Official Luthiers Forum!

Excellent points, once again.

I think a lot about making guitars that will hold up well over time. As just one example, I laminate my bridge plates (using epoxy, btw, but I don't glue them to the top with epoxy) in three layers with alternating grain direction, partly so they won't split, and partly so that they add more stiffness in the long-grain direction of the top, to make a more torque-resistant substrate for the bridge to sit on. I also use unslotted pins (and coat the bridge plate with CA, too) to prevent string ball damage.

I also think a lot about making my guitars easy to work on. For example, my bolt-on, butt-jointed necks with floating fretboard extensions make neck resets a quick and easy job. (Along with that, the body is engineered to strongly resist distortion [neck block/upper bout collapse, etc], and the heel is reinforced also, so it doesn't flex significantly.)

...The point being that I take the issues you raise seriously and very much appreciate your thoughts.

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Todd Rose
Ithaca, NY

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

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

Crystal clear and extremely helpful, indeed, Bob. Thanks a ton!

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Todd Rose
Ithaca, NY

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

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

Bob Garrish wrote:
"The short of it is that there's no such thing as a cleanly sheared surface. At the micro scale, abrasive removal is the same process as planing, but with sharper cutters and less compression of adjacent material. "

The microphotographs I've seen of stock prepared with a sharp hand plane showed clean, sheared surfaces, for the most part. Obviously there will always be some loose ends; stuff that was not supported well enough to cut cleanly, but that's not common. Power planers do tend to leave a burnished surface unless the cutters are really sharp, and they don't stay that sharp for long. Scraping leaves a more burnished surface; with the cell structures bent over and compressed. Sanding with grits coarser than about 150 left a plowed field, covered with rocks. When you get to 220 the sanding scratches get to be about the same size as the features of the wood structure, so if the paper is sharp it's essentially the same as a good plane cut once it's been cleaned up. Naturally, I probably can't scare thos pics up: they may have been in 'Fine Woodworking' many moons ago, or even possibly in the Catgut 'Journal'. All of this becomes moot if the cutter is not sharp. You may be working from a different data set, of course.

Of course... & the 'ultimate' example of a cleanly sheared surface would be sections prepared for (electron or light) microscopy.
Those microphotographs in the 'wood books' look pretty clean, and they are sheared surfaces by either glass or steel 'blades'.

I think we're on the same page, Al, we were just talking at different scales. In mechanical material removal everything ends up either shearing or compressing. Sharper cutters cause material to shear easier, and smaller cutters cut down on compression by shrinking the amount of force experienced locally by the material being cut. Wood is soft and shears nicely, but on other materials (many metals) it can make a big difference. In the end on wood it is semantics past the realization that an equivalent surface can be made by either cutting with big knives or little ones.

The idea of abrasives as cutters is actually quite new to me (last December). There's a group here at the university doing grinding research for Pratt & Whitney and their setup is a wheel (I think Invar) with a single grain from a grinding wheel mounted on it. They need to measure the results with some crazy laser measuring gear, but they model the cutting as normal chip formation (the Merchant model) even at that scale. They definitely got to see my 'extra-surprised' face when I first found out about that one!

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Todd wrote:
"I don't feel the slightest bit compelled to try to build a guitar to be hot-car-proof. Anyone who's going to spend the $$ on one of my guitars knows you can't subject them to extreme heat or dryness (or I make sure they know it!)."

Sometimes they don't quite 'get it' though. I had a person trying out one of my classicals once on a few day's loan; a nice box in BRW/Euro spruce. He decided he didn't want it, brought it back on his lunch break, and left the case on the bench. The case was hot to the touch: clearly it had been left in a car seat in an unshaded parking area all morning on a bright, warm Septmber day. Fortunately, there was no damage to the guitar (surprise!), but the strings were dead.

Ultimately, my plea is for 'standard practice', unless the new way is clearly better. I simply don't believe that a well fitted bridge properly glued with HHG or AR will be any more likely to come off than one put on with epoxy over the finish, and maybe less. Meanwhile, you've added a complication that the repair person won't be expecting when (not if, IMO) it does come up.

Yup; abrasives cut. What else would they do?

Heh, I suppose, eh?

I guess I thought they'd be modeled differently, with some form of friction/shear equation or somesuch, but surprise is my favourite emotion!

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I can't even begin to read all these posts. I do not recommend gluing on bridges with epoxy or Ca. I may not be up on the development of epoxys,but back in the 50's I glued on a few classical guitar bridges. They flew off after a year or so. I think the epoxy kept on hardening until it got brittle enough to let go,then,WHAM. One hit me in the back as I was sitting with my back to the guitar when it let go.

I think modern epoxies are better, but you do raise a good point: there's a lot of variation within the family.

