Three Finger Box Joint Using a Tenon Jig




Introduction: Three Finger Box Joint Using a Tenon Jig

Never be embarrassed by ugly screws and butt joints again! A simple finger joint is strong enough for most projects, and with a jig doing the heavy lifting it couldn't be easier.

I made mine at TechShop

Tools needed:
Table saw
Tenon jig (shop fox)

Materials needed:
Wood Glue

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Step 1: Cut Lumber to Size

For this example we will be using techniques that assume that both pieces being joined have the same height and width as each other. This does not have to be the case, but we'll leave that as an advanced technique for another day.

For now, set the table saw fence and make a rip cut down the edge of each piece to ensure their dimensions are the same. Don't trust anything you buy to have the same dimensions along its length.

The height and width can be different but each piece should be identically dimensioned.

Concerning the length of the pieces, they should be the full length of the finished product. This is in contrast to butt joints where one piece needs to be shortened by the width of the other piece since there is no overlap.

Step 2: Lay Out Your Joint

Since we're making a three fingered joint, we will divide the height of the pieces by three and use the resulting measurement as the height of each finger. One side will have two fingers, one on the top and one on the bottom, while the second piece will have just one finger in the middle. The exact measurement does not need to be precise when making a three fingered joint . . . the middle finger could be a significantly different measurement than the fingers on the two fingered piece, but for aesthetic reasons it is nice to have them appear the same.

It is a good idea to clearly mark what portions of each piece should get cut away during the joint making process to help avoid mistakes.

Step 3: Install the Tennon Jig on the Saw

If you haven't worked with a tenon jig before it can be an intimidating looking piece of hardware.

Lower the saw blade below the table surface to avoid any damage as you install the jig, which is rather clunky.

First locate the guide bar that fits into the miter slot of the table saw. That at least tells us which end is down!

Locate the two handles on the jig. They will be two posts sticking out of the top of the jig. The side that they are closest to is the front of the jig. Slide the jig into the miter slot to the left of your saw' blade.

If it is correctly oriented there will be an adjustment screw on the left side and the handle of a big clamp pointing to the right.

A piece of scrap wood that is the same height as the base of the jig (approx 3/8") should be clamped to the table immediately to the right of the jig. This will act as a stop block when clamping a piece of stock in the jig, providing a consistent clearance above the table as a cut is being made. Using this piece as instructed by the manufacturer will alleviate some of the quirkiness of using the jig, but would not be a readily obvious step if you hadn't read the manual.

Also shown here but omitted from the photos in later steps are backing boards for the back support and side support.

A copy of the very informative instruction manual for the jig can be found here:

Step 4: Set Your Saw Blade to the Proper Height

The jig is designed to hold the lumber vertically as it is drawn over the saw blade, meaning that the length of the fingers is dictated by how high the saw is raised above the surface of the table.

Position one piece of lumber horizontally along the side support of the jig. The face of the workpiece that will be on the inside/outside corner of the joint should be facing up, since that is the depth of the cut we want to make. Notice that the jig has a slight lip on the bottom that prevents the lumber from lying flat on the saw table. This distance needs to be accounted for when setting your blade height or your cuts will be too shallow. If installed properly you should have a wooden spacer clamped to the right of the lip of the jig so your stock won't be floating in mid-air.

With the lumber resting on top of the lip of the jig, loosen any locking knobs on the jig so the clamping portion can move freely to the left and right. Slide the side support of the jig so that the lumber meets the plane of the blade of the saw. Adjust the height of the saw so that the tips of its teeth are flush with the top of the lumber when they're at their highest point in their cutting arc.

Step 5: Cut the Two-fingered Side of the Joint

Clamp the stock vertically in the jig. Since we've already marked the stock into thirds we can adjust the side support (and the stock which is clamped to it) so that the blade of the saw is positioned between the two marks.

Pull the jig away from the blade to avoid inadvertent cuts or kickback, turn on the saw, then push the jig through the blade, completing one cut. Turn off the saw, wait for the blade to stop, un-clamp and remove the stock from the jig, then pull the jig back to the front of the blade.

