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:

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

bent plywood.jpg

[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...



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.