unclamped edge joint

[arbarnhart]

I was watching the DIY "Handmade Music" show and they were at a violin school in Chicago. They did the tops and backs in this episode, which I had recorded due to the similarities to mando carving. One thing that surprised me the most was that they edge glued the wedges unclamped using hide glue. Also claimed they got a better fit if there was an ever so slight hollow. They are cranking out los of very thin tops, so it obviously works. I am curious if any mando builders are doing it that way or if clamping is more the norm.

[sunburst]

I usually use a "rubbed joint". I know that Bill Bussman does too, and probably others.
It's so difficult to evenly clamp a glue joint in wedge-shaped pieces of wood, that I learned to do the rubbed joint. I dont use a "spring joint", (that's with the slight hollow in the center). Those are meant to keep the joint tight if the water in the glue swells the wood slightly, and causes it to curve. It would straighten out, rather than curve. I'm told it is/was? commonly used by furniture makers for long joints like those in table tops, because joint failure usually starts at the end of a joint. When you are clamping, it makes more sence to me. I like for the two halves to feel like there is a vacuum between them when I rub them together, and for no light to show through anywhere when the dry joint is held up to a strong light.

[arbarnhart]

Maybe that is the same thing they mention rubbing:

The way they do it here is with a non-clamp joint. The key is a perfectly planed joint that creates an air chamber between a book-matched pair of pieces of wood stock.

"You push the slightly hollow pieces together, with glue between them, and rub them," explains Thompson, "to create a vacuum in that hollow space that actually pulls the pieces together as the glue squeezes out."

That's from the DIY page about it (http://www.diynetwork.com/diy/hb_musical_instruments/article/0,2033,DIY_13881_3688783,00.html).

[PaulD]

I'm not sure what they mean by the vacuum in the "hollow space". Based on my reading and experience I believe you want as close to perfectly mating surfaces as possible for a rub joint... in fact jointing with a hand plane is better (in my opinion) for a rubbed joint because you don't run the risk of the scallops that can occur with a power jointer. You wouldn't ever want to create a "hollow" for a rubbed joint, as the gap would let in air and spoil the joint.

The idea for a sprung joint is that you don't have to have your clamps all the way out to the ends of the board... clamping the joint closed in the center applies pressure to the ends of the joint as well. You can get a similar result with curved cauls under your clamps; a convex caul with clamps on both ends will apply pressure at the clamping point and the center of the caul. Conversely, clamping in the center of a concave caul will apply pressure at the ends of the caul as well as the clamp point.

Paul Doubek

[sunburst]

Yes, it's the same joint, It's often called a rubbed joint.
It takes some practice, and I still get a bad joint once in a while. If that happens, I redo it.

[sunburst]

Paul, "the vacuum in the hollow space" sounds to me like a journalist's interpretation of something he/she didn't fully understand.

[PaulD]

journalist's interpretation

That was my assumption as well.

pd

[arbarnhart]

It was actually a quote from one of the luthiers that runs the school and I watched the episode and it is an accurate quote, but I thought it was an inaccurate description because the boards sure looked to be flush fit.

Can you give a little more detail on how to do it? How much rubbing? Do you hold it in place or just let it sit? How long?

Also, I assume that once I (hopefully) learn this, it will be a useful general purpose edge joining technique (like making wider blanks for scroll saw or relief carving work).

[PaulD]

I've used rubbed joints for small panel glue ups in furniture construction and staves for my wooden kaleidoscopes. How long.. how much rubbing is just a "feel" thing. For general woodworking I use Titebond or Titebond II and I rub until the joint starts to "bind up" (for lack of a better description)... I'm thinking it's about 15 to 30 seconds. When I feel it get pretty hard to rub I get the parts lined up in their final position and set them aside to dry. I've never done it with hide glue, but I would think that would set up quickly.

The reason I mentioned scalloped machine jointed surfaces is because I've had that problem. If you run the stock too fast over the jointer or if the jointer knives are not all equal height, you'll get very fine scallops to the edge. In that case, a rubbed joint will never "bind up" properly, and if you rub it and set it aside the joint is likely to fall apart because air can get into it. You can create a similar situation by running the board too slowly over the jointer and burnishing the edge. For the joint to work you shouldn't be able to see any light between the joint when the dry boards are held up to the light.

