4.2 Dowel Joints

dowel_joint_overall

The Dowel Joint

Bore matching holes across a wooden butt joint, swab glue into the holes and onto short dowels, then push those dowels into the holes and assemble the joint under clamping pressure. That’s the gist of dowel joinery. Despite its simplicity, a dowel joint is surprisingly versatile and strong. 

The dowels themselves are usually completely hidden, so the joint looks like a regular butt joint from the outside, with no visible fasteners, such as screws, holding the parts together.

dowel_joint_assembly

If I were stuck on a desert island and had to choose just one method to connect project parts, the dowel joint would probably be it. Strong, precise, hidden and versatile, the dowel joint’s only drawback is that its simplicity makes it easy to underestimate the importance of getting key details correct. In particular, successful dowel joints depend on drilling holes that are precisely sized, spaced and positioned. 

dowel_pins

Choosing dowels

You’ve probably seen lengths of dowel, sold alongside other types of trim, in your local building center. While you could saw these dowels into the short lengths you need for joinery, the material is not a good choice for dowel joints. First, it’s not usually sized accurately (¼" dowel is usually slightly smaller in diameter than ¼") and it’s often not precisely round in cross-section. As well, these dowels are often milled from tropical hardwoods that aren’t all that strong. 

Instead, I opt for ready-cut fluted dowel pins every time. Besides being precisely sized, truly round and made of strong wood, dowel pins also have grooves – the “flutes” – compressed into their surface. The flutes provide more surface area for glue and help make it easier to slide the dowel into the drilled hole. As well, the moisture in the glue makes the compressed flutes swell inside the hole, resulting in a stronger, more rigid joint.

fluted_dowelpins_group

Dowel joints usually connect the edge (or end) of a board to another board. The thickness of that edge helps determine the size of dowel you need. The dowel diameter should be one-third to one-half the overall thickness of the board, and the dowel length should be roughly twice the board’s thickness. For example, if you’re joining ¾"-thick x 2½"-wide cabinet door frames, for instance, choose 5/16" or ⅜" diameter dowel pins that are 1¼" or 1½" long. I use ½" diameter dowels when I’m working with 1½"-thick wood. Thin ¼" dowels are perfect for projects with ½" thick parts. Traditional timber frame buildings, however, use 1” diameter dowels, no matter how big the timbers are.  

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Precise drilling is important

For successful dowel joints, the drilled dowel holes must be sized and positioned accurately, and this is why dowel jigs were invented. Jigs guide the drill bits in a hand-held drill as they bore into wood, so the angle and location of the holes matches precisely on both parts to be joined. 

There are a handful of dowel jigs on the market, but the best I’ve used so far is designed and made in Canada. Dowelmax (www.dowelmax.com; 250.764.1770) is the product of the father-and-son team of Jim and Mike Lindsay. Jim, an engineer by training, developed the tool after coming to Canada with his young family. To save money, Jim made beds and dressers for his kids from the shipping crates that brought the family’s goods over from Scotland. He fashioned a lot of dowel joints, which spurred the development of the Dowelmax jig.

A Dowelmax jig kit costs about $200 USD. If you’d rather not spend that much just yet, this self-centering doweling jig costs less than half this much. It’s less versatile, but still works well in fewer situations. I’ve owned this jig for years. 

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Tips for assembling dowel joints

When it comes time to assemble a dowel joint, always clamp the parts together first – without any glue. This testing process, called “dry fitting,” allows you to check whether any dowel holes have been drilled too shallow, which will get in the way of assembling the pieces. That’s probably the most common problem you’ll find, but dry fitting will catch other issues too. In woodworking, you don’t want any surprises; better to discover all the issues now, before everything gets goopy with glue.

Dry fitting also lets you figure out exactly which clamps you need to pull the parts together and how they should be applied. You’ll appreciate the headstart later: you need to bring the joints together quickly after applying glue, since the dowel pins swell. If you take too long because you’re hunting for a clamp, you may never get the parts together. 

You’ll get best results by working the glue onto the sides of each dowel hole with a toothpick, then applying a little glue onto the surface of each dowel. Inexpensive plumber’s flux brushes are great for applying glue here.

As you work with dowel joints, here’s some help figuring the causes of typical troubles you might have with dowel joints and solutions that will get you out of trouble.

Troubleshooting dowel joints

Problem: The joint doesn’t come together fully, even under clamping pressure and with dowels fully inserted.           

Cause: The holes are too shallow for the dowel pins, which are bottoming out and preventing joint closure.

Solution: Make the holes deeper or the dowel pins shorter. 


Problem: Wood pieces split as the clamps pull the joint together.

Cause: The dowel holes are not parallel with each other.

Solution: Remake parts, drilling holes more carefully with a dowel jig. 


Problem: The joint is weak and loose, and the parts don’t align correctly.

Cause: The dowel holes are too large. 

Solution: Remake parts and re-drill with the correct bit. If you have room, you could drill larger holes for dowels that are one size larger.


Problem: Dowels won’t go all the way into their holes during the dry-fit stage.

Cause: If you drilled holes of the right size, chances are the dowel pins have swollen from humidity. 

Solution: Heat the dowel pins in a 350ºF oven for 15 minutes to drive off moisture and shrink the dowel diameter.

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Uses for dowel joints

We could have built the footstool with dowel joints. If we had, the look would be more refined because there would be no screw heads or other visible fasteners. 

But dowel joints don’t have to be hidden. They can also become visually prominent and attractive design elements. I made the bench above for our mudroom in the early 2000s. It uses two 1" diameter dowel joints on each side to connect the top with the legs. The dowels are larger than necessary for strength, but just right for looks. Rather than hide these dowels, I drilled the holes right through, then locked the dowels in place with wooden wedges. The wedges, driven into slots in the dowels, add strength by making the dowels fit tighter in their holes. 

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Notice how the finished wedges are oriented so they exert pressure along the length of the wood, not across the grain. Wood is a lot stronger along its length than across it. If I had driven the wedges at 90º to the way you see them, there’s a chance this misdirected pressure would have split the top of the bench.

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Understanding dowel centers

Sometimes situations arise where it’s difficult or impossible to use a dowel jig but you still want to use dowel joints. The photo below shows a stair railing I built using ½" diameter dowels to connect the railing itself with the newel posts the railing ends at. In non-typical cases like these some simple metal items called dowel centers can help. Drill the dowel hole you’ll need on one side of the joint, slip a dowel center into the hole, then press the mating piece of wood against the one you’ve already drilled. The central point on the dowel center will mark the exact location required for the hole in the other side of the joint. 

railing_dowel_centres-.

The marks left behind show exactly where the hole needs to go, but of course these marks provide no guidance for your drill bit. You still need to drill the hole accurately without deviating from the marked location. If you’re careful you can make this happen with a hand-held drill used freehand. Dowel centers placed in the top of each stair spindle as shown below allowed me to transfer the exact location of dowel holes needed in the underside of the railing that caps these spindles.

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Watch the first video below for a detailed tour of how my favorite dowel jig works to make a simple dowel joint. The second video shows a measured strength test applied to the same joint until failure.

VIDEO: Making a dowel joint

VIDEO: Dowel joint testing

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