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Glue up pressure and number of clamps?????

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Forum topic by oldwood posted 01-18-2018 02:40 AM 786 views 0 times favorited 36 replies Add to Favorites Watch
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oldwood

126 posts in 1153 days


01-18-2018 02:40 AM

So how many clamps are enough and how much pressure do you apply. As I browse woodworking sites I’v seen everything from one clamp every 3” to a spring joint with one clamp.

For uniformity, lets glue up a cutting board 18” long, very smooth, clean joint. I would likely go with 4 clamps, 2 below and 2 above.
how about you, more or less?

Do you keep cranking until you knuckles crack or is a good even squeeze out enough?


36 replies so far

View TungOil's profile

TungOil

844 posts in 404 days


#1 posted 01-18-2018 02:49 AM

I’d probably have 3 pipe clamps alternated up/down on that, just enough pressure to close the joint.

-- The optimist says "the glass is half full". The pessimist says "the glass is half empty". The engineer says "the glass is twice as big as it needs to be"

View Gilley23's profile

Gilley23

489 posts in 291 days


#2 posted 01-18-2018 02:55 AM

I’d probably do the same, 4 alternating top/bottom clamps. You don’t need to kill it but yeah you want plenty of pressure. Remember, the glue is stronger than the wood.

View JohnMcClure's profile

JohnMcClure

147 posts in 549 days


#3 posted 01-18-2018 02:55 AM

I crank down pretty tight, with F-style clamps I go to about 80% of the tightest I could possibly do with my bare hands, I’d say. With clamps with a pin or crank handle, which gives you a lever arm, I guess I go until the bar starts to flex.
But this seems like a great experiment since TungOil just closes the joint – I’d like to do 3 glue-ups on identically jointed boards and do a fracture test on all 3. Varying clamp pressure or number of clamps for each. Maybe someone else would like to try? To cold for gluing in my shop this week.

-- I'd rather be a hammer than a nail

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AlaskaGuy

3915 posts in 2218 days


#4 posted 01-18-2018 02:56 AM

There will be a lot of different answer on this one. I’m in this camp. Don’t over do it.

https://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure

-- Alaskan's for Global warming!

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Rich

2295 posts in 498 days


#5 posted 01-18-2018 04:03 AM

The width of the boards matters too, not just the length. Since the pressure radiates outward from the clamp (I’ve seen diagrams that show 45º but that’s probably just an approximation), the wider the board, the fewer clamps you need for a given length. For example, picture gluing two 1/4 inch strips. You’ll need a clamp every inch or two, or, better yet, cauls to get even pressure along the joint.

And, I agree with AG, too much pressure will starve the joint. I learned to use just enough to bring the boards together firmly.

-- No matter how much you push the envelope, it'll still be stationery.

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clin

798 posts in 905 days


#6 posted 01-18-2018 04:54 AM

As an example, Titebond II clamping pressure, according to Titebond, is 100-150 PSI for softwoods, 125-175 PSI for medium woods, and 175-250 PSI for hardwoods.

While the clamping pressures, in the article listed by AlaskaGuy, may seem really large, the PSI generated can be much less if the area being glued is very large. So if you’re edge gluing boards that are 1” thick and 10” long, you have just 10 sq-in of area and still need a total of something like 1,500 lbs of pressure. Which you can get pretty easy with a handful of lighter duty clamps.

But if you were gluing up something, much larger, like laminating two boards together, something that may have 100 or more square inches, you probably need every clamp you can lay your hands on to begin to get close to what is recommended.


The width of the boards matters too, not just the length. Since the pressure radiates outward from the clamp (I ve seen diagrams that show 45º but that s probably just an approximation), the wider the board, the fewer clamps you need for a given length. For example, picture gluing two 1/4 inch strips. You ll need a clamp every inch or two, or, better yet, cauls to get even pressure along the joint.

And, I agree with AG, too much pressure will starve the joint. I learned to use just enough to bring the boards together firmly.

- Rich

I think the 45 degree thing is not just a rough approximation, I think it has something to do with the way materials react. Though I’m sure it does assume a uniform material. And you’re absolutely right on to bring up using cauls. I think most tend to use them as a way to not crush our projects where the clamp contacts it, but they’re important to spread the clamping force out across the joint. But because of the 45 degrees, you actually need the cauls + stock to be as thick as your spacing between clamps to get truly even pressure.

So if you were edge gluing 6” wide boards, you don’t need cauls if the clamps are less than 6” apart. But if you were gluing something just 1” thick, you probably need cauls.

-- Clin

View AlaskaGuy's profile (online now)

AlaskaGuy

3915 posts in 2218 days


#7 posted 01-18-2018 05:01 AM



As an example, Titebond II clamping pressure, according to Titebond, is 100-150 PSI for softwoods, 125-175 PSI for medium woods, and 175-250 PSI for hardwoods.

While the clamping pressures, in the article listed by AlaskaGuy, may seem really large, the PSI generated can be much less if the area being glued is very large. So if you re edge gluing boards that are 1” thick and 10” long, you have just 10 sq-in of area and still need a total of something like 1,500 lbs of pressure. Which you can get pretty easy with a handful of lighter duty clamps.

