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Old 25th April 2012, 02:27 PM   #1
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Default Enclosure construction technique help: horizontal or vertical x-sections.

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hi,

above are 2 renders of the design i've settled on that will also be able to house my eventual expensive driver.

the way i see it is i have 2 ways to approach the curved sections.
1) horizontal x-sections - multiple sheets of thin ply wood glued into the curve
2) vertical x-sections - stacked already cut into the curved shape

my problem is i have never tried to
a) bend plywood
b) cut plywood into a smooth curve

i've cut plenty of straight edges with minimal edge fraying but know from experience with sanding that edges can fray easily if you treat them incorrectly.

the 2 designs show my 2 thought patterns, the expensive sheoak vaneered baltic birch ply for the multiple ply sheets glued and a naked birch ply for stacks with the ply showing.

my question is, which of the 2 ways to approach this will give the best end result?

can plywood be easily cut with a freehand router into the required shapes moving against the grain without causing all kinds of trouble?

what is the minimum radius various thickness of baltic birch can bend to without cutting slits? the tightest curve has a radius of just under 6 inches. given baltic birch's reputation for being brutally stiff, can this be achieved with it?

obviously MDF is the fallback option here, i'd rather not use it however in an enclosure that will require more than your average effort in the first place.

below is a cross-sectional picture of the problem area.

thanks for your help

Click the image to open in full size.

Last edited by gafhenderson; 25th April 2012 at 02:30 PM.
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Old 25th April 2012, 02:33 PM   #2
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What diameter do you need?

PLYWOOD CYLINDERS

A cylinder may be cut in half to provide the profile you desire...it would also make shipping easier.
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Old 25th April 2012, 02:40 PM   #3
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Quote:
Originally Posted by Ed LaFontaine View Post
What diameter do you need?

PLYWOOD CYLINDERS

A cylinder may be cut in half to provide the profile you desire...it would also make shipping easier.
outside diameter would be 313mm, so just over the 304.8 they offer. i'll bookmark this as a fallback option if i can successfully alter the design without it messing up the simulation etc.

in terms of structural strength, i'd like to keep all the pieces uni-body as far as the curve is concerned (no joint at curve), which is not correctly represented in the supplied picture. so the half cylinder option would be a less desirable course of action.

thanks for the link
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Old 25th April 2012, 02:44 PM   #4
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Aitwood Anderson International - StoreFront

You may also look for "drum shells".

I understand the preference for uni-body construction. From a practical view, a joint can be made there that won't compromise results...IMHO...uni-joint is more of a feel-good thing than a necessity. Wrap veneer over it.
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Old 25th April 2012, 02:51 PM   #5
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Quote:
Originally Posted by Ed LaFontaine View Post
Aitwood Anderson International - StoreFront

You may also look for "drum shells".

I understand the preference for uni-body construction. From a practical view, a joint can be made there that won't compromise results...IMHO...uni-joint is more of a feel-good thing than a necessity. Wrap veneer over it.
that link looks more promising, i'll send them an enquiry.

in terms of making it all myself, do you have any knowledge on which would be the better path to take?

thanks.
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Old 25th April 2012, 03:01 PM   #6
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If you are thinking about 18mm baltic birch, it really doesn't bend much at all without kerfing, and you have to cut the slits through about 9 or 10 of the 11 plys to get the curves that you are showing.

There is bending plywood that has all of the layers oriented in the same grain direction, although it's typically quite thin and requires further layering.....and another product called wigglewood.

Here are a couple of related threads:

Adventures in Curved plywood.

SOME PROPOSED SINGLE-DRIVER SPEAKERS

My curved project is still half-baked, but my plan was/is to kerf the baltic birch, then once the cabinet is assembled fill the kerfs with caulking or liquid nails, etc to add damping.

The main thing I've learning so far is that you need to kerf almost all the way through the panel to bend it without seeing faceting on the outside, and the cuts have to be close together.
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Old 26th April 2012, 09:26 AM   #7
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can anyone comment on the viability/ease of cutting plywood into curved shapes?

will it cut nicely when the blade moves against/through the grain without splintering into a giant ragged mess?
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Old 26th April 2012, 02:10 PM   #8
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Quote:
Originally Posted by gafhenderson View Post
can anyone comment on the viability/ease of cutting plywood into curved shapes?

will it cut nicely when the blade moves against/through the grain without splintering into a giant ragged mess?
Baltic Birch will cut just fine with a sharp router bit and a circle template.....or a CNC router

Are you considering layering up the curved parts ala translam? I think kerfing sheets would be much easier. You can always router a few profiles to use as formers to wrap the kerfed sheet around; as a bonus they will act as stiffeners in the final enclosure. You have to adjust your volumes to allow for these. If you are really ambitious and have strong friends you could wrap the kerfed pieces around both sides of your routered formers and fill the space between with dry sand.

I wouldn't considered using MDF for this. I tried to kerf and bend it, and it quickly turns into ragged cardboard. It doesn't make very robust formers either.

Last edited by boywonder; 26th April 2012 at 02:19 PM.
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Old 26th April 2012, 02:29 PM   #9
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Quote:
in terms of making it all myself, do you have any knowledge on which would be the better path to take?
gaf, I have built things all my life. I wish for a fraction of your rendering skills.

The back could be suitably constructed from layers of 1/8" hardboard glued up around a form. I would lay the back into a rabbet in each side, then rout any overhang. You will need sufficient clamps. Cargo straps with ratchets for tightening may be useful.

Using a half-cylinder, the joint between it and the tangent section cold be bridged with a glued internal splice. Shape the splice to allow for the curvature and maintain tangency at the joint.

Be wary of cutting a cylinder. It may "spring" upon cutting and change dimension a little. I would do my final lay-out with this under consideration.

Sanding the joint will ease the transition in profile before veneer is applied.

Veneer only needs to cover the portion of the curves which remain visible. I would not worry too much about 100% coverage.

Good plywood (baltic birch) produces good results. Lower grade plywood will provide you with lots of frustration...

Should be a stunning project. I have a set of Curvy Changs, myself.

HTH

Last edited by Ed LaFontaine; 26th April 2012 at 02:38 PM.
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Old 26th April 2012, 02:47 PM   #10
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....or if you can get 1/8" bending plywood, lay that up over a form. The trick then becomes choice of glue and how to hang on to everything as Ed mentioned. Lots of strap clamps, pipe clamps, and helping hands....and patience.
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