10 Tricks for Tighter Joints

1. Cauls distribute pressure

It’s not easy to get enough squeeze in the middle of a big box to force home dado or biscuit joints. Big cauls are the answer. 

A caul is simply a thick, straight board. I make my cauls from stiff wood, such as hard maple, but any wood will do. The wider and thicker the caul, the less it flexes and the better it delivers pressure far from the clamps. I made a set of eight, each measuring 1-3/4 x 3 x 24 in., to have around the shop whenever I need them.

Stout cauls like these should provide plenty of pressure, but you can get extra pressure in the middle by inserting one or more shims (I use playing cards). You can also round or taper one of the caul’s edges from the middle to each end to create a crown. I do a dry run with cauls top and bottom, without shims, and place a straightedge on the cabinet to see whether the sides are flat. If one side bulges and needs more pressure in the center, I loosen the clamps, insert shims and retighten. 

Click any image to view a larger version.

2. No-math dado setup

How many shims does it take to make a perfect-fitting dado? You could figure it out with calipers and math, but it’s much easier to use a gauge block. 

The gauge block is the exact thickness of a dado made without shims. For undersize 3/4-in. plywood, make a gauge block that’s 11/16 in. thick. Confirm the gauge block’s thickness by fitting it into an 11/16-in. dado.

Place the block next to a piece of the 3/4-in. plywood. Stack shims, one by one, on top of the block until the stack is even with the plywood. Add these shims to the dado set, plus one more that’s the thickness of a sheet of paper (about .003 in.). The extra shim is the clearance needed for easy assembly.

Source

International Tool Corp., internationaltool.com, 800-338-3384, Freud dado shim set, #FRESS100.

3. Test long joints

Hard-won experience has taught me to test edge joints before gluing boards together. The test is simple, but effective. 

After jointing, I tighten a clamp across the middle of two neighboring boards. I walk around to both ends and wiggle the boards up and down past one another. If they rub together, great. The joint is tight and good to go.

If they don’t rub against each other, I’ve got a problem. One edge—or both—isn’t quite straight, which could result in the joint coming apart after it has been glued together. It may take years to fail, but it’s not a risk worth taking. I rejoint both edges, making sure to put all my hand pressure down on the infeed table until halfway through the cut, then gradually switch to putting all the pressure on the outfeed table. That usually does the trick.

If using the right technique doesn’t solve your problem, your jointer may need a tuneup. See americanwoodworker.com/jointertuneup for more information.

4. Flip faces to make a flat top

I have a tough time getting my jointer’s fence perfectly square. I can’t figure out a way to fix it, so I’ve adopted an old cabinetmaker’s trick that cancels out the error.

Here’s the deal: I alternate the faces that go against the jointer’s fence (see photo, right). Each edge has a slight bevel from the out-of-square fence. Arranged without alternating the faces, the two edges form a V shape. An open joint would result on one side or the other if I were to force the top flat. By alternating the boards, I get a perfectly tight joint and a flat top, even though the edges aren’t square. The only trick is to mark the boards beforehand so I place the correct sides against the fence.

Here's an extreme example: Both these boards were jointed with the fence way out of square. Arranged as shown here, the bevels cancel each other, resulting in a tight joint and a flat top.

5. Perfect tenon shoulders

I thought I had tenon-making with a dado set figured out, but one day it all went haywire. One side of the joint wouldn’t draw tight. The culprit? An inadequate stop-block setup.

I used to grab any old small cutoff for a stop block, holding it to the fence with a spring clamp. Wrong on both counts. The problem with a small stop block chosen willy-nilly is that it’s not necessarily square. If the stop block isn’t precisely square to the table, the shoulders come out at different heights. The problem with a spring clamp is that it doesn’t deliver enough pressure. If the tenon pieces repeatedly bang into the stop block, they can easily knock the block out of square or out of position. 

Now I use a big block, dedicated to the purpose, and a strong clamp.

Uneven shoulders make an ugly joint. These shoulders are way off, but even a small error can create an unsightly gap.

