As I start to build larger peices, I have done more edge glueing. I am a litttle confused as to how much pressure is needed to acheive a good glue joint in an edge glueing situation. I have heard many say “be careful not to starve the joint”, or squeeze the glue out. On the other hand I have read that the thinner the glue line the stronger and better the joint.
The question arises because for the past week I have been assembling a simple mantlepiece and needed to edge glue two boards for the top. I used titebond lll, biscuits every 12 inches, and besey k bodys. The piece laid flat,and the joint dried for about 24 hours. Then I trimmed the peice to a finished length, I looked at the scrap and noticed that that the top of the joint had a skim of glue on it and the bottom of the joint must have had more because I broke it apart and the wood failed at the bottom of the joint, not at the top. It seemed to me that the bottom surface of the piece had a lot of glue squeeze out.
I was wondering if as I applied clamps that I might have rolled or cupped the boards squeezing the glue out at the top of the joint before I could place some alternating clamps on the other side of the peice.
The piece sems fine, no real signs of weakness or cupping and the surface was flat upon curing.
Apologize for the long post. Any thoughts or technique tips would be appreciated.
Webby
Replies
webby
You want to apply good pressure, but not so much that you find yourself really torqueing down on the clamp handles. It doesn't take a whole lot to squeeze most of the glue out of the joint, resulting in a starved joint.
When glueing up large panels, it's very important to apply even pressure on both the top and the bottom of the panel. You should be alternating your clamps (one on top, the next on the bottom, and so on, so forth...) taking care to keep the surface of the newly formed panel as flat as possible. Clamping cauls can assist in not only keep the panels flat, but also in keeping the edges aligned to each other, requiring less planing (or sanding) to achieve a flat panel.
Incidentally, don't waste your time with those biscuits. They add no additional strength to the panel, as the long grain, long grain glue joint is plenty strong enough. Concentrate your efforts on keeping your edges jointed at 90°, and use cauls to keep the panel flat during the glue up.
Jeff
In the last issue (or maybe the issue before the last) of FWW, there was an article about just this topic. There was also a related comment from the author in this month's issue. Bottom line --> Unless you are using hydraulic clamps, you will NOT starve the joint. Crank 'em down as hard as you can. Even at that, you will not likely achieve "optimum" pressure for the glue. You will get a joint that's stronger than the wood, and that's good enough.
Your problem sounds like it may be due to improper jointing -- the edges were not touching equally, top and bottom, under clamp pressure. Check for this carefully before you glue. Joint the two parts clamped together face-to-face so, if you don't get a perfect right angle, the angles will be complimentary and fit tightly. You can even clamp it up dry just to be sure. This may also happen if the clamps are applied in such a way that they don't press parallel to the surface of the part. With K body clamps, this is less likely to be the problem, but it can happen. One way to address this is to use clamping cauls to keep the pieces from flexing at the joint.
Hope this helps.
Mike Hennessy
Pittsburgh, PA
I didn't agree with the article in FW because of a lot of experience with starved joints. I learned how to glue from a cabinetmaker with 40 years experience. His advice, which has never failed me, is to apply glue and tighten clamps until you see squeeze-out then back the clamps off a little. Works like a charm.
~~mike
To the extent the article could be read to mean that you NEED these pressures, I'm with ya. To the extent that joints were actually tested for the article after being clamped at the stated pressures, figures don't lie. I have no doubt that, with perfectly milled joints, the glues perform as stated.
For me, if a joint is as strong as, or stronger than, the wood, that's all I need. I don't care if the joint is 2X or 15X as strong as the wood since, hey, it's only furniture, not a space shuttle and, if the wood breaks, it doesn't really matter that the joint held!
I don't think I ever had a failure due to a "starved joint". I think all my joint failures were actually due to poorly worked joints, not poorly glued joints.
YMMV.
Mike HennessyPittsburgh, PA
My question is why apply glue if your going to squeeze it all out by overtightening the clamps. Have you ever took titebond wood moulding glue and just rubbed the pieces together, then just let them dry for 24, then try to break the joint?
You are talkin apples and pears there is a big differance.
It's all about the joint. Use a small square to check your work before you glue up as well, if you have a slight bevel this can be telegraffed into the boards you are gluing up as well creating a cup of sorts. In other words make sure the fence on the jointer is 90 degrees to the table.
