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Need help with table top movement issue

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Forum topic by himself posted 02-22-2015 07:00 PM 781 views 0 times favorited 12 replies Add to Favorites Watch
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himself

5 posts in 655 days


02-22-2015 07:00 PM

Hey there everyone I’m new to this site and not sure if this is the appropriate place for this post so I apologize if its not. Anyway my question is I made a coffee table about a year ago, here are some pics of it. I have recently had some trouble with cupping its about an 1/8” on one side and 1/16” on the other the top is two 5/4 book matched maple boards 50” in length and 18” wide. When I attached the top I didn’t have a good understanding of wood movement and I attached the top with with figure eight fasteners instead of buttons. I was told that this would not allow for movement if they were used length wise along the grain. I also only put poly on the top and sides of the top. Are either of these things responsible for my cupping issue and if so is there anything I can do to fix it? Also if you notice because of the design I can only attach the top length wise so screwing it to the base doesn’t really pull it flat.

-- Patrick Hannon, Brooklyn


12 replies so far

View Ghidrah's profile

Ghidrah

667 posts in 687 days


#1 posted 02-22-2015 07:12 PM

for starters the entire object needs to be covered. One of the 1st jewelry boxes I made I only covered the exterior before the summer was done one of the corner joints had pulled apart, I was able to force glue into the joint and reclamp it tight. Within a yr it was open again, a friend asked the stupid question, you covered all sides yeah?

In your case I believe the underside acted like a moisture sponge. Be aware, flat sawn wood tends to cup for GPs 1/4 and rift not so much.

-- I meant to do that!

View mahdee's profile

mahdee

3553 posts in 1232 days


#2 posted 02-23-2015 12:59 AM

I don’t think you have done anything wrong with this project other than not realizing the sap area was going to shrink more than the rest. Not sure how you can correct that other than reinforcing it with some cross boards that is more likely to crack the top. But, I don’t see a problem with that if you can fill the cracks with sawdust mixed with glue or epoxy when it sets. You can also use various stones to cover the voids once they occur. It will require time (several weeks) to correct this but the end result will be beautiful in my humble opinion.

-- earthartandfoods.com

View gfadvm's profile

gfadvm

14940 posts in 2155 days


#3 posted 02-23-2015 01:18 AM

What a beautiful table! Love the bowties! The figure 8 connectors should allow for movement if attached at the ends of the flats of your stretchers. The cupping is most likely due to the lack of finish on the underside. Not sure if this is fixable but I would probably try drying the underside (heat gun?) and the applying finish when flat. I hope you can save this great piece.

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View goochs's profile

goochs

56 posts in 694 days


#4 posted 02-23-2015 01:34 AM

can’t help with an answer but I just had to say I love your work!

View DocSavage45's profile

DocSavage45

7705 posts in 2307 days


#5 posted 02-23-2015 01:38 AM

I’m with the Doc on this. It’s a moisture difference. Also the movement is characteristic of figured wood and humidity changes.

-- Cau Haus Designs, Thomas J. Tieffenbacher

View Rick M's profile

Rick M

7923 posts in 1845 days


#6 posted 02-23-2015 03:12 AM

I also only put poly on the top and sides of the top.
- himself

#1 cause of table top cupping. Rule in woodworking is whatever you do to one side, you do to the other. Uneven exposure to atmosphere allows different moisture content in one side of the board. Moisture loss will cause that side of the board to shrink, moisture gain will cause it to expand; and you end up with cup. This is different than wood’s natural tendency to cup toward the later wood while drying.

-- http://thewoodknack.blogspot.com/

View AlaskaGuy's profile

AlaskaGuy

2406 posts in 1774 days


#7 posted 02-23-2015 03:54 AM


I also only put poly on the top and sides of the top.
- himself

#1 cause of table top cupping. Rule in woodworking is whatever you do to one side, you do to the other. Uneven exposure to atmosphere allows different moisture content in one side of the board. Moisture loss will cause that side of the board to shrink, moisture gain will cause it to expand; and you end up with cup. This is different than wood s natural tendency to cup toward the later wood while drying.

- Rick M.


