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Thanks Carlton, it makes a lot of sense, i also agree by the fact that the HHG i use gives instant bondage, if clamped properly, then there should not be any problems using it for this application especially. Thanks again.

Serge.

I won't comment on choice of glue, but I will comment on another of Al's points - scraping versus sanding. From what I understand, he's right that a scraped surface is best for getting the best glue joint. I might modify my method by ending with a light scraping before gluing. Haven't tried it, but I think I might find it challenging to scrape the concave surface without altering its shape, esp taking a bit more off the edges than the middle, which is exactly what I wouldn't want to do. When sanding, I do go down to at least 220 grit, and I do make sure the surface is really clean before gluing. Were I to try to predict the future, I'd feel pretty confident in saying that the bridges I've glued so far (with PVA, btw) will stay put. Time will tell.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Creep is one reason not to use PVA. I've actually seen bridges with a bare line along the back edge, and a little ridge of finish plowed up in front, but the bridge is still down tight. I've also had to re-top guitars where the top cracked along the sides of the fingerboard, and the top slid along 'popsicle stick' and shoulder braces as much as 1/16", again, without actually breaking the bond.

Properly used, hide glue also makes a joint stronger than the wood around it. I got a good demo of this once. I was showing a friend how to use HHG, and made a little 'rubbed' joint using a couple of pieces of cuttoff brace stock. I just slapped on the glue, rubbed them together until the glue grabbed, and set the assembly aside. Normally I allow HHG 2 hours to cure before stressing it at all, but this time I put one piece in the vice and whacked the other with a hammer only 15 or 20 minutes after spreading the glue. 100% wood shear.

PVA and related glues are emulsions: that's why they look cloudy. The glue itself is in tiny drops suspended in water, and it's not really water soluble itself. As the water dries out the glue drops glom together (that's the technical term). Adding water to a cured PVA joint won't dissolve the glue, or, at least, not before the wood goes to mush. By the same token, a new layer of PVA won't really 'wet' dried glue; in the language we use in finishing, it doesn't 'burn in' well. When a PVA joint fails you have to physically remove _all_ of the old glue from both surfaces before you can re-glue, if you need to have full strength. And if the bridge came up, doesn't that mean the joint wasn't strong enough? There's no way to physically remove the glue without also removing some wood, and, on the top, often a lot more wood than you'd like to remove. This is a real problem with loose braces, too.

HHG will 'burn in': you can often make an emergency repair by just warming things up, slipping in some more glue, and clamping it. It might not be as strong as a new joint if the glue is old, but it often is. This is really useful when you have a brace that's lifted. You can also remove HHG virtually completely by simply washing the surfaces with warm water. When they dry out you've got all of the original wood, so things should just fit back together. Should.... ;)

Dry heat does not effect HHG; Frank Ford did the experiment, and I believe him. PVA lets go at 140 degrees. There's nothing like having a customer leave a hammered dulcimer in the back seat of a car in SoCal in the summer to show you what that means!

HHG does break down over time through microbial action: it's food, and there are things that eat it. Keeping it dry, and keeping air away from glue lines helps a lot.

BTW; the best way I've found to get PVA joints apart when you can't heat them is acetic acid. Hot vinegar works, and photographer's 'Stop Bath' is even better. You must use a stainless steel knife, as regular steel with acetic will stain the wood black.

[QUOTE=Alan Carruth]

Gluing a freshly cut surface does indeed reslt in stronger joints, due to
the higher 'surface energy'.

   [/QUOTE]

In more than one commonly used reference on guitar building Ive read of
scratching up gluing surfaces to increase surface area. I do it on
underside of my fretboards by giving same a rub on a 120 grit sanding
board using circular motions. I also give underside of my bridges a light
scuffing up with 120 grit before gluing up.

[QUOTE=kiwigeo] In more than one commonly used reference on guitar building Ive read of
scratching up gluing surfaces to increase surface area. I do it on
underside of my fretboards by giving same a rub on a 120 grit sanding
board using circular motions. I also give underside of my bridges a light
scuffing up with 120 grit before gluing up.

[/QUOTE]

Alan can tell you where it's from exactly, but there's data from US Forest Labs regarding failure rates of plane props with planed vs. sanded surfaces, and planed is better. That's scientific data, the 'rough stuff up' school is untested empirical data. I've seen the 'rough things up' deal before, as well as even scouring the bottom of gluing surfaces, but honestly, it doesn't make sense to me; other than epoxy, non of the glues we use has much cohesive strength, so we want as thin a layer of glue as possible between the pieces. This translates to as clean and tight a fit as possible, and a good planed surface does that for you (and is also clearer of contaminants, such as sawdust in the grain, f'r example). The glues we use have good adhesive strength, but need to be thin for that to work.