9-"My question is why apply glue if your going to squeeze it all out by overtightening the clamps."How on earth could anyone squeeze out ALL the glue by applying pressure perpendicular to the glued surface?
Thanks for the info and advice. I have read the article and like the other poster said, it seems as, according to the article that starving a joint is much more unlikely than likely due to the limits of clamping pressure available..
I didn't think about the jointing. I will be more careful in the future. I really use the biscuits as an alignment tool more than anything.
Thanks.Webby
Sure. Same thing works for hide glue -- a "rubbed joint". But that's my point. It's strong enough, even if it would be a bit stronger if clamped at a higher pressure.
Mike HennessyPittsburgh, PA
My question is why apply glue if your going to squeeze it all out by overtightening the clamps.
Starving the joint by overtightening clamps is a myth told by cabinetmakers with 40 years of experience. Poor spreading of the glue is a more likely cause.
The pressures needed for an 'ideal bond' far exceeds anything you can do with regular wood clamps. I don't necessarily worry about the ideal bond, but I tighten my clamps to their fullest without fear of a glue starved joint.
Buster
Whenever this topic comes up it seems as though the people advocating the barely tight clamps are the ones most passionate about their methodology. But at the same time, the majority of questions about glue joints on the discussion boards are from woodworkers wanting to know how they can hide their visible glue lines, or what is the best colored glue to make the lines less noticeable.Invariably, someone will state that they have been barely clamping their joints for years and never had a failure, but what they don’t tell you is that they also frequently have visible glue lines. I have seen project pictures posted on the Internet by woodworkers that otherwise do great work, but even in the low-resolution photograph, you can see their glue lines. It’s not about the strength of the joint, but about the thickness of the line. A properly jointed and clamped joint should be completely invisible with the only evidence that it exists is a change in woodgrain. So if you have ever been tempted to buy a colored glue, then you are not tightening your clamps enough.
Rick-Excellent point on the visibility of glue lines. A related issue is that almost all of the normal woodworking glues (PVA and the like) are subject to creep. The fatter the glue line, the greater the potential creep.Don
Mike;
when you said "figures don't lie"
I become curious of the absolute statement and question the figures.
Best,
JimI know less than half of you, half as much as I should Like;
I like less than have of you, half as much as you deserve.
Figures may not lie, but (as my old grandpappy used to say), you gotta pay close attention to the liar holding the pencil. - lol
Dave;
re your comment:
Figures may not lie.............But you have to agree; liers figure!
best,
JimI know less than half of you, half as much as I should Like;
I like less than have of you, half as much as you deserve.
That's why grandpa always said to watch whichever lying SOB was holding the pencil. - lol
C'mon you guys. If ya can't hear the wood scream, they ain't tight enuf!
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
The first great rule of edge joining is to get the edges as straight, square, and smooth as possible.
If the edges aren't straight (e.g. concave, convex, or wavey), you'll need lots of clamping pressure to suck them closed - and they may not close up completely.
If the edges are rough (e.g. sawmarks), you're asking the glue to be a filler. That works, but only up to a point.
The biggest headache is unsquare edges. If the edges aren't square with the faces, the joint will want to slide a little under the clamp pressure. If you look at the geometry of a clamped panel, the clamping forces are directed thru the clamp's pipe or bar which is offset from the wood. Since the clamp pads can tilt slightly, some of the clamp pressure introduces a force perpendicular to the face and the wet glue actually lubricates the boards as they try to slip.
That's why alternating clamps top and bottom, cauls, and/or biscuits work so well. Alternating the clamps balances the perpendicular forces, and cauls, splines, and/or biscuits provide mechanical resistance to them.
Biscuits may (or may not) provide additional strength, but there's no doubt in my mind that they help maintain alignment during a glueup - if they're correctly made. If the slots are misaligned, or at an angle, you're hosed. - lol
Since the OP was asking about clamping pressure, I'll add that I try for just enough to get a bit of squeezeout. I apply the glue with a roller to get an even coat, then clamp to the point where I see a roughly even amount of squeezeout along the length of the joint. If I'm really on my game, there's a neat row of pinhead sized "beads" along the entire joint.
Edited 12/24/2007 11:48 am by Dave45
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