Help Needed: flattening a table top and keeping it flat

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Forum topic by Mortalis posted 01-10-2016 11:57 PM 4399 views 0 times favorited 34 replies Add to Favorites Watch
View Mortalis's profile


14 posts in 1072 days

01-10-2016 11:57 PM

I am currently working on a sofa table style table for a friend of mine. I was provided table legs (Mahogany) and am making a table top to match.
The lumber came finished one side but cupped a bit while allowing it to acclimate so I jointed the finished side to flatten and then jointed the edges to be perpendicular. I then planed the other side to be parallel with the first side. I alternated the end grain to minimize any further cupping and used biscuits to join 4 pcs 1” x 4” x 48” rectangle.
This is where my agita begins.
I tried to use my Stanley block plane and Porter CAble power plane to flatten the resulting glue up. First thing that happened was that when I tried to hand plane across the grain all I did was tear up the faces. I use the scary sharp process to sharpen my plane blade with some relief at each corner. I ended up going out and purchasing a Wood River Block plane withe the same end results. I ended up using my power planer and belt sander to get the two table top faces flat. While I was letting the top rest from weekend to weekend the top cupped rather drastically. I spent another weekend with the belt sander getting both faces flat again. Again, a week later the top cupped.
I have decided I need to somehow structurally hold this top flat. I thought about sliding dovetail across the width of the top and then using mating dovetailed edge grain boards. I have also thought about using stretchers attached with screws to draw the ‘bottom’ flat.

I need to advice on what I need to do to flatten the top and keep it flat.


34 replies so far

View conifur's profile


955 posts in 1356 days

#1 posted 01-11-2016 12:02 AM

Maybe some cleats on the bottom.

-- Knowledge and experience equals Wisdom, Michael Frankowski

View Mortalis's profile


14 posts in 1072 days

#2 posted 01-11-2016 12:17 AM

Cleats…..that’s what I meant to say when I poster ‘stretchers’.

I was concerned the table top was not thick enough to engage enough screw thread to be able to accomplish a flattening effect. Do you have any how to vids or examples to explain the proper method for using cleats?

View conifur's profile


955 posts in 1356 days

#3 posted 01-11-2016 12:57 AM

First I maybe wrong but a ‘stretchers’. run with the grain, a cleat is perpendicular to the grain. An yes you use a screw that goes into the top about 3/4ths of its thickness.

-- Knowledge and experience equals Wisdom, Michael Frankowski

View gfadvm's profile


14940 posts in 2894 days

#4 posted 01-11-2016 01:27 AM

If that wood is that determined to cup/misbehave, I’m afraid that it will crack when you flatten it with cleats. Sorry, but I’ve been there. Maybe inlay some bowties after it is through cracking?

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

View bandit571's profile


21790 posts in 2887 days

#5 posted 01-11-2016 01:37 AM

One other option? Breadboard the ends.

Simple enough to do. Just add glue at the center only. Add pins/dowels/nails from the underside for the outer ends.

-- A Planer? I'M the planer, this is what I use

View tturner's profile


63 posts in 2233 days

#6 posted 01-11-2016 01:39 AM

1-Is the material completely dry?
2-Is it very cold or hot In the shop? Extreme temperature changes can and will cause wood to cup-especially Oak.
3-Is the shop area also dry? High humidity levels can cause wood to ‘walk’

-- I'm him

View Mortalis's profile


14 posts in 1072 days

#7 posted 01-11-2016 02:19 AM

I know I didnt mention the shape of the table top so I appreciate the breadboard suggestion but the table top is oval.

As far as the atmosphere in the shop, it is variable with the weather. I live in Massachusetts near the water and it is most of the time humid in the summer and dry in the winter. The shop is my two car garage. I allowed the lumber to acclimate with spacers in between the layers for two weeks before starting the project. In the winter I use a propane heater.

I have a mock up of the table in my projects section.

View conifur's profile


955 posts in 1356 days

#8 posted 01-11-2016 02:33 AM

Cleats…..that’s what I meant to say when I poster ‘stretchers’.
Well as I told the X, I flunked mind reading 101 in College. How would the heck I know what is in your mind, and vocabulary that is miss directed??

