Is there a rule for mortise depth?

[Dan Karachio]

I was routing some mortises this morning (for floating tenons) and I decided to go to a depth of 1". Why? I have no idea. It "seemed" okay. Is there any rule of thumb, formula, best practice... based on the dimensions of the wood being joined for determining the best or minimum depth for a mortise? Would floating vs cut tenons make a difference?

I don't think I need to worry about this and that most wood woodworkers can do this without rules or thinking, but I was curious.

[Cody Colston]

Here are some standard rules-of-thumb which are good guidelines but actual dimensions can vary a bit, ie, I use a 3/8" mortise and tenon in 3/4" stock.

1) Tenon thickness should be 1/3 the thickness of the stock being mortised to ensure the stock being mortised doesn't weaken.

2) Tenon width shoud be no more than 5 times its thickness, 4" being the max before more than one tenon should be used.

3) For a blind mortise, the mortise depth should be 2/3 the width of the board being mortised.

[Dan Karachio]

Cool, thanks Cody!

[Frank Drew]

Dan,

There's no fixed numerical depth for mortises (e.g. 1" for cabinet doors, 2" for passage doors,...); how deep they're cut should be determined by the width of the mortised piece and the expected load the joints have to carry. In general, and as should be self evident, you get more strength (both mechanical and glue line strength) the deeper the mortise, all else being equal, so, depending on the tooling I'm using, I'd go deeper rather than shallower. If I wasn't going to use a through mortise, I'd be inclined to go to maybe within a half or quarter inch of the outer edge of a cabinet stile, maybe an inch from the outer edge of a passage or entry door.

[Frank Drew]

1) Tenon thickness should be 1/3 the thickness of the stock being mortised to ensure the stock being mortised doesn't weaken.



This is perhaps better expressed as 1/3 the thickness of the piece that has the tenon, since if you're mortising, say, a 1" rail into a 4" table leg you obviously won't make the mortise 1/3 the thickness of the leg. To avoid weakening the mortised piece, don't make either of the mortise cheeks too thin is a better rule.

[Chris Friesen]

This is perhaps better expressed as 1/3 the thickness of the piece that has the tenon, since if you're mortising, say, a 1" rail into a 4" table leg you obviously won't make the mortise 1/3 the thickness of the leg.

Arguably the upper limit is imposed by the fraction of the thickness of the piece being mortised OR the thickness of the tenon once the desired shoulders are cut--whichever is less.

In your example I'd likely put 1/8" or 1/4" shoulders on the rail, leaving the tenon substantially more than 1/3 the thickness of the rail.