Wedged Mortise and Tenon Question

[daniel leslie]

Hello,

I'm in the process of building my first legit workbench (based mostly on Schwartz's knockdown Holtapffel) and have the through mortises and tenons cut so it's time to wedge them. The tenons are 1" thick and the bottom stretchers have a 4" tall tenon with the top stretcher has a smaller 2 1/4" tall tenon. I understand the two wedges only need to be about 1/8" to 3/16" thick at the top so the question is...how far from the top and bottom of the tenon do you cut the saw kerf?

I plan to drill a 1/4" relief hole at the bottom of each kerf to help prevent any splitting. How far from the end would you cut the kerfs?

Also, these have been my first mortises and tenons either neander or not and it's been a great learning experience! Not perfect in appearance but I think they're perfectly structural which for a bench, is all a really care about.

Any help would be greatly appreciated! Thanks in advance...

[Sean Hughto]

Bill and mary start out for the store 3 miles away at the same time, but Bill takes 7 steps for every five that Mary takes and Mary's steps are 1.2134 as large as Bill's. Who gets to the store first?

Seriously, your question shouldn't be a math problem. Assuming you have angled your mortises, you know how much wedge it will take to fill the space. Just makes some wedges and try a dry fit. Drilling the base of the kerf won't hurt anything, but it's not necessary in my experience.

fitting:
http://farm4.static.flickr.com/3052/2850075798_de06a212ea_z.jpg

finished:
http://farm4.static.flickr.com/3239/2871906968_0acaced1e1_z.jpg

[Sean Hughto]

Hmm, I reread your question and now think you might be asking not how far to saw into the tenon, but how far away from the shorter sides of the tenon to cut? If that's the question, not too far or you won't be able to close the gap because you won't be able to drive the wedge far enough to overcome the resistence of the wider set of wood fibers. In short, you need it thinenough to allow you to move it (fill the gap) before you break the wedge from whacking it. In fact, you don't have to cut the tenon at all, and can just drive wedges into the gaps at the ends if you like.

[Adam Cherubini]

The answer is, it depends on the species and how dry it is. If you sawed a single kerf in the middle of the tenon, the wedge would likely compress the wood locally, or split the wood, not spring like you want.

I would say start with no more than a quarter of the width. No more. You can also place the wedges on the outside of the tenon. Skip the stop drill hole, that doesn't work in wood like it doesn't metal- or- it's different- it's like moving the kerf.

Adam

[David Myers]

I'm getting ready to make similar joints in a project so I re-read Schwarz's article in the Autumn 2006 Woodworking Magazine about wedged tenons. His conclusion there is in line with Adam's and Sean's advice: skip the holes.

The length of the kerf also does not seem important. The wedge will extend the kerf if it 'needs' to as it is driven to its maximum depth (which should be a function of the joint closing up).

[daniel leslie]

Thanks for your replies. I'm interested to see if the holes make any difference so I'll drill one of the side assemblies and leave the other assembly un-drilled. I'll report back if there's any difference!

Thanks again!

[Adam Cherubini]

In metal, stress concentrates at the tip of a crack as it flows from one side to the other. Drilling a hole at the tip of the crack allows the stress to flow more smoothly, thus the concentrations are lower, thus the crack stops growing. Wood cracks are different. Wood is non-isotropic (different mech properties in different directions). Transfer of load across grain is never very good. So whether there's a crack with a sharp end or a hole doesn't much make a difference. Different species are different.

In your case the hole WILL make a difference. The material above the kerf is like a leaf spring. If you drill a hole at the base, you just made a thinner (softer) spring. This is something you can do if you made the spring in a planing stop (for example) too stiff.

I'm not sure what bench you are building but I wonder why you would use wedged tenons over drawbored tenons. The advantage of draw boring is that you can disassemble. Also, these wedges will likely need to be glued in. And be careful about what glue you use. PVA is the obvious choice, but if there's too much stress the wedge could creep out (long after the glue is dry). Hide glue would be stiffer, but it may extrude out if your joint is too tight. I have this problem with my saw horse legs (tapered cylindrical tenons). Might be helpful to gouge some grooves in them so the glue has some place to go.

Adam

[Pam Niedermayer]

...I'm not sure what bench you are building but I wonder why you would use wedged tenons over drawbored tenons. The advantage of draw boring is that you can disassemble....

How would you remove the drawbored pins?

Pam

[Adam Cherubini]

Hi Pam. I've not not tons of these (and maybe I should be proud of that), but I have tapped out several dozen at least. Obviously they must be thru and you must have access to the back side. For benches and stuff, I taper riven pegs in a pencil sharpener. I make them long enough that the entire taper comes out the back side. I don't saw the pegs off. Seems to me once you get them moving, you are there. I tried using a metal pin punch once. Ooooh what a bad idea was that! The metal punch sank into the oak pegs end grain, permanent clinching it in the joint. I've also drilled out pegs in the past. You'd think an electric drill would work better for that, right? Made a mess of that one too. Seems a small auger is best. No matter what you are going to end up picking bits out with your striking knife.

Having thought about it, I think I've done quite a few of these and, like my veneering experience, tho I haven't done a lot of it, it seems I've experienced just about everything that can go wrong! Hope this helps!

Adam

[Pam Niedermayer]

Thanks, Adam. Based on Alexander's photos of the internals of drawbored M&T's, I'd have thought it fairly difficult to effectively punch out a pin; but I guess it isn't.

Pam

[Frank Drew]

I agree with Adam's implicit suggestion that it can be advantageous to be able to take a bench base apart at some point, say for storage between shops, etc..

[David Keller NC]

Thanks for your replies. I'm interested to see if the holes make any difference so I'll drill one of the side assemblies and leave the other assembly un-drilled. I'll report back if there's any difference!

Thanks again!

Daniel - In general, Sean is spot-on - drilling the bottom of the kerf really doesn't do anything but weaken the tenon somewhat. However, there's a caveat - the mortise where the shoulder of the tenon is going to go (i.e., the "inside" of the joint) must be a pretty good match in length to the width of the tenon.

In typical M&T construction, one usually doesn't worry about the length of the mortise so much as being concerned by the width of the mortise and the thickness of the tenon. Leaving a little extra length to the mortise allows the tenon to expand and contract to some degree because of humidity changes, and it also allows a bit of linear adjustment during assembly to ensure that the resulting frame is square.

However, if you're going to wedge the tenons, the extra length to the mortise may allow the rail to split beyond the shoulder of the tenon. Don't ask me how I know that... ;)