Edge Joining Woes - Cupping At Clamp Up

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Forum topic by LyttleBryan posted 11-18-2013 05:23 PM 1939 views 2 times favorited 17 replies Add to Favorites Watch
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5 posts in 1614 days

11-18-2013 05:23 PM

Topic tags/keywords: question clamp

Everyone has something they just hate doing. For me, it’s edge gluing boards for panels. I know why I hate it. It’s because I’m terrible at it.

I’m working on Christmas presents. Making cheese trays for everyone since “wine and cheese” nights are a big thing right now. It’s basically a cutting board, only face grain instead of end grain. I’ve got a variety of species of wood that I’m splicing into maple for nice contrasts.

My problem is that whenever I go to glue up the boards, they cup on at least one side, if not both.

I’m using an assortment of clamps right now that include:
3 – Bessy H Series Pipe Clamps (like pony clamps only they have feet)
2 – 36” Irwin Bar Clamps
2 – 12” Irwin Qwik Clamps
2 – 12” Bessy F Clamps

The boards have been run through the jointer and table saw. Comparing them against my machines square (one of those highly accurate ones) they appear square, but that’s to the naked eye. Sitting on the bench, as long as the long boards aren’t warped, they appear to make a nice seal.

I put them in the pipe clamps with clamps spaced every ~8” and then put cauls on each end that I clamp down with the F clamps. No matter what, after I apply even a little bit of pressure with my left hand (my far weaker hand) at least one of the boards in contact with the clamps will start to rise up and create a cup.

I created this highly technical diagram to explain the problem.

Things I’ve tried include:
- Using less pressure. Even with really good seams there is a lack of glue spilling out which tells me there isn’t enough force to make the joint hold. On less good joints that need some convincing it’s a no-go.
- Tightening the cauls with as much force as I can. Seems to work better with the Bessy than the Irwin clamps. Just can’t get as much force on those. But still, the bessy side still rises.

I’m worried that it’s the pipe clamps. Look at the clamp it appears that there isn’t clamping surface touching all points on the boards, just the top. This acts to “tip the board over” if you will. This would be very disappointing as I can’t afford a thousand dollars in new wood clamps right now, which is what it seems it would cost.

17 replies so far

View Kaleb the Swede's profile

Kaleb the Swede

1821 posts in 1933 days

#1 posted 11-18-2013 05:31 PM

Are you alternating your clamps top and bottom? As in put one clamp on and then turn it over and put another on, continuing to the end. I didn’t do that and learned the hard way. I don’t think it’s the clamps, I think you just need to alternate them. Hopefully this helps

-- Just trying to build something beautiful

View BigAL4160's profile


8 posts in 1627 days

#2 posted 11-18-2013 05:38 PM

Bryan, I feel your pain…edge clamping with pipe clamps isn’t the most fun thing in the world. I have also noticed the phenomenon you display in your drawing of the pipe clamp not providing parallel pressure to the edge of the board. I have been getting success lately by alternating my clamps (one on top, one on bottom, one on top, etc.) and using very light pressure until all clamps are on, then alternately tightening each one slowly. Also, when installing each clamp, really press the boards onto the pipe, keeping them flat; I’ve been able to avoid using cauls most of the time just by doing this.

Its also possible that you could be using more glue than necessary causing the boards to slide around excessively. Just throwing that out there.

If all else fails you could pick up a couple new clamps. I got a 4 ft. black pipe at Home Depot for about $12 and the pipe clamps for about $14.

hope this helps,

View pintodeluxe's profile


5620 posts in 2777 days

#3 posted 11-18-2013 05:50 PM

The above suggestions are good, but here is the real key…

You need to compensate for any inaccuracy in the jointer setup/technique. Do this with the “I” and “O” method.
Arrange your workpieces as they will be in the finished project. Label adjacent boards with an “I” and an “O.”
Continue this pattern for all boards… I,O,I,O,I,O etc.
When you joint your boards, the “I” boards face inward toward the jointer fence, and the “O” boards face outward away from the fence. Middle boards will have an “I” and an “O”.
This compensates for any errors in your fence setup, and will result in a perfectly flat panels.
Use your H style clamps, they are great for panel glueups.
Here is a detailed walk-through of the process…

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View djwong's profile


176 posts in 3183 days

#4 posted 11-18-2013 06:03 PM

Did you try clamping your cauls down tight before applying pressure with the pipe clamps? You should be using 2 cauls on each end (top and bottom), and 2 clamps on each end. If you have enough clamps, you could put flattening cauls in the middle as well.

