Pulling my fat out of the fire, or HELP!

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Forum topic by Greg In Maryland posted 03-22-2014 07:17 PM 1322 views 0 times favorited 9 replies Add to Favorites Watch
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Greg In Maryland

553 posts in 2963 days

03-22-2014 07:17 PM

Hey all,

I do not routinely glue up large panels, but a project I am working on is calling for several, so here I am. I’ve glued up my two panels using clamps and culls, and low and behold, they are cupped by a hair over 1/8th of an inch. Whether this is acceptable tolerances or not, is irrelevant. It bothers me that they are not “flat” and I want a flat panel!

Here are my two options, as I see it:

1) cut the joints apart and try again

2) take it to my hardwood dealer and have him drum sand it flat.

Number 1 obviously does not guarantee a flat panel in the next glue up, so I need to figure out what I am doing wrong. I suspect that I am putting too much clamp pressure and bowing the panels, or my stock is not truly square. I use culls, alternate clamps (above/below), but still I get a wonky panel. I have some more panels setting up and I used less clamp pressure, so hopefully the next few panels will turn out.

Fortunately, I have enough “spare” wood in terms of thickness and length/width to take either approach. Since I do not have a drum sander and am not overly familiar with its limitations, I don’t know if a drum sander is a workable solution. The hardwood dealer I go to, Exotic Lumber in Frederick, Maryland are nice folks and will help me. They have an industrial sized drum sander if that helps.

Will a drum sander flatten a panel with a 1/8 cup or is this asking too much of the machine? Does a drum sander press down on the material like a planner and would just follow along with the cup and not remove it?

I appreciate any insight anyone may be able to offer.


9 replies so far

View Bluepine38's profile


3379 posts in 3051 days

#1 posted 03-22-2014 07:38 PM

If you are using cauls and alternating clamps, are you looking at the panel while you are clamping it and after
you clamp it. The cauls used properly should be holding the panel flat. If it is glued, clamped and dries flat,
it should not have a cup in it. Are you using the cauls correctly?

-- As ever, Gus-the 79 yr young apprentice carpenter

View DrDirt's profile


4423 posts in 3708 days

#2 posted 03-22-2014 08:11 PM

You don’t say how wide the panel is for the 1/8 cup.

Drum sanding can get it flat, some sander designs are similar to the planers – where there is a feed roller that presses down, so it can be that the roller will hold your panel flat…. get sanded…. then it still has the same 1/8 cup at the other side, if that is the design, you can do a lot with taking light passes.

Your glue technique was probably fine. Even on a solid board, you can joint a face flat, then plane the other side to be parallel and laying flat…. but come back the next morning and have the single board be cupped, just from the moisture.

-- “The two most important days in your life are the day you are born and the day you find out why.” Mark Twain

View Shawn Masterson's profile

Shawn Masterson

1325 posts in 1914 days

#3 posted 03-22-2014 09:08 PM

it all starts at the jointer. If they aren’t square then that is probably where the problem is.

View jmos's profile


823 posts in 2335 days

#4 posted 03-23-2014 01:44 AM

I’ve found making flat panels much harder than I ever thought it would be. Not quite as easy as you would suspect.

If it is a jointing issue, one way to offset any possible issues with your jointer fence not being exactly 90deg to you table is to first lay out your boards. Then mark each joint with matching I and O’s, one on each side of each butt joint. Then run each edge through the jointer with the I facing in (toward the fence), or the O facing out (away from the fence.) You’ll end up with edges that, if they are off from 90deg, will exactly offset each other and cancel out.

Another thing to watch, if you’re boards are flat sawn, try to alternate growth rings; one up one down. If they are all the same direction the movement can compound.

-- John

View Clint Searl's profile

Clint Searl

1533 posts in 2327 days

#5 posted 03-23-2014 02:35 AM

You’re overclamping; back off.

-- Clint Searl....Ya can no more do what ya don't know how than ya can git back from where ya ain't been

View TheWoodenOyster's profile


1313 posts in 1900 days

#6 posted 03-23-2014 12:42 PM

Just yesterday I finished a dining room table glue up. It was “92’ x 42”. I didn’t really know what I was in for when I started, but I sure have learned a bit. Mine has about about a 1/8” cup in it across the width. I clamped and cauled as well. Today, I am in a position very similar to yours, so here are my thoughts about what I could have been doing to put that cup in there.

1. Alternating growth rings – I didn’t do this on my tabletop because I had to pick the better side of the board for the “up” side of the table. Honestly, I think this has the least to do with the cup, at least the immediate cup. Maybe over time this could cause an issue, but I don’t think this is the likely culprit.

