Table Top Quality Control

      Here's a long discussion on table top assembly, covering grain orientation, clamping, and other quality issues, with some entertaining and informative pictures. December 8, 2012

Question
I'm building a 42" round, pedestal dining table, 5/4" in finished thickness and itís cherry. I'm buying the column and legs so I only have to make the top. No skirt. Just a single center 'stretcher', sitting underneath at 90 degrees to the table top's planks. My boards are (after dimensioning) a bit under 10" wide, no sapwood (believe it or not) and are dead true now (straight and square). They are laid up with an alternating growth ring set up that still affords me seeing only the heart wood showing on the top face (see photos - I've taken a marker to the boards ends to show the ring's directions). I want the top to remain on an even plane, period.

Do I still have to rip the 10's in half, flip one of each half end-over-end and then glue up? I've read a thread in the Knowledge Base and not been fully convinced either way. I had always thought you had to alternate the rings. Now I'm reading a few guys saying that was not the case. Another part of that same thread indicated to always cup side up (smiling faces/as you look at end grain). Now I'm thinking of ripping and joining, all cup sides upward. I spent a c note on the cherry. I want to do it correctly and do it only once.

One other thing. A catalytic finish on both sides Ė will it be good enough for even-sided moisture sealing? I would love to hear from those with a lot of experience in this part of our craft.


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Forum Responses
(Furniture Making Forum)
From contributor J:
If your wood was kiln dried you should not need to alternate growth rings. This practice grew out of using air dried wood which still had a MC that allowed the wood to move. Kiln dried wood is pretty much set. I would not rip the 10' planks in half. You might release some inner tension in the wood and actually make it worse, especially with planks this long.

If it was me I would pick the best face configuration, glue them together regardless of how the growth rings were, and then flatten with a jointer plane. The jointer plane should take care of any minor movement you might experience at glue up.



From contributor F:
Glue up for color and grain matching, growth rings be darned. I also orient the grain direction so it runs the same board to board to minimize tearout if hand planing later. If you must rip a board due to machine limitations, glue it back together the same way it came apart. Facing, jointing, and proper milling is crucial as is acceptable moisture content. Flipping narrow boards will do little good if it wants to cup, you will just end up with an accordion instead of a bow.


From contributor D:
I'm one of the proponents for alternating your grain. However I build custom exterior entries, a much more severe situation. A dining table will be fine. Sam Maloof never worried about grain orientation in his furniture, and one of his rocking chairs is in the White House.

Still, pictures say a thousand words. I have seen tens of thousands of glue-ups, and honestly that one isnít giving me the warm and fuzzy. That is cup city for a door panel. Minimizing moisture variation will be key. Thin, wide boards, flat sawn, all facing the same way actually makes my skin crawl. Take a few end cut scraps, glue them up the same way, and subject them to a moisture change if you don't believe me. Seal this table well.



From contributor B:
If your boards are nice why rip them? You end up creating more seams and more chatoyancy. Glue your boards up the way they look the best regardless of growth rings. You don't say how you plan to hold this top down flat and still allow it to move? You may be asking a lot from this top.


From the original questioner:
I had read that ripping them in half and flipping one of the two pieces and then gluing should cancel the cupping effect. It should work in theory. I've actually made a bunch of table tops, but they're usually larger, thicker, and rectangular. All the round ones have been veneered ply (A-1 grade) whose outside edge received a solid stock edge, either laminated or of mitered segments.

None of my solid top tables (as far as I know) have ever gone out of plane (warped), but for some reason, just before I was to glue this one up, I got insecure about committing to it and thought I might as well check in with the guys. These are 5/4 now. My original intention was to take them down to a finished 1 and 1/8th. I do think thicker would be more stable, though. They were kiln dried. I'd love to keep the wide planks for the aesthetics alone. Below is a sketch of how it comes together. The stretcher (underneath) will have recessed slots for a screw bolt with washer to hold the top down to it and be able to move throughout the seasons. I'm making this up as I go so I'll listen to any and all suggestions.


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From contributor B:
The support looks fine at 10". I might just as well run two rows of slotted holes about 7 or 8" apart for a seemingly wider footprint.


From contributor L:
I assume you've looked at the cupped table top thread? It answers several of your questions. Kiln dried is not necessary better or worse than air dried. It depends on the quality of the kiln drying - tension release, etc. and what's happened since. Current MC (not what it used to be), finishing both sides evenly, and allowing for movement is more important than alternating rings. Since theyíre flat now I'd say youíre safe. Take equal amounts off each side as you approach final thickness.


From Gene Wengert, forum technical advisor:
Almost all the time cupping results when the top shrinks while the bottom does not shrink, or shrinks less. It is a differential rather than overall shrinkage. Certainly a small amount of cup might occur if the entire top is too wet and dries, but proper drying and storage and proper specification will stop this risk.


