How to strengthen mortise & tenon?


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My first foray into fine furniture making concentrates on mortise & tenon joints. There are 10 of these, all of them stub tenons rather than through tenons. 4 are "plain vanilla" joints, and (mainly for training purposes) 2 are draw bore and 2 have split tenons & internal wedges. This leaves me with the final 2, which I have already cut but not yet glued. They fit well but the nature of the design mean these joints need to be particularly strong - they join the rails to the legs on a cantilever table. I am wondering if I should strengthen them and if so, what would be the best approach. The legs and rails are 45mm x 25mm birch. The tenons are 40mm x 10mm, 28mm long in a 30mm deep mortise. The top of the rail is 25mm below the top of the leg. The table top is 500mm square. Any advice?

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I can't think of a better way to reinforce M&T joints than drawboring them.  That's pretty much as heavy duty as it gets.

BTW...jargon lesson...just because a tenon is non-through does not necessarily make it a "stub" tenon.  A stub tenon is one that is only the length of the groove a panel rests in, such as in the case of cope and stick joinery.  All other integral tenons are simply called "tenons."

 

stub.png

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@wdwerker: I did make some test joints using offcuts. I didn't test the joints to destruction, but all seemed strong enough in the sense that I could not move or create any play in them. I suppose I could try hanging more and more weights off them until they broke, but that isn't really what concerns me. I don't anticipate the table top ever holding much weight. I am more concerned with how well the joints will stand up over the years; will the joints work loose under constant use, with seasonal wood movement and so on. Perhaps "strengthen" was the wrong word to use.

@Eric: OK, I will draw bore the joints. & Thanks for the jargon lesson.

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A drawbore or pin is insurance against glue failure.  Yes, it does interrupt the long grain glue surface and probably does weaken the joint slightly, at least in the short term.   However, an M&T joint is cross grain and will eventually weaken on its own due to seasonal movement.   The drawbore will keep the joint strong if and when the glue fails.  

Edit: I just realized that G already made my point.....  I missed i the first time I read his post.  

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With the glues that are available today, if you can create a joint that fits together well and without large fill gaps for the glue, the M & T  joint  will be about as strong as it can be if the flat sides of the mortise and the tenon fit together well. The rest of the fit is relatively unimportant for good joint strength.  I have made M & T joints about every way possible and the only ones that ever failed were the ones that fit together poorly (my first attempts) requiring extra glue to fill the gaps, or the ones that were glue starved.

My first M & T joint was made over 60 years ago using the square mortising bit in a drill press, and cutting tenons on the mating piece using a table saw with a tenon cutting jig. I also tried chopping the mortises using mortising chisels. Making the tenons that fit perfectly always required making them a bit too thick and then carefully hand fitting them into the intended mortise using chisels, a rabbet plane, and sand paper.. This was always a time consuming process and I hated it, but I bought a new mortising machine to use instead of the drill press, and replaced the tenon cutting jig on my table saw with a better one to try to improve the process. I even tried cutting the tenons on my band saw. I was never satisfied with the quality or time that it took to get acceptable (not perfect) results.

Then I tried making mortises using a plunge router and spiral bit with a shop built jig to hold the part and stops to limit the router travel.. The result was fantastic, but I still needed to square the ends of the mortise with a mortising chisel. I then went a step further. Since the router was cutting mortises with their width so accurate and smooth, I decided to make floating tenons to fit them. The first were just cut to thickness on the table saw, but I soon switched to planning the stock to the thickness matching the router bit diameters with my planer. What an improvement this made!!! No more significant hand fitting was needed. Just cut the mortise ends square, then cut the prepared correct thickness of tenon stock to size and assemble with glue. Almost never did I need to hand fit each joint.

Then I decided to leave the ends of the mortise round and round the ends of the tenons to fit them using a bull nose bit and router table. It wasn't long afterwards that I decided to just cut the tenons a bit shorter and leave the ends of the tenons square and just long enough to fit the flat sides of the mating mortise. This made it easier to make the floating tenons and left the 1/2 round ends of the mortise empty. The joints were again, just as strong, and the empty half round spaces at the ends of the mortise left a place for excess glue to go (I have always been sloppy with the glue, but I'm improving). At this point I had cut my M & T joint construction and fitting time to about 1/3 of what it used to take me with the drill press, mortise chisels and rabbet plane, and the result is as strong and frequently stronger than the old method, because the joints fit together so much more precisely. Thick glue in a joint does not make a strong joint.

