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| Go-bar trouble http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=51897 |
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| Author: | banjopicks [ Mon May 06, 2019 7:52 am ] |
| Post subject: | Go-bar trouble |
I'm finally getting back to my build I started last year. I got a little off track but I'm ready to dive back in. First thing I did I screwed up. I've never used the deck I built and the bars are plow markers. They seem a little too stiff for this unless I cut them shorter. Here's what happened. I'm working on the back brace installation and I glued the center line strip in place with HHG. When I came back to check on it I saw the strip looked like a snake. I'll either remove it or just soften the glue and try to realign it and clamp it again. I guess I should have clamped a couple of straight edges to form a channel but I didn't think it was necessary. I have another back to do and I want to avoid this happening again. Are my bars too heavy? Should I use straight edges to keep it in place? I really thought this was going to be real easy using a go-bar deck. Oh well. |
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| Author: | bcombs510 [ Mon May 06, 2019 8:03 am ] |
| Post subject: | Re: Go-bar trouble |
I use a 1/8" thick aluminum strip which is also held in place by go bars to hold the line. |
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| Author: | banjopicks [ Mon May 06, 2019 8:20 am ] |
| Post subject: | Re: Go-bar trouble |
Are these bars to stiff? Attachment: 20190504_181652.jpg
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| Author: | Bryan Bear [ Mon May 06, 2019 8:27 am ] |
| Post subject: | Re: Go-bar trouble |
Shortening the bars will not reduce their stiffness or decrease clamping pressure. For a given thickness the only way t reduce the force is to lengthen the bar by raising the deck height. For a given thickness, the force exerted by the bar does not increase much at all once it is bent past the buckling point. This seems counter intuitive but imagine holding a piece of uncooked spaghetti between your fingertips and compressing it. Now break it in half and try compressing it between your finger tips, now a fourth of the original length. As the spaghetti gets shorter you will find that it hurts your finger tips more before it buckles. I use fiberglass driveway markers for my bars and they are stiffer than the 1/4" bars that are commonly used. Instead of a deck, I use my benchtop and the ceiling which has a much longer span. I end up with about the same amount of force per bar as the typical set-up. |
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| Author: | Clay S. [ Mon May 06, 2019 8:33 am ] |
| Post subject: | Re: Go-bar trouble |
For the back graft I usually use epoxy glue. Because of the way the grain is oriented the pieces seemed to want to curl up and move around when I used water based glues. The other thing I do is wait a week or so after gluing the two back halves together before gluing on the back graft. I use water based glues to glue the back seam and I want the water to be gone before I glue the graft. |
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| Author: | bcombs510 [ Mon May 06, 2019 8:54 am ] |
| Post subject: | Re: Go-bar trouble |
Your gobars seem fine, what is the red thing on the tip? Sent from my iPhone using Tapatalk Pro |
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| Author: | banjopicks [ Mon May 06, 2019 9:00 am ] |
| Post subject: | Re: Go-bar trouble |
Just some plastic squares I use for glue removal. I used them to keep from marring the brace I used as a caul. |
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| Author: | Barry Daniels [ Mon May 06, 2019 9:55 am ] |
| Post subject: | Re: Go-bar trouble |
I have some of those orange bars which are 5/16" fiberglass driveway markers from home despot. They are too stiff for most clamping. I only use them when I need additional clamp pressure. I do most brace glue ups with 3/16" fiberglass kite spars from Into-the-Wind. I love them. Just the right amount of pressure. |
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| Author: | doncaparker [ Mon May 06, 2019 10:17 am ] |
| Post subject: | Re: Go-bar trouble |
I think everyone else has addressed your original questions, so let me give you something else to worry about (sorry about this). The lower deck of your go-bar deck looks to be prone to deflection (and therefore deformation) under load. You would be shocked by how much deformation happens to a single 3/4" thick sheet of plywood when you put 20 go-bars in place. That problem is exacerbated by the fact that you are using go-bars that are stiffer than you need. In addition to getting better go-bars, I would have a torsion box for the lower deck. The upper deck can deflect and it is no big deal. Keep in mind that this is only a problem if you want the radii of the curves you are creating to actually be what you planned. The curves will be curved no matter what; but they might be a little different from what you wanted if you let the deformation happen. |
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| Author: | banjopicks [ Mon May 06, 2019 10:29 am ] |
| Post subject: | Re: Go-bar trouble |
Thanks Don, I am aware of those problems will rectify it at some point. |
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| Author: | SnowManSnow [ Mon May 06, 2019 12:43 pm ] |
| Post subject: | Re: Go-bar trouble |
For the back strip I don’t use my deck. Instead I simply use a strip of oak that is reasonably flat over the strip... then lay 2 35 lb dumbbells along it to clamp. Sent from my iPhone using Tapatalk |
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| Author: | meddlingfool [ Mon May 06, 2019 2:05 pm ] |
| Post subject: | Re: Go-bar trouble |
I also use a straight edge. Clamped in place with go bars, then I tilt the go bars on the back strip to push the back strip into/against the straightedge... |
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| Author: | banjopicks [ Mon May 06, 2019 2:37 pm ] |
| Post subject: | