Popular Woodworking 2003-12 № 138, страница 32
Power-tool Joinery Miter clamping cauls help you apply pressure at right angles to the joint's seam. Screw V-blocks to a plywood base to make a frame-clamping jig. Pressure is applied with pairs of wedges. Positioning the blocks- using frame parts- before gluing the frame takes only a minute or two. Packing tape holds glued miters together while you apply V-blocks and clamps.Apply bar or pipe clamps on the diagonals and alternate tightening to avoid distorting the frame. You also can use four clamping blocks with V-shaped notches cut into them. This works where you want to assemble four joints at once - a complete frame - as opposed to just one joint at a time. One way to use the blocks is to mount them to a base. The base provides a flat surface to ensure the frame isn't twisted, and the V-blocks capture the frame corners to ensure they are square. It's particularly useful for multiples and for very large frames, where the support of the base is beneficial. I set up the board after at least one frame is cut so I can use its parts to position the blocks that are screwed to the base. Note that only two of the V-blocks are screwed to the base. At the other two corners, you fasten cleats to the base. Clamping pressure is applied by twin wedges driven between the cleats and the loose V-blocks. To prevent squeeze-out from bonding the frame to the base or blocks, wax them liberally. For the one-off frame, you can use the V-blocks with a brace of bar clamps and several strips of packing tape. Glue the frame and secure each joint with packing tape. Set the V-blocks at the frame corners and apply the bar clamps diagonally. Alternate from clamp to clamp as you tighten them to avoid distorting the frame. Call for Reinforcement I've found that miters can be reinforced in a number of different ways. • Picture framers commonly use nails. The joint needs to be securely clamped so the hammer's impact doesn't knock the parts out of kilter. Bear in mind that nails driven straight into end grain don't hold that well. You can improve the joint slightly by driving the nail on an angle to the grain. • Splines are pretty easy to incorporate, and you can hide them inside the joint. (Splines were discussed in the October 2003 issue of Popular Woodworking - available at popwood.com - so we won't repeat that here.) • Biscuits also can be used. If the frame members are more than 21/4" wide, you can use standard No. 20 biscuits. You can use smaller biscuits - No. 10s, No. 0s or mini-biscuits - for narrower frames. • Dowels are another option, but they are tricky to align, so I don't recommend using them with this joint. 30 Popular Woodworking December 2003 |
