What is proper mortise depth?

[Robert Opalko]

I’ve been trying to make a project from some plans I bought online for a nightstand. The plans call for 1 1/2” square legs with 2 mortises on each leg (Cherry, btw) for rails. The mortises are shown to be centered on each leg, 3/4” deep, 3/8” wide, 3/8” from top of leg. Being the ignorant plunderer that I am I didn’t realize this would mean the tenons would intersect. (Of course, on another page of the plans they show the side mortises as being 3/4” deep and the back mortises being 1/2” deep, so I should have already been suspect of the plans.) By the way, I see in Tage Frid’s book that he uses tenons that intersect and then miters the ends of them so they will fit together…hmm.

2 of the legs split at the top of the leg above and between the two mortises in different stages of either assembly or disassembly. I did not force any of the tenons before you get on your high horse.

It simply appears to me that there is just not enough “meat” above the mortises that would ever keep them from cracking – am I wrong here?

What is the general rule of thumb for making a mortise in terms of mortise depth vs. stock width and considering mortises that will be adjacent (perpendicular to one another).

Back to square one…sigh.

Cheers

[Rod Sheridan]

I Have often made mortices that are 1/4" or 3/8" from the end of a leg, without problem.

Are you sure your wood is properly dried?

As to forcing the parts together, a tenon should fit in a mortice by hand, a nice slip fit, remember you need a bit of room for the glue as well. There should be no force required to insert the tenon. Think of it as sliding a plain bushing on a shaft, there has to be room for the oil film so the bushing should slide easily, without play, just like your tenon should.

With a fit like that, you can pick up a dry fitted piece and have all the parts stay together..........Rod.

[keith micinski]

I make a lot of 1 1/2 legs and I used to make them 3/4 and then mitre them but now I just make them 5/8ths and haven't had any problems. As far as splitting goes I have no resolution other then the tenons were just to tight even though you didn't think they were. I have never had one split though and I have forced some together so I guess I really don't know.

[Stephen Cherry]

Back to square one…sigh.

Cheers

Not really, if the parts split, they can probably be glued. use a sheet of paper, or something to work some glue into the crack.

And as Rod said, snug, but not tight.

Plus, as is usual, initial failures are part of process of learning any skill.

[Peter Quinn]

I think 3/8" from the top in that case is a little close. That leaves only a bit of pretty weak short grain to resist racking forces, I'd rather see 5/8" or 3/4" if the parts allow it, or at least a haunch near the top to step back that short grain area a bit. That said, my high horse was saddle and waiting, so I'd guess you didn't know your own strength and tweaked the legs a bit more than you realized. It could be the wood, it could be the design, but it could be you. Accepting that as a possibility will lead to more growth than looking for another source of blame. I can tell you I have screwed the same thing my self. Luckily the legs glue up pretty well and the whole thing in my case has been fine for the past two years. The cracks are on the inside or to the top, which leaves them blind, so there they live. Might just be fine if you spooge some glue in there when you clamp up.

[Larry Edgerton]

Look at it as a lesson, and do not get frusterated. People move away from M&T joints to easier methods that are not as strong because of the initial frusterations, but ALL of us went through that learning curve. Keep trying.

Larry

[Cody Colston]

Glue the split tops. Miter the tenons. Make sure the fit is not too tight. (note that moisture in glue or even a change in RH can affect the tenon slip fit)

Most tenons need a bit of hand work after milling to adjust the fit. A shoulder plane is the perfect tool but if you don't have one, you can use a sanding block.

BTW, the rule-of-thumb for tenon length is 5 x the tenon thickness. That won't work in your case with 3/8" tenons. (1 7/8")