Proper Length for Tenons

[Richard Amabile]

I am making a headboard and footboard for a bed from white pine. The posts are 3 1/4 inches square and the horizontal boards (top and bottom) are 4 1/2 inches wide by 1 1/4 inches thick.
My initial design was to make the tenons on the horizonal boards 2 inches long by 1/2 inch thick and 3 inches wide. In the past I have done this with a drill press and mortising attachment. But, it is difficult to get the mortises to have nice clean sides with this process and cleaning them up with chisels seems to take a very long time. So this time I purchased a good carbide upcut bit (1/2 inch shank) that has a full 2 inch cutting depth. But, now I find that it protrudes from all my routers and even the shaper by over 1/2 inch when set at its maximum relief for the first cut. This makes it quite difficult to make the initial cut because you are plunging it into the wood for the first 1/2 inch without much support (even with a jig).
Should I take back this 2 inch cutting length bit and replace it with a high speed steel 1 1/2 inch cutting depth bit?
My basic question is "How long should the tenon be on a 3 inch wide by 1/2 inch thick tenon to insure that it will be a very strong joint?

[Dan Karachio]

Hey Richard, I asked a similar question and got some great advice from our in house Gurus! See: http://www.sawmillcreek.org/showthread.php?t=148018

[Mike Cutler]

Richard

The "rule of thumb" is 3/3/5

A tenon is 2/3 the width of the mortised member.

A tenon thickness is 1/3 the thickness of the tenon member.

A tenon width is 5 times the tenon thickness max, before it should be divided.

These are rules of thumb only. Nothing that is etched in stone.

Your tenons shopuld be ~ 2" in length.
I'd use the bit you currently have, and maybe just make a jig to offset the additional length at first.

[Chris Padilla]

Use another bit to hog out the first 1/2"...it doesn't have to be a 1/2" diameter bit, either. In fact, I would use a smaller bit to remove most of the material and then bring in the nice new one to take to final dimensions. It will work much nicer with less material to remove.

[Chris Friesen]

Chris Schwarz posted a bunch of rules of thumb a while back on his blog.

My take on it goes like this:
1) The mortise thickness can be up to half the thickness of the piece being mortised, as long as there is a bit of a shoulder on the tenon. (If the piece being mortised is much thicker than the piece being tenoned, this gives much different results than basing the size on the tenoned piece. Think table apron joining to a leg.)

2) The tenon strength maxes out with a length of around 5x the thickness. However, if strength is important I'll probably make it a through tenon and maybe wedge it.

3) The width should be as much as possible while still leaving a bit of a shoulder, until you start running into problems with wood movement. I probably wouldn't go much wider than 3 inches for the glued section. If the mortise is at the end of the stock, use a haunched tenon to avoid blowing out the end grain. (Or use a bridle joint instead.)

[Darius Ferlas]

Richard, your scenario, and even dimensions, sound almost identical to mine when I was building a daybed. Except my headboard and footboard are really sideboards. The posts are 4 inches, and so are the top rails.

I used the deepest up-spiral I had to cut 2" deep, 1/4" diamater. The effect was really 1 7/8". I don't have any m/t jigs so this is how I went about cutting 5/8" mortises:

- mark where the mortises are to be cut
- drill 7/16" holes in the center of the marked areas (used a dowel high for this to make sure holes were nice straight)
- clamp the 4 posts together so that marked areas align. This gives a nice, wide and stable surface to slide the router on.
- use a stock router guide (most routers come with one) against the outside edges of two of the outside posts. Then just reshuffle the posts so each of them gets cleaned twice with a 1/4" up-spiral. Do not change the guide setup during the process, just reshuffle the posts and rout from both edges of the whole set of posts clamped together. That way the each mortise will start at the same precise distance form the edge of each piece.

I tried to shoot for the rules of thumbs (3/3/5). I was off a little here and there but nothing that would keep me awake at night.

One mistakes I made was that I did not use any stop to make sure the mortises started and ended (mortise height) at the same precise point so I had about 3/16" spread. I had to fix that with a chisel.

Overall I was very happy with the result and the whole process took me about an hour for 4 posts and the total of 8 mortises.

I am still planning on buying the Super FMT, but for now, if I can't have what I like I gotta like what I have.

[Richard Amabile]

Thanks for all the input on this. It really does help. I also found a reference on the Woodcraft website that indicates the following:

[Richard Amabile]

Sorry, but the table did not show up in the last post. Here is a link to the table they provide for mortise and tenon dimensions:
http://www.woodcraft.com/Articles/Ar...eId=252&page=1
It seems like a good starting point when supplemented with all the additional advice posted in this thread.
Thanks again