Spin the stock 180 degrees from it's original orientation and re-clamp it in the jig. The stock will now be positioned to cut a perfect mirror-image to the first cut.

Turn on the saw again and make the second cut.

If you are going to make multiple joints, it is better to cut all of the two-fingered sides now before changing the adjustment of the fence in relation to the saw blade.

Once all of the two-fingered joints are cut, the jig can be adjusted slightly towards the middle of the stock so that the waste material between the fingers is cut away. Continue adjusting and cutting until all of the extra material has been removed.

Clean up the area between the fingers with a chisel if needed in order to get a flat surface at the bottom.

Step 6: Cut the One Fingered Side of the Joint

It is likely that the cuts made to the two-fingered side of the joint will be slightly off of the the marked lines. Before cutting the single-fingered side of the joint, double-check the marks on that piece. Clamp the piece in the jig and adjust the side support such that the saw blade will cut should be slightly to the outside of these lines. It is better to make the cut so the finger is slightly thicker than needed and then adjust the cut slightly to make it smaller. Cutting too thin on the first go-round makes for a bad day.

As before, make one full cut, then spin the piece 180 degrees and make a second cut.

Remove the stock from the jig and compare the single finger's width to the slot between the previously cut two-fingered portion of the joint. Keep adjusting the jig to shave off small amounts from the side of the finger until the two sides will mate snugly.

Remove the tenon jig from the table saw.

Cut away the shoulders of the finger by laying the stock on its side and adjusting the height of the blade so it reaches the slots cut using the jig. Use a crosscut sled or miter gauge to keep the stock at a 90 degree angle to the blade as you make the cut. Using the saw fence to keep a consistent cut position is recommended, but you have to be careful not to get the stock jammed between the saw blade and the fence. A good way to accomplish this is to clamp a piece of scrap wood to the fence well to the front of the start of the blade and use that as a stop block. Once the stock is pushed past the end of the stop block the stock will not be in contact with the fence, greatly reducing the chance of jamming and kickback during the cut. Your fingers will thank you.

It is possible that once you've removed the shoulders the width of the finger will still be slightly too thick to fit into the slot in the two-fingered side of the joint. If so, be aware that clamping the part back into the jig will require adjustments to the height of the saw blade. The blade has been moved to cut away the shoulder of the joint, AND it was probably the shoulder that was resting on the lip of the jig (remember that space under the wood). It is likely that the stock will now rest flush with the table now that the shoulder has been removed. Double check your positioning before making any adjustment cuts.

It's a good idea to test each cut on scrap wood first so you don't spoil your project with one bad adjustment.

Step 7: Assemble the Joint

Dry fit the joint before gluing to make sure the pieces fit snugly but not too tightly. I like to be able to hand-force them together without any hammering. I've read other recommendations that a one-or-two hammer-blow fit is good, but have had pine stock split while fitting some joints that didn't seem overly tight.

Apply glue to all of the long-grain surfaces of the joint. In this case, that's the inside of the two fingers, and the outside of the one finger. Applying glue to the bottom of the slot between the two fingers or the shoulders of the single finger is not necessary as glue does not bind well to end-grain.

Squirting some glue onto a disposable surface and then applying the glue using a thin scrap of wood as if it were a paintbrush lets you get a thin, even coating of glue right where you want it.

Push the joint together and clamp it in place. I took advantage of a corner clamp to keep the two pieces at 90 degrees to each other. The real important direction to clamp is squeezing the fingers together like a sandwich. Some glue will push out of the joint. Wipe this up with a damp cloth for easy cleanup, or use a hobby knife to cut it off once it has dried.

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    3 Discussions


    Reply 5 years ago on Introduction

    Thanks for the info. I had made a brief attempt to find the actual
    name for this joint, but my search engine skills let me down. Thanks for
    providing the proper name!


    5 years ago

    Yep, bridle joint.