Festool Domino - Tenon Size Question

[Patrick1425]

I'm making some bookshelves for my son in his playroom.  Its going to be a pretty simple design... ends up looking like stacked squares.  In total it will be 6 ft tall, by 3 ft wide.  I'm going to use 3/4 MDF wood.  However, I had a question about which size Domino's I should use?  The 5mm seem about right, but I wanted to be sure.  Any suggestions? 

 

[peter halle]

If you do the typical 1/3 the thickness, then the 6mm would be closest to appropriate.  Many also use the 5mm.  I use the 5mm.

Peter

 

[ridgenj]

Peter, please educate me, what is the rule of thumb to determine the right size.
Many thanks
Luis

 

[Brice Burrell]

Peter, please educate me, what is the rule of thumb to determine the right size.
Many thanks
Luis

I'm not Peter but I can elaborate.  The rule of thumb is the tenon should be 1/3 the thickness of the material.  So, your 3/4" MDF/ply should get a 1/4" (6mm) tenon according to this rule.

Now, I'd advise against the 6mm tenon because the length of the 6mm tenon is 40mm.  The problem that presents is that the mortise in the face will go all the through the 3/4" MDF/ply.  Wit the 5mm tenons the length is 30mm, so the 15mm mortise won't go all the way through the material.  

 

[peter halle]

Thanks Brice!

Peter

 

[Rick Christopherson]

The 1/3 rule of thumb is based on the assumption that the tenon and substrate are similar material. However, in this case, they aren't. The tenon is stronger than the MDF substrate, so the tenon size should be reduced. The tenon should also not be centered vertically in the joint either. The shelf will be stronger if the tenon is kept low in the joint. This is all described in the Supplemental Manual that apparently neither Peter nor Brice read.  [poke] (edited to add Brice too.)  [big grin]

The reason I added Brice, is because the manual also explains that it is not necessary to center the tenon's length in the joint either. So that is not the reason to choose 5mm over 6mm. Even if a 6mm would have been the appropriate thickness, you could still plunge only 15mm deep into the carcase sidewall, but then plunge 25 mm deep into the shelf.

 

[peter halle]

Read but not memorized.  [poke]

Peter

 

[Rick Christopherson]

You'd better get working on that. There will be a quiz!  [scared]

 

[Brice Burrell]

......The reason I added Brice, is because the manual also explains that it is not necessary to center the tenon's length in the joint either. So that is not the reason to choose 5mm over 6mm. Even if a 6mm would have been the appropriate thickness, you could still plunge only 15mm deep into the carcase sidewall, but then plunge 25 mm deep into the shelf.

I'm well aware of the offset mortise approach (even without ever reading your manual [tongue]).  I find the risk of forgetting to change the depth setting isn't worth the minimal, if any, additional strength gained by the 6mm tenon (in this type of application).  If you forget, and you will at some point if you do a lot of work with sheet goods, the piece is trashed.

I'm not sure I agree with not centering the mortise in MDF.  If there is enough weight on a MDF shelf it will deflect and you could blow out the thin(er) bottom side of the mortise.  I've never tested this theory so take it for what it's worth...  That said, I rarely center my mortises.  I use the 20mm preset for 3/4"/18mm sheet goods.  I generally, but not always, leave the thicker portion at the top.  I use the 20mm preset out of ease, not for any additional strength.  Beside, I use plywood almost always and I don't think any strength is gained with an offset from center mortise in plywood.