My desk at work is designed to hold a giant CRT in the corner. But in the 21st century, bulky vacuum tubes were replaced with wafer-thin flat screen displays. While this frees up plenty of valuable desktop real estate, it's hard to reach into the deep corner behind the display. I decided to add a lazy susan back there to hold my reference books. The bottom shelf is 18 inches in diameter and 11 inches high, so it can hold large books, and the top shelf is 16 inches in diameter, with a 6 inch "fence" to corral the smaller books. The project is also suitable as an exceptionally pernicious step stool.
Step 1: Tools and Materials
I used a router, saw, and drill to construct this project. Plus glue and clamps. Lots of clamps. You could probably do it without the router, but I used it to cut out the circles and the mortises. You'll need a basic set of twist drills, plus a 3/4 inch spade or forstner bit.
The basic components of the project are:
2 16 inch wooden disks (the base and top shelf)
1 18 inch wooden disk (the bottom shelf)
1 6 inch lazy susan ball bearing assembly (500 lb rating)
3 ft 1x4 stock for 3 "pillars" to support the top shelf
3 ft 3/4 x 3/4 stock for the top cross-piece
3 ft 3/8 inch dowel
You should be able to find these parts at the local hardware store or home center. Some of them even sell pre-cut wooden disks in plywood or laminated pine. The plywood is strong, but the edges don't finish up that well. I cut my own disks out of some 3/4 inch laminated pine sheet, which was cheaper than getting pre-cut disks, and the rest of the stock is poplar.
A note on cutting disks: if you don't have some sort of circle cutting jig for a router or band saw, don't try to cut out circles. You'll bodge it up. Just make some octagons instead.
And a final note on the lazy susan: it might seem ridiculous to use a 500 lb rated turntable for a little desktop bookshelf, but they're not expensive, and the main criteria is that the base plates have screw holes that are large enough to hold some decent wood screws, and they are spaced widely enough that the screw heads won't collide when the plates rotate. Also, keep the hardware packed away, so it doesn't get dusty. Any grit or sawdust in the ball bearings will impede smooth rotation.
Step 2: Potential Modifications
I used 3 pillars to support the top shelf, but that actually makes the layout and assembly more complicated. 4 supports would be easier. Also, I made the top shelf and the base smaller than the bottom shelf. That lets the bottom shelf "float" over the base, and adds a bit of a taper to the design, but you could just as easily use 3 disks of identical size. That would also make layout easier. Finally, I use 1x4 stock for the supports, but I think it would be sufficient to just use 3/4 or 1 inch dowels instead, which would eliminate the fussy mortise and tenon work. Just double them up and use two dowels for each support, spaced about an inch apart.
Step 3: Cutting Mortises and Tenons
This is the fussy step. I started by cutting a prototype tenon, about 1/2 inch deep, and slightly shorter than the full width of the 1x4. Note that my tenon has round edges because I cut the mortise with a 3/4 inch router bit. You could use square edges if you use a chisel or dedicated mortising tool. Next, I had to layout 3 3/4 inch holes spaced 120 degrees apart (if you're clever you can do this with a compass). So I started with top shelf, and marked the hole centers about 1.5 inches from the edge. I stacked the top shelf disk on top of the bottom shelf and centered them, and then carefully drilled 1/16 inch holes through the top into the bottom to transfer the layout marks. I marked the holes A, B, and C on both disks. I then used these holes to center a 3/4 inch spade bit, which I used to drill some starter holes about 1/4 inch deep. Remember that the mortises go on the top of the bottom shelf, but on the bottom of the top shelf. You want to use A, B, and C markings to keep track of the corresponding holes for the top and bottom shelves. That way, if the initial spacing was slightly off, you aren't compounding the error by changing the orientation of the top and bottom shelves.
Step 4: Cutting Mortises and Tenons (continued)
A note on the holes: because I used laminated pine, I arranged the holes such that the mortises would never be exactly parallel to or perpendicular to the laminations.
Once the starter holes are complete, use the router with a 3/4 inch bit to cut the six mortises that fit the prototype tenon (about 1/2 inch deep). Cut the 1x4 stock into 3 12 inch pieces, and trim the ends to match the prototype tenon. There is a fair bit of hand work and trial and error here. Because each tenon is hand-fitted, you should mark them (A, B, and C), and note the top and bottom, as well as the inside and outside edges. The 1x4 stock is slightly larger than 3/4 inch thick, so you need to round off the corners and thin it out a bit. I use a fine dovetail saw to cut the shoulders, and round them off with wood rasps, and sometimes the careful and judicious use of the dremel sanding disk. A shoulder plane would really be the way to go here.
When everything is done, try to dry fit, and test for level and squareness. Don't glue it yet!
Step 5: Lazy Susan Hardware Layout
The instructions for the hardware recommend mounting the hardware on the fixed base, then drilling down from the top using machine screws to tie to the plate. Yuck. I didn't want visible screw heads on the bottom shelf. I decided to mount the hardware so both sets of screw heads are on the inside. How is this possible? Well, if you position the lazy susan so the plates are offset by 45 degrees, you'll see that you can mount the plate on the base, drill some large holes at the 45 degree offsets to provide access to the screw holes on the top plate, and then go up from the bottom to mount the top plate to the bottom of the bottom shelf.
