invisible glue lines?

[David Wadstrup]

Hello,

I was wondering if you could offer me some advice on achieving an invisible glue line. I have dimensioned 2 35"x6" inch 8/4 ash boards, and need to join the them face to face. I've done all of this with handplanes and have near perfectly flat faces. When I did a dry run of the glue up, though, there were hairline gaps along the seam. They must only be 1/128", but are of course noticeable. Is there a trick to getting an invisible glue line? I'd really appreciate your advice.

Thanks,

David

[Andrew Hughes]

Try spreading the glue with a notched piece of scrap wood.I have had the same thing happen to me.Takes a lot of pressure to squeeze out extra glue.I use a fine tooth hand saw to make my glue spreaders for bigger stuff i use the band saw to cut the notches.Hope this helps. Andrew

[David Keller NC]

David - If you're using PVA glue (the common "yellow" glue), what you're seeing is likely the effect of water absorption. Even if your boards are -perfectly- flat on the face (which is dang near impossible to achieve with a handplane - they will always leave a very, very shallow depression along the cut line), when you smear that much glue on them that's necessary to join two 6" faces, the water in the glue will slightly expand the surface that's face-to-face. That will introduce some cup in the direction away from the glue face, and it's nearly impossible to overcome this force with clamps.

There are 2 solutions I can think of - one is to deliberately hollow the centers of the faces slightly - perhaps one or two 3 thousandths shavings, and dry-fit the boards with clamps to see if you've achieved the fit you want. The shallow dish on the faces will accomodate the glue, and will allow a slight bit of cupping without overly expanding the glue line.

The second way is to dry-fit your boards to ensure that they're a close match, then apply the glue and press-fit the boards together, mist the outside faces with distilled water (distilled to avoid any iron staining), then clamp them tightly together. Depending on the glue and your shop's humidity, this method may require coming back after an hour and misting the outsides again.

[Jeff Wittrock]

...When I did a dry run of the glue up, though, there were hairline gaps along the seam....


So this is before the glue up. What I do sometimes, as David mentioned in (2), is slightly relieve the center to have something of a "spring joint" only in this case the spring is occurring across the face. Then when I clamp I use cauls to make sure more pressure is applied to the center.

[george wilson]

If you squeeze all the glue out with extreme pressure,you will have a "dry joint" with not enough glue in it to hold well.

I have seen some harps by lyon and Healy made of maple that have remarkably tight glue lines. They are not stained either. The glue they use is just dry gelatin mixed with water. You can buy Knox dry gelatin at the grocery store. It is a very pure grade of hide glue,essentially. Their workmen eat it at lunch sometimes.

[john brenton]

I often find that the lines are seamless just below the surface. This could be due to lack of clamping pressure on the very edges.

[Jim Neeley]

When you mentioned face-joining these two boards, I thought of Rob Cosman's method of sharpening 1000 waterstones by rubbing them against each other and wondered if it'd be feasible to apply this approach to the wood.

Perhaps some 80 or 120 sandpaper glued (temporary glue, 3M 77?) first to one board and then the other and rubbing them against each other or else

Gluing several sheets of sandpaper to a piece of glass, (table saw or jointer top) and flattening the board against it?

I know this would probably be overkill for wood but if you had the flat surface, 3 or 4 sheets of sandpaper wouldn't cost much.

Feedback?

[george wilson]

I'd prefer to stay away from sandpaper when making tight butt joints,and just use a carefully adjusted plane. You might could rig up a fence for the plane if you can't keep it real square with the work. LV sells a fence that has powerful magnets in it,I believe.

[David Wadstrup]

Thanks Jeff. I was thinking this might be the solution too. Just tried it, and it worked perfectly. Took a few fine shavings a 1/4" in from the sides of the boards and ended up with invisible lines! Like a spring joint in two dimensions. Thanks for the advice.

David

[Chris Friesen]

If you squeeze all the glue out with extreme pressure,you will have a "dry joint" with not enough glue in it to hold well.

This is a common misconception. It's nearly impossible to squeeze out enough glue to get a dry joint, especially when face-gluing. The recommended pressure for PVA is around 200psi, and on a 6x35" glue area that adds up to 42000 lbs of force.

[david charlesworth]

Using a slightly cambered blade for making edge joints, is a great way of getting really tight, or nearly invisible glue lines.

BTW There is no such thing as a glue starved joint caused by high clamping pressure. Just look at the pressures recommended by Bruce Hoadley and glue manufacturers. This is one of the most prominent of Woodworking Old Wives Tales....

David

[Bruce Haugen]

...BTW There is no such thing as a glue starved joint caused by high clamping pressure. Just look at the pressures recommended by Bruce Hoadley and glue manufacturers. This is one of the most prominent of Woodworking Old Wives Tales....

David

The only exception to this that I'm aware of is epoxy. From the West System site (http://www.epoxyworks.com/13/laminate.html): "Even contact is all that is required between mating surfaces -- too much clamping pressure only pre-stresses joints and forces adhesive from the joint, which could result in a glue starved joint."

[Prashun Patel]

This could work. However, on longer pieces, it'd be hard to 'joint' them this way, because it'd be hard to maintain even pressure on the entire board at once.

[David Keller NC]

When you mentioned face-joining these two boards, I thought of Rob Cosman's method of sharpening 1000 waterstones by rubbing them against each other and wondered if it'd be feasible to apply this approach to the wood.

Perhaps some 80 or 120 sandpaper glued (temporary glue, 3M 77?) first to one board and then the other and rubbing them against each other or else

Feedback?

Jim - You don't actually need to go as far as sandpaper or rubbing the board against a flat reference surface to do this. You can simply rub the two board surfaces together with moderate pressure, then open them up and examine the surface under a raking light. The places where the two boards are touching (in other words, the high spots) will be "burnished", and will appear as shiny spots. The parts of the surfaces where there's a gap will be unchanged. One can then simply take light shavings off of the shiny spots, and repeat the procedure. The end result will be somethign very close to 2 matching surfaces, though not necessarily 2 flat surfaces....

[george wilson]

It depends upon the wood and the glue. I have starved a few joints in my time.

[Chris Fournier]

Using a slightly cambered blade for making edge joints, is a great way of getting really tight, or nearly invisible glue lines.

BTW There is no such thing as a glue starved joint caused by high clamping pressure. Just look at the pressures recommended by Bruce Hoadley and glue manufacturers. This is one of the most prominent of Woodworking Old Wives Tales....

David

In practice it is very easy to starve a glue joint - an edge not a face. As has been pointed out some glues are more prone to this phenomenon than others and wood species can play a role too.

A spruce or cedar guitar top is about .010" thick on a medium sized guitar, when a luthier is gluing up the book matched joint it is very easy to starve this joint. Not letting a joint wet properly also plays into a starved joint scenario.

[jamie shard]

Not letting a joint wet properly also plays into a starved joint scenario.

It seems like this is the biggest cause of my own (limited data set) starved joints. If the glue doesn't have time to penetrate, then it can get starved with enough pressure, but if the wood is saturated, then it's hard to overclamp.

Another thing in all of this... is the final glue joint just needs to be a little stronger than the wood, usually the wood fails first.

[Jim Neeley]

Thanks, David... :) I've seen Cosman demonstrate that technique in his Drawer Making DVD and should have made the connection.