Chest design question: Mortise - How deep?

[rickoman]

I am designing a toy chest for my nephew and would like some feedback on the mortise joinery. The chest has 1/4" ply panels set into a 3/8" deep rabbit.

I currently have the tenons for the rails at 3/8" long too just to make everything easy but is that too short? see detail here:

Is there a general design rule to tenon ratios?

Thanks for the feedback!

--Rick

[rmac]

Practically speaking, longer is better simply because you get more glue surface.

The guideline that I've heard is that a tenon should be 4-5 times as long as it is thick. In your case that would be 1 to 1-1/4". If you made them that long, though, the tenons on the back rail would interfere with the tenons on the side rails. So if I was doing it, I would make the tenons as long as I could without interfering with each other.

However, if you do that, the mortises will meet each other in the middle of the leg and leave a little square of more or less unsupported material (shaded in my picture) on the inside corner of the leg. For that reason, you should not run the mortises all the way to the top of the leg. Instead, stop the mortise some distance (maybe 3/4"?) below the top of the leg, and then make the tenon narrower to fit. That will leave a bit of material at the top of the leg to help support that little square piece. (This is not a problem at the bottom in your design because there the leg extends well below the borrom rails.)

-- Russ

[Onboard]

You could use a haunched tenon. If I understand your drawing and dimensions correctly your rail tenons are 2 7/8” wide, and currently 3/8” long. That’s assuming that you’re using the same 3/8” depth for the groove at the bottom of the top rail for the panel to slide into.

I don’t have a way to draw this, so please bear with me as I try to explain my idea. Picture (or do it in SketchUp) going half way down on the end of the tenon of the front rail tenon shown (top to middle). From this half way point to the bottom of the tenon, or half the width of the tenon, extend the length of the tenon by one inch. So, the top half of the tenon will be 3/8” long and the bottom half of the tenon will be 1 3/8” long. This is a haunched tenon. Then picture a mortise cut out for this extended portion of the tenon. The upper half will still fit in the 3/8” deep slot, but the lower and longer half will continue into the wood to a depth of 1”. This will put the end of the tenon within 3/8” of the side of the leg. If your uncomfortable with a 3/8” wall then you can shorten the tenon length by 1/8” leaving a 1/2” wall.

Now do this to the side rail tenon, but in this case, extend the tenon on the upper half and not the lower half as the front rail tenon. Of course you can flip this around and make the side rail extended tenon on the bottom and the front rail extended tenon on the top. The two tenons will extend past each other in their respective mortises, but since they are 1/2 width with one up and one down, they will pass each other and contact only in a small area about 1/4” square. There will be plenty of mortise to tenon surface area for the glue up. I'm trying to picture this in my mind so I hope I got it right, and that it makes sense. If not, just let me know and I'll delete this mess.

[rmac]

I think this is what Onboard is suggesting.

-- Russ

[Onboard]

Thank you, thank you, thank you Russ. That’s exactly what I was struggling to say. Of course this could be repeated on the lower rails as well.

[Torch02]

You could also move the tenon completely to the outside face of the rail, leaving only 3 cheeks. This will allow you to increase the tenon's length without having the mortises interfere with one another.

[rickoman]

Great information guys thanks for the help. I think will use the nested haunched tennons.