Keeping Cabinet Boxes Square

Plywood often has a bow in it. There are tricks for keeping cabs square, but you may also want to use non-plywood sheet goods. September 7, 2006

Iíve been making hardwood furniture for years and have the tools to mill a piece of rough lumber straight and square. Iíve mainly worked with real wood - very little plywood - and if I used plywood it was small pieces. Recently Iíve tried my hand at cabinetmaking. For the cabinet carcass construction I dado, glue, and screw everything together. Each piece is cut square and I check for accuracy on every piece that I cut. When I assemble the carcass and measure from corner to corner it measure square but if I place a very accurate framing square on the inside of the carcass itís not square. Sometimes itís out of square by more than 1/8th of an inch.

I think the problem is the plywood as I can clearly see itís bowed. Iíve made 8 carcasses so far and all 8 have the same problem. Iím going to use face frames on the front. If I build the frames accurately I believe I can take the bow out of the plywood when I apply the face frame. Is this a common problem and if so how would one ever build a frameless style cabinet and have it be accurate, or am I doing something drastically wrong?

Forum Responses
(Cabinetmaking Forum)
From contributor A:
Don't worry about it. I build lots of frameless cabinets with best quality birch ply and it all has some degree of curvature. If the sheet is badly warped I may decide not to use it, but most times just ensure that the curve is where it won't matter, or will actually be a good thing. For instance, on a cabinet base I will make sure the curve is upwards, which will counteract the sag. To ensure the cabinet is square I measure across the corners.

From contributor B:
Welcome to cabinetmaking. You're on the right track. Dadoed decks and tops help keep the box square and the sides flat. A face frame helps, too. Also, use stretchers on base cabinets and use rabbets in the cabinet sides to accept the back. Fixed shelves and partitions help keep things in line on larger pieces. Because plywood is usually bowed, machining dados and rabbets can be a pain to make the depth consistent. A large, solid and flat assembly table is essential, too. I do mostly face frame, so I can't help you too much with the frameless, although most of the above would probably apply.

From contributor C:
With frameless the most important check for square is your tape measure. As you found out the square will always be thrown off by any little bow in the ply. One trick is to try and keep the bow facing the outside on cabinets which will be against other cabinets. This way, when they are installed, the pressure from the other cab will push both panels flat. I don't do face frame very often, but the Knowledge Base has a ton of useful information about the process.

From contributor D:
We use overlay or inset 1/2 to 5/8 backs on all our cabinets and have a slider to cut all our parts. In the case of overlay, we shoot one straight side, the adjoining top or bottom and then the rest is automatically square. If one other side is bowed in or out we tweak it into place and continue to shoot with a nail gun. After that we screw the back in a few places for added strength. With an inset back, we pocket hole the perimeter of the back if the sides or top and bottom of the cabinet will show or we shoot and screw as previously described. Either way, you are guaranteed a square cabinet and save time making grooves or dados with stretchers behind the 1/4" back. The back dimension is also automatic. It's the width of the top and bottom for an inset or the width of the finished cabinet when it's an overlay. We cut out overlay backs after assembly of the cabinets as plywood thickness varies greatly from order to order these days. Dadoes and grooves compared to this system is like comparing a Sopwith Camel to an F-18.

From contributor E:
It gets even more fun when you move up to inset doors. Then you begin to see all of the sins you used to get by with when using overlay doors.

From contributor F:
All the reasons discussed above are why European style cabinets (frameless) are not typically built with plywood. Frameless casework is best built with a dimensionally stable product like melamine on a quality grade particleboard core. Go look at any high-end European cabinet line like Pogenpohl, Boffi, or Snaidero (to name a few). They are beautifully designed, flawlessly finished, with great hardware, but no plywood. All the cores materials for their casework are wood composites.

From contributor G:
I build frameless. I build almost entirely with melamine, stable, unlike plywood. I use full overlay backs, stapled on. If they are dead square (tape measure, corner to corner) and if the cuts on the saw are dead square, (I have a panel saw), then the cabinet will be square.

From contributor C:
I can never believe the prices when I see how much some of those Euro kitchens cost. Flat panel doors, melamine interiors, stamped steel drawer boxes -those were all invented to be cheap! The prices they get by labeling them "Euro style" seem out of line to me. That's just my opinion as I know many people love those cabinets. Personally I still use ply for my boxes, but composites are fine as long as the quality is there.

From contributor F:
You really should go into a Pogenpohl, Boffi, or Snaidero showroom and take a look at what high end Euro really looks like - especially the finishes. Toto, weíre not in Ikea anymore!