One other technique you can try is to use small blocks of wood as clamping blocks on the pipe clamp pads. The clamping blocks should be only as tall as your board stock. If you find that the board is bowing upwards as in you illustration, then you loosen the pipe clamp, and slightly raise the clamping block on the side closest to the bowing. This will have the effect of redirecting the clamping pressure to help straighten the boards. In use, I find this technique a real pain because it is difficult handling all the clamping blocks and getting them into position.

-- David W. Cupertino, CA

View LyttleBryan's profile


5 posts in 1614 days

#5 posted 11-18-2013 06:04 PM

Thank you for the info.

I am alternating the clamps yes. I may give the above post on alternating joiner pieces a try. Good reading.

View Picklehead's profile


1041 posts in 1893 days

#6 posted 11-18-2013 06:29 PM

I recall a tip I read, although I confess that I haven’t used it, that suggested using a dowel between the clamp pad and the edge of the boards being clamped, the thought being that it would prevent the angle of the clamp pad telegraphing into and tilting the outermost boards in the glue-up.

-- You've got to be smarter than the tree.

View joeyinsouthaustin's profile


1294 posts in 2036 days

#7 posted 11-18-2013 06:38 PM

Much good advice and links already. Here is my advice.

Here is what you glue up can look like. Assuming properly joined and planed boards. Alternating the grain.. Look at the end grain and you should see smiles and frowns. Cauls over the top, alternating clamps top and bottom, not a ton of pressure, just the right amount.

-- Who is John Galt?

View LyttleBryan's profile


5 posts in 1614 days

#8 posted 11-18-2013 08:29 PM

Joey, I could only hope to have that setup…and space…

View joeyinsouthaustin's profile


1294 posts in 2036 days

#9 posted 11-18-2013 09:17 PM

One day right?!? It does work the same way on smaller scale :)

-- Who is John Galt?

View LyttleBryan's profile


5 posts in 1614 days

#10 posted 11-18-2013 09:35 PM

One day!

For reference, below is an example of one I did about 20 minutes ago. It’s pretty text book stuff.
  • Wax paper on all the clamps
  • Packing tape on all the cauls.
  • Alternating over-under clamps.
  • Cauls on each end.

So where’s the problem. Hopefully one day I’ll find out.

Whoops, forgot the image.

View joeyinsouthaustin's profile


1294 posts in 2036 days

#11 posted 11-18-2013 11:25 PM

IMO that quick grip does not have enough pressure to counteract the pressure of the two pony’s below. Also.. your caul clamps should be moved out to the ends of the caul just at the outside edge of the outer boards. I can’t see the endgrain clearly enough to eval if they are opposed. Also How are the cauls made. Do they have the slight curve in them? Other than that. Mostly get even pressure from more similar clamps. Two over, two under.

-- Who is John Galt?

View Clint Searl's profile

Clint Searl

1533 posts in 2325 days

#12 posted 11-19-2013 12:21 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 teejk's profile


1215 posts in 2648 days

#13 posted 11-19-2013 12:44 AM

I agree with Clint. Use a straight edge across the glue-up while it is still wet and tighten using your straight edge on your up side. Push down on the piece as you tighten using the straight edge as your bible. I love the pipe clamps but pipe is not engineered to be perfectly straight (it’s sold for gas piping and absolute precision is not required).

Now the fact that you are not getting ANY glue squeeze out…I always hope to see little beads! I don’t do cutting boards but seems to me that no glue means you have starved the joint and will have little Manhattan projects growing (I use TB II and view it much like a joint sealer…and yes it is FDA approved).

View LyttleBryan's profile


5 posts in 1614 days

#14 posted 11-19-2013 12:53 PM

Teejk, there is quite a bit of glue squeeze out in this photo, it’s just hard to see because it blends so well with the maple. Starving the joint is not an issue.

Joey, I took your advice on the next one and moved things around like you suggested. I also made sure that there are the same number of over and under clamps.

View OldRick's profile


72 posts in 1657 days

#15 posted 11-19-2013 01:29 PM

+1 about over torqueing the clamps. I had the same problem with edge joining. Adding cauls and not torqueing the edge clamps made it a lot better. A fellow woodworker told me that you’re just wanting to get the surfaces to bond together…not try to fuse them into a singularity. Leave extra thickness so you can send it through a planer when dry if possible.

showing 1 through 15 of 17 replies

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