Single vs double tenon

[jnc007]

I'm currently building a toy/blanket box.  The plans call for the rails and legs to be joined with two floating tenons 1/4" thick each separated by 1/8" divider in 1" stock.  So in milling my wood I admittedly am a little under 1" thick, but that still seems like a lot to put in one area.  I guess I have multiple questions.  1)  what is the structural reasoning behind 2 - 1/4" floating tenons vs say 1 - 1/2" tenon.  2) I've heard biscuit joints are not structurally weight supporting, but I wonder if multiple dowels would work just as well?  3) Would a domino jointer work as well or is it too similar to the biscuit?  

 

thanks for your help

[Tpt life]

Domino and biscuit are not the same by function. One is a true floating tenon, the other an alignment guide. If plans call for two 1/4”, a single 1/2” should be fine. Much of that decision making process is dependent on your tooling. 

[Isaac]

as long as you provide the equivalent total cross grain gluing area, I think you will be fine.

[minorhero]

Also dowels are fine as well. Use enough of them and they make a strong joint.

[Isaac]

28 minutes ago, minorhero said:

Also dowels are fine as well. Use enough of them and they make a strong joint.

Yup, weren't they Krenovs go to joint system? I don't have a domino yet, so they work well for me. 

[gee-dub]

Your question is joint strength and joint structure in nature.  The sister question is generally what is 'strong enough'?  The double floating tenon in the plans could be the designer's idea of best practice or simply their preference for that type of joint.  A pic of the joint in question would get you more specific answers but, for a floor setting toy box I think several answers or preferences would be 'correct'; the joint would be plenty strong.

I prefer floating tenons and tend to use them whenever they make sense.  I have a router method that allows me to do them quickly and accurately so I never hesitate to use them. As a contrast, I am not nearly as swift at dovetails and so tend to use those only when the client requests them.  We all develop (and continue to develop) our preferred ways of doing things. 

I will say that if a windfall came along I would go for a Domino 700 as it will make a range of mortise that would meet most of my requirements.  Watching various videos it is obvious that once you get the hang of it, this little machine can be a quick and accurate and can replace a number of other joinery methods.

 

[Mick S]

What gee-dub said. The reasoning behind the double tenon is nearly twice the glue surface of a single tenon. i frequently use double Domino floating tenons for that reason. 

[jnc007]

First and foremost, thank you all for your support.  I've got no one locally to bounce these questions and ideas off of and I just found this forum this morning.  

I've attached to this post a couple of photos of what I'm working on.  As I posted above, during the milling process I dipped below the 1" thickness range.  I'm +/- 7/8".  This is the biggest / involved fine woodworking project I've done so far so I'm trying to get a handle on the best practices component. 

As for the Domino 700...I wish.  I already have their sander and vacuum and the loss of my left arm is felt on a daily basis.  Comparatively I was looking at the beadlock system that rockler sells and was wondering if anyone has had any experience with it or the triton doweling system.  

http://www.rockler.com/beadlock-pro-joinery-kit

http://www.rockler.com/triton-tdj600-doweling-joiner#turnto-reviews-content

Mick S you bring up very clearly the point I was thinking but not verbalising; glued surface area. That also dovetails into what Gee-dub was saying.  So I wonder, instead of half inch floating tenon would a series of dowels make more structural sense as well as more glued surface area?  

On a side tangent how is it that festool is the only one with a tenon cutter?

[wdwerker]

Festool invented & patented the Domino machine.  I think that expires in a few years.

[lewisc]

Remember: L T B D

This could be a relevant video for what you’re trying to do. 

 

[gee-dub]

2 hours ago, jnc007 said:

First and foremost, thank you all for your support.  I've got no one locally to bounce these questions and ideas off of and I just found this forum this morning.  

I've attached to this post a couple of photos of what I'm working on. 

This will get subjective but, given the "foot" present in the design I can see the logic in the double tenon.  One never knows when this item may get used as a step stool or a spare seat for a grown up while playing with the kids.

Personally I think your single tenon or even 4 - 6 dowels would be fine.  The double tenon assures this piece will be around long after the child has grown and passed it down to their own children.  

[Tpt life]

Sorry, it would take empirical data to convince me that the narrow wall between those two tenons adds that much value over a properly executed single. 

[jnc007]

23 minutes ago, C Shaffer said:

Sorry, it would take empirical data to convince me that the narrow wall between those two tenons adds that much value over a properly executed single. 

Gee-dub I get what your saying and that's how I want to build this, but C shaffer that's exactly what I was thinking.

[jnc007]

1 hour ago, lewisc said:

Remember: L T B D

This could be a relevant video for what you’re trying to do. 

 

Lewisc Thanks for that.  I've been watching his videos for a few months.  I hadn't seen that one and its quite helpful.

[wdwerker]

Using 2 edge guides prevents the router from getting wobbled and making a wider mortice.

[Coop]

LTBD. Glad to see it had nothing to do with some of the acronyms used today. Neat video, thanks for sharing. 

