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I am planning on building a block that is about 16" square to mount an exterior light. The block is rimmed with a 4" casing (sometimes called a "molding"). The casing is 3-7/8" wide and 1" high at the outside edge. So, the casing will form sort of a picture-frame-like rim around the block.

The problem is fitting it together so that no fittings are visible and the miter joints do not open up later. I want to minimize exposed screws or nails that would have to be covered up somehow.

In old books I have seen that when similar types of joints are made they recommend both using a tenon of some type to register the miter and also a screw to tighten the miter. The problem with using screws is that it would leave holes in the outer rim of lamp block.

I know that with picture frames they staple the miter on the back, but that seems too flimsy for this use.

Another idea might be to use flat brackets as shown below, but the problem with that is that it will stick out from the back of the casing, so some kind of recess would have to be milled, so that the casing can sit flush on the base block. Also, a bracket like this will just hold the joint, not tighten it.

enter image description here

  • What about wood glue and brads? That should make tiny holes that are easy to conceal and wood glue is awesome – UnhandledExcepSean Nov 8 '19 at 0:41
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I'll admit I'm having a hard time visualizing exactly what you want to do, but it sounds like you have plenty of room for at least two types of joinery. The problem here is that mitre joints aren't very strong compared with other types of joints. So some sort of reinforcement is usually recommended.

I can think of two techniques you probably have room for:

  1. A small biscuit joint, which will be hidden.
  2. A spline can look really nice with contrasting wood, or can match and be nearly invisible.

Alternatively, you can opt for a lap-joint instead, which is a lot easier to make and glue up square, and is much stronger.

I'm thinking that trying to hide some metal hardware in a relief is going to be way more trouble that it is worth. Unless, of course, you were already planning on routing out a channel in the back of the moulding anyway.

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  • I am currently planning to use a spline, similar to a picture frame. – Treow Wyrhta Nov 8 '19 at 20:41
  • @TreowWyrhta depending on the full dimensions, you might be able to insert a spline in the back of the moulding mitre joint (similar to how some frames are stapled) effectively hiding it. – jdv Nov 8 '19 at 20:42
  • @jdv, were you thinking spline or key? Splines can be fully hidden, by installing in stopped grooves or in a part-stopped groove which is slightly easier to form (assuming done using a router) so only one end of the spline is revealed on the inside corner where it then becomes hidden after full assembly. But if a router is available milling L-shaped mortises at the back is a snap...... not that these are exactly hard using a chisel and mallet. – Graphus Nov 9 '19 at 8:21
  • @Graphus I think I use the same word "spline" to mean both of those, though in my mind I was imagining a key. Probably incorrect, but close enough for this discussion I think. My comment was imagining a bit of a hack, with a spline/key like piece just inset into the back of the mitre. – jdv Nov 10 '19 at 19:42

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