The Holy Grail? A zigzag rocker - WIP [Archive]

Rocker

17th August 2007, 11:07 AM

Here are some pics of my zigzag rocker, made of blackwood with rock maple back-slats, mitre keys and heel blocks.

Photo 1. Cutting a 29.5° leg mitre.

Photo 2. Cutting the kerfs for the twin mitre-reinforcing keys using a tenoning jig.

Photo 3. Glueing up the rocker laminations. I used the same former as was used for my Maloof-style rocker, but I truncated the drooping rear end of the rockers.

Photo 4. The dry-assembled rocker.

Photo 5. Side view of the dry-assembled rocker. Note the maple heel blocks.

The ends of the rockers will be tapered, and all the edges rounded over. It was a big relief to find, when I dry-assembled the chair this morning, that the chair's balance point was roughly where I had predicted, and that the chair is comfortable to sit in. TurboCAD enabled me to fluke it again:)

The chair still needs a bit of work in the way of sanding, and gluing, screwing, and dowelling the seat/back assembly to the supporting Z frame.

Rocker

jmk89

17th August 2007, 11:26 AM

I was wondering how you were going to do it! Well done.

I admire the workmanship (as always), but somehow the angularity of the Z-chair put together with the curve of the rocker (and the shapliness of the back slats), just doesn't look quite right to me. I just can't come at the heel blocks squaring off the front of the rocker but leaving the back as a normal curve.

Perhaps if the arm were curved, the arm/leg joint were also curved or radiused, and the leg/rocker joint were more like the Maloof fairing, it would have more aesthetic unity (if you know what I mean).

But that is just me and the most important thing is that it is comfortable.

Tex B

17th August 2007, 11:28 AM

Fantastic, Rocker. Very innovative.

Can't wait to see the finished product.

Tex

bitingmidge

17th August 2007, 11:39 AM

I think you've gone stark raving bonkers! :p

No, in theory I love it to bits. The only negative for me is also the bottom "Z"bit. I'd love to see it just turn into the rocker without the straight.

No, I don't know how to make it work... that's your job Rocker!

I can't wait to see it finished.

cheers,
P
:D :D :D

RufflyRustic

17th August 2007, 12:10 PM

WOW!!!!!!!!!!!!!!!!!!!!!:2tsup:

zenwood

17th August 2007, 01:11 PM

jmk89

17th August 2007, 01:21 PM

Rocker

17th August 2007, 01:28 PM

Jmk,

I take your point that aesthetically it lacks unity, but maybe people could learn to live with it. I could make up some bs about the creative tension between the angularity and curves - but I won't.

Peter,

I racked my brain for a way of joining the legs directly to the rockers, but couldn't think of a way of doing it that would be structurally sound. I suppose that perhaps a bridle joint like that on Lignum's conoid chair might work. But I lack the ticker to attempt it. Probably that would require a very hard and strong wood for the rockers.

Rocker

Rocker

17th August 2007, 02:09 PM

I had a bit of a play with this idea when we raised it in the other thread. I was thinking of a more calligraphic 'Z', with more curves and a flowing structure.

Why have you gone with keyed splines here instead of the dominoes?

Zenwood,

Your concept drawings look very good, but I think they would be very hard to execute. Perhaps you should have a go. Cutting mitres or bridle joints on curved workpieces would require some sophisticated jig building.

I thought I would use spline keys rather than locking tenons to produce a contrasting feature. Also the larger angle of the lower mitre makes the use of locking tenons less effective. There would not be much glue area on the tenon on the opposite side of the joint from which the tenon was inserted; and the locking pins would need to be very close together. The spline keys allow the pins to be much further apart.

Rocker

Wongo

17th August 2007, 03:08 PM

David, I like the creativity. You’ve done well.:2tsup:

Lignum

17th August 2007, 03:34 PM

Rock the chair looks excelent, well done:2tsup:. And the bridle joint would be perfect. fter seeing all the furniture you make that joint would be easy for you to do.