-- Knowledge and experience equals Wisdom, Michael Frankowski

View a1Jim's profile


117342 posts in 3781 days

#9 posted 01-11-2016 02:37 AM

Welcome to Ljs
Along the lines of tturner’s comment,I’m thinking it’s a moisture problem ,one side has more exposure to heat or air blowing on can plan it until the cows come and you won’t get it flat. introduce some moisture to the concave part with say a damp sponge and see if that helps flatten it out ,it may take more than one application to get it closer.Be careful about using cleats you have to allow for wood movement.
The possibility also brought up by tturner is that wood was not dry to build with,you want it to be around 8% before you build with it.
another issue could be the way the wood was milled ,some wood will move all over because of the type of wood it os or it was milled from a tree that had internal stress.
here are a couple links that might help in the future.

-- wood crafting & woodworking classes

View bandit571's profile


21790 posts in 2887 days

#10 posted 01-11-2016 06:11 AM

Oval top….use a spline. Cut a dovetail slot across the width, one near each end. Make up a dovetailed shaped spline to slide across, in the slot. Usually were made of a hardwood. One screw in the center of the spline to hold it in place. No glue.

Been used under the lids of many an old Hope Chest…

-- A Planer? I'M the planer, this is what I use

View DrTebi's profile


267 posts in 3471 days

#11 posted 01-11-2016 08:56 AM

Did you put finish on the top after planing it? If so, remember that you should put finish, or at least a sealer like shellac, on both sides of the table top. I have had even MDF cup severely after putting finish only on one side…

View AlaskaGuy's profile


4814 posts in 2513 days

#12 posted 01-11-2016 09:30 AM

Give us more history about the wood. If it cupped while acclimating in you shop that means there was a moisture change in the wood.

What were/are the dimension of the wood?

I assume it was some kind of Mahogany, which species?

Where did this wood come from and why was it finished on one side? I assume finished means like a film finish and not finished planed on one side.

What was the moisture content when you got it and what is it know?

After jointing the finished side did you keep the wood in a manor that allowed air circulation around all side. Hopefully you didn’t lay it flat on a table with no air to one side. How long between milling and glue up?

Wood moves with change in moisture content. No so much with the temperature. Temperature can have an effect on the moisture in the air but hot or cold by its self won’t make it warp?

What part of the country are you in?

When you glued it up did the mating pieces fit together good so you didn’t have to force the joint together with clamp pressure?

Are you sure it was flat when you took it out of the clamps? It it was, when (how long) before the top cupped?

I can’t help but think this is a moisture problem, the wood appears to still be losing or gain moister (the later being most likely) and doing so unevenly.

I reserve the right to ask more questions later.


-- Alaskan's for Global warming!

View rwe2156's profile


3174 posts in 1685 days

#13 posted 01-11-2016 01:04 PM

As you’ve already learned, you used the wrong tools to flatten the top. My advice is to have someone hide that electric plane out in the wood shed somewhere and swear them to secrecy ;-D

How close to finish dimension is the top? This will determine what you can/can’t do.

You’ve got two problems:
1. Cupping: a moisture issue. When you planed the board you opened up new grain and differential in moisture content on each side causes the cupping. I would try a1Jim’s rec first, but I’ve had poor luck with that.

Mechanical solution to cupping many times doomed to failure down the road. One remedy is to rip the top into narrower strips and re-glue. Another is to simply start over.

In the future, never leave the wood in an unconditioned environment like a garage, especially certain times of the year. Bring wood inside the house to acclimate, or put in plastic bags. On a wide top I will usually clamp or strap it between two flat boards with stickers and leave for up to a week. Sometimes a particular piece of wood is just rascally and has internal tensions do to grain you can’t do anything about it.

2. Grain tear out can be a very difficult fix. A scraper is your best bet here. Perhaps turn board over?

Welcome and good luck!

-- Everything is a prototype thats why its one of a kind!!

View Mortalis's profile


14 posts in 1072 days

#14 posted 01-13-2016 12:50 AM

I really appreciate all the comments.

I want to first say to Conifur, my comment after your explanation of trying Cleats was in no way meant as a slight or to be snide. I appreciate your correction as I sometimes get mixed up in my terminology.

Now, back to the topic at hand.