Just to ad to the confusion.
What you think about this article by Bob Flexner (I assume you know who he is)

Click here

-- Alaskan's for Global warming!

View Rick M's profile

Rick M

7923 posts in 1845 days


#8 posted 02-23-2015 05:11 AM


Just to ad to the confusion.
What you think about this article by Bob Flexner (I assume you know who he is)

Click here

- AlaskaGuy

Ask Bob if he finishes the bottom of his tables. ;) If the bottom is unfinished, it will lose/absorb moisture faster than the topside; Bob can’t deny that without contradicting his earlier work and he outright states it in his opening paragraph. But he then claims it is the finished top that is losing/gaining moisture faster because people wipe the top with damp cloths. It’s an utterly ridiculous explanation that contradicts what he knows is true. [note: I’m not saying his compression cupping doesn’t or can’t happen. It’s basically the same thing that happens to hammer and axe heads; but I’m not convinced it can happen in a table top unless you are really aggressively washing it down with water.]

I have an example for you. The very first table I ever made, I only finished on the top and sides and the top stayed flat for about 8 years. Then one winter the top cupped so much it snapped the buttons. Now logically it should have gone convex but didn’t, just as Bob says it cupped upward as if the top had dried or the bottom has absorbed moisture but clearly that isn’t what happened. The humidity level in the house isn’t going to change over 3/4” in altitude and the wife normally uses Pledge to wipe down the tables, the only variable is the finish. But none of my other tables, finished top and bottom, have cupped. And you can look to the many posts on LJ of people that have built tables that cupped overnight or within a short time and invariably they only finished one side. So I don’t have all the answers. I don’t know why they cup upward (I’m taking Bob’s word since he has refinished thousands of tables) but I’ll accept the obvious explanation over one that contradicts what we know is true.

If anyone is still on the fence, you can test the theory by finishing a tabletop on one side only and placing it bottom down above a heating vent in your house. If Bob is right, it won’t cup. Anyone want to take that bet?

-- http://thewoodknack.blogspot.com/

View AlaskaGuy's profile

AlaskaGuy

2406 posts in 1774 days


#9 posted 02-23-2015 05:17 AM

I don’t think placing a table over a heating vent would be a valid test. Place it where it’s going to live would be a better test.

I’m not saying Bob is right or wrong. I do know expert who change sometimes their minds on issues over time as new evidence come to light so to speak.

-- Alaskan's for Global warming!

View Rick M's profile

Rick M

7923 posts in 1845 days


#10 posted 02-23-2015 06:39 AM

The ability to change your mind with new evidence is an admirable trait but Flexner is not changing his mind, he’s contradicting himself. Which is the more likely explanation for a top to cup upward: moisture moving into an unfinished bottom, or moisture moving through a film finish into the top from wiping with damp rags and creating compression cupping? Occam’s Razor. I have mass respect for Flexner’s work in finishing and I still recommend his book for the hobbyist wood worker but he’s slipped a gear on this one. Incidentally there is another example of something that cups upward … a sponge, because the top dries first.

-- http://thewoodknack.blogspot.com/

View DocSavage45's profile

DocSavage45

7705 posts in 2307 days


#11 posted 02-23-2015 06:55 AM

Hey Guys,

Even without a finish, wood will cup due to moisture release. It’s like bread? when you cut it. You can see it in plywood wxposed to sun on one side? There is also the natural release of tension?

Common sense says what we do to one side we do to the other?

Deems my two cents. Ive read flexner and Charles Neil. Good luck on the project.

Good Night all.

-- Cau Haus Designs, Thomas J. Tieffenbacher

View Jorg Zimmermann's profile

Jorg Zimmermann

24 posts in 1071 days


#12 posted 02-23-2015 08:27 PM

Hi there,
what I know about wood is, there is a right side and a left side on every piece of wood. the side towards the center of the tree we call the right side and it is this side that formes convex when the wood is drying. Why? that is because of the length of the year rings. The actual stretch the longer rings on the left side are shrinking is longer so there is a pull towards the left side meaning the board is cupping. You cannot avoid it completely, dovetail housing is the traditional way of minimising the cupping. I have also seen inserted metalrods and the like.

Jorg

-- Jorg

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