Thread: Oak Coffee Table Problem

My initial response is, the reason for the raised pieces have been in general to...

A) The thickness of the material used for the squares.

and

B) the construction of the solid rails places around the table. (Which do look fantastic by the way.)

Sadly timber being a living material i.e. it still expands and contracts, as such with the solid border containing it, it cannot expand in the usual flat horizontal plain as it would normally do so.

Therefore the expanding pieces have no other choice but to pop up.

Added to which the rails around the table, having the same tendency to expand, seeing they also are locked together, the expansion creates more pressure on the pieces in the middle of the table compounding the problem.

I suspect that in the dryer months all will go back to being flat, but with rain and humidity sorry it's goin' to buckle.

On future builds to eliminate this situation either use a floating top which allows for expansion and contraction, Build a top which it not bordered or inlay with standard veneers .6 thick.

As a rule of thumb, when looking at furniture in a furniture store 99.9% of tables displaying a solid rail all the way round the top are either Chipboard or MDF.

Sorry no quick fix, Keep it enjoy it and in time to come, you can look back and see how much you woodworking skills and knowledge have grow.

Just to let you know I have been a Furniture maker for the past 25yrs+ and in that time I have make a lot of expensive and time consuming firewood.

Keep making those shavings


Cheers


Steve

What Steve has suggested may well be a contributing factor, but the fact that the problem is confined to random squares, brings up the question could the random problem pieces be due to grain oriantation? For instance, as the squares are pretty thin, it would be quite easy for them to want to cup if they are backsawn. Alternatively, if quartercut they would remain more stable. As they are glued to a thin layer of veneer who's glue line is of unknown integrity, it well may be that your glue line has not been as successfull as you may have wished. Add to this the fact your squares are end grain to long grain adhesion, you would have to have used an end grain suitable glue. Now its all getting complicated. May I suggest your problem is a combination of causes, the most predominant being edge bond failure and backsawn material. Oak is a very stable timber. But only when cut on the quarter.

Thanks to both fineboxes & rustynail for your input on my problem. I had suspected that it had something to do with expansion but was not sure.

I am going to take Steve's advice & wait for more favorable weather and see if the wood contracts back to it's original state.

I was wondering if cutting a 1/4" deep slot with a 1/16" straight router bit around the perimeter of the field of 3 inch squares just along the inside of the rails would give enough relief for the expansion of the field.

Of course, then I would have to come up with some sort of flexible filler to prevent the accumulation of debris in the slot - another problem. I would be interested in your opinions of this idea.

My vote is for natural expansion and contraction of the timber exacerbated by the surface essentially being a very thick veneer.

As the 'veneer' is so thick, any percentage change in its height will be much more noticeable - potential for movement in your tabletop will be roughly six times greater than for movement in a standard veneer (that's assuming a 1mm veneer vs your 6.35mm veneer).

I would be in total agreement with what others have said,that main issues would seem to be the thickness of the veneers used 1/4" is very thick for veneer 2-3mm maximum is what would be recommended for hand cut veneer, machined veneer is only a fraction of this.

Another poor choice is to use plywood as a carcase,it may be ok for really thin veneer 1/2mm or so with the grains running at right angles but using really thick veneer and the consequent expantion and contraction may cause delamination within the ply wood,Regardless of what glue you used, the whole structure is only going to be a strong as that use in the manufacture of the plywood.

Measure2cut1,

The below sounds like good advice. Assuming the glue seams are OK, the most significant issue seems to be the thickness of the veneers. This is something I understand, having done the same thing myself. Wongo, Ian and a couple of others set me straight. Thicker veneers behave like real wood, expanding and contracting. Thin veneers do not. This irritates me, because if the surface needs sanding and refinishing, thick veneers offer greater security against sanding through to the substrate ... but wood does what it does.

In my case, I cut away the borders, sanded the panels down on a drum sander until the veneers were only about 1 mm thick, and then re-attached new borders.

It worked. Even the recent high humidity in Brisbane has not affected them.

All timber moves, but some moves more than others. With a 10% change in moisture content She Oak moves tangentially by 5.6%, Black Bean by 4%, NG Rosewood by 2.4%, Red Cedar by 2% and Kauri by 1.7%. Radial movement is about half these figures (see Issue 51, Australian Wood Review). I use a lot of cedar and rosewood because of all the timbers I like, they have low rates of movement. This minimises movement problems like sticky drawers.

Before I understood wood movement I made several solid cedar doors with borders ... very naughty. A tabletop made of solid Vic Ash with borders has buckled slightly and the finish has cracked around the borders. However, after about five years I do not yet have a problem with the cedar doors. I am guessing that the low rate of movement, along with the softness of the wood (allows it to compress a little) has saved my bacon ... so far.

Hi Measure 2Cut 1

All good analysis and advice from previous postees. I'm sure your problem is differential wood movement in the radial, tangential and longitudinal planes. The relative wood movement figures are 2.2% 1.2% and 0.4 %. Entrapping the panel in a rigid frame just adds to the difficulty. Is there a solution? I'm not sure but using veneer of the standard thickness of 0.6mm works for folk who do fine veneering (not me!). Don't do anything in a hurry, the migration of finished timber to EMC % takes many months perhaps 18 to 24 months.

I wouldn't assume either that MDF and plywood or chipboard are necessarily supplied with no variation in MC%. It's good practice to straighten and square the edges of sheet materials when making commodity cabinets and its not uncommon to see stress being relieved in the form of edge distortion from straight when this is being done.

Cheers Old Pete

Pete,

Ain't that the truth.

I no longer believe that MDF hardly moves at all, because sometimes it does. Fitted an MDF back to a cabinet a couple of weeks ago. Then we had lots or rain ... warm humid weather. The MDF panel buckled in places as it tried to expand. In places it buckled upwards between two screws, only about 100 mm apart. The cedar will have moved, but not a lot, and cedar movement does not explain the buckling between two screws.

Worse than that John, different brands move differently!

Artme,

I'd guesses that, because I've never had this problem before. I bought that MDF from Bunnings, but may need to look for a new supplier.

In particular, if we are veneering, we need a nice stable substrate. Any clues on a brand that has consistently low movement?

Do you use HMR MDF - stands for high moisture resistance. Its usually has a green tinge. Considering how little extra it adds to the cost of a project I dont know what case can be made for ever using ordinary mdf - except maybe in temporary things.

One of many makers:
MR MDF - MDF - Building Products & Surface Materials | Gunnersens

Yes, I have used HMR, but although I've not seen it in the thinner sheets, it must be around.

Not sure about that. I've never seen it thinner then 12mm. Ply is better for a cabinet back or anything else you would need thin moisture resistant mdf for, so maybe there is no incentive to make it in the thinner dimensions.

When I make cabinets, I just use the 16mm HMR stuff for the backs - its overbuilding but not worth chasing around for thinner.

Cheers
Arron