Making Lock Miter and Finger Joints
You may wish to copy and paste or print all the editions that will help you with all the different techniques for the joints. Either by copying or printing you will start to build a reference booklet that will assist with your woodworking library. 

FINGER JOINTS

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Finger joints on all sides of a project make a box that is very nice.  To cut the joints it requires a dado blade and a simple jig for your miter gauge on your tablesaw.  This jig provides a guide for stepping off the fingers and slots as you move the stock through the blade.

This jig can be made from a piece of plywood with a slot in one edge that matches the slots you want to make. A hardwood key ¼” thick or as thick as the joint that you want to make, that fits snugly in the slots and registers the cuts are glued in place.  The height of the jig should be 4” to support the longer front and back pieces.

To cut the slot for the jig, adjust the height of the blade to slightly less than the sidepieces that you are cutting.   Push the jig over the blade to make the slot, and glue a ¼”or bigger key that is 1 ½” long and will extend past the work piece.  Chamfer the top edges and sand the sides of the key for a snug fit.

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The key to successful finger joining is to locate the jig precisely one saw kerf away from the far side of the blade.  You may have to make some adjustment to get this right and even a slight error will compound across the board to ruin the joint. Now adjust the blade so it extends above the table about 1/32” more than the thickness of the stock.  The end grain of the fingers will suck up glue and it will be easier to sand or plane the tips off than to level the entire length of the project.  Be sure to lock your tablesaw blade so that it does not change in height, as it will ruin the joints.

Here’s where a test piece will assure a perfect project.  Cut a few finger joints on the ends of two scrap pieces and see how they come out.  A nice easy fit, neither too tight nor too loose is what you want.

Ideally, each joint will start and end with either a slot or a pin.  This isn’t absolutely necessary, but it will allow you to flip each piece end for end and cut both sides at the same time with the same set up.   Always cut the first piece so that the joint begins with a finger.  This way you can butt the pieces against the key.

After cutting the first slot, move the board so the new slot can be slid down on the key and the next joint can be cut.  You do this until you reach the end of the board.  Use the first finger on the piece as a stop to cut the first slot on the mating piece. Dry fit all the machined parts and glue up and make sure that the project is square.  When dry, sand all the finger joints flush with the sides.

LOCK MITER JOINTS

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This joint is called a lock miter and is very strong in plywood.  Its advantages are that it is hidden from the outside and that it requires clamping in only one direction because of the built in locking action.

Although the joint looks complicated, you will find it easier than you think.  The following drawing will detail the steps that you will need to take to be successful with all four corners.  Before you begin, use some scrap pieces of plywood to check the accuracy of your cuts.  Great care should be taken when marking the wood and setting up your tablesaw.  First make sure that your fence is set for the exact thickness of the plywood, with the outside of the blade flush with the outside thickness of the plywood.   The following diagram details the cutting sequence.

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 (1) Raise the blade so it makes a rabbet about 1/4 the thickness of the wood and make the first cut in the front section.  (2) Mark for the second cut.  This is done with a dado and should be about half the thickness of the plywood.  After setting the dado blade, raise it up to the full thickness of the plywood.  A tenoning jig will help hold the sidepiece in position.  (3) The third cut will relieve the front section the width of the outside tongue of the sidepiece.  (4) The fourth cut will waste the material of the inside tongue of the sidepiece.  (5) Lower the saw blade, move the fence and tilt the blade 45 degrees and make the angle cut for the sidepiece.  (6) Reset the fence and make the same cut for the front piece.  The joints are now finished.

Brian Murphy
American Furniture Design Co

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