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Using a GoBar deck to glue tops and backs to the sides.
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=20223		 Page 1 of 2
Author:  SteveSmith [ Mon Dec 29, 2008 12:29 pm ]
Post subject:  Using a GoBar deck to glue tops and backs to the sides.
I have 23 of the 3/16" x 48" fiberglass rods on order that I will cut to length. When I glued my top bracing I used about 25 or so of my hardwood rods.

I really liked using the GoBar deck for the braces and would like to use it for glueing the top and back. Based on comments from a previous thread I should limit the deflection to <1" so shorter rods will be needed. About how many of the shorter rods are needed to glue the tops or backs?

Using HHG I found that everything worked much better if I preheated the wood with a heat gun before glueing. Would this technique provide adequate open time for using HHG to glue on the top or back?

Also, would anyone care to share their procedure for glueing the tops and backs. Do you glue the back first? Then the top? How do you protect the top from the GoBar ends or do you put the top in a radius dish? Do you use registration pins or are the cutouts for the braces adequate for the task?

Sorry for all the questions but these are just some that have been bouncing around in my head.
Thanks!
Author:  Michael Dale Payne [ Mon Dec 29, 2008 12:46 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
On OM-Dreadnaughts size I use 14 per side so that is 28. back before i rebuilt my deck to adjust between 24 and 42" I used 6 20-1/2", 6 20" and the rest 19-1/2" long. I always ued my radisus dish as back up with a piece of mylar over the sand paper. Depending on the body depth some of the rods flexed a bit more or a bit less than 1". Once i moved to my new adjustable height deck I use all 24" rods as I fan tilt the upper dect as much as an inch one end to the other so I no longer need short rods.

But if your deck clearance is fixrd then you will need varing lengths to avoid adding too much spring loading in the rods. 1/2" deflection is all tht is needed and in my opinion more than 1-1/2" is too much due to the risk spring out.
Author:  SteveSmith [ Mon Dec 29, 2008 12:53 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Michael - Do you use the GoBars directly on the top and back surfaces?
Author:  Michael Dale Payne [ Mon Dec 29, 2008 12:55 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
No!!!! I use a hard board cut out 1" wide the shape of the body as a clamping caul to protect the top and or back. You want to be sure to clamp directly over the lining.
Author:  SteveSmith [ Mon Dec 29, 2008 1:01 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
I've got some 1/8" ply that should work well for that. Thanks!
Author:  Daniel Minard [ Mon Dec 29, 2008 7:31 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
I tried the plywood edge guard & slipped. Nice big divot in the top. It steamed out nicely but....
Now I use a piece of foam core (cheap from wal-mart) 1/2" bigger than the guitar top, with short pieces of wood taped on around the perimeter. No more worries about slipping go-bars.
Author:  SteveSmith [ Mon Dec 29, 2008 8:06 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Daniel, are you talking about the stuff that's about 1/4" thick that people use for signs and such?
Author:  Todd Rose [ Tue Dec 30, 2008 11:38 am ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
I use my go bar deck to glue on tops and backs, and I do it "upside down". I glue the back on first. With the rim back-side-up, I apply the glue, position the back on the rim and tape it down in a couple spots to hold it in place. Then I flip it over so the back is down, sitting in the radiused dish (the dish has wax paper in it, though I never have enough squeeze out that that would really be necessary). Then I put the bars in place, directly onto the linings (no additional caul) on the top side of the rim. I also put one bar right in the center of the back (before all the others, actually), to help force the back into the intended radius.

The top is done pretty much the same way. This time I am putting the go bars directly onto the back, over the linings (again, no caul). Also, this time I cannot put a bar in the center, because the back is in the way. Someday I'll get around to making a radius dish with a vacuum thingie in the middle so I can suck the top down into the radius dish while gluing it on.
Author:  Mike OMelia [ Tue Dec 30, 2008 12:12 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Just curious, and I am sure I could sort this out... But how do any of you insure the top of the deck does not warp? I only used one piece of 3/4 MDF. I should have known better. Bracing? More MDF (ouch that gets heavy!), or other material?

