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Hi,

I am designing/building my go-bar deck, and have been through alot of the archives for ideas.  I have the following scenario, and was hoping I could get advice/recomendation/critique on my approach.

Set-up: I have a small room that I have retro-fitted for guitar building.  I built an "L" shaped bench to work on.  On the "bottom of the L" part of the bench is where I want to build my go-bar deck.  I have 2 walls a right angle.  I want to build the deck utilizing the 2 walls, which will give me better access to the deck as I wont have dowels in the way.  I want to put the top of the deck slightly higher than I am tall (don't want to keep hitting the darn thing in the head).

By doing this, I will have a 38" clearance from workbench to deck.  I was planning on building an ~ 4" platform to work on which would make the distance ~34"

My Question: Given this 34" space, I wanted to use 36" go-bars.  What would be the best material to use assuming the go-bars will be 36" long?

Thx,

Glenn

3/16" fiberglass rods from "Intothewind.com"

Ron

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OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

"ditto"
but I get my rods at HD by buying driveway markers. However if you use 36" lengths you only get 1 rod from a drive way marker.

Thx for the feedback.

Actually, I am looking at Goodwinds, and they have very good qty discounts.  If I buy 12 1/4" x 72" rods, the cost is $1.45 (which yields 24 36" rods).  Adding .24 ea for 24 end caps, that works out to $0.965 per rod.

Todd-
Thanks for the Goodwinds tip- excellent prices. Just got off the phone with them.
BTW, they will cut (for a modest charge- about 15 cents per rod- they cut 3 at a time) the 72" FG rods in half for shipping.
This can be a factor for shipping cost- especially to Canada or other 'international' destinations.

Good price on carbon bar as well.

Cheers
John

You may wish to go to Home Depot and purchase the fiberglass strips they use for chain link fencing. I bought 48" pieces and cut them in half.

[QUOTE=PeterP] You may wish to go to Home Depot and purchase the fiberglass strips they use for chain link fencing. I bought 48" pieces and cut them in half.[/QUOTE]

Peter-
Thanks for the HomeDepot tip.
I actually have a bunch of those FG tension bars (they're about 1/4 x 1/2 or so in cross-section). With 3 foot working length they exert 'a lot' of force, and tend to 'dent' braces and such. I've got a bunch of wood strips I use for the more 'delicate' stuff right now, and hopefully the 1/4 inch FG round rods will add some more possibilities.
You must be getting a lot of force out of the HD bars if you are using them at 24"!

BTW, not all HD stores seem to carry those FG bars- I found them in Ottawa , but not in Victoria, in Canada.

Cheers
John

Walmart 36 inch hardwood dowels in 5/16 inch cost .48 and build great guitars. And you can cut two inches off if you want with no regrets. I love mine, of course I copied an old timer.

Good luck building your shop. Are you sure you want to do this?   

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Round Go-bar rods can (will) exert force unequally on braces when bent (one end may lift slightly). I'm looking into dowels with springs, so the force is purley vertical. From experience.

[QUOTE=Bruce Dickey]Good luck building your shop. Are you sure you want to do this?    [/QUOTE]

 Yeah, I am sure.  I have wanted to for a long time - Been playing for almost 30 yrs. I am sure alot of challenges ahead.... but I look forward to it.

Glenn

[QUOTE=ToddStock] ......Euler's column buckling formula says that the force required to buckle a column varies inversely with the square of the length, so if a 24" 3/16" diameter bar will apply about 7 lbs of force, a 36" 3/16" bar will apply just over 3 lbs...not that much. The formula also suggests that for the same material and length, force applied varies to the fourth power of the diameter, so going to a larger diameter rod can help balance ease of use with applied pressure....
[/QUOTE]

Minor detail...but if I remember right, I think Euler's buckling formula predicts the amount of axial force need to buckle a column when it is straight. I don't believe it predicts the tension required to hold a column in a bent position. Should that instead be derived from standard beam bending equations?