But if you were gluing up something, much larger, like laminating two boards together, something that may have 100 or more square inches, you probably need every clamp you can lay your hands on to begin to get close to what is recommended.

The width of the boards matters too, not just the length. Since the pressure radiates outward from the clamp (I ve seen diagrams that show 45º but that s probably just an approximation), the wider the board, the fewer clamps you need for a given length. For example, picture gluing two 1/4 inch strips. You ll need a clamp every inch or two, or, better yet, cauls to get even pressure along the joint.

And, I agree with AG, too much pressure will starve the joint. I learned to use just enough to bring the boards together firmly.

- Rich

I think the 45 degree thing is not just a rough approximation, I think it has something to do with the way materials react. Though I m sure it does assume a uniform material. And you re absolutely right on to bring up using cauls. I think most tend to use them as a way to not crush our projects where the clamp contacts it, but they re important to spread the clamping force out across the joint. But because of the 45 degrees, you actually need the cauls + stock to be as thick as your spacing between clamps to get truly even pressure.

So if you were edge gluing 6” wide boards, you don t need cauls if the clamps are less than 6” apart. But if you were gluing something just 1” thick, you probably need cauls.

- clin


And what sophisticated expensive equipment do I need to buy so I know how much pressure I’m applying?

-- Alaskan's for Global warming!

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Rich

2295 posts in 498 days


#8 posted 01-18-2018 05:05 AM

And what sophisticated expensive equipment do I need to buy so I know how much pressure I m applying?

- AlaskaGuy

A simple gluepressureometer will do the trick. $8.95 at Harbor Freight (less if you have the 20% off coupon).

-- No matter how much you push the envelope, it'll still be stationery.

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AlaskaGuy

3915 posts in 2218 days


#9 posted 01-18-2018 05:17 AM


And what sophisticated expensive equipment do I need to buy so I know how much pressure I m applying?

- AlaskaGuy

A simple gluepressureometer will do the trick. $8.95 at Harbor Freight (less if you have the 20% off coupon).

- Rich


Is that one of HF’s hidden jewels I keep hearing about?

-- Alaskan's for Global warming!

View unclearthur's profile

unclearthur

162 posts in 1697 days


#10 posted 01-18-2018 05:44 AM

In theory: For the OP’s cutting board, 18” long, say 1 1/2” thick, would make it 27 square inches of glue area. For an average pressure of 250 psi, you then need a total of 27*250 = 6,750 lbs clamping force. So about 12 F clamps (or 6 bar clamps).

Which would make it an F clamp every 1 1/2” ....... which seems over the top to me.

View Rich's profile

Rich

2295 posts in 498 days


#11 posted 01-18-2018 06:01 AM


In theory: For the OP s cutting board, 18” long, say 1 1/2” thick, would make it 27 square inches of glue area. For an average pressure of 250 psi, you then need a total of 27*250 = 6,750 lbs clamping force. So about 12 F clamps (or 6 bar clamps).

Which would make it an F clamp every 1 1/2” ....... which seems over the top to me.

- unclearthur

You are aptly named. I’m a degreed engineer, and I have never concerned myself with square inches, PSI or any of that when gluing up a panel. Besides, to complicate matters, what if you used psi clamps (Ψ)? That would throw the psi WAAAY off. It’s a slippery slope. Tread carefully.

-- No matter how much you push the envelope, it'll still be stationery.

View Andybb's profile

Andybb

758 posts in 512 days


#12 posted 01-18-2018 06:27 AM

I know you guys are engineers but, is it just me or are you guys putting way to much thought into this? What ever happened to, “you can never have too many clamps”? I am an internet trained woodworker thanks in most part to lj’s and youtube and a few others. In the 10K woodworking videos I’ve watched and articles I’ve read I have never once heard this discussed or anybody saying, “This is what happens when something is clamped too tightly”. Clamp it good and tight (not over-tight)with as many clamps as seem appropriate then add a couple if there’s room. If there’s no squeeze out I know I haven’t put enough glue in the joint. If there’s a lot of squeeze out I know I put too much but I don’t take it apart like I might if I thought I put too little glue since, as someone already said, “the glue is stronger than the wood”. I might use a little extra glue with softer woods as they seem to absorb more than hardwoods.

Again, I have nowhere near the skills and experience you guys do but is clamping too tight really a thing?

To answer the OP’s question, IMO 4 should be good over 18”, using a long caul on each side and a clamp at the ends of the seam.

Rich, go ahead and have your way with me as I sure do respect your opinion.

-- Andy - Seattle USA

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TheFridge

8908 posts in 1395 days


#13 posted 01-18-2018 06:27 AM

A lot. And a bunch.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View AlaskaGuy's profile (online now)

AlaskaGuy

3915 posts in 2218 days


#14 posted 01-18-2018 06:31 AM



A lot. And a bunch.

- TheFridge

But what kind?

-- Alaskan's for Global warming!

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TheFridge

8908 posts in 1395 days


#15 posted 01-18-2018 07:07 AM

The yellow kind and the long kind.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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