I did a Google search on "rubbed joint woodwork" and came back with several articles, none of which I agree with 100%... so maybe I don't know what I'm talking about? http://www.mandolincafe.net/iB_html/non-cgi/emoticons/rock.gif For the most part they say that rubbed joints only work with hide glue... not my experience and I've know others who use them with Aliphatic Resin and PVA glues. One page even goes as far as to state (http://www.inthewoodshop.org/2005/hideglue.shtml) "For joints where strength is not important you can skip the clamps creating a rubbed joint." I haven't done any destructive testing of rubbed vs. clamped joints, but I don't believe there is any compromise in the strength of a rubbed joint assuming the joint is properly prepared.

It will be interesting to read other's experiences.

Paul Doubek

[sunburst]

Paul, I would have said you probably can't do a rubbed joint with anything but hide glue. Thanks for filling me in that it will work with aliphatic.

My procedure:

"Shoot" the halves of the plate with the jointer plane. I want no light to show through anywhere, like Paul said. That means no light at the ends, no little spot in the middle. Furthermore, when the joint is done, you can feel the vacuum between the pieces. You can hardly rub them together because they grab.

Clamp one half in a vice, jointed edge up, and level it. I use a little torpedo level, and check that it is close to level both ways. (That really means all ways.) Gravity will be holding the other half on top, and I've had them lean to one side and spoil the joint, so I level them.

Warm the pieces for more working time with the hide glue. I work under one of those red bathroom heat lamps bulbs in an archetects lamp, and I use a heat gun too.

"Slop" some glue on the edge of the piece in the vice. I want plenty of glue, and I don't want to waste time, so I brush on plenty. I don't care if it runs down the sides of the wood, and any excess will be squeezed out.

Position the second piece on top of the one in the vice, press down firmly, but not too hard, and rub back and forth. At first, it feels like there's grease in there, because the glue is so slick, but it soon squeezes out, and you feel the vacuum of the close fit. Now I need to slide the piece to the position I want it quickly, because the glue will start to tack. As soon as it does, let go and step away. I check the glue line to see that there is squeeze-out everywhere, and if there is not, I take it apart and start over.

Leave it in the vice to dry. Sometimes, if I'm doing several, I'll wait several minutes, and carefully transfer the piece to a hand screw clamp, so that I can set it out of the way and use the vice for others, but I keep them in the same upright position for 24 hours.

Next day, I plane down the surface of the glue joint enough to see the glue line, and if it's not invisible, I rip the joint out and start over.

I agree with Paul. There is no sacrifice in strength. The strength of a glue joint comes from the fit of the parts, not the clamping preasure.

[ShaneJ]

Thanks for the great info guys! I would add that I used Titebond II and had a good "no light showing" joint on my first top last year. After getting the instrument strung up in the white, the glue joint on the top split apart. I likely carved the recurve between the tail block and bridge the same thickness (thinness) as the rest of the recurve - maybe a little thinner than it should have been. But, the glue joint split apart - not the wood. It wasn't one of those "the wood will break before the glue joint" deals. The glue joint was the weak link. After reporting my experience here, others mentioned that Titebond II was not a good glue for such important joints under stress. The original Titebond is apparently used with success - when it's fresh.

I made the rookie mistake of assuming that anything that was "II" must be better than the original.....wrong.

[Stephen Perry]

I think the "vacuum" myth comes from first a tight fit. There's certainly an attraction, but I suspect it is just friction and the air tightness that gives a dry fit resistance. Once glue is on, when the layer gets thin (the extra is squeezing out) the glue no longer flows, just grabs hard to each side. Before that, there's enough liquid to lubricate.

Just some thoughts. Does feel like a sudden suction thing.

