Cupping table help

[Vikash]

About 18 months ago I had a 2" thick bubinga coffee table top made which eventually cupped. I finished it with finishing oil and put quite a few more coats on the top side then the underside. I guess this was the root cause that led to the cupping.

How can I make it straight again without cutting it up and starting over?

I was thinking of using two T shaped steel profiles inserted into a routed groove on the underside.

I was also wondering if there was a simple way to fix the MC/ release the stresses to make it bend back the right way before adding more finishing coats (if that is indeed the problem)

Vikash

 

[OPJ]

Do you have any photos showing the severity of the cupping and how is the top currently fixed to the frame? If it's not much then, you may be able to pull it out with wooden buttons/steel shrinkage plates and screws... :?

 

[Vikash]

Over the whole width (1.4m):

The worst of it over a shorter distance (36cm):

There is no frame. There will be a simple square base for it to sit on (to to replace the cardboard base it's currently on)

V

 

[matthewwh]

I don't think it's your finishing technique, otherwise it would have cupped the other way as the fibres swelled after taking up the oil. If anything you have reduced the problem.

I'd live with it for a minimum of three months and let it do all of its settling down in the environment where it will be used, then rip it at the worst spot, joint the edges and rejoin the pieces.

If you're careful it should be an almost invisible joint.

 

[matthewwh]

....or plane a similar curve into the long grain edges and call it a pagoda inspired table with a barely discernable hint of oriental mystique!

 

[Imperial]

It may have been the person who made it causing the problem by removing uneven amounts of timber when flattening the original material, or not alternating the boards when gluing up ( hard to see the end grain in the pics ). It pays to use smaller width boards when making a large top and not attaching it to a base which would have helped it stay flat while settling down.

After all that, making the base and pocket screwing it to the top in quite a few places may keep it flat after pulling it down flat to the base with clamps. Remember to allow a large space in the screw hole to allow for movement!

Hope that helps

 

[Steve Maskery]

This sort of problem is common and very difficult to avoid altogether. There is a reason why tables are usually made with legs and aprons, and it is because that construction method offers better protection against this sort of problem than the (otherwise very aesthetically pleasing) design you have chosen.

It's never going to stay flat on its own.

A few months ago I was charged with the task of flattening a beech kitchen table. It was quite large, as ugly as sin and normally I wouldn't have looked at it. But I owed this guy a favour. The top (roughly 6' x 4') was cupped by, I can't remember, but it was a lot. Let's say 20mm.

I sawed a number of relief saw kerfs, each about 3/4 of the way through the top (from the underside!). stopping about 100mm from each end. It was then flexible enough to rout (well Domino) a series of button slots on the inside of the apron and pull the top top down reasonably flat. Whilst not perfect, it was PDG, and the top didn't have to be refinished.

Now I realize that you do not have an apron to offer that support.

So, if you make the kerfs, either of two tings may happen. You could relieve the stress in the timber, allowing the boards to cup even more, more easily

Or you could release the tension and allow the top to be pulled fairly flat against a pair of battens on the underside

Or you can live with it.

It's a judgment call (or maybe a judgement call, I can't remember, you'll have to ask Jake) but what ever you decide, the outcome is uncertain (apart from the option of doing nothing, of course).

Cheers
Steve

 

[Modernist]

I don't think it will ever stay flat without some substantial battens.

Maybe if you put it somewhere damper for a while it would go back and you could then attach slotted battens which should hold it while it dried again.

The low design would make the battens invisible unless you were lying on the floor.

 

[studders]

Turn it up the other way, put some books under the low corners and some weight on the high part, then leave it for a while. If it goes back straightish you can then rout some grooves on the underside and epoxy in some hollow rods to try and keep it straight. You won't see the rods unless you're very very short. :lol:

 

[Modernist]

Whatever you do must allow for future movement but this could be incorporated into the base and be invisible, depending on the size of the base.

 

[woodbloke]

V - looking at the pics, I think that realistically this is one that you're not going to win...that top will want to move whatever you do to it. If as you say the top is 50mm thick, then in my view, there's no amount of cramping or fiddling around with it that will provide enough grunt to pull it flat.
Steve has offered a very good suggestion by kerfing the underside...but if it don't work? ..your then left with a top that you can't do anything with.
There are some useful sized pieces of timber in that top so I'd be inclined to rip it into constituent parts and make a smaller, equally chunky table that can be secured to a substantial frame. The top also should be made from smaller section timber with alternating grain.
I know this isn't the solution that you wanted to hear but at least with a bit of forward planning you ought to be able to get something out of it which is better than the original. My two euros, as ever - Rob

 

[Argus]

.


I make lots of edge-jointed table tops and as has been said, you need an undercarriage of some kind to keep the top stable - but even so it will move over its life; it's what wood does and you need to make allowances for it in the construction.

As a general rule I never joint boards greater than 4" and, unless they are really, really quarter sawn (with the growth rings at right angles to the face) I alternate the direction of growth rings so that any slight cupping in one board will be opposite to its neighbours.

Looking at the detail of the edge, the two boards with the greatest deflection that you identified are both aligned with the growth rings in the same direction, whilst the one on the left is opposed.

Frankly, it needs to be cut up and re-done, but an unsupported composite board of that width will always move. You can't avoid it.

If it were mine, here's what I would do:

• 
  1 - Cut along the original joins to get the original boards back again.
  2 - If they are about 7 or 8" wide - re-saw them in half.
  3 - Get rid of the original finish and take them all into the house where they will experience the finished environment and allow them to settle down for at least 3 to 4 weeks, longer if you can swing it.
  4 - Re-square all the edges and joint them again, alternating the growth rings.
  5 - Fit some kind of bracing undercarriage, aprons with loose buttons, dovetailed rail or even bread board ends ... whatever.

Good luck.



.

 

[bugbear]

I don't think it's your finishing technique, otherwise it would have cupped the other way as the fibres swelled after taking up the oil. If anything you have reduced the problem.

I'd live with it for a minimum of three months and let it do all of its settling down in the environment where it will be used,

Could the (present) curve be down to the top surface getting more drying than the bottom surface from the sun? If so, a little more time may eventually even up the moisture content, at least a bit.

So - yes, seconded on the "wait" advice.

BugBear

 

[Modernist]

Having looked again at the picture I think, unfortunately, you are on to a loser. I think Argus is right and drastic surgery is called for.

If you follow his suggestions and also introduce some substantial (say 60 x 30) battens, all will be well. 5 or even 6mm roundhead screws, with thick flat washers, running in couterbored routed slots should hold it flat. You could even taper the battens a little towards the ends but don't overdo it.

It works for Ercol

 

[Modernist]

You could even make it a design feature in a contrasting timber such as maple or sycamore

Sorry if you find that presumptious

 

[Vikash]

Hi,

I never ended up doing anything about the cupping for fear of messing it up. The time has come where I want to just get rid out of it now, but I don't know what it's worth. Could anyone help me calculate the cost of the Bubinga so I can put it up for sale in the for sale section. It's 1400mm x 1400mm x 50mm in size.

V

 

[marcros]

well, i calculate something IRO 3.6 cubic feet in the top. No idea of the cubic footage price of bubinga though, so have only answered half of the question