Question about Loose Tenon Size

[Adel Heiba]

What are the rules for sizing loose tenons? I am interested in constructing a table by connecting a 7/8” apron to a 2”” table leg with loose tenons. What should the thickness off the tenon be? How would things change if the apron is 3/4”?

[John Goodin]

Many people follow the 1/3 rule and make the tenon 1/3 the thickness. Centered, the tenon and walls would be the same thickness. When the math does not make a common fraction I like to make the tenon a little thicker. Since a 7/8” apron would dictate a 7/24” tenon I would probably go with 5/16.” For a 3/4” apron it would be a 1/4.”

[Mick Simon]

When the thickness of the apron stock is the same or close to the thickness of the mortise piece, the 1/3 rule applies. Think face frame. Another guideline applies to your case. The tenon thickness can be up to roughly ½ the overall apron thickness since it's going into a thicker leg. So, a 3/4" thick apron could support a 3/8" tenon. The strength of the joint is more dependent, however, by the width of the tenon, followed by the length, as there is more glue surface to bond to. Tenon thickness is the third most important factor.

[Adel Heiba]

Thanks — Mick and John!

On my next table of 7/8” aprons, I will try to up my float tenon size to 10 mm from 8 mm. I have been using the former.

It is difficult to translate the old rules for mortise and tenon to float tenons. Does the 1/2 rule or 1/3 rule apply to the tenon (thin piece) or to the leg thick piece.

Regards

Adel

[Adel Heiba]

I meant I have been using 8 mm not 10 mm — correction

[Jim Dwight]

I would use a 8mm thick tenon in 7/8 and either a 6mm or an 8mm in 3/4. 7/8 is about 22 mm so 8mm is only slightly over 1/3. 6mm would be way under 1/3. 6mm is pretty close to 1/3 the thickness of 19mm or 3/4 stock but a lot of 3/4 is really 13/16 and I just like my tenons to be if anything a little over 1/3.

It is also not hard to overlap holes to make wider mortises with a domino. It can also allow you to center the mortise easily by cutting from both sides. But that won't work on table/apron joints.

[Adel Heiba]

Hi Jim

Thank you for your response.

Adel Heiba