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There's a handy go-bar calculator at
Left Brain Go-Bar Spreadsheet

I found it useful.

Cheers
John

[QUOTE=ToddStock] Absolutely right - Euler is for an idealized column in pure axial loading. That said, Euler is accurate to within a few percent for a surprising number of practical applications - one of the first things I got stuck looking at as one of my senior year projects in structures at MD.

To get a better answer with a unidirectional composite rod, you'll have to get into laminate micromechanics re: S. Tsai or possibly classical energy methods to account for the differences in strength of fiber and matrix, temperature effects, fiber volume fractions, etc. I have not had to do much of that in close to 20 years, so I'm certain there are numerous additional methods that any doc track structures grad student might have handy. In any case, you'll see good agreement between Euler's buckling prediction and more accurate methods past buckling for the case of a round uni fiberglass rod.

If you have a decent load cell-based scale, it's pretty easy to demonstrate this with a deck, a selection of 1/4" masonite plates to work the deflection, and a blown afternoon.   [/QUOTE]

I'm a regular old plain jane ME and I got it. You DR's and Master's folks don't get to have all the fun. I did study Tsai in an advanced mechanics of materials class that I took as a senior. For a composite you would certainly need to know all the volume fraction info as well as the mechanical properties of the fiber and the matrix...but given that you should be able to easily derive relations for a unidirectional laminate. In the end it's all classical beam bending and resolving the reaction loads at the end into their orthogonal components.

Todd - you aren't from the Bel Air, MD area are you?

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Wrong Todd I guess...I guess MD is a big school...there are probably a few Todds doing doctoral work at MD in engineering.

Dude, that's cool stuff. I've been meaning to derive an analytical solution for soundboard stiffness (assuming zero deflection along the periphery). You don't have anything like this handy do you? I saw some stuff on the Left Brain site, but that was pretty simplistic sandwich type stuff. I'm looking for a full solution for a braced and glued top. Maybe I'll dig out some books and write a little old school FEA code or something..

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[QUOTE=ToddStock]Now that we've established ourselves as pinheaded, pocket protector-wearing geeks, it's all downhill from here, eh?[/QUOTE]

I resemble that remark!

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Steve Walden
Aspiring Builder,
Bartlesville, OK

[QUOTE=ToddStock]BH -

Applied force is always in line with the endpoints whether standard go bar, spring-bar, or other. If the ends are in vertical alignment, the force is vertical.

I can see the attraction of spring-loaded bars for helping with vertical alignment, but the standard go-bars' property of uniform load through the entire range of usable deflection (buckling to ends meeting) is something I'd be reluctant to give up.[/QUOTE]

 

I would go with rectangular Ash then, and go along the long axis of the glued up brace, rather than round fiber-glass, which is more difficult to control. I dont even really like spreading them out, one X one Y one X etc. on the X braces. The load transfer is most definetely uneven. Fiberglass is sloppy go-bars. They didnt use fiberglasses 100 years ago, they would have used (rectangular?) bendy-woods.

 

I still like spring-loaded dowel rods. But I like the idea of Ash rectangles, real go bars like 'they who came before me' used, also.

 

Blackheart says,.........dump the glass or dont buy it in the first place. No engineering degree to figure that out.

Applied force is always in line with the endpoints whether standard go bar, spring-bar, or other. If the ends are in vertical alignment, the force is vertical.

Ooops, I would have to say this is way way wrong with gobars. I cant even imagine explaining this to an undergrad in engineering. If you made BOWS, like a bow and arrow thing, THEN the forces may be vertical to the ends, as long as the cord/bowstring  is taught?? (But not ALL the force)

 Gobars have applied forces in the bend area as well, a SIDEWAYS force is also acting, the OUTWARD expansion of the bar that wants to bring the fibers back in LINE,  hence the virtue of [rectangular] to make it more controllable in one direction, or springloaded, to eliminate it altogether.

