Crack your knuckles, it’s finger joint time!

Dremel 4000 blueprint - finger joint

A finger (box) joint

After writing about mitre joints yesterday, I had a look at Wikipedia’s Woodworking joints page to educate myself a bit more. The dovetail joint, which I am still very keen to try, does seem to be rather advanced, as evidence by this quote:

…the cutting of dovetails by hand is regarded as a mark of skill on the part of the craftsperson.

So today, not being entirely confident in my masterful skills, I settled for the dovetail’s simpler counterpart – the finger joint. Also known as a box joint, thanks to its extensive use in boxes, drawers, etc., the finger joint aptly suits its name its name by resembling interlocked fingers of two hands.

The theory here is not overly complicated: sets of “fingers” are cut into the edges of the two abutting pieces, their depth corresponding to the material’s thickness. On one piece, the fingers are offset with respect to the other, so that the fingers “entwine” when the two pieces are put together.

While Wikipedia states that the spacing of the fingers is typically less than or equal to the material’s thickness, I went with something simpler. On two 5cm-wide pieces of 4mm plywood, I marked out five 1cm fingers. The two pieces were held together with C-clamps, with a 1cm offset, and I went to work with a fret saw. For the first cut, I once again forgot to take into account the kerf’s size and sawed right down the lines. The fingers came out significantly narrower than the spaces between them, with many unsightly gaps showing up when the two plywood pieces were joined.

I cut off the edges and tried again. This time, I erred too much on the side of caution, and most fingers would have required significant sanding had I been determined to put them together. By contrast, one finger was still too narrow for its designated slot.

For the final attempt, I gave up on sawing the two pieces simultaneously and did them one-by-one, going slow and steady and staying to the inside of the cut lines. This third joint worked very well and, had I been working with solid wood, I would have been able to knock it together right away. However, the plywood’s layers started coming apart from the friction, and I used the Dremel 85422 grinding stone to do some light smoothing of the cuts. The final fit was still quite tight, and the joint would have stayed together with friction alone, but I PVA‘d it anyway. Since I had measured a 5mm depth to give myself some leeway, the 4mm-thick plywood fingers protruded slightly, but it would be easy to sand them flush to the surfaces.

While nowadays finger joints are usually made with machine routers and special jigs, there is a certain amount of satisfaction of doing it all by hand. I’m looking forward to doing these in thicker wood and using them in my projects.

Dremel 4000 - finger joints

Left to right: attempts 1, 2 and 3

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