Diff between a good M&T joint and bad one?

[lcm1947]

Well I guess there's no doubt that this is the strongest joint but if it's not done properly I take it that it's not. So what causes a bad M & T joint. This joint seems to be tough to do but surely with practice one can learn to do it properly so why so many problems with it. I don't hear people talking that much of a learning carve on anything else. I was thinking about learning dowels but it sounds like the only one to get is the Dowel Max but heck that costs as much as a M & T setup it looks like to me.

[DUD]

I started out on dowels years ago, they were okay, but not up to what I wanted. I would rather use biscuits than dowels, much faster and easier.

Now to you question on M & T, to me the secret is correct measuring, the secind thing is leave the mortise slightly tight. I then carefully work on a good fit. Bill

[drumpriest]

Yellow glue doesn't fill gaps well, so you need to get just the right fit with a mortise and tenon joint. That's the hard part. Mostly with the tenon these days. You can cut the mortise in a few different ways, but it's not terribly difficult to cut an accurate mortise. The tenon of an integral tenon is one of those "error is doubled" scenarios. You have to cut away exactly the correct amount from either side to fit snuggly into the mortise.

A tenon that fits too loose in the mortise won't provide good contact with the mortise walls for a strong glue joint. Also, if you have a centered mortise, as you remove material from the tenon, you can easily offset the faces of the 2 pieces you are wishing to joint. This isn't always a problem, as in an apron stepped back from the front of table legs, but in a face frame application, it results in additional sanding.

I have found loose tenon to be much more forgiving. You can always run the tenon through a drum sander, or a planer, and tune the thickness to the size of the bit used to excavate the mortise.

[Russianwolf]

2 things make a strong M&T joint. The right fit (already mentioned) is one and the other is correct ratio. If the sides of the moretise walls are too thin, or if the tennon is too skinny, you lose strength. So you have to find that happy median (usually 1/3) where the sides of the mortise and the tennon all have ample strength and the right fit makes it perfect.

Another thing. Glue is a fairly recent invention in regards to M&T joints. The wedged M&T was much more common and much more forgiving. When you cut the mortise angle the top and bottom a couple degrees (like a dovetail almost) cut the tennon and make two slices in it parralel to the top and bottom. insert wedges that will spread the slices as you pound it home.

Done right, the joint will never come apart without a saw and drill. I've seen quite a few antiques that used this style joint. No glue needed. And a 16th off doesn't affect this joint as much as a glued one.

[lcm1947]

Well it sure appears to be a pain. Let me ask is it still difficult with the machines they have out now to cut these or only difficult without them? I suppose the machines would take out the guess work or am I incorrect? Oh and as far as the Dowel Max, will it do edge to surface dowels?

[drumpriest]

Forming the tenon can still be a bit tricky, and can involve a little handwork to finese the fit. Even with a tenoning jig for the TS. Loose tenon is pretty darned simple. Cutting the mortise is pretty easy with a mortiser or a router. I swear that in the next week or so, I'll get a mortise and loose tenon video up. I'll do my level best to get one up on friday.

[lcm1947]

Thanks I'd like to see what all is really involved before making my mind up about M & T or dowels. With that Wood articule about joint testing I pretty much lost my interest in biscuit joining. Glad I hadn't spent $200.00 on one.

[cabinetman]

IMO, a good fitting M&T joint for glueing should be where the tenon will slightly touch the sides of the mortice, and not bottom out. There has to be a minute space for glue. I usually brush glue on the tenon, and on the surfaces of the mortice, but not to the point that there will be pressure built up in the assembly allowing pressure to remain under the tenon. Some of the glue will be scraped off the upper portions of the mortice, and the lower portions of the tenon, and you will most likely have some squeeze-out. As the tenon gets pushed, pressurized air may keep it from fully seating, same as a loose tenon, like a dowel. So there has to be a slip allowance for glue and air to get out.



"I'M NEVER WRONG - BUT I'M NOT ALWAYS RIGHT"

[lcm1947]

Man! that sound like it could get difficult to get right everytime. Course I wasn't aware that dowels needed to do that either. My pocket holes are sounding better and better. How much time does it take to do both the M & T? Say on an average.

[Ken Weaver]

So far I've done mostly through tenons, and the Delta mortiser made a difference there. On the tenons, I've found that I have to sneak up on each one to make the fit where's its snug but not to tight that the glue doesn't do its thing. The same process works on the "regular" M&T and for joining two (as in a corner post) mitering the ends makes the difference. As far as time - as long as it takes, what else I have got to do - right?

[Jeffrey Schronce]

I didn't find M&T to have a tough learning curve. As stated before the key is measuring. The mortising machine makes quick, easy and accurate work of the mortise. For the tenon I mostly have been using a dado blade and Incra miter gauge on the TS. I have slowly started incorporating the tenon jig into my projects, particularly if I have quite a few to cut.

BTW, the highly regarded Delta 34-184 Universal/Deluxe Tenoning Jig is currently on sale at Amazon for $99 - 10% plus the whole $25 off $150 thing. Pretty good deal on what usually ranks as the best jig out there.

Drumpriest (and others) there is a pretty decent video here for loose tenons :

http://www.workbenchmagazine.com/mai...killbld01.html

I have never used loose tenons. Looks like once you have the jigs made and set up that it would be relatively quick. It would also be a good use for scrap wood for the loose tenon.

[onedash]

but not to the point that there will be pressure built up in the assembly allowing pressure to remain under the tenon.

At first I thought you were joking...Trapped air??? Even if you tried to compress air in there by jamming an oversized rubber covered tennon I don't think there will be enough pressure to have any effect. And if there is I think 500 ft lbs of clamp pressure wont care about an air bubble..

[cabinetman]

Quote: onedash
At first I thought you were joking...Trapped air??? Even if you tried to compress air in there by jamming an oversized rubber covered tennon I don't think there will be enough pressure to have any effect. And if there is I think 500 ft lbs of clamp pressure wont care about an air bubble..
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You haven't had a joint that was too tight or too much glue yet. I've had joints that would not seat, and dowels that oozed out. I wasn't kidding. A good joint does not need 500 ft lbs of pressure. Over clamping is another reason for joint failure.



"I'M NEVER WRONG - BUT I'M NOT ALWAYS RIGHT"

[drumpriest]

While you are quite correct about overclamping being a problem, M&T is somewhat naturally immune to some of that problem. The reason being that you can't really actually clamp the mating glue surfaces, as they are entirely inside of the assembly. Therefore if you have allowed for enough room for glue (5 thousandths or so), you can only clamp until the 2 pieces come together. The tenon should be slightly smaller than the mortise anyway, so the shoulders keep you from doing too much damage from overclamping.

New thread with the video I promised.

http://www.bt3central.com/showthread.php?t=24604

[onedash]

I think the only way the glue was comming off is because the tennon was two big. I always tend to clamp things as hard as I can...I haven't done to many M&T but the tennon has almost always been too big so I shave it down a little, bang it in, bang it out shave a little more off and so on way to many times because I am scared I am going to get it to small and then its to loose. Then I probably put way to much glue in it, clamp it up and call it a day. I made about 38" tennons I think they were on my kitchen table for the bread board edges. I didn't have clamps long enough so I used two pocket hole screws underneath.