From the original questioner:
Well, I did my best to take into account everything said by all of you. I left the boards standing on edge in my shop with space in between for two days. I further re-jointed and re-planed to get all down to 1and1/8th. I left them at 10" wide and ended up alternating the growth rings (I got the grain look I wanted in-spite of that). It is glued-up with double stack biscuits and sandwiched with pipe clamps as we speak. No matter how I place a straight edge on it it's dead flat (found one angle where the edge was off the surface by less than a 1/16th). Not a drop of sapwood top or bottom. I think itís going to turn out fine. (In the photo below I photoshopped the growth rings in).

Itís interesting to hear some conflicting ideas about how to avoid the problem. I think there are a number of variables in this equation, though all of them have to do with wood's inherent ability to absorb and lose moisture and expand and contract. Everyone agrees that sealing it on both faces is of utmost importance. I'll be handing it to the finisher with the top separate from its 'stretcher'.

I'm thinking that if you had some heartwood (away from the center) of a tree whose trunk was 24" plus in diameter, growth rings wouldn't be very severe and therefore, less of an issue. Cherry is pretty costly these days and all the rough-cut lumber I see has the end grain sealed to prevent 'checking'. The problem is it stops me from being able to select from a stack based on growth ring pattern.


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From Gene Wengert, forum technical advisor:
We do need a clamp near the end of every joint (which means two clamps minimum). It appears that the top is not clamped near the end and so you are increasing the risk of an open joint at that end. Where we often see a problem is when pressure is applied and then as other clamps are tightened, the pressure on the first clamp decreases. As any excess glue has already been squeezed out, this means that the joint no longer has adequate adhesive. So, in addition to good pressure along the entire joint and especially the ends (especially in the wintertime), we also need to avoid any possible decrease in pressure.


From the original questioner:
I don't disagree that four (five maybe) clamps (though I always like to use an odd number of clamps) would have been more of a guarantee. I wish to offer two mitigating circumstances that allowed me to get away with using fewer clamps.

1. These were 5/4, solid cherry heartwood boards that were ten inches wide, a factor that weighs in my favor.

2. We need to remember the final object is a circle within this roughly square glue-up of planks. The center clamp was doing 70% of the clamping. (Now that I think of it, that is where I could have had two clamps on opposite sides.)

Anyway, my son had put it together when I was out, but I know the edges met air-tight. I squared the jointer's fence before we began. He coated both faces well and made sure a small amount of glue beaded up from every seam. With accurate machining, a board has much less reason to deflect.

I also want to say that the end clamps never get the benefit of another clamp on the outside. They have more important positions and going back to check tension on all the clamps - points taken, Gene.



From contributor P:
A couple practical notes about this: If the moisture content changes, those boards, with their grain orientation are going to move. The width will make the movement more severe. Ripping wide boards down and alternating grain makes the movement less severe across one piece. If you are confident in two things you should be ok.

Is the moisture content of the boards now on target 6-7%? Does the end user have a well climate controlled home for the table? If the answer to both is yes, then you should be ok. To my knowledge there is no finish (other than an unbroken coating of wax) that will eliminate moisture change, at best, finish only slows it. A finish molecule is a large one, and a water molecule is very tiny, it will find its way through; like tossing a BB into an erector set construct. If you have any doubt about moisture content now, or in service, I would glue and screw that top to a plywood backer and hope that restrains any future movement.



From contributor B:
Forty two inches of wood whether it is one piece or twelve pieces will have the same seasonal movements. In some cases flipping and ripping may help flatten otherwise warped pieces to a final assembly, but the wood will move the same in width regardless of how many pieces or which way the growth rings are going.


From contributor O:
ďIf the moisture content changes, those boards, with that grain orientation are going to move. The width will make the movement more severe. Ripping wide boards down and alternating grain makes the movement less severe across one piece.Ē

The statement contains lots of variables:

1. Moisture content changes - by how much? Sudden or slow?

2. Why does the grain orientation make them want to move?

3. What does 'move' mean? Cup? Change in dimension? How much?

4. Why does the width make it more severe?

5. Do wider boards at 42" total move more than narrower boards at 42"?

6. Why does rip and flip work as you say?

Sorry to be so argumentative, but those statements are not very precise. I first ran into 'rip and flip' 30 years ago as part of Jerry Metz's rules for successful solid wood panel processing in large scale furniture manufacturing. Rules developed during the 50's to mitigate the problems when one errant board crept into a panel. By ripping to 2-3" widths, the damage one board could cause was limited, and could still squeak by quality control. This is hard to explain/justify to a woodworker with some fine 6/4 cherry at 10-12" wide, contemplating a table top.



