M&T “Rule of Thumb” Confusion

[Sam Lucas]

I’m trying to understand the mortise and tenon “Rule of Thumb,” specifically when using machinery (not chisels). Everyone quotes the Rule of Thumb differently, and I’m left . Below are various “rules” I’ve come across, with [brackets] indicating variations in the “rule”. Please tell me which “rule” you follow and, if possible, the source of your rule.

Thickness
Should be [1/2] [1/3] the [tenon] [mortise] stock


Height/Length
Should be [4x] [5x] tenon thickness
Should be [1/2] [2/3] [3/4] the width of the mortise stock

Width
Should be 5x tenon thickness, not to exceed 4” or the tenon is divided
Should be full width of tenon stock, less ¼”

Now, tell me your M&T sizes for these 2 scenarios, each having the M&T in the center of the stock, assuming no haunched tenons, etc.

Scenario 1
Rail stock: ¾” thick by 3” wide
Stile stock: ¾” thick by 3” wide

Scenario 2
Rail stock: same as Scenario 1
Stile Stock: 3” thick by 3” wide

[John Coloccia]

For thickness, I'll grab a chisel (or router bit....or whatever) that's somewhere around 1/2 thickness of the tenon stock. Just whatever approximately closest fit I have is what I use. For the rest, I'll pencil it in so it looks 'bout right for a tenon. I mean, we've all seen so many tenons, even if only on New Yankee Workshop, that I doubt anyone would run into serious trouble just using their eyeball and guesstimating to the nearest convenient measure for whatever tool we're using.

Anyhow, for your case, I'd probably make the tenon about 5/16 or 3/8", or whatever convenient chisel/bit I happened to have, and maybe an inch or two long. So for example, I wouldn't make a 1/8" tenon, and neither would you because it would look completely stupid and out of place. Ditto re: a 1/2" tenon. It just wouldn't look right with a 1/8" shoulder and it would look obviously out of place to you.

I'm not sure what importance the width is other than to say the wider it is, the stronger it is, so I can't imagine why you wouldn't go almost to the edge every time. If you want number, maybe leave about the same shoulder as you did on the sides, or just a little bit more. It's really not critical.

All that other stuff makes my head hurt.

[Kent A Bathurst]

For me -

Tenon at least 1/3 the thickness of the stile/rail it is cut into. I often make it 1/2 the thickness.

Width of the tenon - that is easy for me I set up my TS to make the shoulder cuts on the tenon faces, and then go around all 4 sides. That defines the width of the tenon - I want 1 setup on the TS.

Length of the tenon - I dunno. When it is, say a door frame with rail tenon into stile mortise, I want the mortise to be at least 1/2 the depth of the stile's width. Maybe 2/3.

When I am going into a leg from both sides, then the length of the tenon is decided for me - 45* cuts on the ends of the two intersecting tenons, and so the geometry of the leg defines their length.

[Tom M King]

My rule of thumb is to eyeball it. I have an eye that I can trust.

[Sam Lucas]

Thanks for the responses so far. As a beginner, I have to rely on standards rather than instinct, which is why I posed the question. Comments regarding specific measurements would be most helpful. Maybe one day I'll be able to have a "feeling" or an "eye" for what's right. But guidance until that day is appreciated.

[Tom M King]

google found this: http://www.woodworkstuff.net/KnipMT.html

[Pat Barry]

3/4" rail = 3/8" tenon regardless of the stile size (assuming the stile is equal or greater than rail). Height = 2 5/8" for a 3" rail height. Length of tenon = at least 3X the tenon thickness at a minimum up to 5X (eyeball it).

Edit : bad math - 2 5/8 allows for an equal stock removal on all four sides of the tenon so it is an easy single setup stacked dado blade and 90 degree bevel gage setup operation for both shoulders and sides

[Sam Lucas]

Tom - Yes, that link was one of the dozen or so references I read through before posting. Each of which had a slightly different spin.

Pat - Thanks, setting the dado only once will certainly save some time.

