Plywood Bowing on Cabinet

[Bobby]

Hello everyone,

I need some direction on this project. I am building a plywood cabinet. I have used 3/4" BCX plywood and I have attached all the sides and the back with 1-1/4" pocket screws. The cabinet is 6' x 4'x2'. The vertical board on the right side (see photo) has warped inward and now the shelving doesn't fit. I rarely work with plywood so I'm not sure of the correct procedure. I thought I could frame the sides with 1x3's and hopefully pull the bow out. I hope I don't need to pull this all apart and start all over.

I also noticed that the bottom shelf (where the wheels are) has also bowed. I'm thinking there I could frame the entire bottom with 2x4's and that might help that situation. Any help would be greatly appreciated.

Thanks,

Bobby

cabinet.jpg

[Jay Boutwell]

Hi Bobby, what I see is typical of use of the wrong grade of plywood.  What you are using is a general construction grade plywood that is used where it will be attached by studding such as in a wall.  It is strong with a exterior glued core.  This is what you will see at constructions sites where you will see sheets of plywood that will be bowed up like a ski.  This is why on the better construction sites you will see the plywood covered and weighted down until it is used.

The best plywood for cabinet construction is the cabinet grade of which the cheaper but a good choice is the cabinet grade birch. 

In your case I can offer some helpful suggestions to improve and hopefully cure your problem.

The first thing I would do is to install a fixed mid shelf.  Locate this about mid way keeping in mind that in cabinet work for use in a residence or commercial use, cosmetically the rule of thumb is to keep place the mid fixed self at a point where the shorter doors will be on the bottom and the taller on the top.  I try to do this making them about 2 inches difference in height.

Cut the mid shelf to an exact length by measuring the inside dimension (both the top and bottom should be the same) and install it in its location using  glue and screws.  This should push the inside back out to an almost correct position taking out most of the bow.  Once installed lay the cabinet on to its back and using a tape measure insure that the cabinet box is square and if not you must pull it to square.  To check if it is square take a tape reading from corner to corner diagonally and compare the two measurements. The should be equal, and if not you will need to remove the back and pull them square.  Once they are square secure the back.  With the cabinets on its back you will or should use a face frame since this cabinet is bowed.   Cut face frame stiles (up and down) the same length as the outside of the cabinet.  Measure the cabinet width and and deduct the exact width of the two stiles and cut the rails to this length.  The rails are the horizontal members of the face frame. Install these to make the face frame and be sure to cut one for the middle shelf.  Once the face frame is build glue and attach the face frame pulling the cabinet sides, top and bottom to where they will fit the face frame.  This should make the cabinet member stiff and should pull out any uneven bowing out of the cabinet box and maintain its location.  I would recommend using the pocket hole system to attach the face frame and also use glue.  Remember when you are building the face frame mark and install the mid rail in a location that will insure that it will fall on the mid shelf so that the shelf will be even with the top side of the rail so that it will not leave you with a higher face frame or a lower face frame.  it should be level with the shelf.

I don't not know if you plan on making adjustable shelves in this cabinet or not but what ever you do put in the mid shelf and make it a fixed shelf.  It is possible that you may have to cut and fit each shelf in this manner depending on the amount of bowing that is left in the plywood.

Although it is hard to make this cabinet the perfect cabinet it can become a very nice looking cabinet.  Your idea of using 2x4 material on the bottom is good providing that you use flat material.  You can also use 3/4 plywood by cutting straight strips and screwing and gluing them onto the cabinet so that the 3/4 ply is against the flat plywood.  You gain strength doing it this way.

I hope this will help solve your problem however is you still need help please feel free to contact me.

[Bobby]

Jay thank you so much for your reply. To answer some of your questions, I do plan on putting a FF and the pocket holes have already been drilled. I do plan on using adjustable shelves and that has been done using the Kreg shelf pin jig. I think you have a good idea with the mid shelf and will definitely give that a try.

The BCX was not my first choice, I wanted to use birch but the person I am making this for didn't want to spend the extra money. Good for them but more work for me.

If I understand you correctly, I'm going to have to do a little grunt work to get this mid shelf in place in order to push this bow out a little.

[Jay Boutwell]

Hi Robert, you are most welcome,  I know the way it goes as customers always seem to have their own idea as to how something is to be built and will defiantly not want to spend money.   However as you are most likely seeing why they should allow you to make the decision on what you use.  I deal with this all the time in my profession.  I don't know where you live but at the home depot's in my area carry a imported birch line of 3/4 material that is about $40.00 per sheet and I have had to use it a few times building some garage cabinets and found it to be stable and free of voids.  currently I am build a cabinet similar to your project and I am using melamine as the customer is wanting an addition to the current cabinets that they have. Melamine is not my favorite but sometimes you must do as the customer wants.

 I always install a fixed mid shelf as it is pretty much a necessary thing in European cabinets and it is a definite plus in the face frame type as well.    That face frame should do wonders in supporting the front part of the cabinet and the fixed shelf should take care of the middle bow.  I think in this case I would also add a cleat under the shelf and make it only as wide as you can and still hide it with the mid rail.  I also have one just like it that is made of oak ply wood the a guy had me build 10 years ago and after I built the box and installed the face frame he picked it up with the intent of fixing it himself and found that it was a bigger job that he could do so he brought it back for me to finish.  It had been stored in a garage for these ten years and it is still rock solid and straight and square just like I built it.  It had the fixed mid shelf.  Because of the size of the cabinet it is the best  policy to do and I have been doing this for better that 25 years and is the only way I will build one.

I have seen some made in the European design (without face frame) that had no fixed mid shelf and saw the shelves that were adjustable fall out because the sides flex outward.  I have fixed a few and done it by installing a mid fixed shelf and have cured the problem that way.

 The main thing that the CBX is used for is in building sheer walls and other general construction.  The AC ply woods are even better.  I would love to see the photos after you finish.   

 
Robert said:

Jay thank you so much for your reply. To answer some of your questions, I do plan on putting a FF and the pocket holes have already been drilled. I do plan on using adjustable shelves and that has been done using the Kreg shelf pin jig. I think you have a good idea with the mid shelf and will definitely give that a try.

The BCX was not my first choice, I wanted to use birch but the person I am making this for didn't want to spend the extra money. Good for them but more work for me.

If I understand you correctly, I'm going to have to do a little grunt work to get this mid shelf in place in order to push this bow out a little.