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I don't like introducing that much flex. It doesn't add clamping pressure but definitely makes it more difficult to maneuver the rods. Instead of using different rod lengths, I raise or lower the height of the bottom deck.

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George :-)

My old method had me using different rod lengths, but for my new deck I just made a simple riser for the bottom dish.

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You might look at Goodwind Kites for rods and tips - the same 1/4" driveway markers that run about $2.50 per plus tips (another $0.61) can be had from Goodwind at $1.69 in quantity of at least 48 plus $0.20 for two caps, so $3.11 per rod from Home Depot versus $1.89 per rod from Goodwind. Less expensive that Into The Wind as well. For 3/16" bars that work best at about 24" length, a set of 100 bars will run about $.95 each, or about $110 delivered for a set of 100 bars with tips.

For a 1/4" diameter rod, the correct length is about 42" for a 7-8 pound force at moderate deflection. Although the theory is that once critical buckling pressure is reached, the applied force is constant, that is only true for uniform materials - fiberglass go-bars are not uniform because they are made with fibers running in one direction, so expect to see an increase in force as the rod is bent to greater deflection. We see about a 33 percent increase in applied force between an inch of deflection and six inches with 3/16" x 24" bars. A 1/4" bar should show at least that much change in clamping pressure, so we use a 4" platform and 18mm MDF spacers to adjust the height of the dish to give uniform deflection and clamping pressure.

We use many more rods in an average glue up than would seem necessary, because the force applied to a brace by a go-bar is concentrated in a 90 degree wide cone from where the tip of the bar contacts the brace. This means that spacing between bars should not exceed twice the height of the brace if uniform glue line pressure is desired. For a 00-size guitar, this would mean that the 1/2" tall x-brace would take about 18-20 bars and the 1/4" to 5/16" high fingers would need about 8 to 10 bars to get uniform pressure.

When closing a box, we use a 1/4"/6mm MDF caul to spread clamping force more uniformly; for a guitar with 60 inches of perimeter, that can mean 120 go bars, each applying about 6 pounds of force over a 25 square inch area for a glue line pressure of about 30 psi...which is in the range of recommended glue line pressure for a range of glues in well fitted joints.

That 720 pounds of clamping force will need a stout go-bar deck, which is why a 50-60 pound air cleaner may not be heavy and rigid enough to keep more than a dozen or so bars bent to the critical buckling load. A doubled sheet of plywood fixed to the ceiling directly might be a better bet for the job, leaving room for the air cleaner to be suspended from the ceiling independent of the deck.

Another consideration might be vibration - if using Titebond with go-bars, initial tack for that adhesive is very poor, and is more of a lubricant than glue until the glue line thickness is reduced to a few thousandths, so even a token side load generated by poor vertical alignment of the bar tips can be exacerbated by the sort of vibration seen from an air cleaner (we have a large unit hung near the table saw)

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A constellation only takes shape when one maps the whole.
- Beth Brower

Thanks, Woodie. Great point about doubling the 3/4 plywood. I may do that. My intention was not to use the air cleaner as the surface for the go bars, but to use it as a proxy in the picture. Hopefully you saw my later picture. What I ended up doing was fixing a 3/4" plate to the ceiling and drilling positioning holes every 1". I do think that doubling it up may be in order.

Regarding the price, I used the fiberglass rods from HD that were 1.99 each. They do not have plastic tips on both ends. The end opposite the tip is pointed. So my thought was to make the board with the positioning holes to let the point rest in to reduce the possibility of slipping.

I'll do some testing this weekend with a scale to understand the force being applied and tweak as needed.

Thanks!
Brad

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"For a 00-size guitar, this would mean that the 1/2" tall x-brace would take about 18-20 bars and the 1/4" to 5/16" high fingers would need about 8 to 10 bars to get uniform pressure. "

Woodie, do I really need 80 go bars for the four back braces? I used 5 on each brace. I use a uniform and fairly light coat of LMI glue, and get squeeze out on both sides of the brace. Not much, but pretty uniform. Visually the braces are dead tight against the back plates. It doesn't seem likely the joint will fail. I tries "rocking" the dried brace and can't perceive any movement. Do I run a risk of failure somewhere down the line or are you only referring to tonal aspects?