WEST system has been mentioned, and I've been meaning to point out that it is specifically designed to penentrate deeply into the wood before curing. It's original use was to bond fiberglass to wooden boat hulls, and the intent of deep penetration was probably to decrease the sudden change in hardness from the 'glass layer to the wood, which would help keep the 'glass from peeling off. I'm not sure I'd want that on a top/bridge joint.

Some epoxies harden much harder and more brittle than others. Some don't set up at room temperature at all. Some contain wax or other ingredients that are there for specific reasons, but may not be what you want. I've had some epoxies that won't harden properly when exposed to air.

In general, the longer the cure time of room-temperature epoxies the harder, more durable, more water resistant, and less irritating/allergenic they are. All epoxies have a limited shelf life, and may not cure reliably once they've passed it. All of them are also temperature sensitive: a brief time in a hot store room could eat up all of the shelf life of a new batch.

Whatever brand you use, KEEP THE PART OF THE MIX YOU DIDN'T USE. If it doesn't harden properly, you've got a problem. Uncured epoxy can be cleaned off with alcohol or acetone, in general.

I hate to drag this out of the coffin but.....

Back to the old, tired argument about sanded vs planed gluing surfaces, I had a thought.

A sanded surface essentially has much more surface area than a planed one. All those grooves, however small, really add up. So even if glue forms a chemical bond, not a mechanical one, wouldn't that bond be improved if there is more surface area to bond to?

Just a thought. Really, can't we all just get along

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http://www.chassonguitars.com

I will not, I will not!!!

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Kent-
I think that microphotographs show that with a sanded surface the 'pores' in the wood are blocked more, more torn fibers lying on the surface, etc.
Otherwise, I'd agree with your point- a smoothly/cleanly 'serrated' surface would have more surface area.
Cheers
John

It's pretty much as John says. If the surface is clean (ie: the abrasive junk taken off) then a surface of equal surface energy but more area is more wettable and achieves a better bond...sometimes.

After reading up on this one more, I'm now completely convinced that a clean and finely abraded surface is superior to a sheared surface (but always with a caveat!): small-scale roughness increases wettability and makes better bonds...and large scale roughness makes worse bonds! This stuff is crazy interesting, though I'm sure the actual field work is about as exciting as watching paint (glue) dry.

The 'why' requires a warning:

<Nerd>

It has to do with the ratio between the part of the adhesive in contact with the air and the part in contact with the substrate, and it makes absolutely no sense unless you think of the adhesive (or any liquid) as having different properties at the micro and macro scales. It's like the capillary effect in both basic principal and execution: when the ratio between the surface area of a tube over the volume of the tube gets small enough, the surface tension of the liquid can pull it 'up hill' even though it makes no sense when considering its macroscopic properties. Wetting, surface tension, and surface energy are all very closely related.

</Nerd>

And...for the truly, truly nerdy:
This has some good technical info on wetting; it's from a course on metals but the mechanisms and formulae are the same across the board: http://www.ami.ac.uk/courses/topics/0122_mos/index.html

You could actually use the information on there to calculate the exact right grit to use if you knew the target thickness of your glue line, though I'm much too burned out right now to go through the motions (from helping build a race car all week...booyah!)...where's the dead tired emoticon?

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Fascinating.

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Todd Rose
Ithaca, NY

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

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

I have been gluing my bridges this way since '06. Works great. With the wetting out process, the epoxy soaks into the spruce a little, and totally penetrates through the rosewood bridge. I know this because I sometimes see little dew drops of epoxy show up in the saddle slot or on the bridge wings, as it wicks up through the pores (they get removed once cured). In other words - the top and bridge very much become one! There is no way my bridges will ever lift, unless there is some sort of integrity loss in the epoxy years down the road. Guess I'm trusting West Systems on that one. The caution is that if you have bad runout on your tops, a big ol chunk of spruce is going to come off if you get any lift - although with that much runout, you were doomed anyway. I cut my own tops, and know I have zero runout where the bridge goes, so I have no concern with this. The reason I switched to epoxy, is because when I grind the dome into my bridges to match the top (25'), I wasn't comfortable using titebond on the 80 gr surface. I razor scrape all my titebond joints just prior to gluing, and was concerned this would be difficult with the concave surface. Also, I think there are bound to be slight voids somewhere in that fairly large joint - so I wanted something with gap filling properties just in case.

