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I've seen dowels, dovetails (and other joinery techniques), glue and many more used for joining pieces of wood but metal screws, brad nails and metal, in general, is usually avoided.

Why is this?

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    I would challenge your impression that metal fasteners are actually usually avoided. In the sum total of all wood-related work, including construction, there is probably more that is held together with metal fasteners (commonly screws and nails, but bolts too) than there is with glue-only joints. In furniture work while glue joints are now probably most common, screws are regularly used for certain things and some use of nails has continued through to today from the medieval period when they were one of the primary fixing methods. – Graphus May 8 '18 at 11:57
  • Alas, construction is not “woodworking” anymore than harvesting timber is, and in activities commonly referred to as “woodworking”, specifically with an eye for technique, joinery, and traditional techniques (as tagged), there is a general aversion to utilizing metal when joining wood. – coreyward May 11 '18 at 19:39
  • @coreyward: That's what I sensed. – Andrei Rînea May 13 '18 at 23:02
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Tradition plays a large role in it, but generally metal fasteners aren’t ideal:

  • Screws are strong, but unattractive.
  • Brads and other small nails are too weak to hold joints under much stress
  • Metal dowel will expand and contract at a different rate than the wood eventually loosening the joint
  • Wood glue doesn’t work on metal

There are plenty of workarounds and alternatives to the issues presented, but wood is plenty strong to join with and the techniques are well established.

  • FYI thermal expansion and contraction of metal fittings is not a concern in woodworking, the dimensional changes are too small to matter. Also, 'wood glue' isn't one thing, and some common woodworking adhesives do indeed bond very well to metals (e.g. epoxy and polyurethanes). – Graphus May 8 '18 at 11:51
  • @Graphus Oh good, you're still around nit picking. 1) When wood fibers swell with moisture and squeeze a steel dowel the fibers compress. When the moisture evaporates and the fibers contract, the joint becomes looser. Over decades, this can and will cause the joint to become loose and fail. I've repaired a number of chairs with this issue. 2) Your comment on “wood glue” is pedantic and would get you points docked on a test. It's plainly clear that in the context, “wood glue” is referring to PVA (white, yellow, waterproof, doesn't matter…it's all PVA). – coreyward May 8 '18 at 15:36
  • Re. metal "dowels" I'm not aware of much construction that relies on them. Nails, screws and bolts on the other hand... and with those the fibre-compression problem you explain is not universally an issue. – Graphus May 9 '18 at 12:08
  • Re. the glue, that's not a pedantry (I'd argue at all, because PVA use is far from universal across all woodworking) but especially here on SE because of the nature of this venue and the type of reader the Answers are geared towards — including many for whom it is not "plainly clear" that wood glue = PVA. Newbies ask about this all the time and there is even a Q&A on that exact subject. – Graphus May 9 '18 at 12:14
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    @WhatRoughBeast You must have missed the first 27 words of my comment that ended in “case closed”. It was about glue. Not movement. And my original answer, above, available for your reference, wasn't about thermal expansion and contraction. But hey, here's a sticker for your achievement: a “well actually” that didn't use the words specifically! – coreyward May 11 '18 at 19:39
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A lot of woodworkers enjoy the challenge of building stuff using only wood. There are greater and lesser degrees of this. Some people go so far as to build hinges and latches and locks out of wood, for example, though by tradition, most woodworkers metal hardware for these elements.

To expand on the points Coreyward made in his answer, the traditional techniques are largely what they are because of what was available at the time the traditions were started. Mass produced screws are a relatively recent phenomenon; the Robertson head screw was invented in 1908, and the Phillips head in the early 1930s. The flat head screws available before that were more expensive and harder to use. (source)

Traditional joinery uses tools commonly available in a wood shop for other purposes already. For example, dowels are easy to use and aren't hard to make. Even tricky joinery like dovetails that benefit greatly from a special saw to cut them still only need that one saw that can be used for every dovetail you cut, unlike screws, which would become a material cost. Glue is much cheaper.

But screws aren't universally eschewed. Pocket hole joinery has become more common in recent years. Lots of woodworkers are perfectly happy building designs using pocket holes as the primary joinery method. The wide availability of jigs for making pocket holes from Kreg have made this much more popular (though it is by no means necessary to purchase a jig to make pocket holes; like most jigs in woodworking, you can easily build one yourself). This post may be of interest to you if you want to read more about pocket holes and how to use them.

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