Help! Is plywood supposed to warp after cutting a dado?!

[Kent Adams]

I didn't know this could happen. So, I have this baltic birch plywood I purchased from Lowes several months ago and its been sitting in my shop for a month. I cut out a side panel for my first cabinet 2 weeks ago. Today, I got around to cutting a 1/4 inch dado for the back panel. I went to start putting my shelf pins in (with the Kreg jig, thanks for the recommendation from SMC members) and I noticed this (see picture). The other side panel is flat, but this one bowed after I cut the dado. What the????

The panel bows out toward the face side of the panel. I will be using pocket screws to attach the panel to the top and bottom panel, no plans for a rabbet cut just butt jointed.

I'm thinking this will straighten out when I attach it with the screws, but will it effect the squareness of my frame? I intend to put a door on this cabinet. I didn't know plywood would bow like this after cutting a dado. Like I said, this is my first cabinet project. The wood hasn't been subject to swings in humidity, its been laid on a flat surface and it wasn't bowed before I cut the dado.

To summarize my questions are:


Will the bow simply straighten out after I attach it to the top and bottom panels enough for it to be square?
Is it ok to store plywood sheets (uncut) on their side, or do they need to be stored flat on the floor?
Why would it bow after cutting the dado?


323596

[Matt Day]

The general consensus is that all plywood from the BORG is junk for ww'ing. Their birch ply is not Baltic birch, count the number of plys. The best luck I've had is with Columbia Forest Products material from HD, and that warps too. For casework there are some ways to make it work like using the backs, tops, bottoms, and shelves to get it squared back up.
When cutting dados, I always cut with the crown up and push down so the dado is cut to a uniform depth.
"Good" plywood is $100+ per sheet, and you get what you pay for.

[Kent Adams]

We don't have any source around here selling very many $100+ per sheet plywood, not even The Hardwood Store near me: http://hardwoodstore.com/hardwood-plywood

There was no crown until I cut the dado, it was perfectly flat.

[Rick Potter]

You are lucky if that is all it does. Like Matt says, that is not Baltic birch. I made some shop cabinets from HD birch ply that looked pretty good at first, but after cutting dado's I found the thickness varied, and not just between sheets. There was enough overlap at the ends of plys that thickness varied enough in one sheet to throw off all the dado's I had made....using a thickness gage from the same sheet. There were also gaps over three inches long, as well as delamination along the edges.

Oh yeah, there was also birch veneer that was so thin it shattered back as much as an inch when crosscutting it with a decent blade.

It looked pretty good in the store, and was clearanced at under $40 per sheet. Being a cheapskate, I had to learn why by myself.

[glenn bradley]

Not to beat a dead horse but, BORG ply is poor at best. 3/4" BB ply in a good grade is about $48 for a 60" x 60" sheet or thereabouts. The higher cost disappears when you don't have to remake parts or force things together which causes the carcass to go out of square, then you have to deal with that, etc. etc.

[Kent Adams]

Well, lesson learned on this. I wonder what in the world this stuff is good for then?

Anyway, I'm told over at another forum that we do have some local suppliers (within 50 miles) that sell cabinet grade plywood and I'll give them a call on Monday. I may have to buy a 100 bd. ft of this stuff though :(

[Dan Neuhaus]

Kent,
I've purchased a lot of sheet goods from The hardwood store and have been very satisfied. Mostly use their prefinished maple but have gone through several sheets of baltic birch and never had an issue.

[johnny means]

Almost any sheet material will bow if grooved on our side. Better materials will bow less, but even the best materials are susceptible.

[Kent Adams]

Kent,
I've purchased a lot of sheet goods from The hardwood store and have been very satisfied. Mostly use their prefinished maple but have gone through several sheets of baltic birch and never had an issue.

Thanks Dan. I was just there this weekend but didn't think to look at their plywood. I'm going to call them tomorrow as well as Steve Wall Lumber, which is about the same distance from me as well.

[M Toupin]

We don't have any source around here selling very many $100+ per sheet plywood,

Sure you do.

Wurth Wood Group
201 S. Pendleton Street
High Point, NC 27260
336-431-2712

Quality sheet goods, hardware finishes and general supplies like abrasives, fasteners etc. Just keep in mind their customer base is pro shops. They will sell small quantities to walk ins on a cash account but they expect you to know what you want/need. I find if I go during the slower times, around 10-11am and 2-3pm they are much more amenable to answering questions.

Mike

[Kent Adams]

Mike, is this a place I can walk into the warehouse, like The Hardwood Store or do I have to go to a desk and place an order?

[M Toupin]

You will have to go to the desk and no, they won't let you cherry pick through the bunks of plywood. But, if you go there during the slow times they will often let you walk though and look at the stock to get an idea what you're getting. They have always been good with me about selecting quality materials though and the one time I got a bad sheet they exchanged it no questions asked. But like I said, their costumer base is the pro shops who buy multiple bunks of ply etc at a crack and know exactly what they want. They aren't used to dealing with the hobby/general public but if you act professional and polite it goes a long way towards a good experience.

