounting created a dramatic effect.

The secret was torsion-box construction. A torsion box is a simple grid of slender ribs glued between thin plywood skins. It makes these shelves stiff and flat, yet incredibly light. This rigid architecture means torsion-box shelves won’t sag or twist, and they can be mounted without any external support. 

We’ve made these shelves easy to build and install by using simple shop-made jigs and dividing the process into four steps. We’ll make the torsion boxes, attach the face moldings, then glue the C-shaped units together and, finally, hang them on the wall. To create the wall of shelves shown here, you make four identical C-units and hang every other one upside down.

We wanted our shelves to be a uniform light color, so we chose hard maple instead of birch. The cost is the same, but maple lumber and plywood colors are easier to match. The Multiply brand of underlayment (available at home centers) makes great rib stock, because it’s inexpensive, stable and exactly 1/4 in. thick. We spent $220 to make our four C-unit shelves. 


Build the Torsion Boxes

1. Cut the torsion-box skins (Fig. A, E1, F1 and G1, below) from 1/4-in. maple plywood (see Cutting List, below). To get skins for all four C-units from two sheets of plywood, rip each sheet into five 9-1/2-in. x 8-ft. blanks. Cut eight of these blanks into 51-in. and 34-in. skins for the shelves. Cut the eight 18-1/4-in. skins for the uprights from the remaining two blanks.

2. Cut the sheets of 1/4-in. Muliply plywood underlayment into 7/8-in. x 48-in. ribs (A).

3. Plane poplar edging stock to the same 7/8-in. thickness as the ribs’ width. Cut the front and end edging pieces (C and D) to width and length.

4. Build the notch-cutting jig (Fig. B, below). 

5. Cut notches in the ribs (Photo 1). Butt the ribs against the indexing piece to cut the first notches. Use these notches to index the ribs so you can cut the next notches, and so on. By using a hold-down, you can cut notches in several ribs at once.

6. Cut some of the long ribs into short ribs that are consistent in length (B, Photo 2).

7. Assemble grid sections (E2, F2 and G2) by fitting the long and short ribs together. These sections don’t need to be glued; the half-lap joints hold them together. You’ll need two 48-in.-long grid sections for each C-unit. Use one full-length section for the lower shelf. Cut the other into a 31-in.-long section for the upper shelf and a 15-1/4-in.-long section for the upright. 

8. Build a pair of box beams (Fig. C, below).

9. Glue the torsion boxes, using the box beams  (Photo 3). We used Titebond Extend wood glue so we didn’t have to rush these complex glue-ups. Lay the edging and grid on the bottom skin and check the fit. The edging should be flush at the front and on the ends. Make sure the grid is snug against the back of the front edging. A 1-3/8-in.-wide cavity should extend across the back of the two shelves; the ledger boards (E5 and G5) will occupy this space when the shelves are mounted to the wall. The back of the upright is solid. Roll glue on the edging and grid. It’s important to not get any glue in the back cavity. Flip the pieces over, position them and apply glue to the second side. Place the top skin in position. Make sure all the edges are flush.

10. Clamp the torsion boxes between the box beams (Photo 4). Clamp the middle first; then work outward. 

11. True up the torsion boxes. After the glue has dried, remove the torsion boxes from the box beams. Joint the front edges, after scraping off any excess glue. Then rip the boxes to the final 9-3/8-in. width.

 

Make the Beveled Molding 

12. Mill your 6/4 maple to 1-1/4-in. x 1-3/8-in. molding blanks (E3, F3, G3, E4 and G4).

13. Orient the shelves and uprights for each C-unit and mark the fronts. Mark the open ends of the two shelves, where the end moldings go, too.

14. True the open ends of the two shelves by crosscutting. Remove just enough to leave clean edges. Make sure the cut is square to the front edge.

15. Cut and fit the miter joints on the molding blanks. Then glue them on (Photo 5).

16. Trim the molding edges (Photo 6). It’s OK if routing leaves the molding a hair proud. You can sand the surfaces flush later.