Mike
Author:  Michael Dale Payne [ Tue Dec 30, 2008 12:24 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Mike O'Melia wrote:
Just curious, and I am sure I could sort this out... But how do any of you insure the top of the deck does not warp? I only used one piece of 3/4 MDF. I should have known better. Bracing? More MDF (ouch that gets heavy!), or other material?

Mike


I assume you are speaking of the top deck of your bo-bar deck. First a small bit or warp is not a big deal but I prefer to use birch ply not MDF. Ply is stiffer and resists the tension better but i also used 2 laminated sheets of 1/2" birch ply.
Author:  Mike OMelia [ Tue Dec 30, 2008 12:45 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Oh, I get a little bit more than a little flex!

I have go-bars falling down while installing others... then after all done, some will fall again.

I will swap out the MDF for two birch sheets later. For now, I will just brace it.

Thanks Mike,

Mike
Author:  Zach Ehley [ Tue Dec 30, 2008 2:01 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
I made my original deck similar to the ones from Stewmac, with threaded rod. I used nuts and washers on both sides of the top plate (2x3/4 ply – no flex). I beefed it up a bit and used 5/8” threaded rod instead of ½”. The nuts on top and below allow you to adjust the height of the deck. This is useful when gluing on the top and back. You can adjust the top to accommodate both the increased height as well as the angle due to the slope of the back. This way you can fine tune the force with what you are clamping.

In the new shop, still in progress, I made two permanently installed decks inverted from the ceiling, still using the 5/8 threaded rod. This allows for the same height and angle adjustment as well as leaving the entire work area free from the threaded rods. Those rods always seem to be in the way when cleaning squeeze out. [headinwall]
Author:  Michael Dale Payne [ Tue Dec 30, 2008 2:49 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Zach Ehley wrote:
This way you can fine tune the force with what you are clamping.

[headinwall]


This is not true. You can not add more clamping force by deflecting the rods more. Once the column starts deflecting it has reached its max load force all other added deflection is only potential spring force. It does not add to load exerted by the go-bar in the vertical plane at all but becomes just stored energy looking for a path to escape. It is dangerous to think of additional deflection in the go-bar as adding additional load or clamping force. It aids nothing to the clamping but potential danger. We need to add some deflection to our go-bars to insure that the maintain a constant load as we add more go-bars to a clamping project but by doing so we add no more clamping force than was available the instant the rod started deflecting.
Author:  Mike OMelia [ Tue Dec 30, 2008 3:25 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Uh, here we go again. ;) I think I have to respectfully disagree with Michael. If one considers a rod with opposing forces at both ends (go-bar deck for example), one can reason that a small amount of force will cause a small deflection in the rod and a larger application of force will cause a larger deflection. The main thing is the spring constant which says that F=kx where k is the spring constant in force units / length units and x is the compressional displacement. Straight rods can be thought of as springs. Replace the straight rod with a spring (a rod twisted into a helix). The more you compress the spring, the more force gets applied (opposing force).

Michael is correct on one thing, there is danger in overflexing the rods. Potential energy increases with deflection.

reference material: http://solidmechanics.org/text/Chapter1 ... er10_4.htm (see section 10.4.3)

Mike (duck and cover!!) pfft

(then again, I may be looking at this problem incorrectly)
Author:  Michael Dale Payne [ Tue Dec 30, 2008 4:02 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
You are certainly welcome to disagree. But it is a fact that the column load will not exceed the critical buckling load. If the material is elastic enough deform without stress fracturing then all added loading goes into deforming the column. If the load is constrained between two stationary points and the axial load is constant the then any addition force above the critical buckling load as defined by Euler’s formula is held as potential spring load as long as the additional load does not exceed the elastic stability limit of the columns material.

If we were dealing with helix wound coils then we have a different scenario where the coil has a compression loading capability as well as the axial column loading . At least till the maximum compression of the coil reached, and then becomes a simple column. Go-bars or at lease simple one piece go bars are columns not helical springs. In the way we use them on a go bar deck they are simple columns that we force beyond their critical buckling load. They in no way resemble in mass or configuration helical coils.