[Wimayo]

2 hours ago, C Shaffer said:

Sorry, it would take empirical data to convince me that the narrow wall between those two tenons adds that much value over a properly executed single. 

I tend to agree. My gut tells me that the thin material left on either side and middle is too little. If I were doing it, I would use a single 1/2" tenon leaving 1/4" on either side. If the part with the mortise was 1 1/4" thick, then I might use the double tenon to get max glue area. The single 1/2" tenon should have more than enough strength for this purpose. IMO the amount of material remaining should be about the same in both the mortixed piece and the tenon.
 

 

[Ronn W]

Since  you are putiing  the mortises into 1" (now 7/8" material),   your mortises will be only about 3/4" deep.  That is an aguement for the double tenons but with only 1/8" bewteen, well that is an arguemnt for a single, larger tenon.  Since your tenons are a healthy 1 1/2" tall,  I would go with a single 3/8" to 1/2" mortise and tenon or equivalent domino or loose tenon.  Can't see the advantage to the double tenon in this application.

[Jean [Fr]]

On the drawings the double tenons are too thick IMHO and the area in between the two tenons is too thin. Multiple tenons just allow a larger gluing surface, but are not stronger. Do you really need a larger gluing area ? Well, except oily woods or questionable glue type, you would probably not. The main thing to consider is the proportion  mortise/tenon compared to your stock dimensions. Do a simple math : take your stock width, divide it by 3 and you'll get almost the good mortise/tenon size. (works both width and height). I agree with @gee-dub, I would use dowels on this kind of project.

I usually use loose tenons or dowels, because you waste less stock. I was always disappointed by the amount of wasted wood removed from the stocks. When you use floating tenons or dowels, the needed stock is shorter and you minimize waste. The tenons and dowels can be made out scrap wood, whatever the size. I use real tenons only when pull dowels are needed.

Choosing the right dowel is easy : you need at least the equivalent of the dowel diameter all over the dowel (the 1/3 rule again...). The more dowels you can put, the stronger. You can multiply dowels using staggered arrangement. The dowel or floating tenon hardness should be equivalent to the wood hardness. Sometime I see people using beech Dominos in pine projects : poor joints I'm afraid...

[jnc007]

On 1/1/2018 at 9:26 AM, Jean [Fr] said:

On the drawings the double tenons are too thick IMHO and the area in between the two tenons is too thin. Multiple tenons just allow a larger gluing surface, but are not stronger. Do you really need a larger gluing area ? Well, except oily woods or questionable glue type, you would probably not. The main thing to consider is the proportion  mortise/tenon compared to your stock dimensions. Do a simple math : take your stock width, divide it by 3 and you'll get almost the good mortise/tenon size. (works both width and height). I agree with @gee-dub, I would use dowels on this kind of project.

I usually use loose tenons or dowels, because you waste less stock. I was always disappointed by the amount of wasted wood removed from the stocks. When you use floating tenons or dowels, the needed stock is shorter and you minimize waste. The tenons and dowels can be made out scrap wood, whatever the size. I use real tenons only when pull dowels are needed.

Choosing the right dowel is easy : you need at least the equivalent of the dowel diameter all over the dowel (the 1/3 rule again...). The more dowels you can put, the stronger. You can multiply dowels using staggered arrangement. The dowel or floating tenon hardness should be equivalent to the wood hardness. Sometime I see people using beech Dominos in pine projects : poor joints I'm afraid...

So a couple of questions.  Any partictual doweling setup you'd suggest?  I'm using birds eye maple.  is there a type of wood I should use for the dowels?  or would any hard wood would work?  Finally, If I make a 3/8" hole, am I filling it with a 3/8" dowel or smaller dowel so there's room for the glue?

Again, thank you all for your feedback. its hugely appreciated. 

[wdwerker]

I would still go with the floating single tenon over the dowels . I have fixed too many failed dowel joints. Too much of the hole is end grain and drill bits can tear the grain which weakens the joint. Seen plenty of glue starved dowel joints. Usually it's the fear of squeeze out.

Having said that if you use a very sharp brad point bit , an accurate jig and good dowels with plenty of glue (not those spiral grooved things).It can work with close attention.  Fit is important, not too loose, not too tight. Got to have room for the glue.

[Jean [Fr]]

In Europe a simple doweling jig have a great success, the Joint Genie. It's easy to make your own out of scrap wood if you're accurate enough. But I guess any jig from Rockler or whatever will do the trick.

You can make your dowels from your Maple stock scraps, but Beech, Oak, Ash, Mahogany can work too. Please refer to a wood hardness chart to find species with a close hardness. Just avoid sap wood for dowels.

Glue acts as a lubricant, so the same diameter, hole and dowel, is perfect.

[Immortan D]

I use dowels a lot in my projects. I make my own jigs using a couple of drill guides. You can see them on this pic, also some drilled parts, ready for assembly:

That said, using dowels for joinery can be frustrating. Needs some practice to get it right, but once you got it, it's a cheap and convenient joinery system that works.