17. Rip bevels on the front moldings (Fig. A, Detail 2; Photo 7). To avoid kickback, make sure the blade is tilted away from the fence. Your final pass should leave a 5/16-in. square shoulder at the top.

18. Crosscut bevels on the end moldings (Photo 8). After your final pass, the square shoulder should match the one on the front.

19. Remove all the saw marks by sanding the bevels. You’ll get the best results if you use a sanding block. A power sander is likely to round over the crisp edges. It’s safer to smooth the joints between the moldings and the plywood with a block, too. 

 

Build the Mitered C-Unit 

20. Miter the corners (Photo 9). These angled cuts won’t tax your saw the way the molding did, because the core material isn’t nearly as dense. However, these miters have to be dead on, so make test cuts and be sure of these three things:

- Your miter gauge slides smoothly, without any side-to-side play.

Your miter gauge is set at exactly 90 degrees to the blade.

Your blade is tilted exactly 45 degrees.

21. Check your miter cuts with an accurate square and cut again if they aren’t right. In the end, it doesn’t matter whether your shelves are exactly the same size as the dimensions in the Cutting List. What’s important is that the joints fit.

22. Cut slots in the mitered ends for biscuits (Photo 10).

23. Make a pair of fir plywood braces to square and support the C-unit during assembly. Their height must match the upright’s inside length (between the miters), and their outside corners must be dead-on at 90 degrees.

24. Assemble the C-unit and check the miter joints (Photo 11). It’s easy to make adjustments, because the biscuits and braces keep everything in place when you loosen the clamps.

25. Glue clamping blocks onto the shelves (Photo 12). Grocery-bag paper and triangular offcuts from beveling the molding are perfect. Be sure to glue both sides of the paper. Don’t use clamps; just rub the blocks back and forth on the surface until they stick. Wait at least 15 minutes before using them.

26. Disassemble the C-unit and apply glue to the miter joints. Reassemble the unit and clamp the corners, using the temporary blocks (Photo 13). Be fussy when you fit the joints. 

27. Remove the clamping blocks by tapping the end grain with a hammer. The papered joints will break, leaving half the paper on the block and half on the shelf. (You can also split a papered joint with a chisel.) Moisten the paper residue on the shelves to soften the glue. After a few minutes, the paper will rub off and the glue will turn white, so it’ll be easy to see. Gently scrub off the glue using a paper towel or a fine nylon abrasive pad. Don’t use steel wool; it’ll discolor the wood. Remove excess glue from the mitered joints the same way.

 

Mount the C-Unit on the Wall

28. Mill ledger boards for both shelves. They should fit the cavities firmly, but without binding. 

29. Locate the C-unit on the wall after marking the stud locations (Photo 14). 

30. Fasten the lower ledger board to the wall (Photo 15). It’s shorter than the cavity, for side-to-side adjustment. If you need to be fussy about height, install the ledger a bit below the line, to allow for the shelf’s plywood skin.

31. Locate and install the upper ledger board by using the C-unit. It’s much easier than measuring on the wall (Photos 16 and 17). It helps to sand this ledger down a bit, so it slides easily in and out of the cavity. Use a brace to support the C-unit and keep the shelves parallel during this process. If the upper ledger bridges only one stud, use a toggle bolt for the second anchor.

32. Install the C-unit to determine whether it needs to be scribed (Photo 18). 

33. With the C-unit firmly pressed against the wall, drill pilot holes for the mounting screws. Keep the countersink shallow—the plywood skins are thin and you just want the screws to install flush. Fasten the C-unit to the wall (Photo 19). If you’re installing multiple C-units, mount each unit separately to drill the pilot holes. Then go back and install them, using a stubby screwdriver to drive the screws where the shelves overlap. 


Oops!

My face molding doesn’t cover the shelf’s plywood edge! The molding was noticeably bowed, and I forgot to check the edges when I was gluing it on. Now I’ll have to cut off the molding and start over. 

Molding that isn’t straight can be a real pain to glue on. The solution is to use a spline. Registering bowed or twisted molding with a spline guarantees it’ll glue on perfectly. Cut shallow grooves in both pieces, using your dado set. Be sure to locate the grooves off center, so the spline remains hidden after you cut the bevels.

