Adventures in Curved plywood.

[boywonder]

I've been experimenting with curved plywood in an effort to build cabinets for my Tang Band W8-1772's ("Curvy Tangs"....haha ).

The goal is to end up with a 45L BR cabinet tuned to 45Hz or so. These cabs will end up on tubular stands with the port and binding posts on the bottom of the cabs.

So far I've tried 3/8" wiggle wood, which didn't work because un-kerfed it will not bend tight enough, and after kerfing halfway thru at 1/2" spacing, it cracked since the grain runs parallel to the bend.

Similar results with kerfing 3/4" MDF. I cut almost all the way through every 1/4" with an 1/8" width blade, and the board broke apart when making the tight rear bend. 18mm baltic birch took a couple of attempts; the trick is to kerf down to the last lam, and make sure the last lam grain is running perpendicular to the bend.

The two strips on the flat face are fixturing to keep the formers aligned in the same plane when wrapping and gluing the sheet around the sides.

The formers were fabbed by printing a full size paper template, then carefully jig-sawing out the first one, and using it as a template to make additional formers with a router and laminate trimmer bit. They are also 18 mm BB.

Next step is to laminate 1/8" poplar bending plywood on both faces (inside & outside) of the curved skin and then trim everything, including cutting the ends of the curved panel flush with the front baffle (which may be a bit of a challenge).

[evanc]

boywonder very cool project.
I'd skip the kerfed wood and use multiple layers of the bending ply. You could use formwood 2ply veneer for a nice outer skin.
Two-Ply Wood on Wood Veneer Stock List
The multiple layers of glue will make a very stiff final product.

Tight bond will work, But for the ultimate in glue I like
Vacuum Pressing Systems -- Veneer Glue
Its kind of nasty, and dries slowly, but will allow no creep when its dry.
good luck...Evan

[LineSource]

Your cabinet shape is becoming popular. I have seen stacked laminate or translam construction, as well as frame+bended board similar to your efforts on several commercial speakers. The old school construction discusses steaming thin plywood sheets and laminating them together around a frame.

Some speakers attempt to reduce cabinet edge diffraction, reduce speaker side-cone reflections, and taper the rear to reduce standing waves and rear cone reflections.

White Light speakers use an airfoil shape cabinet front in a effort to reduce diffraction effects. The bluge in front provides some space between the speaker side cone and cabinet walls. The tapering reduces standing waves. I wish I had the cabinet skills to build an MLTL for my Tang Band W8-1808 shaped like the White Light cabinet. I was surprised how much cleaner the bass from the MLTL was over past wide-and-shallow bass reflex cabinets.

[badman]

Hey, nice work! Are you putting a curvature on the front panel? That's where it's most critical for diffraction. I need to get down your way soonish.

[boywonder]

Originally Posted by evanc

I'd skip the kerfed wood and use multiple layers of the bending ply. You could use formwood 2ply veneer for a nice outer skin.

Evan: Thanks for the links! I've been considering layering up 1/8" bending plywood if the heavily kerfed BB didn't work out. My hope with the kerfed 18mm BB is that I'll end up with zillions of little 7/8" tall I-beams once I glue the thin bending plywood to the inside surface. At least on paper, it should be lighter and stiffer (or at least as stiff hopefully, I haven't done any bending calcs) than one thickness of 18mm BB. With a layer of 1/8" ply on both the inside and outside surfaces, the walls will be 1" thick.

This is presently an experiment so who knows what I'll end up with.......

[boywonder]

Originally Posted by badman

Hey, nice work! Are you putting a curvature on the front panel? That's where it's most critical for diffraction. I need to get down your way soonish.

The present plan is for a two layer (1 1/2" thick) front baffle with 3/4" roundovers everywhere. The baffle will be flat where the driver mounts, as curving the whole front would be another level of complexity for my tiny brain.

Since this is an experiment, feel free to stop by and make some saw dust (and some router dust)...I'm game to do more curving.

Yes, you'll have to stop by and check out my latest setup; TubeLab 300B and Half Changs........Send me a PM and let me know your availability.

[boywonder]

Originally Posted by evanc

You could use formwood 2ply veneer for a nice outer skin.
Two-Ply Wood on Wood Veneer Stock List
The multiple layers of glue will make a very stiff final product.

What advantages does the 2 ply veneer offer on curved surfaces compared to the conventional paper backed stuff?

[evanc]

sorry i'm slow getting back.
The 2 ply veneer doesn't transmit whats under it as much. and bends well without cracking.

[chrisb]

Originally Posted by evanc

sorry i'm slow getting back.
The 2 ply veneer doesn't transmit whats under it as much. and bends well without cracking.

the downsides are restriction in attainable radius ( i.e larger ), and more exposed edge core than with the paper back

[boywonder]

Originally Posted by chrisb

the downsides are restriction in attainable radius ( i.e larger ), and more exposed edge core than with the paper back

The edges will be covered with solid ends and baffle....this experiment is continuing..had a few failures... but I'm slowly moving forward. I'll have more time to build these shortly.

I've been building some fixturing that will allow trimming the front edges on the tablesaw. The test piece shown is also twisted about 1/4' end to end, so I've make some fixturing for that also.

My present plan is to still use kerfed BB (1/2" presently), then contact cement 3 or 4 layers of 1/8" bending ply around the BB, then veneer.

I have a 4x8 sheet of paper backed quarter sawn cherry, so that will be first up once the design is up and tested.