Wedged tenon question

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adowden

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Amy
I need to attach a couple of through tenons with double wedges for the cherry buffet I am building. I have never done wedged tenons before. When I researched it, it appears there are two different ways to do it: 1) angle the mortise to allow for the expansion caused as the wedges are hammered in 2) cut angled slots in the tenon to accept the wedges. The plan that I am using shows the latter way. It shows a square mortise with wedged chunks cut out of the tenon.

Here are my questions. 1) Which method is better? 2) How much do you vary the wedge angle for solution #2? - I couldn't find any details about this method. I am assuming if you made the wedged hunk the same size as the wedge, there would be no advantage to doing this.

Right now the tenon is a very snug fit in the mortise. I don't want to do anything to weaken this joint. Any suggestions would be greatly appreciated. Here is a picture of the situation. The mortise is about 0.70" deep.



Amy
 

James Davis

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James Davis
I am far from an expert, but IMHO there is no advantage to cutting the tenon away to accept the wedges. I feel the mortise needs to be enlarged to accommodate the wedges. I may be totally off base here but if we are not opinionated can we really call ourselves woodworkers? (and yes, I do remember the joke about the piccolo player in church)

James
 

DavidF

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David
Hi Amy, it depends on the reason for the wedges. If like mine in the table top they are primarily for decoration, then I simply cut a slot the width of the TS blade in the tenon and make a good fitting "almost" parallel piece to go in there. It probably has a max of 0.5mm taper over the length. hammer in and cut flush. If, however it is done for structural reasons as in timber framing or a traditional trestle table then the mortice must be made tapered outwards at the top. This way the tenon becomes a dovetail as the wedge is driven in and mechanically locks the joint. Be aware that I have seen several examples on the web were they suggest you can open up the tenon by 1/8" with the wedge, trust me, unless this is green wood joinery or the tenon is 4" long, it won't happen without breaking something. On the last table I made where the stretchers had wedged through tenons, I did some tests on sample joints and the max I could open up the joint was about 1mm total (1/16" near enough) this will depend on wood species, but I would limit it to taking 0.5mm off each side of the mortice at the top, tapering down to about half way down the tenon. Drill a small hole through the tenon at the bottom of the slot to avoid the grain splitting.

In you application, I would say this is the decorative approach, in which case, don't taper the mortice, just make a nice tight, very slightly tapered wedge and glue the joint in the normal way, drive in the wedge, let the glue dry and cut the whole thing off flush.

Hope that helps.
 

adowden

New User
Amy
Shh, don't tell anyone. It was perfect, but there was the smallest gap where the drawer dividers were tenoned into these vertical supports so I fiddled with it to close that more noticable gap.

I am so torn. I agree it is kinda decorative, except for the fact that the top will cover it anyway. I am so scared that when I try to wedge it, it will shift the position of the vertical supports and affect the drawer divider where you would definitely notice a gap.



Thanks so much for all the suggestions. My biggest frustration with doing something new is that the books are so generalized that you can't get a feel for the specifics. I sure don't want to split the tenon or mess up the alignment at this point.

Amy
 

scsmith42

New User
Scott Smith
Good morning Amy!

I don't have personal knowledge about wedged tenons, but I do have a lot of books!

In The Complete Illustrated Guide to Joinery by Gary Rogowoski, there is a section on wedged through tenons on page 368 - 369. In it, he recommends against widening the mortise. He does recommend drilling a relief hole about 2/3's of the way back from the end of the tenon.

The article is pretty straightforward and understandable. (There is also an article on an end-wedged through tenon that won't cause tenon splitting. End wedged tenons utilize a tapered mortise.)

If you'd like, I'll be glad to either loan you the book or make a copy of this portion of it.

Here is the text:

"A tenon wedge should be aligned so it applies pressure against the end grain of its mortise rather than the long grain. This will avoid any tendency for the long the long grain to split out. First, cut the mortise and fit the tenon cheeks.

Next, cut the tenon to height so it just fits inside the length of the mortise. Use the bandsaw to trim it down to size. Check the fit from end to end of the mortise. Then round the tenon ends if the mortise has round ends.

Because of the pressure that a wedge exerts on a tenon, especially in dry wood, don't widen the mortise. This increases the risk of splitting out the tenon when you apply the wedge. Keep the mortise at one length throughout and concentrate on a smaller wedge doing its job of creating pressure.

To avoid the risk of splitting out the bottom of the tenon with the wedging pressure, drill a 3/16" relief hole about two-thirds of the way back from the end of the tenon on the drill press. This hole will spread out the wedging pressure around a circumference instead of concentrating the pressure at the bottom of the wedges slot, where cracks will occur. Cut the wedge slot down to this hole on the bandsaw, using a fence to guide the cut. Make the kerf about 3/32" wide.

Make the wedge from a harder wood than your tenon. Cut the wedge to about twice the thickness of its slot and make some extra wedges that are a bit thicker. If you find that your wedges are going in too easily, use the thicker wedges for the remaining tenons. Cut the wedges to length at about three-quarters of the full length of the wedge slot. You'll find that the wedging force will tend to fill up most gaps that show at the edge of a through joint.

Put glue in the wedge slot and use a hammer to drive the wedge. When the sound changes from a thud to a ping you've gone as far as you can with the wedge."

I'm making a run to Greensboro today to bring back some lumber for North Carolina Woodworker members that purchased at the recent auction, and won't be back online until later today. Let me know if you'd like a copy of the article, or to borrow the book.

Scott
 

Gregory Paolini

New User
Gregory Paolini
Amy,

I make a fair ammount of wedged tennons as well as tusk tennons with the Art & Crafts pieces that I make -

When I make a wedged tennon, I make it just like a standard through tennon, concentrating on the fit at the cheeks. When my basic "through" tennon looks good to me, then I shoot over to the band saw and just hit a couple of kerfs in line with the tennon, stopping about a 1/4" from the shoulders. Then I grab a drill bit that's roughly double the width of the bandsaw kerf - I can't remember what size I use, it may be an 1/8", but it's not really important - I drill holes through the tennon cheeks, where the sawkerf ends. These holes are just for stress releif, so that the tennon cant split as I drive the wedge in.

Then when it comes time to assemble, I like to use contrasting wood species for the wedges for accent - Anyway, Just Assemble as normal, and a drop or two of glue on the wedge, and tap it into place - Don't drive it home, let it stay proud - Simple wedges are a lot stronger than we give them credit - Afterall, they use tiny little wedges to split granite boulders.

Ok, so the joint dries with the wedges sticking out - Now just flush trim, and clean it up with a block plane - Presto, your wedge joint is done

Another point is, when designing, always think about the stresses a wedged joint are placing on the mortised piece - Always place the wedge perpendicular to the grain of the mortise, otherwise.... Well, see my note about splitting granite

Hope this helps
 

DavidF

New User
David
Are you sure you need to wedge this? It would seem that a standard M&T would be plenty strong enough. If it's being covered then you don't need to wedge for decorative reasons. If you want to add a little something for strength that would be decorative as well, how about glueing the tenon as normal, while holding the piece in place so as not to move and open a gap elsewhere and then pop a nice dowel through the face or maybe two? they won't be draw bored, but they would do everything I think you are looking for. I am away at the weekend, but I could come over one evening this week and take a look if you wanted?
 

woodnick

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Nick
IF you use two wedges they MUST be driven in together DO NOT drive one in and then the other. I agree there more for looks then function but there sure look good when finished in contrasting woods.
 
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