Why bother writing an article about tasks as simple as mitering and beveling? Because there are a number of refinements to the process that can make the difference between mediocrity and quality. Furthermore, the techniques described here are useful for any quality woodworking project that requires precise joints-from the simple picture frame to the most elaborate piece of furniture.
Precision mitering begins with selection of the raw stock. Obviously, it must be straight and true with no warpage or other defects evident. If the work is large enough to require more than one piece of stock, be sure that the pieces are identical in width and thickness. At times, pieces of millwork that are nominally the same differ noticeably, owing to wear on the cutters at the mill or the cutters being sharpened between runs.
Having selected proper stock, the next step is to sand and stain it before cutting. This is the proper time for these operations because of the difficulty of sanding properly or staining evenly near the corners of an assembly. If the work is glued up before staining, it is impossible to avoid having some glue soak into the surface of the wood near the joint, interfering with the penetration of the stain in that place and so producing an uneven finish. Staining after cutting but before gluing is also undesirable because the stain soaks more heavily into the disrupted wood fibers right at the cut than into the rest of the wood. This gives rise to a dark line right at the joint that spoils the appearance of even the most precise joint. Furthermore, if you are using a penetrating stain, the stain will seal the wood pores, producing a weak glue joint. So, better to sand and stain first, then cut and glue.
Before cutting the stock, it is important to see to it that the saw is properly set up. The tilt and miter protractors on most saws cannot be relied upon for accurate adjustments, because the index marks are too coarse and the flimsy sheet-metal pointers are too easily knocked out of alignment. One method of setting up the saw is to remove the blade and replace it with an alignment disc. An alignment disc may be made from a suitable piece of metal (e.g. 1/8″ aluminum) that is about the size of the blade. It need not even be round, but must be perfectly flat. Drill a hole in the center of it for mounting on the arbor.
The saw tilt must first be adjusted. Hold an accurate square against the alignment disc and the table, and adjust the tilt until the disc is exactly perpendicular to the table surface (not the insert around the blade). The adjustment is correct when no light can be seen to shine between the square and the disc or between the square and the table. THe miter is adjusted next. A suitable standard is held between the miter gauge and the alignment disc, and the gauge is adjusted for the exact angle. A draftsman’s triangle is excellent for establishing angles of 30, 45, 60 or 90 degrees. For other angles, a precision machinist’s protractor is recommended. These items can also be used to set the saw tilt for a precise bevel. You can check your miter adjustment by cutting two scrap pieces at your chosen setting, assembling them, and measuring the total angle that results. That way, you’ll be sure your assembled angle is as precise as desired.
A jig to enable two more more pieces to be mitered to identical lengths should be used. Here a straight bar of wood or metal is bolted to the miter gauge. (It is important that the bar have accurately parallel surfaces. If this technique is to be used, it would be well to gauge the miter as previously described with the bar in place.) A block of wood or metal is clamped to the bar to define the length. Glue a piece of 80 grit sandpaper to the bar on the side facing the wood to help hold the wood against the bar. After each piece of stock has been mitered on one end, the mitered end is pushed very gently against the block and the other end cut. The stock must be clamped to the bar to keep the forces exerted by the saw from moving it and destroying the accuracy. A block, rather than a clamp alone, must be use to define the end because the constraining surface must be perpendicular to the bar. A C-clamp, for example, is not suitable because the edge of the clamp pad is rounded. This would allow the point of the miter to dig in under the pad and would impair the accuracy. It is helpful to remove the slight featheredge at the point of the completed miter (do this uniformly on all pieces) before pushing it gently against the block.
These techniques produce a joint that can be glued up without clamping. All that is needed is to supply the glue, put the two pieces against adjacent sides of a right-angled object to define the proper position, and hold them in place for four or five minutes.