Official Luthiers Forum!

I just came in from the shop where I was gluing up a laminated neck. As I was twisting the Bessey's for all I'm worth, I started hearing a faint voice say 'not to tight or it will be starved for glue'.

So as I was cranking away I got to wondering, how tight is tight when you are clamping a AR joint? I know epoxie is a whole different animal and you CAN starve them for glue, but AR?

I still remember what my wood shop teacher said way back when, 'tighten them as much as you can and use a lot of clamps'. I don't think we had AR then so maybe things have changed?

What do you think?

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Joe Beaver
Maker of Sawdust

Back in WW II the Forest Products lab figured out that 125 PSI +/- 10% was 'right' for aircraft glues on soft woods. The resorcinol they use often has some wood flour filler in it, but it's not all that much thicker than AR. So that's one answer.

My feeling is that the main purposes of clamping are to get the wood parts together and squeeze out the excess glue. If the parts are large, or thick, and don't fit perfectly, you may have to bull them into place with a lot of clamps to get the excess glue out. For the thin parts and good fit we work with you might not need that. I've used 'rub' joints on fiddle and archtop center joins, with no clamps ar all, with good results: the excess glue is rubbed out.

In the end you have to develop a feel for it, I guess, but it's generally harder to starve a joint than it is to leave it too thick.

Joe, I'm with Todd and Alan here, i like to use a lot of clamps and tighten as hard as i can within reason and see squeeze out, worked well for me so far.

One can never have to many clamps on a piece. A lot of it has to do with the quality of fit I think. Like Alan I have done rub joints on arch top plates and they're doing great after 10 years or so.
I vacuum clamp my braces and that's a whole lot less than 125 psi, like 14 psi or so. So I think a lot of it has to do with the fit and distribution of the pressure.

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Jim Watts
http://jameswattsguitars.com

   Same here. If you have glue everywhere on both surfaces being matted, you probably would never be able to put enough pressure to critically 'starve' an AR joint.

   I feel very confident when I have a nice constant squeeze-out bead... Not so much when there isn't any...

   

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I'd like to be able to prove, just for once, that money wouldn't make me happy...

I usually clamp until I'm seeing squeeze out all along the joint and the joined woods themselves appear flush. So the pressure varies.

Best,

Michael

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Live to Play, Play to Live

There's a book all about gluing - "Gluing and Clamping" by Patrick Spielman, that covers almost everything about this topic - the properties of different glues, woods, clamping methods, etc.

He suggests clamping pressure of 100 to 150 psi for "most softwoods" and says that harder woods, like oak, require more pressure - as much as 300 psi.


He also lists woods as to ease of gluing, ranging from 1 ("glues very easily...") to 4 ("requires very close control of gluing conditions, or special treatment to obtain best results")

Sitka spruce, WRC, and redwood, for examples, are #1 (easy) and hard maple is #4 (difficult). He doesn't have many exotic woods in this table, though.

This is a great book to have in your shop- I refer to it all the time.

There are several copies (used, good condition) on half.com right now for less than two bucks.


Phil

I tighten it as much as I can and I use enough clamps so there is only 2 to 3 inches between clamps. That way I getsome really tight seams that are hard to see.

The history of instrument building seems to have shown that, within reason, the amount of pressure isn't really something we have to worry about too much. What we do seem to need is: 1. Even pressure along the joint 2. Even squeeze-out 3. A thin, even distribution of glue between the mating parts.

For a long time, people have been applying hide glue to tentellones and holding them to the sides/top with just finger pressure for a few seconds. Because of HG's quick set, it seems to be enough. As has been mentioned, it's possible to join plates by applying glue and rubbing the parts together (which creates a vacuum) and not clamping at all. I bet if you wanted to hold a brace with firm, even hand pressure for 45 minutes, you could stick it to a top with Titebond, and it would be just fine.

I know that tutorials and manufactures tend to suggest quite high clamping pressures, but lutherie practice seems to show that we don't have to worry too much about applying hundreds of pounds of squeeze with our clamps.

Carlton, i agree with you on how to proceed, that is what i did for my laminated neck and i just can't find any seam anywhere on all sides, i also used wood scraps to protect my wood from the amount of pressure i applied with all the clamps i used.

I don't fret about not having enough pressure, but find that, generally, a boatload of wooden cam clamps are pretty ideal. Quick, apply plenty of pressure, and won't ever apply too much. I do have metal clamps for the heavier work/where I don't have enough cam clamps, but still.

Thanks guys....

About cam clamps... It seems to me they are on the weak side of the clamping pressure range. I've been investigating a little more and have read that the denser the wood the higher the clamping pressure should be as others have said here. (100 psi for soft woods and up to 300 psi for dense woods). That means if you are clamping 1 sq ft of mahogany at 200 psi you would need 28,800 ft pounds of force.