With ROUND  Fiberglass go bars, you are dealing with the pain of round endpoints and 'uneven' pressure distribution, which can be moderated by offestting to one side of the brace or the other, 'good enough' I guess, but for a trade/skill that deals with thousandths of an inch, I think fiberglass is in the wrong place.

Ok, me post picture or something. We'll go to 'visual' mode and use .ooX gauges. - Im using HHG now, maybe that was an issue. I just slapped as much on as possible for one side (id rather do both but the time issue.) Noticed that #@#@@)__@(# the dang brace was tilted to the 'left'. Had to take off and do over.

A go bar rods forces will not all be taken in the pure vertical at the ends. There are side forces at work. This is obvious just as a bow and arrow work. When the bow straitens out, it propels the arrow in the same direction as the curve righting itself. The force is the same idea for a gobar. This can cause problems with glue up. No doubt.

Im talking measureing with the gauges here. I try for .002 before glue- up. Less than that is difficult for me using 150 grit and given wood variation. 

 

If your not careful where you place gobar rods, you may end up with uneven joint/surface cohesion. At least Im pretty sure of it.

 

Glue- up is anther issue. Its common with wood working to glue BOTH surfaces. Not sure why critical strength joints in the guitar would be less critical than furniture joints. Im not an expert, so I cant tell you why, but rules is rules.

 

I disagree about side pressure with gobars. There just isn't any. The analogy of the archer's bow is not relevant here as we do not attach the end of the gobars to a string and then apply force with our arms. For a gobar to have a sideways force, there would have to be an equal and opposite force applied to the gobar, but last time I looked, nothing is pushing my gobars in that opposite direction. There is only force on each end of the gobar (from the deck and work table) that is completely vertical (one up and one down) that cancel each other out. Basic engineering statics here.

Ok Todd, thanks,

 

maybe my gobars are not "in line",and thats what causes unequal pressure. No one said you need a plumb-bob to glue up braces.

I'm still making the spring ones.  I actually spent tonight making 1/2" dowells be 15/32" so they fit smoothly into CPVC pipe.  Seems to work just fine.

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Waddy

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What kind/size of springs? Do you get them at Ace hardware?

I got them at a local Hdw company here in Charlotte, called Little Hardware.  What I liked was that they were about 5" long, small enough in diameter to fit in a CPVC 1/2" pipe, have a fairly even compression rate, that gives me a range of from 5 - 9 lbs in the last 1 1/2" of compression, with 7 being right in the middle of that,.  I posted a picture before, of my prototype, here it is again.  You can see the springs just above the prototype Go-bar.



I went out to the shop and checked to see of the package was labeled. The springs are "AJAX Springs" - Part # 54, maybe you can find them on the internet.  It took me a long time to find what I was looking for, and I couldn't order them because I couldn't figure out what would work and not work with the specs I could find on line.  I paid like $1.50 each for the first 8 or so, then I ordered 15 from them, and they gave me 3 packs of 6 and they ended up costing me about $0.75 each.  If you want to try one, I'll send you one and you can play with it to see if it will work for you.  PM me if you want me to do that.

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Waddy

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Another thing, the Charlotte Pipe CPVC doesn't work as well as Genova.  It is a little smaller ID, and doesn't stay as straight.  The difference is enough that the spring is just too tight to work.  Also, you'll have to resize the 1/2" dowel rod.  I had some cold rolled steel 1/4" thick, and drilled a 12" hole until the bit had cut almost all the way through, leaving only about a 3/16 edge around the inside.  I then drilled that out with a 15/32 bit, leaving a thin lip around the edge.  Then I filed surface where the bits cut through flat with a fine file, leaving a sharp edge on a 15/32 sized hole.  Driving the dowel through from the bottom as it was drilled, scrapes off the excess, and the 1/2" back cut below allows for clearance as the dowell comes past the sharp edge.  It takes a while to do a bunch of them, but it works.  I got some tearout, but was able to sand it out pretty well.  They don't need to be perfect, just small enought to slide freely in the pipe, and big enough to not fit inside the spring.

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Waddy

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