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According to the article in fine woodworking on gluing up joints:

The formula for number of clamps required is (square inches of glued joint x PSI for the wood in question) divided by PSI of the clamp you are using.

I am gluing up a desktop made of 2x2's of different hardwoods (red oak, cherry, walnut, maple and poplar. According to the article, the required gluing pressure averages about 350 psi, so for my project (6' long joints on 2x2 which are actually 1-13/16 squared) comes to:

(72" x 1.81) x 350 = 45,675
divided by ~1000 (pressure exerted by 3/4" pipe clamp.

] That works out to 45 3/4 inch bar clamps for a 6' length!

This means I need a clamp every 1.6"!

That seems really nutso. Is it accurate? Do I really need basically as many clamps as I can fit on the board for the full 6' length?

  • Can you post a link to the article? Something seems very fishy in your calculation – mmathis Oct 2 '17 at 20:57
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http://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure

According to this Article the makers of Titebond recommend 175 to 250PSI for hardwoods, and they also say that a C-clamp can exert about 2000 lbs of pressure. Plug those numbers into your equation and you get 72 x 1.81 x 250 = 32580lbs total pressure along the length, divided by 2000lbs per clamp = 16.3 clamps, rounded up to 17 total or about 1 every 4 inches. Seems a bit more sensible.

FWIW though I don't think putting too much pressure into a joint (as mentioned in the article) is an issue. I've never managed to do it. I work at a commercial joinery and we laminate hardwoods up all the time and use big, seriously chunky sash cramps, tightened up to the max, and have never had a joint fail from being "glue starved" - you'll crush the wood before you starve a joint of glue from overclamping.

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You will be fine even if you use fewer clamps. Alternatively build yourself a few clamps out of 3/4 pipe. They put a lot of pressure

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  • Pipe clamps are quite pricey. Following the reccos in this article would cost over $1,000 to buy all those clamps and 3/4 black pipe. – GGizmos Oct 8 '17 at 4:09
  • Use 20% off coupon in HF to get harborfreight.com/… then get 3/4 galvanized pipe and cut with the angle grinder to size. This is as cheap as it gets. – Igor Katkov Oct 9 '17 at 5:06
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That works out to 45 3/4 inch bar clamps for a 6' length!
This means I need a clamp every 1.6"!
That seems really nutso. Is it accurate?

In terms of their recommendations in this article you can usefully extrapolate out from this:

Clamping recommendation, FW article

Source: How to Glue-Up Joints: The right number of clamps on Fine Woodworking.

The above is going to seem like complete overkill to many (probably most) woodworkers who glue up stuff all the time. But to put it into some kind of perspective, in factories that assemble 'butcher block' countertops the clamping is done in hydraulic clamping rigs that can be capable of exerting over 100,000 lb (45,000 kg) for precisely the reason that that is the kind of pressure testing has indicated IS needed for maximum joint strength.

Do I really need basically as many clamps as I can fit on the board for the full 6' length?

While with PVA glues it wouldn't hurt to use more clamps than typical, there is a practical limit in what can fit and a definite upper limit to the number of the largest clamps most people can afford (more than a dozen? no way) and, let's be honest, be bothered to deal with during a large assembly!

One does have to be serious about clamp pressure where you want the strongest joints. In the recent past and continuing on to today woodworkers do have a tendency to under-clamp, in part because of a mistaken but unfortunately widespread belief that you can starve joints made using any glue with clamp pressure (you can with some adhesives, not with PVA). Since we don't see widespread failures in PVA glue joints* it indicates you don't have to go crazy on the number of clamps and how hard you crank them down to get joints that are strong enough for service.

Now that said for any clamp capable of exerting enough pressure to crush wood you should use a scrap piece of softwood as a clamping block so that they can take the damage, not the edge of the workpiece, and then tighten the clamp hard enough that the scrap block does dent.


*We'd all know about them if they happened, given the Internet!

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