Mortise and Tenon Rule of Thumb???

[Boilermaker]

Is there a general rule of thumb about what dimensions to make a mortise and tenon? I've dug around a bit and I've heard that your tenons should be 1/3 the width of the thickness of the board it's going into. I couldn't really find a good answer on how long or deep they should go. If there is a rule of thumb, are there certain instances when you would stray from this?

I'm wanting to build a bed within the next year, so the mortise and tenon would have to be pretty strong. I'm just worried about sizing it so it's strong enough to support the weight.

 

[DaveO]

I generally don't make my non-thru tenons longer than 1/2 the width of the stock that is mortised.
The 1/3 rule is a good one especially if you chopping the mortises by hand. The additional mortise wall thickness helps survive the chopping and levering process better. But if you are cutting the mortises by machine, either router or chisel mortiser you can increase the mortise width to around 3/8" on 3/4" stock.
Dave

 

[walnutjerry]

Is there a general rule of thumb about what dimensions to make a mortise and tenon? I've dug around a bit and I've heard that your tenons should be 1/3 the width of the thickness of the board it's going into. I couldn't really find a good answer on how long or deep they should go. If there is a rule of thumb, are there certain instances when you would stray from this?

.

I like to keep the thickness of the tenon 1/3 or less the thicknes of the material it is on. Usually the material you put the tenon on will govern the size of the mortise. As far as length goes, nor more than half the width of the piece being mortised, as in table legs being mortised on adjacent sides, any deeper than half will result in the tenons meeting before the sholulder meets the leg. As far as width of a tenon, I like to have a 3/8 to 1/2" shoulder at the top and bottom of an apron/rail. Whatever is left is the width and also what the length of the mortise is to be.

Most important is a good fit regardless of the sizes. I have heard it put this way: close enough friction fit that you can push it together by hand but need the mallet to tap it apart. Make sense?

Good luck on the bed----------looking forward to seeing pics of progress!

Jerry

 

[jeff...]

guess I'm a little more aggressive. 1" stock I do 1/2" all the way around, 3/4 stock I do 3/8, keeps things easy, ya know?

 

[walnutjerry]

guess I'm a little more aggressive. 1" stock I do 1/2" all the way around, 3/4 stock I do 3/8, keeps things easy, ya know?

What do you mean "all the way around" ?:-? If you are talking about making the cheek cuts around the tenon and you make all cuts 1/2" deep on 1" stock you would not have anything left for a tenon. Or am I misunderstanding the statement?

Jerry

 

[jeff...]

Tendon sticks out 1/2"

 

[Gofor]

Bottom line: The longer the tenon (assuming a close fit) the stronger the glue joint. End grain glue does not provide a lot of strength. The side area provides most of the adhesion, and the cross section of the tenon and the mortise sides provide the resistance to split out/breakage from cross stress.

Thrown out for discussion: What is the advantage of a square sided tenon? I have started drilling the mortise out with either bradpoint or forstner, and then just rounding the tenon to fit with a four-in-hand file. How is this less effective than a square tenon, floating tenon, or dominoe?

Go

 

[Joe Scharle]

Bottom line: The longer the tenon (assuming a close fit) the stronger the glue joint. End grain glue does not provide a lot of strength. The side area provides most of the adhesion, and the cross section of the tenon and the mortise sides provide the resistance to split out/breakage from cross stress.
I agree and 3rds and long are the best balance.

Thrown out for discussion: What is the advantage of a square sided tenon? I have started drilling the mortise out with either bradpoint or forstner, and then just rounding the tenon to fit with a four-in-hand file. How is this less effective than a square tenon, floating tenon, or dominoe?

Go

Square sides are more common and therefore more pleasing to the eye on visible tenons. The strenght of any M/T joint is the friction fitted glued-up area. Even FWW has admitted that a glued half lap is the strongest joint! So whichever tenon fits more snuggly all way 'round will be the strongest, don't you think.

 

[DavidF]

Generally I go 1/3rds on the width and 2/3rds on the length. On a corner joint I mitre the ends of the tenons to get maximum depth.

 

[Keye]

You mentioned you will be making a bed. If you are going to use bed bolts you will need to keep the size of the bolts in mind.

I always find it interesting when the discussion turns to the shape and fit of the joint. I always cut the mortise about 1/16" deeper than the length of the tenon (someplace for the excess glue to go). I also cut the mortise about 1/16" longer than the tenon (allows me a little wiggle room for adjusting the fit). In other words, I am obtaining all my joint strenght from the sides of the mortise and the checks of the tenon. I have never had a problem with this.

Someone mentioned rounding over the ends of the tenon when using a drill bit to cut the mortise. I see absolutely nothing wrong with this. This is easier and quicker than squaring the mortise.

I worked in an antique shop while in college. Anything that came in the back door that needed work I did it. I do not remember every seeing a M&T joint failing for any other reason than the fit was to loose between the sides of the mortise and the tenon.

 

[garymuto]

Bottom line: The longer the tenon (assuming a close fit) the stronger the glue joint. End grain glue does not provide a lot of strength. The side area provides most of the adhesion, and the cross section of the tenon and the mortise sides provide the resistance to split out/breakage from cross stress.

Thrown out for discussion: What is the advantage of a square sided tenon? I have started drilling the mortise out with either bradpoint or forstner, and then just rounding the tenon to fit with a four-in-hand file. How is this less effective than a square tenon, floating tenon, or dominoe?

Go

A square tenon and especially a square mortise should be weaker than rounded counterparts. The sharp edges act as stress risers. I've used square and rounded and don't see an advantage to either.