Mike

[Erik Christensen]

if the edge does not look something like this...

http://www.woodpeck.com/media/main_bb_plywood.jpg

then it is NOT real baltic birch

[Dan Neuhaus]

I'll second Wurth as well, though we use the Raleigh store. We're a very small business and they have been great to us. Also, their screws are rebranded deerwoods, and they stock abrasives in Festool hole patterns.

[David Eisenhauer]

I run a rabbet around the inside of my cabinet sides/top/bottom and insert a checked-to-be-square plywood back in my cabinets. That, plus adding a face frame to the front side will pull and hold the cabinets into square and they will stay that way. I don't worry about the bowing at all. Main thing is to apply even pressure to the ply as it passes over the dado/rabbet blade so that the depth of cut stays consistent. I use cabinet grade ply (paint grade) for my utility cabinets to stay away from the Borg ply. Typically, either ash or maple is cheapest at around $30-$40 per sheet. I promise that the cabinet shops in your area get at least some of their ply from somewhere other than having it all trucked in. Ask them where they buy and tell the vendor there that you want/need to pay tax rather than supply a tax number.

[Peter Quinn]

There is probably almost no place on earth that both has really good plywood and lets the public access said plywood freely and at will. Half the problem with big box plywood is that often it is ungraded junk, rejects, off shore garbage, dumpster fodder....etc. Every sheet I get from a major hardwood plywood distributor has a grade stamp on it...the big box? Never. The other half the problem is storage. Those places are vaguely climate controlled, lots of spots where the giant HVAC machine is blowing on the sheets or lumber, the public goes through the stacks like animals, stuff all over the place, never a cover sheet, dents, scratches. The last place i worked had a retail store and sold plywood and hardwood, mostly to professionals. Every lift of plywood had a cover sheet on it. The top sheet always warps, being exposed unevenly for some period to the surrounding air. So they left a single sacrificial piece, like the ends of a loaf of bread, and often reused the same sheet on the next lift when they opened that. Full lifts always came wrapped in heavy cardboard, then general a 1/4" cover sheet, all strapped together.

We use quality sheet goods at the cabinet shop where I'm employed, it still warps, particularly the top sheet in a pile which sits a while, construction methods generally help work out the errors, backs squaring up cases, sides getting squared up by face frames, etc. But I've seen that big box stuff warp as it went through a saw, and to watch plywood react like that is frightening. I gladly pay $85/sheet for quality sheet goods versus $45 from the borg whose quality falls between dubious and unknown.

Good luck in your search for a better supplier, realizing the problem is half the battle. When you work with real baltic birch you will be pleasantly surprised, its a much better thing than whats in your picture.

[Art Mann]

I have worked with material like that many, many times and that bow is no problem if the design and assembly are good. The final assembly will remove the cupping. The things to watch for with big box store plywood are voiding and extremely thin outside veneers.

[Kent Adams]

Erik, I bought two of those 5x5's today. :)

[Kent Adams]

Thanks Art. I think I can get it right with assembly but I've learned my lesson and won't buy this stuff again.

[Kent Adams]

There is probably almost no place on earth that both has really good plywood and lets the public access said plywood freely and at will.

Peter, I was lucky and found a place about 30 minutes from my house, called The Hardwood Store. Its a wood worker Disney World in there and they actually invite the retail public to come in and purchase. I found the true 13 ply 3/4 5x5 Baltic Birch there as well as 3/4 A1 cherry that has veneer about 1/8 of an inch thick on both sides.

I'm going to try to correct the borg stuff through the assembly process, but now I've got some good material to make the next 2 or 3 cabinets. Thanks for your encouragement.

[Wes Ramsey]

I ordered 2 1/2"x5'x5' sheets of real BB plywood from my local Lowe's. They'd never heard of it in the store, but called the supplier in Little Rock and they had it on the next truck headed north. Came in a couple of days after ordering and was something like $78 with tax for both. Free shipping! I don't know if any Lowe's can get it, but around here it appears to be available.

[Mike Heidrick]

I was shocked at how reasonable sheet goods were to ship. I called a place in Chicago that dropped off three sheets of LDF (low density fiberboard used as a CNC vacuum clamping substrate spoilboard) on their way to Springfield IL (trip they make 3 times a week) free shipping. Its worth calling around. Found them on a google search.

[J.R. Rutter]

its been laid on a flat surface

Personally, I think this is the root cause, not a dado cut.

Same reason top sheets on a pile warp, as Peter mentioned. Better plywood will definitely help, but moisture imbalances get emphasized when one face is exposed to air and the other is not.

[Kent Adams]

Personally, I think this is the root cause, not a dado cut.

Same reason top sheets on a pile warp, as Peter mentioned. Better plywood will definitely help, but moisture imbalances get emphasized when one face is exposed to air and the other is not.

J.R., my shop is small and no room to lay the sheet goods flat. Should I wrap something around them, perhaps like a moving blanket to keep the air balanced?

[J.R. Rutter]

Best practice is either to allow air flow on both faces if standing upright (lets you spot internal tension issues as the whole sheet dries) or cover the top face if laid flat - a scrap piece, or plastic sheeting/tarp would work. This applies to cut parts as well. Eventually the sheet will reach equilibrium, but will always be susceptible to this sort of cupping because the ambient humidity is always changing and the surfaces are large compared to edges.