Thread: Destructive testing of Rietveld z-joints

Stubchain,

I'm making side tables, not chairs. And they're just for home, not sale.

Rocker,

I'm not familiar with the creep phenomenon. Would it happen even with keys?

Zenwood,

In the Encyclopedia of Wood, there is a table giving the properties of various adhesives. For PVA, they say: "Generally high in dry strength; low resistance to moisture and elevated temperatures; joints tend to yield under continued stress." A Z-jointed table is going to be under continued stress, and if the table is subjected to high temperatures and high humidity, the tendency will be exacerbated.

You may have noticed that panels glued up with PVA glue often develop ridges along the glue lines due to this tendency of the glue to creep.

750 ml of Techniglue epoxy will only cost you 33.50, and you will have plenty left over for other projects that require slow-setting, high-strength glue. You might also consider using a resorcinol glue, if you want to make a feature of the dark red glue line in the mitre.

Rocker

You can allways cheat a little.

Like in the chair on the bottm joint there is filler peice for more support. Make a wedge what fits inside the the joint for more glueing surface. Just cut them a little shorter them the width and sand them to the point ( 3 face point ) to help hide them.

Junkboy,

I considered putting wedges in the angles of the joints on my chair, as you suggest, but I think the bridle joints are strong enough without them. They definitely detract from the look of the chair, so I did without them. If the chair had mitred joints they would probably be essential, though.

Rocker

How dare anyone think of such a joint as this attachment :eek: , but seeing as we are considering a contemporary chair - why be so traditional with the joint.

For the sake of lateral thinking see attachment. I can take the criticism but be gentle.

Teejay,

Harry72 made a similar suggestion in another thread. See http://www.woodworkforums.ubeaut.com...t=21448&page=1 . I am sceptical about whether setting a bracket inside the wood would actually do a great deal to strengthen the joint, unless the wood were laminated on either side of the strengthening bracket, as Harry suggested. Cutting the slot for the bracket from solid wood, and drilling the wood for the bolts would weaken it significantly, particularly if the bolts are in line with one another rather than offset.

My idea, before I thought of using bridle joints, was to stengthen a mitre joint with a floating tenon and with a bolt perpendicular to the mitre faces. But I think my solution of using a bridle joint instead is structurally much sounder.

Rocker

Might it be worth bringing in some of 'those people' over on the Boat building section?

Isn't this applicaiton (Z joint) similiar to 'knees' in traditional wooden boat building? If I remember correctly the knees are "traditionally" one piece i.e. from a crotch or fork.

Those salty types may be able to speak up and offer some excellent opinions.

Rocker,


What you say is correct if my detail is taken to scale as drawn - however with the outer pin remote enough from the hinge so that the steel plate accepts the lever action or moment then it works. See attachment.

It is really a balance of steel size to lever arm to section of timber in place. A balance can be struck but really the steel needs to be sufficient to be rigid as a flexing joint is not serviceably acceptable either just as suffient timber has to remain to work through the joint.

There is even the option to simply, from the hidden side of the timber drill a numer of smaller pins epoxied in place that travel 3/4 through the timber and are not visible on the outerside for the sake of discrete fixings numerous and small.

I confess traditional joints showing the intracacies of timber snuggly fitting together is part of the appeal of fine workmanship.

Cheers

G'day Clinton1!

You're correct in that traditional wooden boat building uses one-piece knees - but lattererly these have been supplanted with fabricated members - partly due to necessity as it's pretty difficult to source the timber required, and also because modern glues (e.g. epoxies, and such) together with loose tenons, for example, make the member sufficiently strong - but still with the 'give' necessary in any wooden boat structural element.

Actually, that 'give' is the real challenge with epoxies, so the design becomes even more critical.

Cheers!

Teejay,

As Zenwood's photo shows, the part of the mitre joint that is going to fail is on the apex side of the joint, where it is under tension. The other side of the joint is under compression, and will not fail, so long as the wood is hard enough to resist crushing. I think the answer to reinforcing the mitre joint effectively is to do so with keys in the part of the joint under tension. With this in mind, I have made a jig to cut 22.5° mitres on 35 x 40 jarrah (see http://www.woodworkforums.ubeaut.com...894#post204894 ), and am gluing up a mitre joint with epoxy. I will then cut a couple of kerfs in it on the table-saw tenoning jig, and will epoxy-glue maple keys the same thickness as the saw kerfs in the slots. I will bring the resulting joint to Midge's place on Sunday, and we can try testing it to destruction.

If it proves as strong as I think it will be, I might get motivated to make another of my chairs in jarrah with mitre joints instead of bridle joints.

Rocker

Originally Posted by Rocker

750 ml of Techniglue epoxy will only cost you 33.50, ...

Where do you source that glue - Bunnings? Does it set clear?

Zenwood,

Carbatec stock techniglue; but be careful to get the one with a slow hardener, which gives a working time of about 45 minutes. The fast hardener gives a working time of about 10 minutes, I believe. Techniglue does set clear.

Rocker

Rocker,

Looking closely at the failure of the joint it appears to have bond failure for about the outer 1/3 of the timber section in the tension zone then once this occurred the section resisting the force was sufficiently small that the timber failed with longitudinal splits.

Yes ensuring the joint doesn't open up in the tesion zone is necessary, but perhaps having your keys also enter and bridge the zone there the timber has split would be advantageous.

What would look good is a double sided dovetail key following the joint in a contrasting timber - maybe ebonised.

Cheers

The other thing that could look very classy while reinfocing the joint is a strip of brass around the outside of the knee. Even a brass or metal hinge could be used.