Straightening a bowed cabinet on legs.

[mrFinpgh]

I've been working on and off on a 5' wide cabinet on a base for a few months.  The construction is box joints on the corners, a divider in the middle, and one shelf.

The base is a simple 6" tall affair with ~1 5/8" rails joined with pinned bridle joints.  I've got 4 stretchers running between the front and back, about 1 5/8" square.  The base attached to the box via 8 threaded inserts (2 per stretcher) that have some play for expansion/contraction.

The issue I'm running into is, the bottom of the carcass has bowed over the past few months and the base is not rigid enough to force it flat.  If I throw a 6' level on it, there's about 1/8" of an inch in the middle.  The top panel is still flat.

The carcass sides are already 3/4" thick, so I don't really have room to plane out the issue. I'm trying to figure out how i can increase the rigidity of the base so it will be able to bring the bottom flat.  I was thinking to let in 1/8" thick 1/2" angle iron, but wasn't sure if that would be sufficiently rigid to resist the bending.

Does anyone have any thoughts/ideas on how i could get the base straightened out?

 

[squall_line]

I guess a lot of it depends on whether or not the aesthetics of the current base need to be maintained or not.  For example, adding a center leg or other reinforcement that would be visible vs. angle iron which would be able to be hidden for the most part.

What is the cabinet made of?

Do you have any pictures?

 

[Packard]

With a 6” tall base, the line of sight probably goes no further than 8” to 10” under the base.

That means you can probably add one ore two additional legs in the middle that will be hidden from sight.  One leg would mean the legs are spanning 30”.  Two legs mean they are spanning 20”.

I would make a pair of temporary supports and see if the additional legs will address the issue.

Or you can make the additional legs as part of the design and have them visible.

Five feet is a long span for any sheet goods.  Years ago I tried making a shelf to span 6 feet from 3/4” particle boards glued together to make 1-1/2” thick shelf.  It sagged, so I added a support half way (3 feet) and that worked.

 

[Michael Kellough]

You need to provide photos.

Do you want to push the bottom of the carcass up or pull it down?

 

[Packard]

You need to provide photos.

Do you want to push the bottom of the carcass up or pull it down?

I saaumed he meant it was sagging in the middle.  I should have asked for photos too.

 

[Crazyraceguy]

I fixed a similar problem my daughter had a few years ago. The unit was about 60" long, and sagged significantly in the middle. I made a center platform, laminated it black and it held the whole thing up, from the middle, for a few weeks, until it sagged back the other way. The feet did make contact with the floor again, after a while. Not the most elegant solution, but it worked, you can't see it under there. The item in question was not really worth more effort  [unsure]
I'll get around to making something better someday, but she's happy with it, for now.

 

[mrFinpgh]

Thanks, all.  I've added some photos in this reply to show what I'm talking about.

The entire cabinet is made of Sassafras.

I don't know if I could call this sag, per se - that might be a whole other issue.  I think this is more wood movement than succumbing to gravitational force.

Some details of the base:

View attachment 1

View attachment 2

If I put the bottom of box on, without fastening it down, and push one end flat to the base:

View attachment 3

Fastened down:

View attachment 4

In the middle of the base, after fastening, with a 6' level across it.

View attachment 5

Putting all 4 sides of the box together, just using hand pressure:

View attachment 6

The point about a center leg is well taken ..  I think maybe that's a different issue from the need to straighten it out, though.  My hope was that the dadoed in divider/shelves would help resist some of the potential for sagging, kind of like blocking between joists.  I could probably find a way to add a center support if I did need to.

 

[Packard]

If it was straight originally and it sagged, then it will “sag” back to flat if you support it correctly.

I would add a leg to the middle of the two cross members.  They would remain out of the line of sight unless you were lying in the floor or genuflecting.

Test the concept out.  Put a temporary support under the two cross members and see what effect it has.

What I can’t tell is if those cross members are securely attached th the long pieces.

If they are not, then try adding the extra feet to the rear long piece only.

Or add a through dowel from the back to add strength to the joints before adding the feet.

In fact through dowels may be the easiest way to attach the additional feet.

1.  Cut the feet to length.

2.  Add glue to the joint.

3.  Attach with a screw.

4.  Wait for the glue to dry and then remove the screw.

6.  Drill a 5/8” or 3/4” hole where the screw used to be.

7.  Fill the hole with a long dowel and glue.

The dowels will not show.  And this makes a strong joint.  Plus alignment of the dowel holes is guaranteed.

But test the concept first by placing a temporary support under the middle of the base.

Note:  The new, and very aggressive Apple spell checker changed “holes” to “hoes”.  Are “dowel hoes” some new sort of street walker?  I really don’t like this spell checker.  It changes correctly spelled words to different words if it wants to.

 

[smorgasbord]

The issue I'm running into is, the bottom of the carcass has bowed over the past few months and the base is not rigid enough to force it flat.  If I throw a 6' level on it, there's about 1/8" of an inch in the middle.  The top panel is still flat.
...
Does anyone have any thoughts/ideas on how i could get the base straightened out?