6. Dovetail gauge block

It’s no secret that setting up your router for a half-blind dovetail jig can be frustrating. Extend the router bit up too far and the joint will be too tight. Lower it down too far and it will be too loose.

I made a gauge block to record the perfect setting, once and for all. To make the block, I first used the standard trial-and-error method to make a tight-fitting dovetail joint. After I figured out how far the bit must extend, I rough-cut a notch in a block of wood using a bandsaw. I made the notch about 1/16 in. less deep than the bit’s height. I turned the router over and used the dovetail bit to recut the notch’s bottom. Now when I rout dovetails, I simply raise the bit until it touches the notch, lock the router down and start cutting. 

7. Reference from the base

Biscuits are great for aligning edge joints. I once used my plate joiner’s fence to make these cuts, but after a few bad experiences, I’ve switched to referencing from the base. It’s a little more work to set up, but the improved results are worth it. 

Frankly, I have trouble with the fence method. If I don’t hold the fence and the machine just so, my slots aren’t consistent. Referencing from the base is less risky.

Here’s the setup: Instead of my bench’s uneven top, I use a big piece of MDF as a flat reference surface. I put each board face-side down and clamp it to the MDF with a pair of 12-in. hand screws. This creates a wide ledge for the plate joiner’s base. I press down hard on the handle above the base to keep the base flat on the MDF.

8. Joint with your router

I learned this trick about jointing extra-long boards from a woodworker who only had a short-bed jointer. Jointing with a router isn’t new, but most methods require an absolutely accurate straightedge to guide the router.

Actually, a perfect straightedge isn’t necessary. All you need is a pretty good straightedge that’s longer than the boards. A little bow in it doesn’t matter. The trick is to rout both boards at the same time, so the edges mirror each other (see photo, above). Bowed or not, they’ll always fit tightly.

To set up, mill three blocks 11/16 in. thick. Use them asspacers to position and clamp the boards to a pair of sawhorses. Chuck a 3/4-in. bit in your router. Clamp the guide board so the bit takes an equal amount off both boards, about 1/32 in. Ride the router tightly against the guideboard, removing the spacers as needed.

This down-the-middle method joints both edges at the same time. The two boards will fit perfectly together, even if your guide board isn’t perfectly straight.

9. Make more room in the mortise

I well remember the day when I couldn’t get a mortise-and-tenon joint to come together no matter how hard I tightened the clamp. I had ignored a fundamental rule: Always make mortises 1/16 to 1/8 in. deeper than the tenons.

There are two reasons for doing this. First, you don’t have to obsess about making a perfectly smooth bottom in the mortise, which saves time. Second, space is needed for excess glue. This was the reason my joint didn’t go home. The tenon had a tight fit on all four sides. Too tight, really. It had pushed all the glue to the bottom of the mortise. A deeper mortise would have given the glue space to pool and allowed the tenon to go home. 

10. Make tight mitered edging

My father used to tease, “I cut the darn board twice and it’s still too short!” Of course he was kidding—or was he? When I make a mitered tabletop, I start with one or two pieces that are “too short” and a bit too wide. Jointing their inside edges effectively lengthens them until I get a perfect fit. This is much easier than trying to cut the boards to exact length. 

The center of this top is a piece of plywood. (Solid wood won’t work here because the center piece is unable to expand or contract with the seasons.) I often use biscuits or spline to align the edging with the plywood and to reinforce the corner joints.

1. Miter and glue the short pieces first. Rip the long pieces about 1/4 in. wider than the short pieces. Cut the long pieces a bit short, so they each have a 1/32-in. or so gap at one end.

2. Set your jointer to take a minimal cut, about 1/64 in. Joint the inside edges of the long boards. This lengthens the distance between the miters. In effect, the board becomes longer.

3. Check the board’s fit after each jointer pass. When the miters come tight, mark the excess width, rip the boards and glue them on.

This story originally appeared in American Woodworker January 2007, issue #126.

January 2007, issue #126

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