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I kind of assume that dadoes, grooves, rabbets / rebates should be no deeper than half the thickness of the material. So, in 3/4" stock, dadoes should be no deeper than 3/8". Anything deeper would compromise the integrity of the wood. Is that correct?

And, going the other way, is there a minimum depth of a dado before it's either not adding any strength, or not assisting with alignment? Maybe 1/8" or 1/16"?

Finally, is there an optimum depth of dadoes? In some of the youtube videos I've seen, it seems that 1/4" dadoes are fairly common in 3/4" material. Is that because it gives the optimum strength, or is just easier / faster to cut, or something else? Does it depend on the thicknesses of the pieces being joined?

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    This really depends on the materials, the dimensions of the materials, and what you are trying to accomplish. – keshlam Nov 18 '16 at 21:23
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I kind of assume that dadoes, grooves, rabbets / rebates should be no deeper than half the thickness of the material. So, in 3/4" stock, dadoes should be no deeper than 3/8". Anything deeper would compromise the integrity of the wood. Is that correct?

Broadly speaking yes.

In just about any wood, with any joint, if you end up removing more wood than remains you're obviously undermining the strength of the material. So with a dado (UK: housing joint) going deeper than half way should be avoided.

And, going the other way, is there a minimum depth of a dado before it's either not adding any strength, or not assisting with alignment? Maybe 1/8" or 1/16"?

This is less clear cut I feel because so much depends on the inherent strength of the material. While a 1/8" (3mm) dado wouldn't be particularly strong in any wood in stronger hardwoods it could be more than sufficient, while in something like pine the same joint depth would probably represent a noticeable weak point.

The expected load and the outright thickness of the material obviously tie in here as well. In a small campaign case in pine 1/16" dados may be appropriate and provide suitable strength, but obviously not for a pine shelf in a sizeable bookcase or cupboard where you want to give the ends of the boards much more support.

I suppose one way of looking at it is if there's no good reason to do a shallow dado then don't. Many joints in woodwork are actually stronger than strictly necessary, but erring on the side of greater strength is rarely a bad thing. And as it may be no more or less effort to create a weaker version then why do it anyway?

In some of the youtube videos I've seen, it seems that 1/4" dadoes are fairly common in 3/4" material. Is that because it gives the optimum strength, or is just easier / faster to cut, or something else?

I suspect some of this will come down to habit or ease of setup rather than being carefully considered and done with best strength in mind.

But it is actually quite a common rule of thumb to have a joint be 1/3 the thickness of the material, although that's by no means set in stone and variation either way is possible and acceptable.

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  • My reasoning for doing a shallow dado would be if the pieces were already cut to length, but I wanted to cut a dado for alignment purposes. Then the option is a shallow dado vs a butt joint, and whether the effort to cut the shallow dado would be worth it or provide any alignment benefit. – mmathis Nov 19 '16 at 0:25
  • @mmathis Obviously the best thing here is not to cut the shelves too short first :-) but once you have if you need to strengthen the butt joints you can toenail in from the underside using fine finishing nails or small brads. Screws in from the side would work too but obviously harder to hide those. If you don't mind the small change in dimension a shallow dado or rabbet is worth doing in this sort of situation, but strength of material matters as well as the expected load. I don't think I'd rely on very shallow rabbets or dados in pine/spruce but in e.g. white oak or maple prob. fine. – Graphus Nov 19 '16 at 8:30
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    If you need alignment and a bit of strength but not change in length, look at biscuit joiners. They are readily available used. – Aloysius Defenestrate Nov 19 '16 at 15:04
  • @AloysiusDefenestrate Yes +1 for the suggestion. They're a good solution for this type of thing if one already has one (I don't feel they're a must-buy if you don't). – Graphus Nov 20 '16 at 19:04
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    @mmathis But just to add, dowel joints work as well or better and don't require a specialist power tool to form (or any power tool since you can drill the holes manually). Also, less appreciated is that you can make the dowels yourself in various ways, including simply hammering slightly oversize scrap pieces though a dowel plate so money-saving in more than one way. – Graphus Nov 22 '16 at 7:52

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