[Chris Baird]

There certainly can be a vacuum type effect between two well mated surfaces. If two mating surfaces fit so well that all air is forced out it creates a suction cup like effect which is vacuum. The force that holds two "suctioned" bodies together is the difference between the outside and inside air pressure. You don't have to have anything "sucking" if there is a tight fit the vacuum will remain until the seal is broken. The seal on two perfectly mated surfaces could be broken quite fast but the faster you try to pull away the more you realize the vacuum force. I'm sure you all have tried to pull a well planed board off the outfeed table of a jointer and know that momentary vacuum effect. In my estimation the glue acts to increase the effectiveness of the seal and hence creates a very fine vacuum between mating surfaces. #It is commonly misunderstood that vacuum is a "sucking" or pulling force, it is not. The force that holds two vacuumed bodies together is the exterior air pressure pushing into that vacuum. You only need the sucking force to overcome leaks in the vacuum system. A well prepared joint with glue in it is probably nearly leakless.

[Chris Burt]

The next Guild of American Luthiers Journal will contain an article, with photos, that describes creating a rubbed joint.

[markishandsome]

Hmmm. You can't really have a vacuum with no air volume. There isn't lower air pressure inside the joint because there just isn't any air in there. The feeling of suction is probably more similar to the "glug" you get when you pour out a soda bottle. The soda can't come out of the bottle unless air is let in to replace it. And since there's only one hole, they have to take turns! If a joint is well planed there's no hole for the air to get in through, until you pull one open.

[Chris Baird]

The glue will act as a seal which will maintain its integrity slightly beyond the point of the parts coming apart, which means that if one trys to pull the parts apart you will come against the vacuum effect until the seal is broken. #
#The above stated coke bottle glug effect is also associated with vacuum and air differential pressure. The reason the glug occurs is because, as the liquid exits, a vacuum is formed inside the bottle which causes air to come in until that vacuum is equalized at which point more liquid will come out until the force of gravity again becomes less than the force of the vacuum and again you have another glug.

[arbarnhart]

Hmmm. #You can't really have a vacuum with no air volume. #There isn't lower air pressure inside the joint because there just isn't any air in there. #The feeling of suction is probably more similar to the "glug" you get when you pour out a soda bottle. #The soda can't come out of the bottle unless air is let in to replace it. #And since there's only one hole, they have to take turns! #If a joint is well planed there's no hole for the air to get in through, until you pull one open.
You are partially correct; you can't have a vacuum without volume, but you can certainly have one without air (that is sort of the point of a pure vacuum). When you try to move two surfaces away from each other, there is only air at the edges. The surfaces toward the center will offer more resistance for that brief instant before air from a side gets there. I guess it's more of a dynamic vacuum than a static one if that makes any sense.

OTOH, I am sometimes warned that I am compromising my vacuum when I yawn, so I amy be full of well, nothing, I guess...

http://www.mandolincafe.net/iB_html/non-cgi/emoticons/biggrin.gif

[Chris Baird]

I see it somewhat like a ziplock bag that had all the air sucked out and then zealed up. There would be no internal volume and no air, but try as you might it would be very difficult to create an open volume inside that bag by pulling on the sides of it. That is until the seal is broken and then it is very easy to create space between the walls of the bag.
I believe the glue acts as a somewhat flexible seal. In a vacuum system the only lateral stability you have comes from the friction created between the mating surfaces as a result of atmospheric pressure. When you first begin to rub the joint is slips around quite easily because there is a lot of glue still in the joint which acts as a lubricant. As you rub them further the glue is squeezed out and they begin to "grab" as the friction increases. The friction, however, is only lateral; the force that holds a rubbed joint together, if you turn it on end and only hold one side, is vacuum (and likely the intial "tacking" of the glue).

[sunburst]

I think so too, Chris.
I think I can feel the difference between the "vacuum" or "suction" effect, and the glue starting to tack.
If I resurface a Japanese water stone with my diamond 'stone' by rubbing the two together in water, sometimes they'll stick together like two suction cups. There's no glue in there to tack, just water.
That's what it feels like when the glue squeezes out of the joint. When the glue grabs, it grabs, and if the parts aren't where I want them, too bad. Time to start over.

