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I am remodeling my kids sandbox and shrinking it from 12'x 8' to 8'x8' square. It is approximately 25" in depth. I will be using two "boxes" on top of each other made of redwood 2x10s.

I am wondering how easily (or even possibly) I could make finger joints to join the corners of the wood in order to make them as strong as possible.

If this is a possibility, would be using a jig saw and some chisels be my best bet to make it happen?

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  • Welcome to Woodworking.stackexchange! – Ashlar♦ Mar 8 '16 at 11:59
  • Some form of finger joint — I'm thinking beefier 'box joints' but same basic thing — should form a very sound structure. Just to echo two points from the Answers, your chisels should be as sharp as possible and do ensure you use a waterproof glue. [contd] – Graphus Mar 8 '16 at 16:43
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    [contd] One thing I would do differently to the recommended process in Eli's response is to do your marking on just one piece per joint, not both, It's better to cut the joint on one board then mark off it directly onto your mating piece, same way that dovetail pins are marked from the tails (or vice versa). See here about halfway down, and more detail here if needed. – Graphus Mar 8 '16 at 16:48
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A jigsaw and chisels are quite adequate for making finger joints in thick stock like that. Some tips:

  • Carefully mark both sides of the joint, making sure to mark the waste clearly.
  • When cutting the waste with the jigsaw, cut away from the line, into the waste area. This will make the waste area a little too small, and can be fine tuned with the chisel.
  • Fine tune the joint by dry-fitting, trimming a little and fitting again. Do this until the joint is snug (meaning you can join the parts by hand with some effort, but don't need a mallet to force the parts together).
  • Use the jigsaw to cut away as much of the waste as possible, reducing the amount of wood you need to trim with the chisel.
  • Keep your chisels sharp. Even though it's just redwood, a sharp chisel is always much easier (and safer) to work with than a dull one.

Also, since this goes outdoors, make sure to use a water-proof wood glue, such as Titebond III.

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  • If you have the time consider extending the joints beyond the adjoining face, rounding the edges a bit and adding some pins with peg covers at the joints and you will have the design of a giant Greene and Greene style box. – Ashlar♦ Mar 8 '16 at 12:04
  • Keep your chisels sharp. Even though it's just redwood, a sharp chisel is always much easier (and safer) to work with than a dull one +100 – FreeMan Mar 8 '16 at 13:15
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I would be wary of this approach myself. Joinery works best when you have a near perfect fit with no gaps. In my experience My jigsaw is terrible at cutting vertically through a simple 2x4's. I expect you might have gaps that won't make good glue joints. That is unless you make sure you leave a good chunk of waste for the chisels to clean after so as to be sure you stay in the lines as well.

Given the size of your wood I can't imagine that your fingers are going to be too small. Obviously you are not against hand tools so I would suggest a simple crosscut saw. You would still need to clean with chisels but some might consider this more fulfilling of a project using just hand tools!

Can't emphasize enough the importance of outdoor glue if you choose to go this route. Finger joints expose more of the wood to the elements so you are creating more potential crevasses for water an potentially ice depending where you live. Both can reduce the effectiveness of the joint and appearance over time.

For added strength you could drill down into the fingers (from the top and after assembled) and glue a dowel in there. That is most likely not necessary though. Also could be difficult making a ~10 inch hole and keeping it straight.

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    My jigsaw improved tremendously as soon as I bought some decent blades for it. Now it can even cut veneered plywood without ripping the veneers. In retrospect, it only makes sense: saws are always only as good as the blades on them. I think I was hesitant to fork out the money because I thought of it as a rough cutting tool (which it still is, but one that makes much nicer cuts now). It transformed the jigsaw's utility for me. – Charlie Kilian Mar 8 '16 at 15:20
  • thanks for the advice Matt, this is good supplementary information to Eli's answer above. Hopefully this all works out in the end! – Ode Mar 9 '16 at 2:10
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Skip the box joints and consider some type of joinery which offers mechanical resistance in at least one direction. For example, mortise and tenon or hand-cut dovetails resist movement along one axis. The amount of effort you'll expend using a jigsaw and cleaning up the cuts with a chisel will almost certainly exceed the effort required to join the corners with hand-cut dovetails. With the box joints, you won't gain the inherent mechanical resistance; and if you have a cheap jigsaw (<$100), your cuts will require a good amount of cleanup, regardless of the blade you use, simply because the guides on cheaper jigsaws allow the blade to deflect more easily.

Also keep in mind that, as a sandbox, the corners will have to resist a tremendous amount of pressure and movement as the humidity changes day-to-day.

Be careful with your selection of glue, as PVA glue can "creep" or allow movement under constant tension and/or with high temperatures. Polyurethane glue hardens during curing and does not melt or creep once cured.

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