That seems to let cam clamps out but then as Carlton, Mattia and others have said history and accepted practice say otherwise.

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Joe Beaver
Maker of Sawdust

I've actually gotten away from the 'all the pressure you can muster' school over the past few years.

It seems to me that the important thing is how much actual glue there is in the dried joint. Glue itself is not a strong material, and most of the strength in the bond comes from the chemical attraction between the glue and the substrate. Thus you want enough glue, but no more than that. The purpose of clamping is to get the excess out.

If you think about it that way you'll see that there are a lot of variables that interact: the fit of the joint, how hard and how porous the wood is, the viscosity and setting time of the glue, and so forth. A well fitted joint in mahogany, with all of those pores, will practically suck up the glue, while tight grained and hard maple, particularly if it's not fitted well, can give trouble, particularly if the joint area is large and the glue sets up fast.

All of this is particularly germaine when you're talking about hot hide glue. If you mix it as thick as the label directions on the can say you probably will need a fair amount of force to get the excess out before it gels, but you'll get pretty good gap filling. However, thinning the glue a bit will both extend the working time before gelling and reduce the viscosity. The down side there is a greater liklihood of a starved joint. with glue that's not too thick a 'rubbed' joint, which is essentially what the old boys did with tentellones, leaves just about the 'right' amount of glue.

Another consideration is that even a pretty poor joint that is lightly clamped can still develop a fair amount of strength. Glue is not strong compared to cellulose, but most of wood is air, and the glue fills in those air spaces somewhat. There is some strength there, at least on side grain. Some joints on the guitar, such as the bridge to top, are pretty highly stressed, but others, like the back to the edge, are not. With enough area even a 'weak' joint will hold up pretty well to a distributed load. It might be interesting some time to try to figure out just how much glue area we actually _need_, at least in the short term. We might be surprised.

I try to be very careful when gluing laminations such as a neck to keep the clamps as perpendicular to the work as possible. For one thing the piece can slip out of position which is readily noticeable. I use alignment dowels on some glue-ups but again I try to keep the clamps perpendicular to prevent inducing any sideways stress in the boards.
I guess that clamping a fretboard is a good example of this as I really don't want to introduce any lateral stress in the fretboard,neck assembly.
Nelson
npalen38893.4498148148

I don't like saying tighten the heck out of a joint. I prefer to say prep the surfaces as tight as possible. It does not take much glue to make a good joint and it should not be difficult to get the extra glue out. Use a lot of clamps to me means even distribution of pressure. Clamp very firmly, but don't use so much you compress or crush the wood. Harder woods can handle more pressure (applied evenly it would be hard to apply too much pressure, well I guess if you got crazy with it anything is possible). I really hate to see a sloppy joint that has been forced into place, or a piece that has been distorted by uneven pressure. It just seems like it is introducing stress to a joint that could have been avoided by cleaner preperation. A mistake that can be avoided by doing our due dilligence.

Peace,Rich

I'm beginning to get the feeling that some of my confusion is because I have not notice which glue people are using when the say a good fit and vacuum clamping is all you need, or clamp it as tight as you can.

I now think maybe a good fit and modest clamp pressure (vacuum or cam clamps) is all you need for a gap filling glue like HHG or expoxy but with an AR or white glue you should have higher clamp pressure (100 - 300 psi). I could be wrong about that but it does seem to make sense. (finaly)Joe Beaver38893.5734027778

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Joe Beaver
Maker of Sawdust

The only gap filling glue I know of is epoxy. Everything I've read leads me to believe HHG is even less tolerant of big gaps than AR/PVA glues are.

I've been vacuum clamping with AR glue with no problems for a long time. When I see these high pressure requiremnets for clamping I can't help but wonder what type of clamp density they have or are they really implying that you actually need 10,000 - 30,000 pounds for a 10 inch X 10 inch area?
Does any one know? Seems like you'd need a hydraulic press to perfrom glue ups.
Just curious.

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Jim Watts
http://jameswattsguitars.com

Just to throw a few numbers out there...

According to Understanding Wood by Bruce Hoadley, a 4" c clamp will give you about 2100 lbs and a 3/4 pipe clamp will give you around 1100 lbs. He doesn't have bessy clamps in the book I have but I'd guess they give you around 900 lbs. Most joints we do are probably right in the ball park, the exception may be things like bridges. That may account for some of the failers.

Let me parphrase a little of the book, it is interesting.

'If mating surfaces were perfect in terms of machining and spread, pressure wouldn't be necessary. The rubbed joint attests to this.... But unevenness of spread and irregularity of surface usually requires considerable external clamp force.'