And Zen, that second sketch is a cracker. That would look stunning as a rocking chair. Have a go and make it. Make it a WIP for all us to enjoy:)

bitingmidge

17th August 2007, 04:20 PM

I racked my brain for a way of joining the legs directly to the rockers, but couldn't think of a way of doing it that would be structurally sound. I suppose that perhaps a bridle joint like that on Lignum's conoid chair might work. But I lack the ticker to attempt it. Probably that would require a very hard and strong wood for the rockers.

I think if one was to attempt it, it would mean a brace, or perhaps a welded steel T shaped insert. I can't get it out of my head at the moment, it looks terrific to me in ever other respect, and proportionally you've hit the sweet spot again!

Cheers,

P

Christopha

17th August 2007, 05:10 PM

What colour are you going to paint it?

zenwood

17th August 2007, 05:33 PM

Having never made a chair before, I think starting with this one could put me in a world of hurt.

The positioning of the chair over the curve of the rocker would be crucial to achieving balance. A scale model trial would definitely be in order. I think a full scale model in plywood would also be useful before attempting the real thing. Then there's the question of joinery. I'm pretty sure keyed mitres would be strong enough, but as Rocker pointed out, they'd be very awkward to cut.

Happy if someone else wants to have a go...hint, hint!;)

Skew ChiDAMN!!

17th August 2007, 05:40 PM

Rocker, perhaps if you shortened the bottom sections of the Zed, to maybe half the distance from the front of the rockers, and curved their tops in such a way that it smooths the change from linear to curved? Maybe mirroring the curve of the rocker?

And, to add a bit of visual balance, add a similarly shaped fillet to the back half as well?

Just a thought... I hope you understand what I mean. :shrug:

bitingmidge

17th August 2007, 05:48 PM

The positioning of the chair over the curve of the rocker would be crucial to achieving balance.

One of the beaut things about Rocker's rockers is that they balance perfectly, both empty and with an occupant.

Some tilt way too far back at rest, they look a little more elegant perhaps, but are very difficult to get in and out of. If they tilt too far forward, they become too difficult to rock.

If you are having a go yourself, his geometry would be a good place to start.

Rocker isn't called Rocker for nothing! :D

Cheers,

P
:D

Rocker

17th August 2007, 06:06 PM

David, I like the creativity. You’ve done well.:2tsup:

Scott,

I have to say I like your rockers better than this one, but then this one took less than a week to build. I am not really happy with its aesthetics - so I am in two minds whether to make it or a Rose Gum zigzag dining chair my second entry in the AWR competition.

I really like Zenwood's sketches; but, as he says, they would be really challenging to build and get right. I have been so lucky so far that I don't fancy my chances of being able to be so again. I also am not sure that that Zenwood's design is actually feasible in wood. The stress on the front portion of the rockers would be huge.

Rocker

Harry72

17th August 2007, 11:15 PM

The stress on the front portion of the rockers would be huge.

What if the design included a curved brace(or triangulated) from the rockers to the center rear of the seat?

Rocker

18th August 2007, 07:19 AM

Skew,

I see what you mean, and had thought of something similar; but I wasn't really happy with the aesthetics of that either.

Harry,

I think an arched or triangular support would defeat the objective of providing a springy chair, and it would tend to push the rockers outwards unless a stretcher was incorporated.

Zenwood,

I think the straight portion of the Z would need to be a bit more upright than your sketches portray. If the front end of the rockers projected that far in front of the seat, the sitter would tend to trip over them as he got up from the chair.

Rocker

zenwood

18th August 2007, 08:32 AM

I agree Rocker. The straight section could be made more upright, but that would move the centre of gravity forward, hindering the rocking action. Or the rockers could be splayed outwards at the front (adding huge complexity). Or the shape of the lower curve could be changed (hard to do while keeping a visually pleasing line).

The whole design needs some real-world experimentation.

Rocker

18th August 2007, 11:30 AM

Zenwood,

I have had a bit of a play with TurboCAD, using your basic idea of attaching the legs to the front ends of the rockers, and I am beginning to think that it would actually be quite feasible to build this design. I have kept the arms straight, for the sake of simplicity. It would probably need to be built in jarrah, rather than blackwood, though.