Wait - the base is straight, right?
It's just not rigid enough to force the ¾" warped plywood bottom flat - right?
When just laid on top, the ends of the plywood bottom curve up.
When fastened to the base, it gets flatter, but the plywood warp is strong enough that the middle pops up about ⅛", which means the plywood is bending the base. That is, the base isn't strong enough to force the plywood bottom flat, the combination of the two is flatter, just not flat enough. I'm assuming you have a way to attach the bottom to the base not just at the ends, but also in the middle. That there's no gap between the base and the bottom anywhere.

You said the top is flat. You did a temporary assembly, but that didn't include the vertical middle divider. What happens if that's in place? It would connect the flat top to the almost flat bottom, so might move things enough.

If not, then I would look into shimming between the base and the carcass bottom. The base is recessed, so any small gap from the shimming won't be noticed. Other than that, I think your original idea of getting some long steel angle iron and figuring how to attach that to the bottom of the carcass and the base would help, but doing that seem problematic to me.

 

[mrFinpgh]

If it was straight originally and it sagged, then it will “sag” back to flat if you support it correctly.

It didn't sag, it's bowed. I don't think it's a function of a load, but a function of the 5' long board having tension in it/uneven moisture issues.

The stretchers are already fixed in place w/ 2 dominoes per end.  A dowel is probably going to be the easiest way to insert something.  Well, a few GRK screws might be a bit easier.   I don't know for certain how it will sag under load, but given the current performance, it seems as if a center leg is likely.

Wait - the base is straight, right?
It's just not rigid enough to force the ¾" warped plywood bottom flat - right?
When just laid on top, the ends of the plywood bottom curve up.

When fastened to the base, it gets flatter, but the plywood warp is strong enough that the middle pops up about ⅛", which means the plywood is bending the base. That is, the base isn't strong enough to force the plywood bottom flat, the combination of the two is flatter, just not flat enough. I'm assuming you have a way to attach the bottom to the base not just at the ends, but also in the middle.
That there's no gap between the base and the bottom anywhere.

You said the top is flat. You did a temporary assembly, but that didn't include the vertical middle divider. What happens if that's in place? It would connect the flat top to the almost flat bottom, so might move things enough.

If not, then I would look into shimming between the base and the carcass bottom. The base is recessed, so any small gap from the shimming won't be noticed. Other than that, I think your original idea of getting some long steel angle iron and figuring how to attach that to the bottom of the carcass and the base would help, but doing that seem problematic to me.

 

You've got it. The bottom is solid wood, but yes, it's an issue where the base is not sufficient rigid to pull the bottom flat.

I did look at shimming. My only misgiving about that is the visible gap it would create.  Not really a critical issue, but personally I would know about it. 

What about the steel seems problematic to you?  I think the main issue I have with it would be that I don't particularly enjoy metalwork.

 

[Packard]

If it was straight originally and it sagged, then it will “sag” back to flat if you support it correctly.

It didn't sag, it's bowed. I don't think it's a function of a load, but a function of the 5' long board having tension in it/uneven moisture issues.

The stretchers are already fixed in place w/ 2 dominoes per end.  A dowel is probably going to be the easiest way to insert something.  Well, a few GRK screws might be a bit easier.   I don't know for certain how it will sag under load, but given the current performance, it seems as if a center leg is likely.

Wait - the base is straight, right?
It's just not rigid enough to force the ¾" warped plywood bottom flat - right?
When just laid on top, the ends of the plywood bottom curve up.

When fastened to the base, it gets flatter, but the plywood warp is strong enough that the middle pops up about ⅛", which means the plywood is bending the base. That is, the base isn't strong enough to force the plywood bottom flat, the combination of the two is flatter, just not flat enough. I'm assuming you have a way to attach the bottom to the base not just at the ends, but also in the middle.
That there's no gap between the base and the bottom anywhere.

You said the top is flat. You did a temporary assembly, but that didn't include the vertical middle divider. What happens if that's in place? It would connect the flat top to the almost flat bottom, so might move things enough.

If not, then I would look into shimming between the base and the carcass bottom. The base is recessed, so any small gap from the shimming won't be noticed. Other than that, I think your original idea of getting some long steel angle iron and figuring how to attach that to the bottom of the carcass and the base would help, but doing that seem problematic to me.

 

You've got it. The bottom is solid wood, but yes, it's an issue where the base is not sufficient rigid to pull the bottom flat.

I did look at shimming. My only misgiving about that is the visible gap it would create.  Not really a critical issue, but personally I would know about it. 

What about the steel seems problematic to you?  I think the main issue I have with it would be that I don't particularly enjoy metalwork.

If you have not yet applied a finish…

You may be able to “bow” it back by “painting” water on the inside of the curve. 

The one time I got a bow like you describe, I applied poly on one surface.  Because the other surface was mostly hidden and since “Minwax” referred to their stain as a “stain and finish”, I left it with just the stain.  I did not try to correct it.  It was a shelf.  I finished the other side and let the load take the bow out.

I don’t know if painting with water will work.  In my mind it does. Usually when wood bows it is a moisture (or lack of moisture) issue.  But water is cheap.  It might be worth a try.