Clamping Pressure

Sadly there’s no real way to know exactly how much pressure you’re applying. What I usually do is tighten the clamps until I get a an even amount of squeeze out along the joint. Not very scientific, I know, but I’ve had some good joints that way.

-- "At its best, life is completely unpredictable." - Christopher Walken

Interesting – I did not know you could over-clamp.

-- James - What's your excuse this time??

Hummmm, is that why the treads on my clamps a strpped? lol

-- Bob, Rockford IL, http://www.woodandwax.net

The rule of thumb I’ve always heard is just enough pressure to close the joint. If you are edge-gluing two perfectly jointed boards, in theory, very little pressure is necessary. You are really only holding the boards in place till the glue sets up.

-- Charlie M. "Woodworking - patience = firewood"

I have learned that just tightening the clamps till there is an even squeeze out works as previously stated. When overtightening the clamps, the joint will not be as strong.

-- Julian, Park Forest, IL

I’ve read a couple articles that tested wood joints saying they could not get a weakened joint from glue starvation not matter how hard it was clamped.

-- Jim from Heirloom Woodshop Southern Oregon, custom furniture,woodworking school,heirloomwoodshop.com

I only tighten the clamp until i feel resistance in the clamp screw. Then I check that I have uniform squeeze out.

-- Drink once, cut twice. New website up.... lazylarrywoodworks.com.au

I use Tite Bond glues, and they specifically say clamp for X amount to time and do not clamp overnight. l also look for a little squeeze out and have had no problems. In fact, I once tried to take apart a joint after an hour or so (miss matched board) and could only crack the wood near the joint. The joint went with one piece…

-- jack -- measure once, curse twice!

Different glues at different set conditions will exhibit different results. I solved the problem here with samples and destructive testing .It is part of learning wood working.

Hey jack1—- I’ve used Tite Bond for years and years, but I don’t remember reading anywhere where it says not to overclamp at night—did I miss something somewhere on the label?
I’ve done dozens of furniture pieces, and had to let the glue dry overnight. No harm done to the piece that I could see. Rick D.

-- Remember--- one good turn-- gets most of the blanket!!!!

Thanks to everyone for responding. Great info and very helpful.
I think I have to break the bad habit of using my gorilla grip when clamping.

An addendum question -
As many of you pointed out, you look for uniform squeeze out.
What if you don’t get “uniform” squeeze out ? Do you take the joint apart and re glue ?

The problem is that some of my boards look fine, I can see a fine glue line along the joints, but then when I sand the board and finish with mineral oil (sometimes days to weeks after gluing) the glue line develops gaps like there were bubbles or something. This is why I think the joints are starved. I’ll try to get a picture loaded.

Lisa

-- I inherited the woodworking gene and it's gone into overdrive.......

What is the moisture content of the wood at layup ? Do you condition the wood to ambient prior to construction ?

I always put clamp pressure on untill its snug…how much pressure that is I am not sure, but I dont crank hard on the clamps.

-- Wayne - Plymouth MN

Allway thought you had to go tight as possible. Then found about “Tapeing” miter corners to hold for glueing. Not near as tight as the clamps and still works great.

-- Scrap Wood's the best...the projects are smaller, and so is the mess!

No, don’t pull it apart if it isn’t perfect, only if there is a pronounced gap. But to save any real concern, make sure you have a nice even coat of glue to start with and rub the board edges back and forth lightly. The oil problem could be because you haven’t cleaned it up properly. Always, where possible, clean up any excess glues while it is still wet. Some say do Some say don’t. I do and have had no problems up to now.

-- I cut the damned thing twice and its still too short

It’s just so hard not to tighten the snot out of that clamp. I was guilty of this but have changed my ways. Having well jointed material is the key so you don’t have to force things together. Excellent advice in this post!

-- There's many a slip betwixt a cup and a lip.--Scott

Very helpful to all of you. Being I have been in Aircraft Mx for 25 yrs, over kill has been beaten in to me. Everything I do at home weather remolding our house or a project in the woodshop, I apply over kill. I broke one of the rockler Alum. clamps the other day tighting it down to much. Opps. The Jet clamps that are on sale 1/2 off are amazing!
Ken

-- Ken, "Everyday above ground is a good day!"

If you’re getting a gap when you sand, it sounds like your edges aren’t mating perfectly. Make sure that there’s no bevel or gap before you glue. Your clamps shouldn’t be used to squeeze gaps together, just to hold the wood in place while the glue sets.

-- Steve, Webster Groves, MO

I use a simple board clamp to keep the edges from buckling and then the appropriate hand clamp to keep the glued edges together.
I think lot of trouble starts out with too much pressure in the glue ups.

