Gluing cherry without glue line

Is glue type, color or amount responsible for prominent glue lines? September 24, 2002

What's the best glue for cherry? I've always used regular Titebond, which works fine, but once in awhile there is a fine glue line. Should I use a dark wood glue?

Forum Responses
Titebond works fine. I use a MPA white from CP Adhesives.

I use good old Elmer's dark version. Works great and to me, no visible line.

From contributor G:
A good glue line is about 3 thousands of an inch thick. I don't know about you, but I can't see that. If you can see a glue line, I suggest looking into the surface prep of the mating surfaces and ensuring that they do meet well.

If you can see the glue line, you're using to much glue or not enough pressure or poor surface prep. Any good industrial PVA will do a good job with cherry.

Jeff Pitcher, forum technical advisor

To put the above into perspective, a one dollar bill is .004" or four thousands thick.

From the original questioner:
I've never had any problems edge gluing cherry, but have when face gluing it. I've had the problem when I wanted a 1 1/2" top and glued 2 layers of 4/4 at the edges. I must not be getting enough pressure. I think I'll try some dark glue next time.

From contributor G:
Gluing pressure should be between 100 and 200 pounds per square inch, with denser species requiring pressure near the top of that range. If you do the math, on a typical face board, say 12" x 40", that's for 480 sq inches, which, if you applied 150 psi, would result in a total force of 72000 pounds over that panel. The only real way to obtain such an even pressure is with a vacuum press. The 100 to 200 psi rule is from my adhesives class that I took at SUNY ESF in Wood Products Engineering.

From contributor L:
I also like the vacuum press, but isn't it limited to 15 psi rather than 150?

From contributor G:
I don't have the exact figures with me, but I'm pretty sure that the units on the vacuum press are not psi, but are in/Hg. This is the length in inches that the vacuum would pull mercury up a tube of a given diameter. The equating psi is much higher than this number. I don't have a reference book here, but I will look up the corresponding psi at 15 in/Hg when I get home tonight and get back to you.

From contributor K:
14 to 15 PSI is the most that the atmospheric pressure is at sea level. This goes down as you go up. A vacuum press cannot give you more pressure than the atmosphere has to give.

And yes, that is pounds per square inch.

I recommend a glue line pressure of 150 to 200 psi for gluing cherry. The most pressure that a vacuum press can deliver is the barometric pressure at the time of gluing. Typically, this is 14.7 psi or 30 in Hg at sea level.

A vacuum press is able to apply the pressure very evenly to a panel. This will permit veneers to conform to the core, but it would have a lot of trouble doing the same with 4/4 cherry. I have rarely seen anyone using a vacuum press for face gluing.

The resulting glue line should be no more than about 0.005" in thickness, and will be difficult to see. Thick glue lines can be a result of uneven surfaces and long assembly times as well as insufficient pressure.

From contributor G:
Contributor K, you are absolutely right - the gauges are measured in inches/Hg, but at 22, the pressure works out to 1550 pounds per square foot, which is 10.76 psi. You would have a difficult time gluing solid lumber with this. I always thought that the pressure was greater. However, the true miracle of a vacuum system is the distribution of the pressure throughout the work piece.

I wouldn't suggest the vacuum press for solid wood gluing. As we've seen here, it doesn't develop the necessary pressure. It's a great tool for veneer, however. With today's modern substrates and some of the better cut veneers (as well as backed veneers) the pressure requirements of the past are less. Interestingly, the vacuum effect also helps the adhesive penetrate the substrate which often results in a better bond.

Jeff Pitcher, forum technical advisor

From contributor L:
I'm not challenging, just curious. Why not face glue with a vacuum press?

With a veneer between the lower platen and the core, isn't the glue line in effect a joint between two rigid planks? *If* the surfaces are well jointed, why wouldn't face gluing be the same?

If the issue is that man-made material is flatter than raw lumber, would abrasive milling both surfaces flatten the lumber adequately?

Granted I wouldn't want to face glue structural beams with a vacuum press, would it be adequate for the loads seen in furniture?

I've been considering face gluing cherry in my vacuum press, so your experience might save me from ruining some stock.

You'll develop far greater pressure with bar clamps. If the cherry were machined very well, you might get okay results, but my experience gluing solid wood has proven out the need for greater pressure than a vacuum press can provide.

Jeff Pitcher, forum technical advisor

From contributor L:
Jeff, I'd done a test laminating 3 1/2" MDF sections and it worked fine.

I put a 1-1/2" square, solid stock edging on a 3/4 birch ply panel before veneering it so that the top looked solid. I had some failure at the edging's outer corners where I'd hand planed the surface "level". Everywhere else worked okay (but it's not yet been in service). Someone had suggested that I may have not left the surface planar.

I'd hoped that the press would have been sufficient. Oh well; back to the parallel bar clamps.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

From contributor A:
Getting back to the original question... What's the best glue for cherry?

A yellow carpenter's glue will do just fine for cherry. If the joint is visible enough to catch the eye, it may be the method or technique that needs improving upon. A visible joint can be caused by several factors. It may be that the grain figure in both pieces of wood just happened to line up together, causing this effect of a dark seam. Another reason for a visible joint is having too much glue between the seam. You see, sometimes stain will dye the glue darker or it may do the opposite. Let's talk about material preparation. The more precise your cuts, the better the jointing will be. You donít need a fancy smancy edge joining machine. Your two boards do, however, need to be of the same thickness.

Comment from contributor J:
If the glue line seems dark when gluing cherry, I would suggest adjusting the PH level of the glue through the slight reduction of catalyst used in the glue.