Replies

1. 
   tom77 | Apr 04, 2011 10:05am | #1

   Trapezoidal joinery

   I can think of three ways, depending upon the shape of your trapezoidal sides.  For sides that are short in height relative to the base, you could use your miter gage and table saw to safely cut the proper compound angles.  Two setups would be needed, right and left.

   For tall sides, the miter gage would probably not be safe.  I would make a rectangular sled larger than the side.  I would adjust the blade angle to 45 degrees and the rip fence so that the blade just kisses the top edge of the sled.  Then align the workpiece so that the angled side is at the sled edge.  Use double sided tape for safety.  Make a rip cut.  Reverse the work piece and cut the other edge.  Keep the same work piece surface against the sled.

   A similar method would work with a router table.  Make four identical trapezoidal sides.  Set up the router table so that the 45 degree bit just cuts to the top of the work piece.  Sneak up on this with shallower cuts.

   Good luck, Tom.

2. 
   hammer1 | Apr 05, 2011 10:28am | #2

   Sounds like you need to make compound cuts, these are a combination of a bevel and an angle. You tip the blade to 45 degrees and set the trapazoid angle on the miter gauge. Your saw must be tuned so the blade aligns with the miter ways. Add a stout wood bar to the miter gauge head that reaches the blade. By cutting into the bar, you know exactly where the desired cut will be. The miter gauge bar should be fit to the ways so there isn't any slop. Gluing some sandpaper to the bar prevents the work from slipping. After making the ahead cut, don't back up. You can clamp a stop block to the wood bar to make repetative cuts, so all parts are the same. You will need a right and a left set up. Whether you cut face up or face down, left or right of the blade or back or forward angle on the miter bar is up to you. Make a few sample cuts with the blades you have, one may cut this tough compound better than another.

   You won't have a fun time trying to cut the miters on a router table with a 45 degree bit. You have to cut all the way to the edge of the material. This is like a jointer cut. The outfeed side of the fence will be different than the infeed side and you will have to keep tight to it, just like jointing. Often, you get tearout, splintering on the fine edge and it's not sturdy enough to apply pressure to. Doing end grain will likely tearout on the exit, too. Even with incremental cuts, the last one is full width of the material, that's a lot on a 45. Been there, done that, save yourself the pain.