From the original questioner:
"In some cases flipping and ripping may help flatten otherwise warped pieces to a final assembly...''. I agree, though I'll always want to make a table top from wider planks if I can. Itís better/richer in appearance. Layers swell inside the plank and wood expands (mostly) at 90 degrees to the growth ring lines. Anyway, the top's still very flat. Here's a few shots of making the top and its stretcher assembled.


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From the original questioner:
I guess you'd call this a cleat?


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From the original questioner:
Sitting in a good, dry place, waiting to be finished.


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From Gene Wengert, forum technical advisor:
More about flipping pieces: First, the tendency to cup is almost zero for an individual piece if the rings have little curvature (when viewed from the end of the piece; that is, the end grain). Any cup would be so small as to be ignored, even when the MC changes considerably. However, as the rings exhibit more curvature, the bark side will shrink more than the other side, so cup results.

In narrow pieces, the rings will tend to be fairly flat, with only a rare piece having curvature. Note that curvature of the rings means the piece was sawn from lumber that was located close to the center of the tree where curvature occurs. Such a piece is more prone to being knotty, so is not likely to be included in a clear, narrow strip.

A wide piece of lumber sawn from today's tree is likely to have the center section (width-wise) from near the center of the tree; getting wide lumber with flat rings is just not likely today from trees and logs in the U.S. Hence, wider lumber is prone to cupping when the MC changes. If a wide piece of lumber is ripped into strips and then the strips glued in the same order as they were in the lumber without flipping, cupping tendency will not change. In this case, flipping would be prudent. If several pieces of lumber are ripped then ripped pieces randomly assembled into a panel, cupping risk is small. Individual pieces with ring curvature will possibly cup with MC change, but it will be small indeed.

Fifty years ago, wider pieces of lumber were more common, so the feeling that flipping was essential may have had some merit, as we did not mix q-sawn grain with flat-sawn. So, it was likely to have more strips with lots of ring curvature.

So, the bottom line is, if a lot of your individual strips have strong ring curvature, then flip to obtain the more stable glued panel. Maybe you also want to figure out why your lumber has such ring patterns. Perhaps someone else has taken the good pieces of the same grade with flatter rings, so you are getting inferior quality.

For reference, most furniture wood can expect 1/2 to 1 % size change seasonally in width, while no change in length. However, it is so common to see the MC of purchased lumber to be too wet and so the size change initially after the piece leaves your shop and goes to the customer to be 2%, especially in the winter. This is why humidifying a shop above the humidity level of the customer is not the best idea. Oftentimes the customer will have 30% relative humidity or lower in the wintertime which is 6.0% MC or lower. Such MC change from shop to customer will result in more change and more shrinkage, cup, etc. than the yearly cycle.



From contributor E:
For a flatsawn tabletop I orient the growth rings in the same direction, so if it cups at all it will be in a smooth curve instead of a roller-coaster, which is a nightmare. The attachment to the base should hold the top flat and all will be well.

From Gene Wengert, forum technical advisor:
Contributor E is correct about roller coaster effect if you flip. However, if the MC change is small it will not be a factor. The glossier the finish the more noticeable any defects will be.


From the original questioner:
The table top is still as flat as can be, but here's something I did which almost ruined the whole top. Having made the plate upon which the top sits, I drilled 1" deep holes in this 1and1/4" thick top and I started to insert those bolts that are wood thread on one half and machine thread on the other (this is what will hold the top down and still allow for some expansion/contraction). I was using a vice grip in the middle of the bolt and talking with my son. I kept turning the bolt thinking I was going to feel some additional resistance and I stopped, looked down, wondered if I was over-doing it, reached under with my hand to feel the underside (which is the top side as I was working with it upside down), and my heart sank.

The bolt didn't come through all the way but I did lift and split some of the cherry (1/4"). I placed some glue in the hole on the bottom side, blew on that hole until a tiny bit of glue showed on the face and then I turned the table top good side up and placed a block of wood to the area and used a hammer to knock it back flat. I wiped it good with a water soaked rag and left it to set up. You can't even see the spot, but I'm not out of the woods until some stain goes on and we see if any glue has stopped stain from taking in a tiny area. I should have marked the bolt with tape to indicate how far in I was allowed to go.


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From contributor L:
Lucky you stopped when you did! That's why I use brass bushings. They hold well, are shorter, and the bolts can be removed over and over. One problem a friend of mine had though was after shipping a table to the customer, the customer was going bolt the top to the base. He lost one bolt and when to the hardware store and the bolt he purchased was too long!


From the original questioner:
It's finished. Here are two shots (base and joined). The break on the table's top is absolutely invisible. Thanks for everyone's input. I'm very happy with how it turned out.


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