When the lumber was delivered I unwrapped the boards and put them to one side in my garage/workshop using sticks I cut from some scrap plywood that spanned the width of each board placed every 12” or so along their length. The lumber was stored horizontally one on top of each other. The wood was purchased a dried finished all sides cut to 48” lengths of Santos Mahogany. Sorry to have misled…the lumber arrived as finished all sides. I paid for finished 2 sides if I recall correctly. Purchased from AdvantageLumber
I ran the lengths on my Jointer to flatten and then my Planer to parallel.
I biscuit joined 4 boards with the end grain alternating.
I used the power planer (which actually did a good job) whereas the hand plane tore the crap out of the grain, then belt sander to flatten both the top and bottom and then left for a week to come back the next weekend.
I had the top sitting on my workbench when I first noticed the cupping. I did some online research and was informed that the cupping is due to moisture. More on one side than the other. When I went to check, lo and behold, the sided that was faced down was very moist.
I placed the wood strips I used during storage under the top with the moist side up. The cupping seemed to minimize after a week of sitting and then the next weekend I saw that there was slight cupping and started in the flattening process again. After sitting again for a week till the next weekend the cupping was back.

I dont own a moisture meter so I cant tell you what the moisture level is in the boards but this winter here in Mew England has been rather wet. I have not applied any finish or sealer.

Long story but that’s the history.

View JBrow's profile


1366 posts in 1124 days

#15 posted 01-13-2016 03:25 AM


I will share my thoughts, although I am not sure how much help I will be. I believe as you and others stated, the problem appears to be a moisture problem. I eliminated internal stresses being the cause of the cupping because I assume the edges were fairly straight during the glue-up and the cup is fairly uniform. Additionally you mentioned one side of the glue-up was noticeably damp. You also stated you heat the shop with a propane heater.

First I wonder whether you are inadvertently introducing a lot of moisture into the shop. A propane heater (or kerosene heater) not vented to the outdoors generates more or less a gallon of water for every gallon of propane burned. This could perhaps be the source of your moisture. If your propane heater is not vented outdoors, then opening the garage door for a few minutes when you are done for the day (and a few minutes throughout a long day of shop time) may help reduce the moisture level in the shop.

If moisture is the culprit, the glue-up cupped because the wood fibers on one face were relatively moist while the wood fibers on the opposite face were relatively dry. The greater the difference in moisture content of one face compared to the other face, the greater the cup. The cup appears because the moist wood fibers swell and thus increase the width across the face, while the relatively dry side width remains mostly compressed across the width. The result, if my reasoning is correct, is that the crown of the cup is on the moist side of the wood.

If this analysis of cupping is correct, then it suggests what I would try to return the glue-up relatively flat. Unfortunately, it will take mostly time. This effort is intended to squeeze the moisture ladened fibers together to force moisture out of the fibers. I would position sets of straight cauls across the width at each end and at least one, maybe two, in the center evenly spaced along the length of the glue-up and on each side of the glue-up. In other words, cauls would be on each face of the glue-up with the glue-up between the cauls. Then I would draw each set of cauls together with clamps (squeezing the glue-up toward flat), being careful to avoid overtightening the clamps. I would worry that overtightening the cauls would crack the glue-up. With the cauls clamped in place, stand the panel up on edge (not on end) and place the panel where it can receive good air circulation around the entire panel. Placing a piece of plastic on the floor will reduce humidity from the concrete floor. About daily, check the glue-up, and tighten the clamps if they become loose. The loosened clamps indicate the panel is returning to flat. Continue the process, tightening clamps as needed until the panel is as flat as it will go, which suggests moisture content has stabilized. Introducing a fan and/or de-humidifier in the shop environment to circulate and dry the air would speed the process, but I would not blow air directly on the panel, just mix up the air in the shop. Since winter has finally come to Massachusetts, you may find the process to go fairly quickly unless somehow moisture is being introduced to the shop.

If this process gives you a flat enough glue-up, applying a coat of finish along the end grain edges may help reduce further moisture uptake, but also can impede moisture trying the escape the panel. Immediately before applying final project varnish, mill the edges to remove the applied “stabilizing varnish”, sand the edges and get the first coat of finish on the table top as quickly as possible. Having said this, I would resist doing this step, preferring to avoid the problem of later dealing with removing the “stabilizing varnish”.

If this prescription fails to work, rwe2156 suggestion of ripping the glue-up and starting over with the milling process after the mahogany has acclimated would be my next step. If you go this way, now may be the time to add a moisture meter to the workshop. You could then check moisture content of the faces to ensure the lumber is balanced before milling. A moisture meter is probably cheaper than the final fix. I believe the final answer is probably as rwe2156 suggested, start over. I am sure the mahogany was not cheap, so this is the answer of last resort.

Regarding any form of mechanical fasteners or joinery to get and maintain a flat panel when the root cause was not addressed, is, again as rwe2156 opined, likely doomed for failure. The forces of wood movement are simply too great to restrain.

Good luck, and let us know how you solved the problem. After all, I may encounter the same problem on my next project.

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