A few years ago I was about to start a project that was going to require me to make slightly over 1600 M & T joints and I decided that I needed yet a better and faster way to do it.  I ended up buying a Leigh FMT jig. This jig has to be the ultimate way to make mortises and tenons. The same setup allows you to make both the mortise and it's matching tenon, but it's so precise that you can make all of the mortises of that size on all of the parts, then go back and cut all of the mating tenons to fit them, and they will all fit together without any hand fitting. The FMT has a dial setting that allows you to adjust the gap between the mortise and its tenon by a few thousandths at a time, so the end result is as tight or as loose as you want it to be, and both the mortise and it's tenon have perfect fitting round ends, so unfortunately, no more gaps at the end of the mortise for the glue. This has forced me to be a bit more careful about how much glue that I apply, but otherwise, it hasn't been a problem. If truly square ended mortises are needed for Greene and Greene type construction, the rounded end mortises can be squared with a chisel, and there is a pattern set option for the FMT to make square ended tenons to fit them. 

I now do all of my M & T work using the FMT if I can, but I still like to use floating tenons occasionally for certain projects. I will never go back to square mortising chisel/drills and hand fitting the tenons to them. It may be OK for you neanderthals, but I want more precision and faster joinery than can be made that way. I find it much more fun to make things now too.

Sorry, I don't like the Domino floating tenon system. To me the Domino just makes thicker biscuit slots for their expensive thicker biscuits.. If you like it, great!  I don't care for it, or the standard thin biscuits. To me, they have their place, but not in fine furniture.

Charley

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12 minutes ago, CharleyL said:

Everyone is entitled to their own opinion. I'm only expressing mine, and you, yours. I will not debate this.

Charley

You don't have to like the domino joint.  That is perfectly acceptable.  But to say it is just a thicker biscuit is factually incorrect.  

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46 minutes ago, CharleyL said:

Everyone is entitled to their own opinion. I'm only expressing mine, and you, yours. I will not debate this.

Charley

If you "don't like the Domino," that's fine....I can't argue that.  But it's in no way comparable to a biscuit - objectively - and no one can argue that.

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1 hour ago, CharleyL said:

I have made M & T joints about every way possible ...

Thanks for your insights Charley. Interesting reading, but you and I are operating at very different levels and with very different goals, so my view is rather different to yours.

The time taken to do the work is not really important to me, as this is just a hobby. My project had 10 M&T joints. I cut some of the mortises on a mortise machine and some with a chisel. I cut all the tenons with hand tools. Next time I do a M&T joint I will probably do everything by hand. Totally my choice of course and I do understand that it would be the wrong choice for you.

I make no comment on dominoes etc as I have zero knowledge to draw on.

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  • 4 years later...
On 2/4/2017 at 7:23 AM, Eric. said:

I can't think of a better way to reinforce M&T joints than drawboring them.  That's pretty much as heavy duty as it gets.

According to the paper published on; research conducted by Schmidt and Scholl, 2000, University of Wyoming.

Draw-bore only serves to make the joinery tighter (aesthetics).  It does not serve to strengthen an M&T joint in any way.   All-the-more, draw-bore, load duration as well as seasoning, increase the requirements for mechanical detailing requirements by .5 as the multiplier of the diameter of peg, (end, edge and i.c. spacing).

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On 2/4/2017 at 9:08 AM, gee-dub said:

I have not seen a joint test where doing anything that interrupts the tenon; pins, pegs, wedges, etc. does anything but make the joint weaker when tested to destruction. 

 

I do agree with all you wrote there.  However, when "housing" or "pocketing" a tenon board into the mortis board, load cap. increases significantly and can allow for the use the pins, pegs, etc. without interrupting the strength of the tenon, as long as mechanical detailing requirements are met well.  

This is an application that can be employed with adjustable modular tenoning.  For example, cantilever racking, where support arms need adjustability to accommodate varying load masses.  The tenons, mortises and pockets are shaped, to hold position against the force of gravity, while the pin is a safety catch against accidental reverse deflection of the load.  For example; when moving or adjusting a material load on the cantilever rack arms.

According to the paper published on; research conducted by Schmidt and Scholl, 2000, University of Wyoming:

Draw-bore only serves to make the joinery tighter (aesthetics).  It does not serve to strengthen an M&T joint in any way.   All-the-more, draw-bore, load duration as well as seasoning, increase the requirements for mechanical detailing requirements by .5 as the multiplier of the diameter of peg, (end, edge and i.c. spacing).  

 

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55 minutes ago, The Carpenter said:

According to the paper published on; research conducted by Schmidt and Scholl, 2000, University of Wyoming.

Draw-bore only serves to make the joinery tighter (aesthetics).  It does not serve to strengthen an M&T joint in any way.   All-the-more, draw-bore, load duration as well as seasoning, increase the requirements for mechanical detailing requirements by .5 as the multiplier of the diameter of peg, (end, edge and i.c. spacing).

Eric has not been around awhile now. That said, anything that forces haunches and shoulders to register tightly resists racking forces much better. 

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