Darn it; I may have to build another rocker.

Rocker

bitingmidge

18th August 2007, 11:38 AM

I think with that one, you'll find the lever arm of the rocker is now very long, increasing the load on the joint immeasurably. Well ok I'm sure someone could measure it, but it's a lot!

I like your first version better aesthetically too, it seems to balance better visually (except for the foot bit that we're talking about!)

cheers,

P

Rocker

18th August 2007, 12:03 PM

Peter,

You may well be right that the strain on the leg/rocker joint would be large, but, on the other hand, I think a mitre joint with pinned twin keys is virtually unbreakable, as we proved at your place a couple of years ago. The advantage of the Zenwood design would be that the rocker would be springy, as well as rocking. I am afraid I am not going to be able to resist giving it a go.

Rocker

bitingmidge

18th August 2007, 12:11 PM

I am afraid I am not going to be able to resist giving it a go.

That's what I like to hear!
:D :D :D

P

zenwood

18th August 2007, 01:33 PM

Good on ya Rocker. Would be great to see the master have a go at this chair. I agree with you that the keyed mitre joint is very strong: the test with 4 or 5 guys standing on your joint proved that.

I like the balance you've achieved in your design, and the sweep of the curved rocker is very nice. And, as you say, the whole thing will be nice and springy.

One point with my design was the fairly deep recurve at the front. Yours has it at the rear, which does look nice, but having it at the front would lower the angle of the mitre, and increase the gluing area available (but increase the leverage). No free lunches with this design!

Rocker

18th August 2007, 01:51 PM

Zenwood,

The basic problem is that I am too slack to make a new former for the rocker laminations; so I am making do with the one I have always used for my rockers.

With regard to the glueing area, I think the glue on the faces of the mitres contributes very little to the strength of the joint, which derives by far the greater proportion of its strength from the glueing area of the keys, where the glue is subject to shear forces, rather than tensional ones.

Rocker

zenwood

18th August 2007, 02:58 PM

With regard to the glueing area, I think the glue on the faces of the mitres contributes very little to the strength of the joint, which derives by far the greater proportion of its strength from the glueing area of the keys, where the glue is subject to shear forces, rather than tensional ones.

I agree. I was thinking of the key area rather than the mitre face area when I made my comment. But now I'm not sure if narrowing that angle actually does increase the area of the keys. I'll have to think about that some more...

Rocker

21st August 2007, 06:18 PM

I glued, screwed, and dowelled the seat/back assembly to the supporting frame today. Pictures to-morrow. I have curved the tops of the arms and feet, whiich improves the look of the chair a bit, but I am afraid this is not the Holy Grail. The fact that the rockers are parallel to one another makes it look too chunky, and the straight underside of the arms does not look right. I will have a go at the Zenwood design next week.

Making the rockers converge towards the back will be challenging, but I think it can be done, if the stretchers connecting the Z-assemblies are eliminated (their joinery would be altogether too hard). I am considering substituting housing joints for the dowel joints between the Z-assemblies and the seat/back assembly. I am not sure if eliminating the stretchers would weaken the chair unduly, but I think that, if I used glue blocks to strengthen the joint between the legs and the seat side-rails, it should prevent racking.

Rocker

jmk89

21st August 2007, 06:26 PM

I look forward to the photos. I suspect that you are now being too critical, but we shall see...

Big Splinter

21st August 2007, 07:48 PM

Hi David,
Great looking piece!:D
Really looking forward to seeng the updated version.
Cheers
Tom

bigAl

22nd August 2007, 04:51 AM

Kudos Rocker.

I think it's a great technical design and I look forward to seeing the next version.

Al.

Rocker

22nd August 2007, 09:50 AM

Here are pics of the completed rocker, before any finish has been applied. The minwax wipe-on poly will darken the blackwood considerably and give a pleasing contrast with the light-coloured rock-maple back-slats.

Rocker