From my woodfolder

-- A mind, like a home, is furnished by its owner

I remeber reading some where you want to use about 250- 300 lbs for soft wood / pine ect and hard wood
at least 350 and then starting looking for the even amount of glue squeeze out I now use parellel clamps seems to be much easier than bar clamps with gluein up pannels But we all start out somewhere usally with cheaper clamps in the beginning I know i did.

-- christoper Blanchard, Michigan

Todd Clippinger (one of the local experts on this site that seems to know what he’s talking about) says there is no such thing as glue starved joints. Check his video on how he tests his glue joints.

If you are getting glue squeeze out and bad joints I’d say you have some sort of contamination (foreign material) on the edge of the board. Usual suspects could be stuff like wax, varnish, silicone, oil, etc.)

-- Joe

poroskywood said It’s just so hard not to tighten the snot out of that clamp. My question is—What kind of clamp are you using??? That sounds messy!

Seriously, I have always tended to overtighten, and can’t tell that it’s ever been a problem.
Good joints, however are a must in glue-ups. If you have to increase pressure to compensate, you should work on a better fit.

-- Kent Shepherd * The goal is-----More Tools!

clamping pressure all depends on the type of wood. The higher the specific gravity, usually the more force is needed. It also depends on grain direction. For hardwoods, gluing flat sawn grain takes twice as much pressure than gluing quarter sawn grain. (it’s the opposite for softwoods) The basic principle is to get a gluleine of .003”-.005”, that is what makes the strongest joint!

Oh, and you can never starve a joint with hand clamps, only hydraulic ones.

-- Childress Woodworks

If memory serves me today, most modern glues exceed the fracture strength of most commonly used woods.
What remains now is to make sure that the seam between the two mating surfaces is not obvious when the piece is finished.
Good jointing is probably the first consideration.

Bob

-- A mind, like a home, is furnished by its owner

My theory is that the limit of clamping pressure should be determined by the strength of the wood. If the area of clamp contact will be a part of the finished piece, then the clamping pressure should not be exceeding the yield strength of the wood. Distortions created by clamping that will be later cut off are fine.

PVA glues bond by electrostatic charge and by mechanical conformation. Mechanical bonds are created when the glue hardens and conforms to the woods pores, open cells and the deviations from a perfectly flat surface created by the machining process. Electrostatic bonds are created by polar affinities. Like sticking a balloon to the wall after rubbing it on your dog’s back. These glues contain water, and so raise the gain while gluing. Its almost impossible to have too much pressure without damaging the wood. Thick glue lines (005”) are relying on the mechanical strengths of the glue, while thinner and nearly invisible glue lines are relying more on the electrostatic attraction.

PVA mechanical strengths are high, but the glue is prone to creep, is softened by many solvents in finishes, and thermoplastic, so I favor thinner glue lines, particularly under gloss finishes that expose sinking or moving glue lines.

Now stadler has given us the scientific approach to the question. Lord knows we could all use a little more polar affinity in our lives. :-)

-- Charlie M. "Woodworking - patience = firewood"

That’s what I meant to say. You’ve got to keep those mating surfaces tight.

FWW has a really good article about this in the Nov/Dec 2007 issue. The article is called “Get Serious About Clamping”. It goes into the different pressures for different woods (generally the denser and tighter grained the wood is, the higher the clamping pressure is needed). It also discusses grain orientation and related clamping pressures. And it discusses the amount of pressure different clamps can apply. In the end the author says that it is hard to over tighten to the point of a glue starved joint without also starting to crush the wood.

-- Lancaster, PA

Read the FWW article mentioned. It gets a bit crazy about the amount of pressure that is considered “good” (you tend to run out of room) but its accurate about what glue can take.

Glue gets stronger the thinner the layer. Ideally you would probably want a layer 2 molecules thick (just to cover any holes).

Hey Bob #2,
I like the board clamp set up you made. Consider your design stolen, I’m making one.

-- I inherited the woodworking gene and it's gone into overdrive.......

I read a great FWW article that said to use your weak hand to tighten the clamps as tight as you can..so, if your right handed use your left hand to finish the clamps as tight as that hand can turn them. There is a large debate on weather you can actually “starve” a joint of glue…I don’t think it is really possible because you really only need a few millimeters of glue for a proper bond.

-- David, South Windsor, CT "I love the smell of sawdust in the morning"

I try to use enough pressure that the excess glue oozes out of the seam, whatever pressue that takes, but I’ve always focused on making sure there is no gap between the glued pieces. I preclamp dry to make sure the seam is just a pencil thin line between the pieces.

-- A tree is our most intimate contact with nature. - George Nakashima