You are right that the potential energy is being generated. But it is constrained and will not become kinetic unless the constraint of the column is released or if the spring load becomes enough to over come the friction of the axial load, but while restrained it does not ad to the axial load.


I don’t know what else to say. See Euler’s forum
Author:  Mike OMelia [ Tue Dec 30, 2008 4:32 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
I stand corrected. I was wrong. This is one place where intuition fails. [uncle]

Mike
Author:  Hesh [ Tue Dec 30, 2008 4:40 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
OK not to be Oprah and spend too much time on the bathroom scale but I put this together just now for you guys. I did this a while back too but could not find the thread:

Standard LMI go-bar with 1" deflection:

Attachment:
DSCN3037.jpg


And she weighs in at....:

Attachment:
DSCN3036.jpg


Same go-bar - 2" deflection:

Attachment:
DSCN3033.jpg


And the results...:

Attachment:
DSCN3032.jpg


Finally the same go-bar with 4" deflection:

Attachment:
DSCN3035.jpg


And the reading....:

Attachment:
DSCN3034.jpg
Author:  Michael Dale Payne [ Tue Dec 30, 2008 4:48 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Mike O'Melia wrote:
I stand corrected. I was wrong. This is one place where intuition fails. [uncle]

Mike


Fear not I made that same intuitive assumption many years ago
Author:  Zach Ehley [ Tue Dec 30, 2008 6:00 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
I guess Heshs experiment debunked the non-constant force assumption. Thats no my main reason for adjusting the desk height. I still like to have all the bars deflecting the same distance all the way around the top/back. It also keeps the force perpendicular to the top/back surface.
Author:  Hesh [ Tue Dec 30, 2008 6:38 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Don't feel bad guys I thought the same thing you were thinking until I got out the scale and tried this a couple of years ago.

Also, while we are on the subject it's not a bad idea to know exactly how much clamping force what ever you use as go-bars puts out. I also have the Stew-Mac fiberglass go-bars and they put out around 17 - 21 lbs of clamping pressure.
Author:  Mike OMelia [ Wed Dec 31, 2008 1:42 am ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Hesh, those are some wussy bars. Mine weighed in at 23.4 pounds regardless of the deflection. Yes, I did the same test.

I bet there are more than a few OLFers scratching their heads right now.

Michael, I am never afraid to admit I am wrong. Since I am married, I do that a LOT. BTW, still waiting for the final conclusion on the refractive properties of wood. ;) beehive

Mike
Author:  Michael Dale Payne [ Wed Dec 31, 2008 9:58 am ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Well if a member 2 years or more they have read this same argument at least 10 times. Because of the intuition factor of thinking of the bars as springs there is always someone that feels they can prove the bars are exerting more force the more they are bent. It is a naturally intuitive assumption unless you have column engineering experience of some sort. I am sure we will have this discussion a time or two more this coming year.

I hope you are not clamping with 23.4 lbs of load on your tops/backs or braces. 6-8 lbs is all that is needed much more runs the risk of glue starving the joint.
Author:  Hesh [ Wed Dec 31, 2008 10:02 am ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Michael Dale Payne wrote:

I hope you are not clamping with 23.4 lbs of load on your tops/backs or braces. 6-8 lbs is all that is needed much more runs the risk of glue starving the joint.


Maybe Mike could just raise the roof on his go-bar deck and not bend the go-bars as much? gaah laughing6-hehe :D
Author:  Barry Daniels [ Wed Dec 31, 2008 11:11 am ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
No go bar is going to caused a glue starved joint. They have no where near the pressure that manufacturers recommend for clamping.

For example, Franklin recommends 175 to 250 psi clamping pressure for hardwoods using Original Titebond. It would not be possible to get that many go-bars in one square inch, so this should not be a cause for concern.
Author:  Mike OMelia [ Wed Dec 31, 2008 1:57 pm ]
Post subject:  Re: Using a GoBar deck to glue tops and backs to the sides.
Michael, just curious, what boundry condition do you believe applies here? Hinged-hinged? (pinned-pinned)

Mike
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