For both edges of the molding to stand proud, the molding’s groove has to be slightly offset from the groove in the shelf. Creating the offset is easy. Make sure the top face of each piece rides against the fence when you cut. First, cut the groove in the molding. Before you cut the groove in the shelf, simply move the fence a bit closer to the blade. If your molding is 1/16-in. thicker than your shelf, a 1/32-in. fence adjustment centers the molding.


Cutting List


Fig. A: Exploded View



Fig. B: Notch-Cutting Jig 



Fig. C: Box Beam


This story originally appeared in American Woodworker November 2004, issue #111.

November 2004, issue #111

Purchase this back issue.

Click any image to view a larger version.


1. Cut notches in the ribs, using a shop-made indexing jig and a 1/4-in. dado set. The notches allow you to assemble the ribs into the grid sections that comprise the core of the torsion boxes.


2. Cut short ribs from some of the long ribs. Use a template (Fig. A, Detail 1) and an indexing piece to make sure you cut them consistently.


3. Glue the torsion boxes together by sandwiching the edging and the grid section between the plywood skins. This is a complex glue-up, so use glue that won’t dry too fast and a roller to spread it quickly and evenly.


4. Clamp the torsion boxes between box beams. Box beams distribute clamping pressure evenly and guarantee your glued-up shelves will be flat. They’re well worth the effort to make. 


5. Glue on molding blanks that are slightly thicker than the shelves. Make sure you feel ridges on both sides of the blanks as you tighten each clamp, and again after all the clamps are tight.


6. Rout the edges flush. Use a second shelf to support the router and a piece of scrap plywood as a spacer.


7. Bevel the molding. It’s difficult to get good results when you cut thick, hard wood at an angle, so make two passes. First, make a slightly oversize rough cut. Adjust the fence and make a second light pass to clean the edge. 


8. Bevel the end molding with your miter gauge and a long support fence. Rough-cut the bevel and then make a light final pass. Caution: If your shelf and miter gauge are unstable in the starting position because they hang off the front of the saw, use a sled to make this cut.


9. Miter both ends of the upright and the inside ends of the shelves. Unlike the previous beveled cuts, these go all the way to the tip. 


10. Cut biscuit slots. Biscuits align and strengthen the miter joints.


11. Clamp the three shelf components together without glue, so you can check the fit of the miter joints. Use plywood braces to support the upper shelf and hold everything square. 


12. Glue temporary clamping blocks at the corners. They’ll allow you to clamp the miter joints effectively, without using long, heavy clamps. Pieces of heavy paper glued between the block and the shelf make the blocks removable. 


13. Clamp the miter joints to glue the C-unit together. After the glue dries, the clamping blocks knock off easily because of the paper, and the residue cleans off completely with water.


14. Position the C-unit on the wall and mark the inside corner. A piece of tape can be used to mark stud locations. Installation is easiest when both ledger boards anchor in two studs.


15. Install the ledger board for the bottom shelf, using a level and your corner mark for reference. 


16. For a perfect fit, use the C-unit to locate the upper ledger board. Attach double-faced tape to the ledger and slide the ledger into its cavity, using spacers so it protrudes. Install the C-unit on the lower ledger and press the upper shelf against the wall.


17. The upper ledger board remains when you remove the C-unit, thanks to the double-faced tape. Anchor this ledger to the studs.



18. Scribe the shelves to remove gaps. First, transfer the wall’s uneven shape with a compass. Remove the shelf and sand the back edges to the lines. It’ll be easy because you’re sanding 1/4-in. plywood skins. Reinstall the shelves. Voilà! No gaps!


19. Fasten the shelves to the ledger boards with screws. This is strong enough for most applications and it makes the shelves removable. For maximum strength (to hold your anvil collection, for example) glue and screw the shelves to the ledger boards.


Comments

Old Salt wrote re: Floating Shelves
on 06-27-2010 6:40 PM

Just to cool

Corinne12 wrote re: Floating Shelves
on 07-03-2010 7:23 AM

this is nice information need to know more