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Explosive Gobars - arggggg!
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Author:  Ric Hollander [ Wed Aug 15, 2007 2:24 am ]
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Help!

I’m using the 1/8” round gobars from LMI and am having a real difficult time using them. It seems that no matter how carefully I place them they are a hair away from exploding off the piece I’m gluing. My gobar deck is 2’ tall so that shouldn’t be the issue, i.e. the right amount of preassure. Just this morning I was taking the gobars off an x-brace I was gluing to a top and after removing a few of them the remaining collection starting flying off on their own. The top didn’t seem to shift a bit on the dish so I’m at a loss as to how to reliably and safely use these darn things. They made a few dings on the inside of the top that I'll need to repair.

Do the flat ones that Stewmac sells work better?
Author:  Sam Price [ Wed Aug 15, 2007 2:28 am ]
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Could be that the gobar deck roof is shifting with the pressure of all the rods? If there is significant upward pressure in one area of the gobar's top, it can weaken the pressure on another area?

To translate: Is the gobar deck top strong enough? My makeshift gobar deck has a load of marble cutoffs to weight it down enough for the upward pressure.

Just a thought.
Author:  Mike Mahar [ Wed Aug 15, 2007 2:28 am ]
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Do they have plastic tips on them? That helps alot. The other thin that I have noticed is that the top of your gobar deck should be very stiff. The combined pressure of all the gobars will cause it to lift up and cause the earlier bars to fall over.
Author:  Ric Hollander [ Wed Aug 15, 2007 2:31 am ]
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[QUOTE=Sam Price] Could be that the gobar deck roof is shifting with the pressure of all the rods? If there is significant upward pressure in one area of the gobar's top, it can weaken the pressure on another area?

To translate: Is the gobar deck top strong enough? My makeshift gobar deck has a load of marble cutoffs to weight it down enough for the upward pressure.

Just a thought. [/QUOTE]

Thanks for the reply Sam.

I have a 3/4 thick piece of plywood on the top and I bet you're right that it's bowing under the preassure. Do you think laminating another 3/4" piece would do it, i.e. total thickness would be 1.5"?

Author:  Ric Hollander [ Wed Aug 15, 2007 2:33 am ]
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[QUOTE=Mike Mahar] Do they have plastic tips on them? That helps alot. The other thin that I have noticed is that the top of your gobar deck should be very stiff. The combined pressure of all the gobars will cause it to lift up and cause the earlier bars to fall over.
[/QUOTE]

Hi Mike,

Yes, I have the plastic tips on them. It looks like the top of my deck needs to be thicker.
Author:  letseatpaste [ Wed Aug 15, 2007 2:33 am ]
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Yup, sounds like the upper deck is shifting. I didn't understand the importance of a rock solid go-bar deck the first time I made mine. I just used 1/2" allthread to connect the upper and lower decks. I redid it with 2" PVC with the ends cut on a miter saw, using the allthread to pull everything tight together. It made a world of difference. I use 3/16" fiberglass rods from Intothewind.com .

I also only used one layer of 3/4" ply at the top, so it flexes if you're using a lot of gobars. I need to add a layer, but in the meantime I just stored some heavy items up there and that did the trick.
Author:  Bill Greene [ Wed Aug 15, 2007 2:34 am ]
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Ric, that's what I had to do. I'm using 3" of glued/screwed OSB. 3/4" just wasn't enough. I've seen go-bar decks flex and ZING, bars go flying. Not a good thing.

Good luck.
Author:  letseatpaste [ Wed Aug 15, 2007 2:35 am ]
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Also, a related safety tip/reminder... Always wear eye protection when using gobars!
Author:  Ric Hollander [ Wed Aug 15, 2007 2:38 am ]
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Thanks everyone for the quick and helpful replies. I'm off to Home Depot to get some supplies so I can redo the deck and strengthen the top.
Author:  Michael Dale Payne [ Wed Aug 15, 2007 2:39 am ]
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Sound to me like you are over flexing the bars.