So there you have it! Perfect joint, little pressure, average joint... clamp it tight.Joe Beaver38893.8881481482

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Joe Beaver
Maker of Sawdust

Thanks for the quote, JoeBeaver: that pretty well says it. In this business we always make 'perfect' joints, right? :)

One thing that clued me in was a book called 'Making Scientific Apperatus'; a sort of manual for scientists who need to understand something about the practicalities of the tools they design. In discussing glues the authors tended to refer to 'glue joint allowances': how much thickness you need to allow in the design stage for each cured glue line. In our terms; if you glue two .5mm veneers together the resulting layup will be a little more than 1mm thick, no matter how much pressure you put on the clamps. I find with AR or HHG the joints tend to run about .05mm each, and I take that into account when making up stacks of veneers for trim.

I got the 125psi number from an old Experimental Aircraft Association publication on making wooden aircraft. Those boys really want to get it right, given the consequences if they don't. One fellow actually wrote about borrowing all the C clamps he could find to lay up a 30 foot spar. He greased them all, and then used a torque wrench and a bathroom scale to calibrate them, finding the exact ratio of torque to pressure, which he wrote on the clamp, so that he'd end up with the correct uniform value.

BTW, in aircraft construction the governement figures that the clamping force spreads out from the face of the clamp at a 45 degree angle on either side. Thus, on a 1" thick piece of wood the clamp force is spread out over an area about 2" wider than the clamp face. This dictates the clamp spacing you will need: the pressure has to overlap a little. If the clamp will put out more pressure than is required for the resulting area you can use a well-fitted caul to spread it out further and use fewer clamps.    

[QUOTE=Jim_W] I've been vacuum clamping with AR glue with no problems for a long time. When I see these high pressure requiremnets for clamping I can't help but wonder what type of clamp density they have or are they really implying that you actually need 10,000 - 30,000 pounds for a 10 inch X 10 inch area?
Does any one know? Seems like you'd need a hydraulic press to perfrom glue ups.
Just curious. [/QUOTE]

That's exactly what they are saying - 10000 lbs for softwoods, and 30000 for woods like oak. (according to Patrick Spielman's book)

It may be - as others have said in this thread - that, if the fit is very good, you can get by with less pressure.

But, if you can, it makes sense to use the clamping pressure that the experts recommend. Since a 4"C clamp is good for about 2000 lbs, it would take 5 of them to give you the recommended 10000 lb clamping force for a 10" by 10" joint .

That sounds about like what I would use. Or, I might use a larger number of smaller clamps in that situation to get about the same total clamping force but distribute the load more uniformly.

Phil

All thats very interesting, thanks guys.
I wonder how much pressure go-bars put on?

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Jim Watts
http://jameswattsguitars.com

[QUOTE=Jim_W] All thats very interesting, thanks guys.
I wonder how much pressure go-bars put on?[/QUOTE]

You can easily measure this with a bathroom scale between the go deck and the go bar. Mine, which are 1/4" fiberglass and about 30" long put out about 9-15 lb of force if I remember correctly.

You also may find it interesting that as you compress the bar, the force goes up somewhat linearly, until you get to the buckling point, and then it stays rather steady even if you increase the amount of bow in the go-bar. I believe this behavior is described by Euler's buckling equations.

Sounds to me like go-bar and vacuum clampers need to follow the part about getting a perfect fit and spreading the glue evenly. There is no way you could clamp a 5/8" x-brace with go-bars without a great fit. Each x-brace is about 10 square inches of gluing area requiring 1000 lbs of force. That is around fifty go-bars and impossible with vacuum clamping.

No wonder we have brace failers...

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Joe Beaver
Maker of Sawdust

[QUOTE=Joe Beaver] Sounds to me like go-bar and vacuum clampers need to follow the part about getting a perfect fit and spreading the glue evenly. There is no way you could clamp a 5/8" x-brace with go-bars without a great fit. Each x-brace is about 10 square inches of gluing area requiring 1000 lbs of force. That is around fifty go-bars and impossible with vacuum clamping.

No wonder we have brace failers... [/QUOTE]

You're right - the very low clamping pressure could explain why people sometimes have problems with braces coming loose.

Another reason could be that people often prepare the braces curved side by sanding it in a radius bowl.

A sanded surface (especially with a coarse grit like #60 that some use) makes for a poor glue joint. The rough, sanded surface, combined with low clamping pressure might be resulting in an effectively thick glue line - even if enough glue is used. None of the glues usually used for bracing are good gap fillers.

Spruce-on-spruce glue joints should NEVER fail - it is one of the easiest, most reliable woods to glue.

Phil