Author Topic: Strategies for keeping plywood straight?????  (Read 5178 times)

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Offline fritter63

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Strategies for keeping plywood straight?????
« on: April 15, 2018, 11:38 PM »
Ok, so I'm wanting to build more plywood cabinets both for storage in the shop as well as the house. Esp. now that i have the CNC machine going, mucho easier to just slap a piece of ply down and start cutting. I've decided to just go with nice 13 ply baltic birch ply (higher quality), but in longer lengths the stuff still doesn't alway stay straight.

What are your techniques for keeping the panels straight and true in cabinet construction?

Thanks.

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Offline Alanbach

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Re: Strategies for keeping plywood straight?????
« Reply #1 on: April 16, 2018, 12:22 AM »
I don’t know what your “typical” cabinet construction looks like but if you rely strongly on movable shelves and you are experiencing a lot of movement and twisting in the sides you might want to add a fixed shelf or more lateral support between the sides. Things like a bottom that is dadoed into the sides instead of butted, a four sided frame support frame or a top, etc. Also to design your back and face frames in such a way as to provide more structural support for the sides.

Also, if you are in a climate where your shop environment is very different than your environment either where you assemble or install can really make a difference. I live near the Gulf Coast in Texas where it is very humid and we use air conditioning a lot. The humidity difference is huge. I assemble immediately as I cut pieces. I never cut more than I can assemble during that session. The moment I make a new cut I assume the humidity differential is making things move immediately. If possible I make sure that the material is acclimated in the assembly area and I try to insure that environment is as close as possible to final resting place’s environment.

These are guesses bordering on shots in the dark. I hope they help.

Offline lwoirhaye

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Re: Strategies for keeping plywood straight?????
« Reply #2 on: April 16, 2018, 12:34 PM »
You've hit on one of the reasons pb-core melamine is so often used in frameless cabinetry.   I used to think it was just because it was cheap, easy to clean and so on.   It's also because cutting it seldom results in the movement we get with plywood, especially larger pieces like tall pantry sides.

The more I researched frameless cabinet making the more I saw the virtues of melamine.  I used to brag about how I always used plywood, not that junk particle board stuff that falls apart when it gets wet.   Clients generally expect something pretty close to perfection, no matter what kind of bargain they're getting.  When I realized this I started looking at the difficulties plywood presents more seriously.

Baltic birch is a terrific material in a lot of ways. Reliably flat it is not though.  You can try stacking and weighing-down panels after they've been cut for a day or two.  I doubt it will help much but its worth a try.
« Last Edit: April 16, 2018, 12:38 PM by lwoirhaye »

Offline aloysius

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Re: Strategies for keeping plywood straight?????
« Reply #3 on: April 16, 2018, 08:06 PM »
Seal both faces.  Movement "off the saw" indicates the presence of internal stresses.
FOG-wit since '95:  Some say since birth...

Offline fritter63

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Re: Strategies for keeping plywood straight?????
« Reply #4 on: April 16, 2018, 08:23 PM »
You've hit on one of the reasons pb-core melamine is so often used in frameless cabinetry.   I used to think it was just because it was cheap, easy to clean and so on.   It's also because cutting it seldom results in the movement we get with plywood, especially larger pieces like tall pantry sides.


Ah, good information. So, "it's not me"! Yes, was wanting to frameless 32mm system cabinets. That's too bad, I love the look of (shop) cabinets made out of the good 13 ply stuff. And Melamine is so dang heavy!

What about laminating too pieces together? Like two 1/2" pieces vacuum pressed together? would that cancel out the warping and making them act more like a stressed skin panel?

Offline Bohdan

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Re: Strategies for keeping plywood straight?????
« Reply #5 on: April 16, 2018, 08:36 PM »
Cut three times. Oversize once to relieve the internal stresses, and then trim both edges to straighten at the correct size.

Offline fritter63

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Re: Strategies for keeping plywood straight?????
« Reply #6 on: April 16, 2018, 08:40 PM »
Cut three times. Oversize once to relieve the internal stresses, and then trim both edges to straighten at the correct size.

so, just to be clear, we're not talking about unstraight edges after they've been cut. We talking about it warping in the faces out of flat.

Offline aloysius

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Re: Strategies for keeping plywood straight?????
« Reply #7 on: April 16, 2018, 09:55 PM »
You can plan your way around ply's tendency for movement.  Glue laminating multiple layers of thinner material to build up stable larger panels. A bunch of stopped rebated fixed shelves next to/adjoining/attached to large, unsupported vertical or horizontal spans.  Rigid box construction with backs.  A small-ish rectangular hardwood horizontal member (girder) immediately below wide shelf spans as a door stop.  Rectangular hardwood drawer runners or even as a vertical stile/door meeting stop.  Substantial hardwood lipping of exposed unsupported edges.  Torsion box construction where all else is impractical.

Most of these remedial techniques can be cleverly hidden by cabinet doors, hardware & drawer fronts.
FOG-wit since '95:  Some say since birth...

