How Many Dominos for a Large Rail?

[TomInNC]

I am in the process of joining rails that are 1.75 inches thick by 3 inches wide to 4x4 legs. These rails are being used to support some slats. I am going to use the Domino to attach the rails. My initial plan was to use 2 of the 14mm dominos. After doing some layout, it looks like using two of the 14mm here will leave less than 7mm between the dominos and the edge (and between the dominos themselves). At least as per the manual, this is cutting it dangerously close regarding the amount of material around the domino. Would 1 centered 14mm domino be enough to support the 3 inch rail? I used traditional MT on the top and bottom rails, and this rail is located in the middle. 

 

Based on the 1/3 "rule, it seems like the 14mm tenon is close for a 45mm workpiece. Since the tenon is only 28m wide, however, this is creating a huge "cosmetic shoulder" in the terminology from the Cosman discussion below. Will a cosmetic shoulder this large significantly weaken the joint?

 

 

https://robcosman.com/pages/newsletter-article-mortise-and-tenon-joints#:~:text=In its most basic form,The tenon tongue.

[Chestnut]

Do 2 7mm is a good amount of material ive done less.

[BillyJack]

You can use two Dominos, but I suggest a metal Bracket or similar for additional support..

[TomInNC]

Thanks. Just tried the 2 dominos on a test piece, and it does seem perfectly sturdy. Still getting the hang of this thing.

[Chestnut]

Their manual provides a good guideline but you have to remember that the engineers play it extremely safe. They want to ensure that 95% of situations aren't going to cause issues. Those "rules" I"ll call more guidlines and you can use them as a guide keep in mind they are also made to play it safe and work in 95% of situations.

Putting a domino into end grain close to the edge is not problematic. When getting close to the edge is problematic is at the end of a leg when the mortise goes into the face of the leg. This could lead to the leg splitting. This is not a problem exclusive to the domino many times with traditional M&T I've split legs sending a slightly oversized tenon home.

As far as strength goes 2 14mm floating tenons in a 1.75" wide rail will be nearly as strong as integral M&T. I made an L bracket to hand some yard games in my garage. The stock was 1.25" x 1.25" and I used 2 8mm dominoes. I have about 65 lbs hanging from the end of a 12" post with no concerns. If you extrapolate this to a shelf held up by the 4 corners the shelf would be able to reasonably hold 260 lbs.

[TomInNC]

On 4/4/2023 at 10:38 AM, Chestnut said:

Their manual provides a good guideline but you have to remember that the engineers play it extremely safe. They want to ensure that 95% of situations aren't going to cause issues. Those "rules" I"ll call more guidlines and you can use them as a guide keep in mind they are also made to play it safe and work in 95% of situations.

Putting a domino into end grain close to the edge is not problematic. When getting close to the edge is problematic is at the end of a leg when the mortise goes into the face of the leg. This could lead to the leg splitting. This is not a problem exclusive to the domino many times with traditional M&T I've split legs sending a slightly oversized tenon home.

As far as strength goes 2 14mm floating tenons in a 1.75" wide rail will be nearly as strong as integral M&T. I made an L bracket to hand some yard games in my garage. The stock was 1.25" x 1.25" and I used 2 8mm dominoes. I have about 65 lbs hanging from the end of a 12" post with no concerns. If you extrapolate this to a shelf held up by the 4 corners the shelf would be able to reasonably hold 260 lbs.

Makes sense with the engineers. I was initially concerned because of the suggestions that Festool had in what they label the supplemental manual (https://www.festoolusa.com/service/owners-manuals). Is this the manual you are referring to? I didn't see anything on tenon sizing in the main manual with the 700. 

I ended up with 7mm between the dominos and the edge, and 6mm in between the dominos. I'm guessing this would be pushing the envelope with something like MDF, but it seems fine on the Sapele.