To achieve the max. clamping pressure you do not need a 4" deflected in the bar. In fact all you achieve by over flexing the bar is more spring loading or recoil loading not downward force. when I built my newest go-bar deck I built it with 3/4" all-thread posts, a nut and washer above and below the top plate. When I use the deck I adjust the tops height from the work piece so that I have about 1/2"-1" of deflection in my bars at clamp up. I use 3 different lengths of bars for brace clamping and even more for attaching backs and sides to the rim. But even with one length you just adjust to the longest reach.

Also the direction of the deflection in relationship to the loading of the rod is a factor in spring out.

But once again over flexing is the #1 cause of rods springing out.
Author:  Ric Hollander [ Wed Aug 15, 2007 2:39 am ]
Post subject: 
One quick point of clarification - I mistakenly said I was using 1/8" gobars when in face they are 3/16".

Thanks again.
Author:  Ric Hollander [ Wed Aug 15, 2007 2:43 am ]
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[QUOTE=MichaelP] Sound to me like you are over flexing the bars.

To achieve the max. clamping pressure you do not need a 4" deflected in the bar. In fact all you achieve by over flexing the bar is more spring loading or recoil loading not downward force. when I built my newest go-bar deck I built it with 3/4" all-thread posts, a nut and washer above and below the top plate. When I use the deck I adjust the tops height from the work piece so that I have about 1/2"-1" of deflection in my bars at clamp up. I use 3 different lengths of bars for brace clamping and even more for attaching backs and sides to the rim. But even with one length you just adjust to the longest reach.

Also the direction of the deflection in relationship to the loading of the rod is a factor in spring out.

But once again over flexing is the #1 cause of rods springing out.[/QUOTE]

I'm definately over flexing the bars if all I should have is 1/2" to 1" of deflection. Having the deck height adjustable is a great idea and also using different size gobars makes a lot of sense.
Author:  Michael Dale Payne [ Wed Aug 15, 2007 3:09 am ]
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I recommend two 3/4" plywood sheets laminated together for the top plate. you want to eliminate deflection in the top as much as possible
Author:  Colin S [ Wed Aug 15, 2007 3:17 am ]
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What all the others have said. Rock solid top, and rod flex of only about 1". My go bar set up uses 1 1/2" of plywood as the top that are fixed to very heavy 2" angle iron brackets bolted to a brick wall, no nothing moves!

I set the height of my top the thickness of my radius dish plus the depth of my guitar bodies and then reduce an inch. This means that when I glue on the top or back I have an inch flex in the rods. When bracing tops or backs I use a riser platform that the dish sits on.

I use 5mm fibreglass rods from the kite shop. Each rod gives a pressure of about 8lb so multiply that by 30 or so and you have a lot of force to contain.

This picture will show the sort of flex you need.



Colin
Author:  redennis3 [ Wed Aug 15, 2007 3:25 am ]
Post subject: 

Hi Ric, I noticed something with my go-bars, as I put them in I put the top in first and then as I put the bottom end of the rod in I twist it so it will bow the way I want it to bow. If I just stick them in any old way the center of the go-bar will swing around and try to lay the brace (or what ever I am glueing) down and jump out. Sounds kinda like what you were talking about with all the go-bars wanting to jump out.


Hope you understand what I was trying to say.


                                              Red
Author:  Howard Klepper [ Wed Aug 15, 2007 4:19 am ]
Post subject: 
Eye protection. Always when using go-bars.
Author:  Cocephus [ Wed Aug 15, 2007 4:47 am ]
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I lined the underside of my upper deck with 80 grit abrasive. Helps quite a bit.
Author:  Dave Rector [ Wed Aug 15, 2007 5:12 am ]
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[QUOTE=Cocephus] I lined the underside of my upper deck with 80 grit abrasive. Helps quite a bit.[/QUOTE]

I read somewhere that carpet works well too.
Author:  Bruce Dickey [ Wed Aug 15, 2007 5:22 am ]
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I've used 5/16" hardwood dowels all along. Something no one has addressed is length. The longer the gobar, the better. These came standard at 36" so that is what I used.