Offline Gregor

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Re: Strategies for keeping plywood straight?????
« Reply #8 on: April 17, 2018, 04:51 AM »
Glue laminating multiple layers of thinner material to build up stable larger panels.
As this process is the definition of plywood I honestly see no point in that, as it's simpler (and cheaper) to buy it in the required thickness in the first place.

Better is IMHO to get A grade Multiplex (which is made of branch free, equally strong middle layers using waterproof glue - instead of random dimension scraps being pressed together to thickness, as often found in 'normal' grade plywood) from a good source where you can protest sheets that are not flat (which shouldn't happen with good quality - and hasn't for me so far). A tad more expensive than low grade stuff, but as it removes the warping issues (and problems such as finding unexpected holes inside the sheet when cutting it up) it's worth it to me.
« Last Edit: April 17, 2018, 04:57 AM by Gregor »

Offline lwoirhaye

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Re: Strategies for keeping plywood straight?????
« Reply #9 on: April 17, 2018, 11:23 AM »
When I got started 20 years ago or so I went to a dealer in Baltic birch.  The guy in the office told me there were some quality differences with some of the boards coming from Finland and some from Russia.  I think he said the Russians were drinking too much vodka and the Finnish quality tended to be better.

On tall pantries I'd try to put a fixed shelf in the center to fix the width.  If that absolutely isn't feasible I'd try to pick the very best boards to machine the long sides from.  On the installation I'd try to put those tall cabinets with something on either side.   If you're just in love with the edge appearance of the baltic birch, you could use something flatter, that maple stuff with the MDF layers under the veneers for example, and apply Multiplex edge banding which imitates the appearance.

Offline Master Carpenter

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Re: Strategies for keeping plywood straight?????
« Reply #10 on: April 17, 2018, 06:06 PM »
If your trying to prevent tall panels from bowing, I've used shelf pins that lock into the holes. Takes a little longer to build and I'm not sure how much bowing they prevent, but has worked the time I needed it. Lee valley sells the ones I used.

http://www.leevalley.com/en/hardware/page.aspx?p=70535&cat=3,43648,43649&ap=1
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Offline escan

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Re: Strategies for keeping plywood straight?????
« Reply #11 on: April 17, 2018, 08:38 PM »
If your trying to prevent tall panels from bowing, I've used shelf pins that lock into the holes. Takes a little longer to build and I'm not sure how much bowing they prevent, but has worked the time I needed it. Lee valley sells the ones I used.

http://www.leevalley.com/en/hardware/page.aspx?p=70535&cat=3,43648,43649&ap=1

Awesome tip, I do a lot over custom oversized stuff and this should come in handy!

Online jarbroen

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Re: Strategies for keeping plywood straight?????
« Reply #12 on: April 25, 2018, 11:14 AM »
Ok, so I'm wanting to build more plywood cabinets both for storage in the shop as well as the house. Esp. now that i have the CNC machine going, mucho easier to just slap a piece of ply down and start cutting. I've decided to just go with nice 13 ply baltic birch ply (higher quality), but in longer lengths the stuff still doesn't alway stay straight.

What are your techniques for keeping the panels straight and true in cabinet construction?

Thanks.

There have already been some great suggestions but I'll add my experience and non-expert opinion.
lwoirhaye already mentioned a quality difference in Baltic Birch.  I've experienced that first hand.  I can pick up BB from several places locally.  Some of the chain lumber yards offer it in 5'x5' sheets of 'ok' quality.  And 12mm is kinda 12mm and 18mm is kinda 18mm.  Another smaller place has the 4'x8' sheets of 'ok' quality but the same kinda sizes.
My favorite place to get BB sells both 5'x5' and 4'x8' sheets and the 18mm is precisely 18mm - same with the 12mm.  Also, it's really void free when I look at the layers.  The other suppliers are mostly void free but definitely not the same premium look.
My favorite place doesn't cost any more than the other places.  I think they just have a better supplier of BB.

Also, when I've built BB cabinets without frames I've used 18mm.  It's typically much more flat and stable than the 12mm I get.  The 12mm I use for drawer boxes and shelves.
I built a 42"x72" cabinet supporting a Paulk style bench made of 18mm ply.  There's no frame to the cabinet - 1 divider running long way and 2 running short ways.  Basically 6 cubbies.  The cabinet stays square, holds the heavy Paulk top, slabs, stacks of plywood, me, etc.

I also store my ply on a cart with it almost vertical.

Offline Peter Halle

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Re: Strategies for keeping plywood straight?????
« Reply #13 on: April 25, 2018, 11:21 AM »
If you are looking to buy quality plywood, look to see if you can buy for a company that supplies cabinet shops.  Shops will not accept garbage and returns or lost sales are a great motivator to the seller.

Once you get it, store it lying down.  If you can put weight on it even better.  You stand it up (especially at an angle) and you are inviting unhappiness. 

Peter