Woodworker's Journal fall-2009, страница 52
Three holes are required to attach the ledger: 1) a counterbore for the washer and head; 2) a shank hole through the ledger; 3) a core clearance hole in the wall. The photo shows a mocked-up wall with 1/2" MDF playing the role of sheetrock.
A critical measurement: make sure the core clearance hole is the same size or slightly larger than the core of the lag screw.
A Serious Shelf
The length of this shelf captures three studs. It's made using 1/4" ash-faced plywood skins with 1/2" plywood core stock. The ledger is maple held in place with 3/8" x 4" lag screws. The shelf is lipped with 3/8" solid ash. The edges are crisp but with the sharp edge removed. The stark white of the ash veneer was turned only slightly to a softer hue by three coats of salad bowl finish.
Drilling the Holes
You'll need three installation holes: a core clearance hole in the wall and a counterbore and shank hole in the ledger. In the case of a 3/8" x 4" lag screw, the core clearance hole is 9/32" diameter by 4" deep. Don't try to put a 3/8" x 4" lag screw into a stud without the correct clearance hole for the core. If you use a smaller diameter, the strength it takes to turn the wrench is being absorbed by the friction between core and clearance hole. You want the friction to be between the helical thread and the wood tissue.
In the absence of an electronic stud finder, you can locate stud centers by probing with fine drill holes within the area that will be covered by the ledger. Since light switches are usually nailed onto studs, that's a good place to start if one is near to hand. Plot the studs, drill the core clearance holes, then transfer their location to the ledger.
Drill the counterbore 1V8" diameter by 3/8" deep. A spade bit in a drill press with the workpiece clamped in place works well. In theory, the shank hole is 3/8", but in practice it should be a bit larger to accommodate any leveling adjustment (see Ledger Attachment Drawing, page 51) that may be required when attaching the ledger to the wall. Now glue the shelf to the ledger using the instructions on the next page, and you've got an amazing, beautiful and immensely strong shelf. You're ready to pile on your favorite books, sculpture or whatever your heart desires.
Attach the ledger and confirm that it's level. If you make the shank clearance hole 13/32", it provides enough room to move the ledger just a fraction to get it level.
52 Build A Bedroom Shelf