At this stage we only want to do some layout -- we aren't screwing the hardware in place until all assembly and finishing is done. So if your hardware doesn't come with a handy template, build your own. Cut out a square the same dimensions as the plate, and fold it on the diagonals to find the center. Remember, if you don't center the hardware, your shelf will shimmy when it rotates. But not in a good way. Layout and pre-drill the mounting holes on the base and the bottom of the bottom shelf, and drill large holes at the 45 degree offsets in the base. Make sure you have some room to maneuver -- mine were a little tight, and I had to stick the screws in using hemostats. When you're done with layout and pre-drill, put the hardware away in a sealed bag so you don't contaminate it with sawdust.
Step 6: Top Cross-piece
The top cross-piece is made out of 3/4 inch square stock. Unlike the 1x4 stock, which is really about 3/4 by 3.5, or 3/4 inch plywood which is 23/32, the 3/4 square stock is really honest-to-god 3/4 inch square. I cut two 15 inch pieces, then used the router with a 3/4 straight bit to mill a groove across the center of both pieces. Set the depth to half the thickness, ie 3/8. Dry fit the cross piece to make that the fit is correct. I decided to relieve the edge just a tad with a 5/32 roman ogee. All the more fun, it gives you four opportunities to mess up the devilish end-grain cuts. If you want to try this, do each piece separately (but avoid the centers), and use extra 3/4 stock to support the router since the piece is so thin. And try not to mess up the end-grain cuts. Then assemble the cross-piece (I did it dry fit, but I suppose you could glue at this point) and route the tricky intersections at the center (and you have four places to mess this up as well!).
Step 7: Mounting the Cross-piece
The cross-piece stands on nine 6 inch pieces of 3/8 dowel. The best way to set this up would be to construct a template, and use a plunge-router with a 3/8 straight bit to precisely drill a set mounting holes in the top of the top shelf and the bottom of the cross-piece. The second best method would substitute a drill-press for the plunge-router. You might be able to use a hand drill if you have a stop-collar, but I think it would be too inaccurate. So the third best way is to drill a set of holes completely thru the top, then clamp the cross-piece in place, and re-use the same holes to go up from the bottom to make some corresponding holes about half-way thru the cross-piece. Then if the holes aren't precisely the same depth, you can make the dowels slightly long, and trim the excess after gluing. But what if you glued up the shelves before you mounted the cross-piece? Then you are left with the fourth best, or worst method: drill thru the top of the cross-piece about halfway thru the top shelf, and then trim the dowels. And now the dowels are visible from the top, so you have to be extra careful trimming and sanding flush. So choose some method to mount the cross-piece, do the layout, drilling, cut the dowels and dry fit. I drilled a smaller hole first and the 3/8 to avoid splits.
One other point: the top shelf has three "legs", but the cross-piece has four arms. So how should they line up? I just aligned one arm with one of the legs and left it at that.
Step 8: Sanding and Gluing
Now that the pieces are cut, do some final sanding before glue-up. I also used a round-over bit on my circles at this point. I sanded with 80, 180, and then 320 grit. For glue-up I would recommend mounting the cross-piece on the top shelf first, letting it dry, then mounting that assembly on the bottom shelf (I didn't do this, but wish I had). Also, if you use either of the "through-dowel" techniques, just mount the four corner and central dowel first and level the top, clamp and let dry, then glue the four remaining dowels. There is a tremendous amount of tension if you try to deal with all 9 dowels at once. After the cross-piece is mounted, dry fit the top assembly with the uprights and bottom shelf. Last chance!
Because the shelf unit has three legs spaced in a circle, it had some interesting "racking" issues. In woodworking, "racking" is the tendency of a project to spontaneously convert from a perpendicular structure to some less-desirable parallelogram. For a general box structure, it is easily cured with two diagonal pipe clamps. But for the two circles, the top racked and rotated in relation to the base, which led to some feverish clamping gymnastics. I'd recommend a glue with a long setup time, and place the small children out of earshot for this phase.
Step 9: Finishing and Final Assembly
After you survive the glue-up, prep your project for finishing. Remove any excess glue and do some touch-up sanding.
Some hints: sliding books in and out of a book case cause a lot of wear-and-tear on the finish but since the case is full of books, one will notice! So don't worry to much if the interior bits don't finish up well - concentrate on the bits that don't get covered by books. Finishing was a bit of a bear -- all the extra dowels and the uprights complicate your efforts to get complete coverage. Just be patient and do a couple of passes.
After finishing is complete, you can finally mount the lazy susan hardware. Last chance to lubricate the hardware! Mount the plate to the base first (I think I used 3/4 #8's), then rotate the top plate 45 degrees so you can see the screw holes thru the large "access" holes in the base. Flip the shelf unit, center the base, and screw thru the access holes into the bottom of the bottom shelf. Test for spinnyness.