I would avoid the vacuum clamping part, due to having to babysit squeeze out for an hour or so. Also, you do need to have a little bit of epoxy in there to do the job (no it doesn't dampen anything - it's pretty much glasslike when cured). West systems advises against excessive clamping, and I think the vacuum may starve the joint. A few differences from your proposal are indicated below:


1. Mask the bridge area to within 1/16" or so of the actual bridge footprint.

FRENCH POLISH, INCLUDING BRIDGE AREA

2. Send the guitar to Joe White for a top-notch polyester finish; when he's done, he removes the mask, and the finish on the top is VERY thin (I've measured it, but I forget exactly what it's been - maybe .003" or so, iirc)

FRENCH POLISH

3. Sand bottom of bridge to fit curvature of top, using 80 grit

YES, WITH BELT DRIVEN CONVEX DOME TO MATCH THE TOP

3. Position bridge and trace its outline with alcohol-soluble marker; apply either masking tape dams or pins to reposition bridge for gluing

PUT WIDE MASKING DOWN, POSITION THE BRIDGE, THEN X-ACTO AROUND THE BRIDGE. REMOVE THE INNER PART OF THE TAPE, AND YOU HAVE PROTECTION FROM THE GLUE SQUEEZE OUT

4. Remove bridge and scuff-sand the lip of finish that extends within the outline (I believe I could do that easily with a small shop-made sanding stick, much faster than I could score and remove the finish by any method); also scuff-sand the spruce

SAND FINISH OFF TO BARE WOOD W/ 80GR, UP TO THE MASKING BORDER

5. Vacuum and then wipe gluing surfaces down with alcohol to remove dust, other contaminants, and the marker
JUST VACUUM, DON'T WIPE (ALCOHOL WOULD TRASH MY F.P.)

6. Mix up some slow-setting epoxy and brush it on both the top and the bridge
WEST SYSTEMS, WET BOTH SURFACES

7. Thicken the remaining epoxy with silica powder to somewhere between mayo and peanut butter and brush it on the bridge
YES

8. Reposition bridge (using either the masking tape dams or pins applied in step 3); use hand pressure to press it on well enough to get most of the squeeze-out out; wipe away the squeeze-out
YES

9. Put my vacuum clamp on there and apply vacuum
NO - USE THE FOX STYLE BRIDGE ATTACHMENT JIG. MAKE YOUR OWN, SIMPLE CONCEPT. THIS LETS YOU WIPE AWAY SQUEEZE OUT. ONCE YOU HAVE ALL THE SQUEEZE OUT GONE, REMOVE THE TAPE AROUND THE PERIMETER
10. After maybe half a minute (just giving it time to squeeze out any more glue that's going to squeeze out), remove clamp and clean up squeeze-out, using alcohol on a swab to get it really clean
NO ALCOHOL - IT MIGHT SEEP INTO THE EPOXY AND COMPROMISE THE BOND

11. Reapply clamp
DOESN'T APPLY
12. Check back in a half hour or so; if there's any more squeeze-out, clean it up with alcohol

YES, BUT JUST WIPE WITH FRESH TISSUE. PROBABLY USE 1/3 BOX OF KLEENEX DOING THIS. WIPE WITH TECHNIQUE SO THAT YOU REMOVE TRACES OF SQUEEZE OUT, THEN GO TO A NEW TISSUE SO YOU DONT RISK SMEARING EPOXY ON YOUR FINISH.

13. Leave it clamped for a few hours, then let it sit for a day before doing anything else to it
LEAVE IT A FULL 24 HOURS. WEST SYSTEMS SAYS IT CONTINUES TO CURE FOR THREE DAYS TO FULL STRENGTH, BUT I'VE STRUNG UP AFTER 24 HOURS.

IT'S POSSIBLE TO GLUE THE JIG TO YOUR BRIDGE, OR GLUE THE SCREWS/NUTS TOGETHER ON THE FOX STYLE JIG, WAX THE THREADS FIRST, THINK THROUGH IT WELL BEFORE YOU START.


So... there you have it. A lot of messing around, yes, but I am enjoying the peace of mind of knowing my bridges won't lift. Yes - I'll be the guy crying if some odd repair comes up where I need to take one off.

Thanks, Jason. I appreciate your sharing your experience and the details of your method.

As for vacuum clamping, cleaning up squeeze out may indeed be easier with a clamp like you use, but, unless I'm mistaken, I don't think over-clamping and starving a joint happens with a vacuum clamp. The clamping force is not especially strong; it's just atmospheric pressure - 14.7 psi at sea level, less at higher altitudes. The beauty of it is in its evenness. Anyway, this is my understanding of vacuum clamping - someone please correct me if I've got it wrong.

