How big should the holes be? After you clamp things together and squeeze some glue onto those dowels, everything hinges on attention to detail. You let go. You wait. And then… success! The wood settles; the joint is good. It’s invisible, and it’s strong as can be.
Or not so much. When this goes wrong, it means there is a seam along the joint that the fillers won’t cover. Usually, hole size is what went wrong. Woodworking often flies in the face of common sense. So, what do you use for a drill bit? To take the guess work out of it, here’s how it relates to the chart (above).
How to Drill Good Dowel Holes
First thing to know is that snug isn’t tight. If you drill to the same size as your dowels’ nominal size and then try to jam them in place, they will most certainly split the wood. This also means there will be no chance of getting glue to bond. Best practice is to drill down just under the size of the dowel. A gap of roughly one sixty-fourth of an inch gives you a little gap that allows for glue to flow into it and also creates a mechanical lock. The gap helps prevent hydraulic pressure from pulling the joint loose. A small gap are strong.
The diameter isn’t everything; depth is important too. The majority of builder will drill a hole exactly halfway through the dowel to get most surface contact. The problem with this is there’s a cushion of air and glue on the bottom of the hole where the wood pieces don’t meet flush. Take another sixteenth off your depth calculation. A tiny space serves as a place for any excess glue and allows the faces to come together tightly. It is a little change but it keeps you from having to pry boards away to release pressure.
A good hole starts with the right drill bit. If your dowel is under three-eighths of an inch in diameter, use brad point bits; their center spur is sharp so it doesn’t wander off course, while outside spurs score wood fibers before they cut, resulting in a clean edge that better grips the glue. On pieces bigger than three-quarter inch in diameter, especially if you’re using thicker stock, you should of opt for Forstner bits instead, as their flat bottom lets you accurately control how far they goes into the wood. Using twist drills will result in conical bottoms, which won’t seat well at all.
The type of wood being used is one other variable not to ignore. Oak, maple and other dense hardwoods is great for holding their shape when compressed. They are perfect for standard sized hole. Other soft woods such as cedar and pine will have different results. The fibers in them tend to compress easy. A dowel cut to the standard size may simply slip through the board and loosen up over time as the wood relaxes, or it could tear out the board on entry. Here’s where you often want to modify your plan and perhaps go down a bit in bit size, or settle for something less snug so you don’t end up with a split. You’ll also run into problems with oily woods such as teak, whose natural oils inhibits the standard polyvinyl acetone glues, requiring an epoxy adhesive or having to clean up the glue with solvents.
Now for alignment. Your holes may be spot-on but if they’re off by over thirty-two hundredths of an inch, the gap between them will be visible at the joint face. Transfer Pins or a self-centering jig will cause the holes in mating boards to line up exact. It eliminates waste and it works better.
You want the joinery to vanish, only the strength of connection left behind. A good joint is one where the dowel holds things together without calling attention to itself once you have the fit down. Naturaly, you will recieve a better result with practice.
