Jobsite Jig for Cutting a Straight edge on a Crowned Board
An accurate setup made from scraps can help you salvage lumber that is too crooked to frame with but could be great for blocking or furring strips
In a cabinet shop, it is easy to straighten a crooked board: run it over the joiner a few times, and voila, a perfectly straight edge. Jobsites do not typically have large joiners, so carpenters turn to jigs.
This method works well for cowed and crowned lumber, but twisted lumber should be chopped up and thrown into the burn pile. Twisted lumber is dangerous to cut because it does not sit flat on the table and could shift, causing the blade to bind.
Before making the jig, do this:
- Find a factory edge on a long piece of sheet goods, such as 3/4 inch OSB
- Make sure the tablesaw's blade is sharp.
- Make sure the fence is parallel to the blade.
- Make sure you have proper outfeed.
To make the jig:
- Cut a strip of 3/4 inch plywood to be about 2-1/4 inches wide—the thickness of the straightedge plus the thickness of a 2x4 block.
- To attach a push block to the straight edge, Matt connects a 2x4 block first, as backing. Before attaching the backing block, he rips the rounded corners from the 2x4 to get crisp edges.
- Screw the plywood push block into the backing block making sure that the push block is flush on the side that will ride against the fence and the bottom that will slide on the table.
- Safety additions: Screw two hold-down blocks into the OSB straightedge to be flush with the top of the workpiece. In this case, the workpiece is a 2x4, and the straight edge is 3/4 inch OSB. He makes the blocks from 3/4 plywood to flush up to 1-1/2 inches. He puts one block near each end of the workpiece.
- Another block on top of the first one, screwed into the jig and the workpiece, holds the workpiece in place.
To use the jig:
- It is essential that the bow faces away from the jig. If the bow is placed against the workpiece, it can rock, making it less stable to feed through the high-speed, spinning, sharp blade.
- Slide the workpiece against the fence such that there are two points of contact.
- Bump the end of the workpiece against the push block
- Screw the workpiece in place with the hold-down blocks.
- Measure the combined width of the jig and workpiece to set the saw fence. Matt has about a 1/8 inch bow on the board, and he wants to remove the rounded edge of the 2x4 too, so he sets the fence about 1/4 inch less than his combined measurement.
BONUS Video Section: Matt went down to the big box store to pick through the picked-over 2x4s looking for a suitable candidate that could help illustrate his point better. He found one with a 1/2 inch bow in it!
- With one edge straight, remove the jig and flip the workpiece over. Trim off the other crooked edge, and you're done.
—Matt Jackson is a master carpenter, remodeler, SketchUp Wiz, YouTuber, and contributing editor to ProTradeCraft. He lives and works in Rapid City, South Dakota.