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How tight do I need to set my clamps?

2K views 14 replies 9 participants last post by  wuddoc 
#1 · (Edited by Moderator)
I've been doing a bit more woodwork and made a few cutting boards/shelves. I've put the clamps on fairly tight and notice once the project is dry they appear bowed sometimes. Is this non-square cuts on the strips or too much clamp pressure? I tend to test fit/clamp to make sure everything is good but keep having this issue.

Basically: How tight do the clamps need to be?
 
#4 ·
I think Jim has your answer but there is another one to the question.
Different glues require different pressures.
Urea formaldehydes require quite high pressures while epoxy joints can be "glue starved" by too much pressure.
PVAs don't need much pressure if the fit is good and hide glues even less.
 
#5 ·
You also have to take into account that the smaller the project the easier it is to over clamp and get the bow you mentioned. The photo Jim shows would be almost impossible to bow the boards but you start looking at 3/4 or 4/4 boards and it's a lot easier to bow the board.

Don't be a gorilla tightening the clamps - Like Paul said, the clamping pressure can be different depending on the glue you are using.

Just because you can turn the clamp some more, doesn't mean you should.

In general terms, for me anyway, I try to tighten the clamps as evenly as possible, a little on one, then a little on the next and so on. Once all the clamps are even and feel tight I'll spin the screw another 1/2 turn or so. If I can bump the clamp and it moves - it's not tight enough - then adjust as necessary.

Not sure if I explained myself very well.
 
#7 ·
Yes to Cauls
Yes to Clamps on both sides
I tighten my clamps fairly snug, then after 20 minutes or so I give them an extra turn.
That is, when I think about it. I doubt it really needs the extra turn but if I don't I notice the clamps aren't as tight when I take them off as when I put them on.
I'm assuming were talking about using the most common glue, being tightbond.
 
#8 ·
As others have stated and I'll 2nd 3rd etc. In my early days I use to slather as much glue as possible and have a test of my manhood to see how tight I could make it. Its just not necessary.

Now I use a reasonable amount of glue (if more than slight squeeze out its just a waste of glue.

I also clamp 1 up 1 down and always use an odd number with one in the center and the other even.

I use calls if waranted, but if I'm to trim the panel down and its long grain to long grain, I don't worry about it.

Hand tigh and a 1/2 turn more and thats it. Never had one fail on me.
 
#9 ·
I think Jim has your answer but there is another one to the question.
Different glues require different pressures.
Urea formaldehydes require quite high pressures while epoxy joints can be "glue starved" by too much pressure.
PVAs don t need much pressure if the fit is good and hide glues even less.

- shipwright
I have yet to see a "glue starved" joint unless on purpose….
 
#10 ·
I ve been doing a bit more woodwork and made a few cutting boards/shelves. I ve put the clamps on fairly tight and notice once the project is dry they appear bowed sometimes. Is this non-square cuts on the strips or too much clamp pressure? I tend to test fit/clamp to make sure everything is good but keep having this issue.

Basically: How tight do the clamps need to be?

- Tom
Tight enough that there is no more glue to press out. This comes with experience.
 
#14 ·
I built wooden boats for many years and used a great deal of epoxy. There are certainly places where you can drive the glue out of an epoxy joint and cause failure. Trust me on that one please.

- shipwright
I can't agree or disagree with building a boat and its + and - on the use of epoxy. I can only respond to its use in the question at hand.

Building boats your king of the sand box as I don't, but in furniture that's another sand box.
 
#15 ·
Check out Dr. Gene Wengert, he is Professor Emeritus in Wood Processing, Department of Forestry, at the University of Wisconsin (Madison) AKA "The Wood Doctor". I find he is an authoritative voice in the woodworking industry.

He has a white paper on adhesives and clamping pressures. Search on the internet for the PDF sponsored by Franklin Glue. GLUINGWOOD: A STICKY BUSINESS

There are drawings and pictures showing problems in this paper.
 
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