I too had to strengthen my upper deck. I did so by adding ribs across the front middle and back of 3/4 by 1 1/2 material. That really helped. I bet the second lamination of 3/4 inch material would work well too.

I don't have the rubber tips, don't seem to need them since it's wood to wood. Besides, all the braces get shaped anyway and the little marks left by gobar sticks get shaved away.

I remember these go bar sticks costing 48 cents each. Just about the cheapest clamp in the world I'd say. I love the go bar. Somewhere between Hoffman and Matsushita is where I learned to make and use it, about 1999.

Mine uses ripped two by four spruce for uprights, tops drilled for bolts which are adjustable. This is really nice, because guitars are tapered you know. I tilt the top to match the body when gluing rims to back and tops. Otherwise, I use it flat, and since I'm gluing in dishes, I have little cuts of wood sticks, left over brace stock, to bridge tone bars and make up the perfect tension.

How about a couple of pics?



Author:  jonhfry [ Wed Aug 15, 2007 5:39 am ]
Post subject: 

 


I am glad to see I am not the only one using wooden dowels. My deck doubles as a shelf under a table I built for my drill press. Also, you can get the go bar rods cheaper at a hobby shop.
Author:  Michael Dale Payne [ Wed Aug 15, 2007 5:46 am ]
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A dollar a bar if you don't mind cutting them to length at the Borg (and you get a driveway reflectors button free of charge)
Author:  Sam Price [ Wed Aug 15, 2007 6:28 am ]
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I use wooden dowels too!
Author:  Ric Hollander [ Wed Aug 15, 2007 6:39 am ]
Post subject: 
[QUOTE=ToddStock] This is an easy test...place a bathroom scale on the bottom of the deck, place a go bar between top of scale and top of deck. Add 1/4" ply under the scale until the reading maxes out...for a 3/16", 24" bar, should be 8 lbs and about 3/4" to 1" or so reduction in length and 1-1/2" deflection.

Remember, for a constant diameter, the longer the bar, the less pressure is exerted before maxing out. Think in terms of how much force it takes to bend dried spaghetti...try it with a 1/2" long piece (poke holes in fingertips before it bends) and then with a full length piece (very little pressure required).[/QUOTE]

Terrific info everyone. Thanks again!!!

I rebuilt the deck using (2) 3/4" thick pieces of plywood for the top (laminated them together) and did the same thing for the bottom. I'm using 3/8" threaded rods that are 36" long, so lots of room to expand the height of the deck.

The 24" rods seem to be deflecting correctly now but when I used a scale to check I'm getting about 7 lbs rather than 8. When a rod is sitting on an average size brace on top of the radius dish the gobar is about 1" taller than the space from the top of the brace to the top of the deck.

Should I lower the top of the deck to increase the preassure until it reaches 8 lbs or leave it as is?

Author:  JohnAbercrombie [ Wed Aug 15, 2007 6:45 am ]
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If you are interested in seeing how the theory that Todd refers to works, there's a neat little spreadsheet at
http://www.ukuleles.com/spreadsheets/RodBuckle.xls

You can play with some values and see how they affect the pressure.

To add to Todd's (correct) comments, the shorter the bars the more critical the length so you will need more different bars or a bunch of spacers under your work dish. Also, I had a 'shorter' go bar deck and it was more difficult to work 'inside', so I went to the overhead shelf system- 2x4 framing, MDF and lots of junk up there.
Chain link fence tension bars are available in fiberglass from HDepot (in some places only) and they work well in longer lengths- they're about 1/4 by 1/2 inches or so in cross-section . The ends need to be sanded a bit and dipped in epoxy or similar or you will be getting shards and splinters from them- painful. Even with the fiberglass bars on hand, I still use wood sticks for some tasks where less pressure is needed.

Cheers
John
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