Your point about the possibility of alcohol (used for clean up) seeping into and compromising the joint is well taken. My thinking has been that, as long as I just dampen the swab, i.e. use the alcohol sparingly, this doesn't happen, but I admit I can't say that with certainty. I wouldn't think the epoxy would want to draw the alcohol in or that the alcohol would want to work its way into the epoxy; the small amount of alcohol evaporates almost instantly; still, I am not knowledgeable enough about the chemistry/mechanics of what may happen as the alcohol contacts the edge of the epoxy in the joint to really say with certainty. I can say that, after wiping away most of the squeeze out (I get most of it off with a stick, actually, same as with other glues), I've used alcohol to clean up the last bit of epoxy on quite a few glue-ups in various situations, and I've never had a joint show any sign of problems.

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Todd Rose
Ithaca, NY

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

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

Well stated Todd Stock... Joinery rules!

This has been an interesting thread to me on the issue of epoxy in general and I learned a lot. A special thanks to the nerds who took the time to share their epoxy knowledge and experience...you know who you are.

If nothing else, it confirmed in my mind the path I chose 8 years ago in using HHG for bridge adherence. I never had a bridge release in that time and attention to the details has made the aesthetics good as well. Just another incident of trying to fix something that isn't broken. The unintended consequences of using epoxy in this application expose too much risk IMO.

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JJ
Napa, CA
http://www.DonohueGuitars.com

Todd's, of course, quite correct on the joinery. While epoxy will suffer a weaker fit than other adhesives, any of them in common use are more than adequate for every joint on a guitar given a good joint.

The stuff I wrote about applies to all adhesives, not just epoxy, so it helps across the board. Just for the record, though, I wouldn't use anything but hide, CA, and fish glue if I were putting together a guitar tomorrow. Nothing to do with mysticism: hide and CA are the fastest glues I have and I find fish is the easiest to clean up of the 'slow' glues.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Agreed. I plane my edge joints, scrape my bridges and sand my braces. Haven't had a single joint failure yet (and I'm working on #70) so I've come to believe it doesn't matter what the surface finish is as long as it's fresh, well fitted, and correctly clamped with good glue.

I just had not seen that thought about surface area in all the threads I've read about "toothed" vs flat. It always seems to boil down to mechanical vs chemical bond. I thought maybe that could help explain why both sides are so adamant and maybe find some common ground. So much for that.

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http://www.chassonguitars.com

All points well taken. I may or may not draw a different conclusion than some of you; the jury is still out in my exploration of the subject.

Once again, I greatly appreciate all the helpful input on this thread.

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Todd Rose
Ithaca, NY

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

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

Why not bolt on a bridge?? easy to adjust, easy to remove ... cover bolt heads with a removeable plug!!

I don't understand...many applications of different materials being bolted together in many places....would you really know if a bridge was bolted on and could you really detect a change in sound ?? Think of all the construction and finishing hassels that would be eliminated with a bolt on bridge... are not necks bolted to the body? Slotted bolt holes would also allow intonation adjustment problems...

There's a long history of Gibson bolted bridges, and not only plastic ones… That's what the two pearl dots cover on each side of the pins, two brass bolts. IMHO they create more problems than they solve, as the bridges tend to lift from the rear the same. Besides the fact that the Gibson copy of the belly bridge (reverse) is a bad design to start with, the pin line is too close to the rear edge of the bridge. The non plastic Gibson bridges were bolted AND glued BTW.
Not only the bolts add weight, it could be a positive effect though, but the nuts bite into the relatively soft maple bridgeplate.
The bigger problem is that if no glue is applied between the top and the bridge the spruce and bridge hardwood will probably expand/shrink at different rates with RH changes, the bridge most probably will warp one way or another and maybe both, creating unsightly gaps and possible tone sinks because of vibrations.
BTW I think gluing the bridge with epoxy is a bad idea… If you use a tough finish, CA would seem to be preferable.

EDIT:
You can't compare as the forces and materials in play are different. First the neck block and neck are usually made of the same material, presumably avoiding shrinkage/expansion at different rates. Second string tension forces the neck into the body on top of the heel. All you need really is one bolt from the middle down the neck heel. Third a total bolt on neck needs to be carefully engineered and executed, otherwise inevitably the fretboard extension will shrink, cave in because of bolt tension and create all kinds of problems, like an early neck reset, refret and so on. I know, I've seen it enough.
Strings exert sheer force on the bridge so that, in essence, it wants to lift from the rear and go meet the peghead on the other side of the neck. Only the relatively large gluing surface of the bridge prevents that from happening. And in spite of this, a lot of guitars need a bridge reglue at some point in their life. It's considered routine repair, almost maintenance. Which is why I would not use epoxy. OTOH CA on a solvent impervious finish seems much, much easier to repair.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Babicz guitars use bolted bridges, They are adjustable for intonation.

They make bolts from brass, so if what I've heard isn't malarky then that can only improve the tone. Brass is the holy grail of tone, don'cha'know? If brass bolts don't work then we could try turning some from ivory

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services