Popular Woodworking 2005-10 № 150, страница 73
When the back must be completely housed in a rabbet, there are still several choices. If the case is joined at the corners with a miter, the rabbets can be cut quickly with a dado blade on a table saw. For dovetailed cases, you can add a miter before the first dovetail pin and, again, run off the rabbets with a dado. (Although you can cut a stopped rabbet in dovetailed pieces with a plunge router before assembling the case, it leaves a weak area on the corners that's susceptible to breaking.) It's also possible to cut the rabbets after the case has been glued up by using a bearing-guided rabbeting bit and a router table. Corners are cleaned up with a chisel. All of these methods work. Sizing Back Pieces Solid lumber changes dimension-ally as the amount of moisture in the air rises and falls seasonally. Movement is more pronounced across the grain than it is in length or thickness, but unless the relative humidity remains absolutely unchanged you can count on at least a little bit of wood movement. Flat-sawn wood moves more in width than quartersawn material (changes in length are too small to worry about). Wood movement is the reason that solid backs can't be glued into a case. Using shiplapped boards skirts the problem because the narrow gaps left between individual boards is enough to accommodate seasonal movement. The greater number of boards that make up the back, the smaller the gaps can be because you're spreading the total loss and gain in width over a greater number of spaces. If you have an accurate moisture meter, know the range of relative humidity in your area and really like math, you can calculate exactly how much the width of each board will change with a simple formula that Bruce Hoadley includes in his book "Understanding Wood" (The Taunton Press). There are lots of variables: the species of wood, how much its moisture content is likely to change from summer to a heated environment in the winter and whether the board is flat-sawn or quartersawn. A flat- sawn cherry board 18" wide, for instance, will change in width by nearly V2" if its moisture content fluctuated between 5 percent (during the winter in a central-heat environment) and 15 percent (a muggy summer day). If you made a cabinet back with four pieces of this material, gaps of about V32" should be enough to handle the winter-to-summer expansion. Most of us probably won't go as far as to try and calculate the change to the third decimal place. If you start with kiln-dried material and you're mindful of the conditions in your shop at the time of construction, you can make an educated guess about how much room to leave at the sides and between boards. In winter, Adding a miter to the corner of a dovetailed case allows you to cut the rabbet for the cabinet back easily on a table saw with a dado blade. A rabbet for the back can be cut after the case is glued up with a bearing-guided rabbeting bit and a router table. The router will leave rounded corners that can be squared up with a chisel. Alternately, shape the corners of the corresponding back pieces to fit. piece of scrap first. Shiplaps can be run off in a single pass . 70 Popular Woodworking October 2005 |
