Toe Rails Scarf Jig

July 2nd and 3rd, 2015

I began laying out the scarf jig (sled) on Thursday evening.  The scarf joint is often used to increase the length of material required for a particular application, and is used routinely in boat building.  Fabricating lengthy toe rails for a sailboat often requires "gluing" up multiple lengths of material (teak, mahogany, etc.,), and this requires a strong joint, enduring joint that will take the curves of the boat.  The scarf joint is a diagonal cut between two boards that are then bonded together with wood glue, or better yet epoxy for the marine environment.  The rule for the length of scarf, or diagonal cut, is a ratio of either 8:1 or 12:1.  I choose the 12:1 ratio, so that means for every 1 inch of thickness in material, the scarf joint calls for 12 inches in length.  The toe rails for Alva Anne will be right around an inch to an inch and a quarter, so the the 12:1 ratio will be used.

I decide to make a sled jig to run over the table saw.  I started with a rectangular piece of hardwood ply, and installed two runners along the underside.  These runners will slot into the channels on the surface of the table saw, and will serve to keep the board aligned, consistently, as it progresses into the cut area.  I then laid out my cutting, or scarfing angle, onto the board.  Since I needed a 12:1 scarf, I would need an angle that produce this 12" length of cut as the sled ran over the saw blade.  The simplest way to produce this is to lay out a line on the board 12" in length, at one end of the line make a point 1" perpendicular to the original line, finally, connect the point and the opposite end of the original 12" line.  This produces a 12" span over the 1" thick piece of material.  At first thought, my plan was to lay out this cutting angle on the edge of the sled, and so I did install a board on the edge of the sled to use as a clamp board.  But after more thought, I decided to install the clamp board toward the center of the sled, making it much easier to cut, and would also produce more consistency in the cutting.  

Making the decision to move the clamping board to the center, I slid the measurements and angles of the edge board over toward the center line.  After replicating the cutting angle with a pencil line, I secured the new clamping board with screws.  The final touch was to secure a couple push blocks onto the surface of the sled - at a safe distance from the blade.

I tested the sled with a couple similar-sized boards, and was very happy with the results.  The scarf jig for the toe rails is now ready for the mahogany blanks!  Back to preparing the boat for paint...

Total Time: 2 Hrs.