Thread: Zigzag Chair Screws

Welcome to the Forum wolfcj. Please tell us what the reference is to the post on the zigzag chair. I'm interested to see this. As to relative strengths of screw or not to screw joints - I'm with you - the joint should be weakened by the screw because it damages the integrity of the joint. Some people will never be confident in the longevity of their work unless they make it out of metal!

Screws are absolutely unnecessary. Dominos or keys with long-grain to long-grain glueing is sufficient.

Here's a side-table version I made:



which supports my weight at least:



The joinery here is mitres reinforced with 10 keys per joint. Timber is jarrah, about 15 mm thick, keys are also jarrah, 3 mm thick. Epoxy glue.

See here for a load bearing test on single Rocker z-chair joint (a 45 degree mitre reinforced with two splines), that supported the weight of 4 men.

Another alternative (see attached) would be to glue a wedge behind the mitre, discretely run to point at each end. Dowels or screws fixed into the wedge could add some strength.

the first thing I noticed was those screws. They should have made it without the screws and tested it load capacity. Or they should have read Popular Woodworking's test results for different joints and not worried.
Zenwood - great post. I love the cheater wedge for additional strength. Without too much work it could a design element of the Z-table.

Originally Posted by griffithpark

the first thing I noticed was those screws. They should have made it without the screws and tested it load capacity. Or they should have read Popular Woodworking's test results for different joints and not worried.
Zenwood - great post. I love the cheater wedge for additional strength. Without too much work it could a design element of the Z-table.

GriffithPark, welcome to the Forum. You should know that the Woodcraft article was written by our own Rocker, who is the designer of the chair. Since it's his design, he can use pinned tenons if he likes. That's what the screw really does, pin the tenon. You could also use brass rod if available or, as pointed out, do it without. I rather like the feature myself.

It is a great project. I hope you'll show us yours when you've finished it. We love pics.

Originally Posted by Knurl

Welcome to the Forum wolfcj. Please tell us what the reference is to the post on the zigzag chair. I'm interested to see this. As to relative strengths of screw or not to screw joints - I'm with you - the joint should be weakened by the screw because it damages the integrity of the joint. Some people will never be confident in the longevity of their work unless they make it out of metal!

Knurl,

Just search for zigzag and Rietvelt, and you will find several threads on the chair and the joint. I have to admit that the joint would probably be stong enough without the screws, but I thought they looked nice; if you disagree, build the chair without them. I doubt if the joint will fail.

Rocker

Rocker, As I have not seen the design I cannot comment on it. Clearly you wanted a decoration, rather than strength so my earlier comment is irrelevant. Sorry for being critical without understanding your motive. Tell me, though, do you agree that the net result of any screw will weaken a joint or doesn't it really matter if you've got good long grain glue contact area?
Now the BIG question - what is the best brand of glue to use for interior furniture joinery when you want strength and long life?

Knurl,

My intention was to strengthen the joint by pinning the locking tenons. I think the screws do in fact strengthen it, but I am sure the joint would be strong enough without them.

With regard to glue, I believe it is essential to use epoxy for this type of mitre joint. PVA would soak into the end-grain on the mitre face and starve the joint of glue; also PVA is liable to creep, which would be bad news in a joint liable to prolonged and repeated stress.

Rocker

Firstly i can tell you that the joint in rocker's chair is plenty strong enough & I doubt if a couple of bits of brass would make much practical difference at all.

We..... & thats some members of the board have tested rocker's joints a couple of times by getting several large woodworkers to jump up & down on the joint together.

as for the glue
most of the reputable modern glues are great..... I've seen magazine tests and wood failure seems to be the issue.

there are some realy good epoxies out there such as west system, techniglue and any number of others.
a lot of the time it comes down to "other advantages" apart from strength.

like colour of the glue line, gap filling working time.....

there is heaps of discussion about glues here and on other boards and in the magazines.

therew is a lot of choice and even more good products.

cheers

I found the thread where we tried to break rockers joint the first time.

https://www.woodworkforums.com/showth...ghlight=rocker

check out the 12th post.
4 of us on a plank on 1 joint.

Im the bloke in the long trousers.... I was 85kg then & at least 2 of the others wee bigger than me.

We had to jump a bit to get it to crack but it didn't gross fail.

then there was the discussion about the method of failure

rocker is a retired scientist, and there was at least 1 structrual engineer there.

so much discussion we had to do it again at rockers place some time later.

cheers

Soundman,

I have a slight confession to make. The joint in the chair in the Woodcraft article had different joinery to the one we tested.

The joint tested by four people standing on it was made as in diagram 1. It had two rock maple keys 1/8" thick glued into two saw kerfs that were cut using a tenoning jig.

The joint in the chair that was tested by three people standing on it was made with a single 10 x 50 through tenon, as in diagram 2.

The chair in the article was not actually tested, but I believe its joinery with three locking tenons 6 x 28, and an alignment tenon (on the left), as in diagram 3, was stronger than the chair tested.

All three joints were pinned with 4 mm thick brass screws.

Rocker

If you recon the new joint is stronger.... thats good enough for me rocker.
You may have retired years ago, but mate you cant help yourself..... you are still a man of science and look for a better way.

we'll just have to test that joint too next time we visit.

cheers

85 kg? WAHHHHHHHHHHHH. i gotta lose that much. Ya shrimp!

I'm almost 90KG at the moment
The doc says I should be 80kg or a bit less.

cheers