Official Luthiers Forum!

Hi

I've set up a go bar deck with 6mm fibreglass rods . Each rod exerts approx 8 lbs force.

Is this ok for gluing the top and back to the sides or should I get some thinner rods ?

Thanks for your help

_________________
Richard

I'm not sure about the fiberglass rods cause I use wood, but I think most folks use 3/16" rods (5mm), but I can't see there being a problem with it.

The force exerted by the rods is dependent on how much you bend them (ie: the distance between the top of the deck and the piece being glued). So to say that there is 8 lbs of force is only good for the distance you measured. Say you have a space of 20" between the top of the deck and the piece being glued and you have a 21" go-bar, you will only flex/bend it enough to change its height 1", but if you have a 24" go-bar, you will have to flex/bend it more (creating more stored energy trying to get back to it's original shape) to change its height 4".

So depending on the distance between "anchor" points, the force of the go-bar will change.

Having said all this, I think you'll be fine as long as you don't have to bend the rods to much.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

[QUOTE=Rod True]
<snip>

The force exerted by the rods is dependent on how much you bend them
(ie: the distance between the top of the deck and the piece being glued).
<snip>
So depending on the distance between "anchor" points, the force of the
go-bar will change.

Having said all this, I think you'll be fine as long as you don't have to
bend the rods to much.[/QUOTE]

Rod, my friend, I must respectfully disagree. I've measured the force on
my fiberglass rods at different deflections and, lo and behold, found that
the force doesn't change with a change in deflection.

_________________
now known around here as Pat Foster
_________________
http://www.patfosterguitars.com

There's practically no difference between force applied if you bend a rod past about 1-1.5" shorter than its length; there was a big discussion on this, and the why of this, not all that long ago. Check it with a bathroom scale for a rough guesstimate.

For the record, I use 5mm fibreglass go-bars, work just fine for tops and backs (do use an interface caul!).

Your fine 4 rods per brace is ok force is not an issue but I like six rods so that there is a more constant clamping from one end to the other.

you can get thinner rods if you wish. They will flex a bit easier base on length. But 6mm rods will do a fine job

Hey, I don't mind being wrong. In fact, it's a very good way to learn.

I'll look into it more.

The reason I said this is that I read it in a woodworking book some time ago that changing the distance between anchor points will change to force.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Rod,

I thought it would too, until I tested after seeing some discussions IIRC
here and/or the MIMF.

Richard,

What Michael said. 6mm is fine. I started with those, but went to 5mm
(~3/16"). I found that with the smaller rods, I could use more rods
thereby distributing the force more evenly.

From the bottom up, I do this:

radius dish
dense foam pad, with 1" wide outline of body shape
top (or back) face up, aligned on the foam pad so the pad takes the force
rim in external mold
various cauls to protect body wood
rods pushing down on cauls, extra rods over neck block

Works for me, but like everything else, there's a whole bunch of ways to
do it. I'd feel more comfortable using a couple dozen cam clamps, but my
collection is too puny at this point.

_________________
now known around here as Pat Foster
_________________
http://www.patfosterguitars.com

I use the fibreglass fence rods (bars) from Home Depot, bought the longer ones and cut in half - they are about 3/16 by 3/8. $-6 per brace depending on length, and I get about 40 on a top or back with small wood cauls.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Thanks

_________________
Richard