2. Your cauls aren’t straight – This is kind of a goofy hypothesis, but if the cauls aren’t striaight, the top won’t be either. Mine are homemade, and though I feel they are pretty good, they aren’t precision machined. But I am not going to pay $199 for a few pieces of bowed maple. Sorry BowClamp.

3. Overclamping – Don’t know much about this, but Clint may be onto something there. I always clamp the hell out of stuff and it is completely reasonable to think that as you really cinch down on the clamp and it bows a bit, it’ll start to pull the boards up instead of just in.

4. Edges aren’t 90 degrees – This is a tough one. It is very hard to get perfect 90 degree corners, and it gets exponentially more difficult when you get to 8 foot long boards. I did my best with mine and I assume you did as well. The fact is that as your table gets bigger, the inaccuracies of your jointer compound, so that could be a problem. Very possibly the culprit, and definitely a factor in my case. JMos’s answer to this is really cool and I might give it a whirl on the next panel glue up I do.

5. Alternating clamps – I am bad about this. I almost always clamp from just the top side, as it is a hassle to get under the massive panel to clamp from the bottom, especially while your “long open time” glue is turning to concrete before your eyes. So, that matched with overclamping could be the killer. I think this is probably what did me in. I’d like to hear what some other guys have to say about this problem in specific. Don’t know if it is more of a myth or if there is truth to it.

All that said, I can live with a 1/8” cup in a 42” wide table. I honestly don’t expect a whole lot better out of my limited experience and tools. Hell, you could do a perfect glue up and it gets a little humid and you are totally out of whack again. Wood is organic and I would guess that even James Krenov and Sam Maloof and George Nakashima had plenty of crooked panel glue ups. I would let it be and just do your best to use good practice in the first place. I think that is really all you can do. I had a friend go thru the same problem with poplar chest sides. We could never figure out what was happening, but we think the weather was just changing too much.

Also, I don’t think a drum sander would work. I think it would do exactly what you are concerned about. You’d end up with a 1/4” thick panel that was just as crooked as when you started. Either way, good luck and I would try to accept a little bit of bow or cup if you can. Or you could just finish one side and let it straighten itself out. :)

-- The Wood Is Your Oyster

View Greg In Maryland's profile

Greg In Maryland

553 posts in 2963 days

#7 posted 03-23-2014 02:42 PM

Hey everyone, thanks for the great help. I really appreciate it.

I think that my clamping technique and tools are mostly appropriate for the job. The culls I am using are dead square (with a bit of a taper though), I alternated clamps, etc. My approach is to position and clamp the culls first, and then add clamp pressure to bring the individual pieces together.

What was happening is that the panel would spring out of flat once I got the culls off them. It’s not like the panel was sitting around and warping with humidity. So this leads me to the conclusion that my problems are two fold:

1) Too much clamp pressure

2) Edges are not perpendicular to faces (i.e., not 90 degrees to each other). Exactly as Shawn suggested, thanks.

I pulled of few of my extra pieces and checked them and low and behold, a few were out of square. My guess is that with the large number of pieces I was milling, I got mixed up and didn’t square the final edge on the table saw and then attempted to glue them up.

Because of the numerous issues with attempting to correct the problem via a drum sander (which by the way, I am not the operator and do not control), I decided to just rip apart all the individual sections and start over. It was actually quite easy to rip the panel in the center of each glue joint. Word of warning to anyone who tries this: wear your eye protection. The globules of dried glue become little projectiles.

My next steps are to clean up the glue squeeze out and make sure the edges and faces are perpendicular, and then give it another go. I’ll use less clamp pressure as well. Fortunately I have enough stock not to worry about the 1/8th inch loss from the various cuts I am making.

By the way, my panels are approximately 40×30 at the moment. They are a bit oversized and will be cut down. I am making a dovetailed chest, so I don’t want to count on the joint to pull the panel straight like I might with a large table top.

Thanks again for all the great words of wisdom.


View AlaskaGuy's profile


4045 posts in 2275 days

#8 posted 03-23-2014 05:02 PM

+1 for jmos and clint’s post. those two techniques worked together should take care of your problem.

-- Alaskan's for Global warming!

View TheWoodenOyster's profile


1313 posts in 1900 days

#9 posted 03-23-2014 05:53 PM

I figured you might be using these for a chest. The flatness would definitely be more vital in that situation than with a tabletop. I can understand your worries now. I would have worried too. Glad you figured it out.

-- The Wood Is Your Oyster

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