[Chris Baird]

Yes, I have another similiar example that many will have experienced. The sticking of a plane blade to the waterstone when one is trying to polish the large flat side. The water creates a seal and atmospheric pressure pushes the blade against the stone at 7-8 pounds per square inch. If you don't have enough water underneath the stone then the friction quickly gets to the point where you can't move it and it is vacuum sealed to the stone. You either have to overcome the 7-8 pounds per square inch by levering it off or overcome the friction by sliding it off.

[PaulD]

Good discussion... Chris sums up my impression of a rubbed joint (or even a well fitted clamped joint) in that last sentence (edit... last sentence a couple posts back); it works due to a combination of vacuum and the glue tack over a great surface area. If the joint is not properly fitted (i.e. jointer scalloping) you are losing the advantages of a sealed vacuum and losing some of the tack due to liquid glue in the gaps between the boards.

As far as analogies, the one that comes to mind for me having done quite a bit of glass work, is trying to separate two clean panes of glass. Unless you can get the air started between the two, they feel as if they are glued together. If they are wet that effect is enhanced; the water prevents air from getting between the panes and they are very hard to pull apart. The water has no "tack" or friction properties and acts as a lubricant if you slide the two panes in the same plane. If you get your air compressor and inject air between the two they easily separate.

This is certainly overanalyzing the mechanics of a rubbed joint, but I've always found it interesting to understand why things work instead of just trusting that they do if certain steps are followed. I've always tried to emphasize to my kids that understanding a "system" is the first key to troubleshooting, rather than rote memorization of solutions to problems. In my profession (computer networking) it seems that too many support folks want pat answers on how to fix a problem rather than understanding the underlying system.

On a different note, I wanted to respond to SJennings comments about Titebond II. I mentioned it in the context of building furniture. There was a time when I would have considered it acceptable for instrument building, but relating my readings to my experiences I know that it has few desirable properties for luthery and a number of drawbacks. That said, you should still be able to use Titebond or something like LMII's instrument glue in a rubbed joint. I think aliphatic resin glues (generally yellow) tend to build up a tack that assists in creating a rubbed joint better than white glues; but that's conjecture on my part and also a generalization since some "white" glues are chemically equivalent to "yellow" glues.

Paul Doubek

[sunburst]

...I've always found it interesting to understand why things work instead of just trusting that they do if certain steps are followed.
I couldn't agree more with that statement, though I know that everyone's brain works differently, and most people don't really want to know what happens in their car's transmission when they move that lever.

To that end, then, here's the way a glue joint was explained to me many years ago by a furniture maker that I worked for for a short time.

The way glue works is through molecular attraction, or adhesion. In most cases, the adhesion between the glue molecules and the surfaces to be glued is much stronger than the cohesion between the glue molecules themselves. For that reason, a thin glue line is better, because there is more adhesion (stronger) than cohesion. A lot of glue in the glue line, and the joint is no stronger than the dried glue itself.

Now imagine two perfect pieces of glass. OK, that's impossible, but imagine two surfaces flat down to the molecular level. If it were possible to have a one-molecule-thick layer of almost anything in between those two pieces of perfect glass, it would be a virtually perfect glue joint. If it was water in there, at one molecule thick, it would hold tight. The water wouldn't act as a liquid, solid, or gas because each molecule would be adhered to the glass on both sides.
The reason glues have to harden is because it's impossible to get that one molecule thick layer, so they have to form a solid with enough cohesive strength to hold up.

Here's a fun "experiment" that I tried once, and I think is a good learning experience;

Prepare two pieces of wood for a glue joint. Put water in the joint, clamp, and put the whole thing in the freezer. When it's well frozen, unclamp the joint and try to break it. When I tried it, the joint held and the wood broke, so that means water is "stronger than the wood itself".

I don't have to tell anyone what will happen to the joint when the temperature reaches the melting point of water, but a lot of people don't seem to understand that glues that are touted as being "stronger than the wood itself" have melting points of their own, and some can be reached in the car trunk in the summer.

