How many clamps???

[Greg Becker]

According to an article in FineWoodworking by Prof Roman Rabiej, Cherry being glued on a quarter sawn face needs 250psi. My table top is 78" x 1.75 along the seem which calculates to 34,000 psi required.

Using all my large clamps (12 pipe and 5 K Body) and giving a maximum pressure (due to my hand strength) that totals out to 20,000 psi.

Do I really need to go out and purchase 10 more pipe clamps? I am almost ready to glue up the dining table top that I have hand planed 3 boards to prepare. The final top will be 82 x 42 (including breadboard ends) and is my most important project to date. This is beautiful cherry that deserves to be done right.

Any suggestions? Below are photos showing my clamp base with built in panel clamps and a frame to support the pipe clamps. 10 clamps will be from underneath and 7 will be applied from the top.

[george wilson]

I haven't heard that figure. I think that a properly fitting seam should NOT have to be clamped together with great force to make it fit. If it does,it is going to pop open in the future. just make sure neither seam is slightly convex. It should rather be slightly concave,so the ends of the seam do not have a tendency to come open later.

It looks like you have enough clamps if the joint fits properly.

[Gary Max]

We have over a 100 clamps and still buying more----I hate it when I have to stop because I don't have enough clamps to get the job done.
You might as well go ahead and get them.

[Thomas love]

Seems like you have plenty of clamps, I like Georges advice.
tom

[Andrew Gibson]

People use math when it comes to glue up?
I clamp it up untill I get nice even squeeze out along the joint and call it a day.

[george wilson]

The math makes no sense to me. You just need to make sure a joint fits properly. Attempts to CRUSH it together will only come back to bite you later.

Some old Spanish guitar makers only used twine and tightening sticks to tie guitar tops and backs together,and even to tie the body of the guitar together when gluing. They often were too poor to have clamps. They still made some very fine instruments that way,though.

I did the same thing when working in public in Williamsburg. A form the shape of the guitar with nails driven all around the edges was used to lash down the vaulted backs of early guitars. The inlaid guitar I made in the FAQ section here was glued together that way. It was actually better than regular clamps because you could damage the vaulted shaped back with clamps.

Clever ways to clamp things can always be devised. Extreme pressure isn't needed.

Guys who sit around figuring out esoteric math formulae for things like clamping wood aren't getting much work done. Besides,WHO has the equipment to even measure clamp pressure anyway? It's a bit ridiculous.

[David Keller NC]

According to an article in FineWoodworking by Prof Roman Rabiej, Cherry being glued on a quarter sawn face needs 250psi. My table top is 78" x 1.75 along the seem which calculates to 34,000 psi required. Any suggestions? Below are photos showing my clamp base with built in panel clamps and a frame to support the pipe clamps. 10 clamps will be from underneath and 7 will be applied from the top.

Take what you find in FWW with a grain of salt. I don't know Roman, but I can tell you that the new crew running FWW lacks a lot in the "right" department, particularly when they're designing experiments (the "which joint is stronger" comparison from last summer is a glaring case in point).

It may well be that Roman is correct from an industry side of things - more than likely much development work has been done by engineers/scientists to determine the least amount of glue and clamping pressure possible to produce an acceptable joint. However, those results aren't really applicable to home shops and custom built furniture.

In particular, you are very unlikely to subject the joints in your table top to any appreciable force, particularly if you properly allow for expansion/contraction of the top with seasonal changes in humidity. What you should be more concerned with is what George mentioned - how straight are your edges, are they either match-planed or dang near exactly 90 degrees to the face, and are your panel clamps properly set-up to correctly align the boards.

On that last point, your panel clamp bars should not be perfectly straight when they're not under compression. Instead, you should have about 1/16" - 1/8" of convex curve across their length. The reason for this is that the wood will flex when they're put into compression, and if there's no convexity on their edges, most of the force you will be putting on the glue-up will be at the tabeltop edges, which defeats the purpose of the panel clamp.

A good way to introduce this convexity is with what David Charlesworth calls "stop shavings", where you "land" the plane a copule of feet or so from the end and take a shaving all the way off of the edge. Then repeat the process except "land" the plane closer to the end, etc... The final shaving should be the entire length of the panel clamp edge with a short plane (such as a smoother), which will take out any arrises left from the stop shaving process.

No, you don't need any more clamps - you've got plenty.

[David Christopher]

with 78" of lumber and 17 clamps, thats a clamp every 4 1/2 "....that will be more than enough if your edges fit

[James Taglienti]

The FWW clamping pressure article from a few years ago is a complete joke. There are a number of instances where the recommended number of clamps wouldn't even fit on the workpiece.

I'm also utterly unimpressed with their entire issue this month and am considering cancelling my subscription.

[Greg Crawford]

Here's what Titebond recommends for Titebond III;

http://www.titebond.com/IntroPageTB.....asp?prodcat=1

Much different that the specs you mention. One problem many people introduce is starving the joint of glue with too much pressure. I think the previous advice is very sound.

[Tom Henderson2]

Greg-

I couldn't find clamping pressure recommendations in the site you linked... could you point me in the right direction?

-TH

Here's what Titebond recommends for Titebond III;

http://www.titebond.com/IntroPageTB.....asp?prodcat=1

Much different that the specs you mention. One problem many people introduce is starving the joint of glue with too much pressure. I think the previous advice is very sound.

[Sean Hughto]

Good grief, you have plenty of clamps for the job. At just over 6 feet long, and having 17 clamps, you'll have a clamp every 4 or 5 inches, if my math is right. That's more than enough to yield a perfect joint (as with any joint - assuming you've prepared your edges well).

[Frank Drew]

Since Dr. Roman Rabiej has spent the last several decades studying and teaching wood technology, particularly as applied to furniture manufacture, I have to think that he's not a complete idiot and that something is being lost here between his clamping pressure recommendations quoted in FWW (250psi) and what that actually means in terms of type and number of clamps used.

[Tony Shea]

That's a first for me of ever hearing any such calculation. Way too much thought is being put into simple procedures such as glue-ups with articles like the one referenced to blame. Like has been stated above, most of your energy and concern should be focused on your joint and how well they come together without clamps. It's strange they come up with this insane calculation yet in a couple articles back you see a guy writing an article about using tape and rubber bands for clamps. How much pressure can one achieve with tape? Just do what you feel is right and concentrate the energy on your joints.

[glenn bradley]

I gave up crushing things together in favor of good fitting joinery some time ago. As others have said; if your joints fit dry, adequate glue and good clamping will result in a joint that is stronger than the wood in many cases.