[keith micinski]

If I'm making a small table or nightstand I always make the tenons a 1/4 inch thick with 3/4 aprons. I usually use an inch and a half tapered leg so the tenon ends up being about 3/4 long with a mitred end. I can stand on these 15 years later and have no issues with strength. As a matter of fact I'm not sure there is any need for any tenon thickness larger then 3/8ths. Also the longer you make a tenon the easier it is to snap it. I can't think of a reason to have one longer then an inch and a half which is the longest I have ever made one for a dining table.

[Chris Friesen]

The rationale behind the rules are as follows:
1) You want the tenon to be big for strength.
2) You want to keep some shoulders on the tenon. They help with racking resistance but also cover up any "dings" around the mortise.
3) You want to have some thickness to the mortise walls for strength, with both walls together being at least the thickness of the tenon. If mortising by hand, you want the mortise walls thick enough to minimize risk of breaking them while reefing on the chisel (so in that case with 3/4" material use a 1/4" tenon).
4) If you make the tenon too wide or too long then wood movement with humidity change comes into play. (Not a problem at the scale you're talking about.)
5) If you run the tenon all the way through you can wedge it for strength. This doesn't fit all designs though.

So...in your case assuming mortising by machine and non-through tenons, I'd go with:

Scenario 1: 3/8" tenon, 2" long, 2 5/8" wide. This gives 3/16" shoulders on the tenon cheeks, so I'd use the same on the edges to make it easier to cut.
Scenario 2: 1/2" tenon, 2 1/2" long, 2 3/4" wide. This gives 1/8" shoulders all around.

[Jim Rimmer]

For thickness, I'll grab a chisel (or router bit....or whatever) that's somewhere around 1/2 thickness of the tenon stock. Just whatever approximately closest fit I have is what I use. For the rest, I'll pencil it in so it looks 'bout right for a tenon. I mean, we've all seen so many tenons, even if only on New Yankee Workshop, that I doubt anyone would run into serious trouble just using their eyeball and guesstimating to the nearest convenient measure for whatever tool we're using.

Anyhow, for your case, I'd probably make the tenon about 5/16 or 3/8", or whatever convenient chisel/bit I happened to have, and maybe an inch or two long. So for example, I wouldn't make a 1/8" tenon, and neither would you because it would look completely stupid and out of place. Ditto re: a 1/2" tenon. It just wouldn't look right with a 1/8" shoulder and it would look obviously out of place to you.

I'm not sure what importance the width is other than to say the wider it is, the stronger it is, so I can't imagine why you wouldn't go almost to the edge every time. If you want number, maybe leave about the same shoulder as you did on the sides, or just a little bit more. It's really not critical.

All that other stuff makes my head hurt.

^^^^^What he said. Especially "All that other stuff makes my head hurt. "

[Duane Meadows]

Well, when you make the tongue thicker, the walls of the mortise get thinner, unless you go to thicker stock. So, there's kinda of a happy medium thing going on. That's why I would generally go with a tongue about a third of the stock thickness. YMMV.

[Myk Rian]

1/3 to 3/5 of the stock thickness. Or eyeball it. Whichever is convenient.

[Chris Friesen]

...It just wouldn't look right with a 1/8" shoulder and it would look obviously out of place to you.

I'm not sure what importance the width is other than to say the wider it is, the stronger it is, so I can't imagine why you wouldn't go almost to the edge every time.

I wouldn't have a problem with a 1/8" shoulder, assuming that the mortise is in a piece that's thicker than the piece being tenoned.

The width of the tenon comes into play when you're talking really wide stuff, like an 8" wide stretcher on a workbench, or a breadboard end. That's too wide, it'll likely crack due to wood movement. So you either split it into multiples and only glue one of them (pinning the rest with elongated holes in the tenon), or you use a narrower full-length tenon with an unglued haunch to keep the rest from twisting.

[Chris Padilla]

The rationale behind the rules are as follows:
1) You want the tenon to be big for strength.

...but at the expense of a thin-walled mortise? I tend towards the 1/3 rule.