Replies

1. 
   Bepperb | Nov 18, 2005 06:08pm | #1

   That's a kinda vague question, and I'm sure somebody else can do better than I, but here's a shot at it.

   If you're glueing two boards side to side (face grain to face grain) it accomplishes a few tasks, in decending order of importance:

   It forces the two boards as close together as possible, making the film of glue as thin as possible while still maintaining contact over both wood surfaces.  While the wood that is permeated with glue can be stronger than the wood around it, the glue itself is weaker.  You can't (or shouldn't) use wood glue to fill gaps, that creates a weak spot.  This has an additional asthetic benefit of minimizing the appearance of glue lines.

   It stops the boards from moving in relationship to one another while the glue sets.  This results in a stronger joint (since the glue is allowed to 'set' without movement) that dries in the same place you glued it (things get frustrating when things move).

   It forces the glue further into the grain of the boards.

   If you're glueing two boards into a T (face grain to end grain) it will do the same things, but this is a much weaker joint.  If you only know one thing about glueing wood this should be it. 

   Force per unit area?  Generally the more the better, until you get to the point you are compromising the wood around the clamps.  Quick grip clamps work ok, 3/4" pipe clamps work better.  Finish usually doesn't matter, unless there will be moisture involved (like outdoor furniture) and you are using a glue which isn't watertight.    Also, some polyurethane glues (gorilla glue) are UV sensitive and need to be protected.  Time under pressue?  At least until the glue is set, the longer the better, usually printed on the container.

   Hope this helps....

    

1. 
   WillGeorge | Nov 18, 2005 06:46pm | #3

   glued joints why compress?Just like a fine lady.. Ya have to hold onto her or she will move away from ya! At least in my datin' experiences!

2. 
   notDusty | Nov 18, 2005 06:11pm | #2

   Hi jacksa ,

                Well that's a big rather open ended question , with many variables. I would start by stating that different types of glues have different characteristics as far as the pressure requirements . For instance Gorilla type glue is used by a finish carpenter pal of mine and if I am not mistaken he said he uses no clamps only hand pressure for a short period of time . For some applications this may be great .

               If you put too much pressure on a joint you can squeeze almost all the glue out and starve the joint and contribute to a failure .It probably depends on what type of joint we are talking about and perhaps the material being used . Do you have a specific task you need help with ?

             As far as finish , do you mean gluing something that already has finish on it ? Personally I don't glue on top of finish . Maybe when you repair a piece that has already been finished after it was assembled.

            Follow the guidelines of the adhesive you are using , imo the temperature can be most critical for proper curing as well as the time.

               hope this helps some               dusty

          

3. 
   HowardAcheson | Nov 18, 2005 07:04pm | #4

   It depends on the adhesive you are using.  For PVA (yellow or white) the Forest Product Labratory and Titebond recommend 100psi for softwoods and 200psi for hardwoods for maximum strength.   To put that in perspective, a 3/4 pipe clamp will exert about 1000 psi.  Edge clamping two boards 3/4" thick and 12" long will equate to 9 sq.in.  Multiple that times the 200 psi required and you will have to exert a force of 1800 psi for every 12".  This means that you will need to fully tighten two 3/4" pipe clamp every foot. 

   Given the above it is basically impossible to "starve the glue joint" with the clamping pressure available to most woodshops. 

   Howie.........
1. 
   PlaneWood | Nov 18, 2005 07:24pm | #5

   Gees!  Someone actually taking the scientific approach!  How unique!

   But, I have a question.  Where did you come up with the figure of 1000 psi for a 3/4" pipe clamp?

   PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood

1. 
   SteveSchoene | Nov 18, 2005 09:45pm | #6

   I think I've seen a similar figure for pipe clamps, possibly in Hoadley's Understanding Wood.  In the Woodcraft catalog the Gross Stabil clamps are listed for 1,100 lbs. of clamping pressure.  Bessey K-body clamps list 1,000 lbs. on their web site.  The Bessey medium duty F-clamps list 600 lbs. and the heavy duty 800lb. 

   If you want really heavy clamping power go with the Jorgenson I-bar clamps, they list maximum clamping force of 6,000 lbs.  Don't know how practical it is to achieve that level.

   Edited 11/18/2005 1:47 pm ET by SteveSchoene

2. 
   HowardAcheson | Nov 19, 2005 01:13am | #8

   As someone else said, you can fine clamp force info in Hoadley's Understanding Wood.  It's also in the Forest Products Lab's "Wood Handbook" I believe.Howie.........

2. 
   notDusty | Nov 18, 2005 11:59pm | #7

   Hi Howie ,

                What I have noticed to be common on failed glue joints was they didn't use enough glue , you can see where it was. I use a lot of clamps , don't know how much pressure per foot but boards seem to stay together good .

                               dusty