[markishandsome]

I agree that there is some resistive force in a rubbed joint very similar to Paul D's glass example. But it isn't technically a vacuum. I'm not sure what to call it.

There is no such thing as a pure vacuum. Even outer space has some gas in it. The molecules in air are really small and even a joint with no light shining through it has air in it with the same density and pressure as the atmoshere.

[ShaneJ]

PaulD, your comments on the glues match up with others I've read here. There seems to be quite a difference between Titebond and Titebond II, especially for luthiery applications. Titebond II most likely is better than Titebond for other applications. Glueing up a spruce top doesn't seem to be the best use for Titebond II though. http://www.mandolincafe.net/iB_html/non-cgi/emoticons/smile.gif

[billt]

Where is Richard Feynman when we need him...or a physics buff, at the least? #I suspect that John is onto the real "effect." #

During my school days many years ago we were told about experiments in space; the perfect vacuum. #If two "perfectly" flat pieces of steel were rubbed together, they would create an ideal weld (no heat). #This was because of "molecular attraction", or magnetic attraction at the cellular level...same thing. There were no oxide layers to interfere and the molecular closeness made the bond possible.

[oldwave maker]

If you're using fresh hot hide glue for centerjoints its almost impossible to apply clamps before the glue sets up, I use a stanley #7 plane with a ron hock blade and a clifton-sheffield capiron, clamped to the workbench and a shooting board to slide the wood across, keep working the edges till no light can be seen thru the joint with very little pressure holding them together, cook up fresh hide glue, clamp 1 half of the bookmatched plate edge up in a legiron vise, hit both edges with a heatgun, slather the glue on 1 edge, rub lengthwise an inch or so , then back to position, leave it overnight before planing to check the joint. When you have very straightgrained engelmann spruce with minimal runout and fresh glue from JR music, its sometimes hard to see where the centerjoint is. time consuming to get the perfect planed joint with some of the curvy character maple in my pile. a good isometric workout if you're doing a couple dozen plates at a time. I know theres a centerjoint around here somewhere....

[arbarnhart]

I agree that there is some resistive force in a rubbed joint very similar to Paul D's glass example. #But it isn't technically a vacuum. #I'm not sure what to call it. #

There is no such thing as a pure vacuum. #Even outer space has some gas in it. #The molecules in air are really small and even a joint with no light shining through it has air in it with the same density and pressure as the atmoshere.
I think there is something to that old adage that nature abhors a vacuum. Maybe the resistance is resistance to forming a vacuum rather than negative pressure from a formed vacuum.

Yes, there is a tiny bit of air in there. But the passageways it came through aren't big enough for the rush of air that needs to come in to fill the volume created when you pull the surfaces apart (or try to) without a mismatch in pressure (a partial vacuum) occuring temporarily.

But this is all just intellectual flatulence on my part (and maybe it's not even intellectual) because it doesn't really matter why it works. It does and I am planning to start edge joining this way.

[Chris Baird]

It is true that there is no vacuum inside the joint. That is, until a force of somekind acts to try to pull it apart and break the seal; at which point the separating force must first create a vacuum before the seal is broken. The force holding the parts together is atmospheric pressure.
#I use vacuum jigs daily in my shop. The jigs are designed such that there is a seal which encompasses the part. With in the seal are a few lines to spread the vacuum evenly inside the sealed area. As the part is drawn down and the seal is made the part touches the inside of the jig and the only vacuum present is within the few lines which act to help it spread (which is relatively small compared to the total surface area of the jig). However, due to the fact that the seals are slightly compressable/flexible the jig has the holding force of the entire surface area enclosed within the seal and is not constrained to the surface area of the small lines of true vacuum. #The total vacuum force of the jig is enacted instantly as a force begins to lift it up. It works out well because one doesn't need a holding force unless there is a seperating force. #Likewise, I see the rubbed joint the same. The joint has no vacuum until some force acts to seperate the parts and break the liquid seal and then the vacuum is created, if the integrity of the seal holds then you would effectively have an equal but opposite (to the seperating force) clamping force of up to 7-8 pounds per square inch.