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Brad Hall
Mystic Dawg Guitars

This has been my rig for the last 12 years. One 3/4 Melamine board. Two would have been better as it deforms a little with a forest of go bars altering the tension of some. Not enough that I've gone to the trouble to beef it up.

Currently using 1/4" kite rods for lighter stuff like closing the box, center strips, and smaller braces. 5/16 hardware store driveway markers for the rest.

Using rubber thread protectors on each end has surprisingly worked well as to any slippage against the melamine.

I would also be a little concerned about movement of your base. Just a little twist and you could have go bars flying everywhere just as the hide glue is gelling.

I probably use around 12 rods on an X brace and 6-8 on a back brace and it seems to be OK but I hate to argue with an engineer.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Nice shop you have there Terrance. What are the cylindrical cauls you have on the business end of the bars? Cork on the bottom side?

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Brad Hall
Mystic Dawg Guitars

Hmm ...

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George :-)

Hmm...

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Pat

Harambe!!


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When I first started I bought a bunch of spool clamps I wound up not using later on. They are the sliding part of the clamp. If I didn't have them I would have made a more traditional caul.
Yep they have cork on the bottom.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I made some hardwood cauls to keep the rods from slipping off of the braces.



Alex



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"Indecision is the key to flexibility" .... Bumper sticker

Gosh.... I think I'll just stick to using cam clamps

Old school is sometimes the best school.


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I assume the meme above is related to the use of Gorilla Glue for bracing, rather than a self-portrait of Mr. Macaluso pondering his next top bracing glue-up. If I am incorrect in this assumption, let me instead complement him on his rather striking facial hair and distinctive visage.

Just food for thought...

- Franklin suggests 100-150 psi glue line pressure for softwoods and 150-300 psi for hardwoods when using Titebond or their polyurethane glues; they have tested oak to 400 psi, and state that the compressive strength of the wood may be the limiting factor on clamping pressure. Franklin's techs have also said that starving a joint is unlikely with what can be accomplished with 'normal' clamping schemes ...although I have no idea how that translates to our luthiery practices.

- Martin achieves a uniform 13.5 psi or so glue line pressure in their vacuum bagging process, but in so doing, applies over 3000 pounds of pressure on the top to make it conform to the radius dish.

- Rubbed joints have minimal glue line pressure, but still work well for well fitted joints

And reference go-bar clamping, we shoot for about 30 psi, use hot hide glue in nearly all structural and stressed joints, and try to achieve the thinnest, most consistent glue lines possible by using more clamps distributed evenly along the brace.

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A constellation only takes shape when one maps the whole.
- Beth Brower

Ok, here is v2. I doubled up the 3/4 plate and reinforced with 2x4 material. (thanks, Woodie). I then attached the box to the ceiling on sleepers.

I swapped out the mobile cart for a foldable workstation (thanks, Don). The idea here is that when not used as the go-bar base I can use it for other things or fold it up and out of the way.

Then, I made a bottom 3/4" plate with 1/2" black pipe fixed to the corners. I put the locking end of pipe clamps on each corner. These serve as the holder for the plate that the radius dish is glued to. This will let me change the height without a lot of hassle and still is easy to slide under a bench.

I'll keep tweaking but feedback is appreciated. I'm wondering about the foldable station holding the weight. It's stable and I was able to give it a good jostling and all the gobars stayed in place. Safety glasses and rock climbing helmet on of course.

I met the first two goals:

1) 360 degree access to the work.
2) Disassembled it is easy to store out of the way.

I don't like needing the adjustable base but it is what it is.










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I'm liking that a lot. Nice job!

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It's not what you don't know that hurts you, it's what you do know that's wrong.