Either way, for some things, I sand, and it's fine, but I prefer to plane or scrape pre-gluing. And if I sand, to 220 or 320 grit.

I'll bet the old "rough up the surfaces" myth is one of the reasons PVA joints sometimes creep or come apart. I'm not arguing against using HHG at all (in all honesty, the only reason I don't yet use it is because I haven't yet learned to), but I'm not going to lose any sleep over a properly done joint glued with PVA.

Thanks, Al, for continuing to educate us.

_________________
Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

[QUOTE=kiwigeo]



In more than one commonly used reference on guitar building Ive read of
scratching up gluing surfaces to increase surface area. I do it on
underside of my fretboards by giving same a rub on a 120 grit sanding
board using circular motions. I also give underside of my bridges a light
scuffing up with 120 grit before gluing up.

[/QUOTE]
I've never personally seen a reference that suggested scratching or sanding a wood surface to improve a glue bond.

Here are some of the things I've read:


Cumpiano - "A thin glue line is obtained primarily by having smooth, flat mating surface on the parts being glued...

Patrick Speilman's book "Gluing and Clamping" (I recommend this highly) :

"Wood surfaces that are being glued should be smooth, flat, and true. Sanded surfaces, as a rule, do not make good gluing surfaces. ... Also, sanding creates torn fibers that are likely to pull apart under stress."

The Encyclopedia of Wood (A very good general reference):

"Wood surfaces to be bonded should be smooth, true, and free of machine marks or other surface irregularities such as torn or chipped grain or planer skip. ... Experience has shown that a smooth knife-cut surface is the best for bonding. ... Abrasive planing ...causes surface and subsurface crushing damage. Such damaged surfaces are inherently weak and result in poor quality joints."

Can you tell us which guitar-making books suggest scatching the surface to improve gluing?? I'm interested to read what they say.

Phil

I'm a bit late to this party but just thought I'd throw in something. I drew the correct arcs (25' and 16') along the long face of a 4x4. Curves to the outside facing away from each other. A short trip to the band saw and I have both concave and convex curves for both radii all in the space of one 2' section of 4x4. I cover those surfaces with sand paper and while not spherical they have many uses. RCoates38858.6076967593

Back in the Olde Days they used to use tooth planes to rough up surfaces to be glued. I've seen old Martin bridges that were so treated. I think it's one reason Martin had to go to the 'belly' bridge to keep them glued down, where Mario Proulx reports no trouble at all with straight ones using smooth surfaces.

Most of the strength of the glue bond is chemical. Glue itself is not a strong material, but it is very strongly bonded to the wood, and the fact that the glue fills in the open areas in the cell structure helps. Thick glue lines are not as strong as thinner ones, just so long as they aren't so thin as to leave 'dry' areas in the joint. Toothing or otherwise roughing up the surface will result in thicker glue lines and/or gaps, which can't possibly help.

I _think_ that toothing was something furniture makers did in part to reduce the amount of squeeze-out that they'd have to clean up. The gaps gave the excess glue some place to go other than all over the surface. We tend to stress glue joints very highly, and need all the strength we can get, and for that there's no substitute for a perfect fit and smoooth surfaces.

Well, Ive looked over the postings on this thread and it looks like I ned to
change my thinking on gluing surfaces. One reference Ive relied on for
my early attempts at luthiery is Courtnalls "Making Master Guitars".
Scuffing up the fretboard prior to glue up is one of Courtnalls
suggestions and I assumed that the same advice would hold for bridges.
Having read the posts in this thread and also having just bought and read
Spielman's "Gluing and Clamping" I'm convinced that scuffing up gluing
surfaces is NOT a good idea.

Cheers Martin

This is the first time I am putting up photos in this website and I hope they show up fine. These photos were taken at Romanillo's guitar making course in Siguenza, Spain. I asked a forum member and a great friend, John Hall (Blues Creek Guitars) to make me an exact duplicate of this jig. The contour of the jig matches that of the dome/doming built into the soundboard. John has made a number of jigs/patterns, etc. for me over the years.

Yep! I made mine with a 12" X 4" piece of MDF on my radius dish, took 10 minutes...and topped with 80g to start, 120 to finish...then a quick scrape right before gluing...works great, although the comment here about the top maybe not ending up exactly the same as the radius dish is a good one...I just haven't made up my mind about sticking sandpaper to a newly finished top!

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I use a 2x4 sanded in the 28 foot bowl - that gets it close, then I finish up with a scraper just prior to gluing with titebond. I use a chisel to clean that area under the bridge and fingerboard - scares the hell out of students when they see me do this to a freshly buffed out lac finish (I learned this from watching Serge De Jonge one day I visited) but it gives a clean area - and with the sharp chisel you KNOW when you hit the wood fibres. No failures yet.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"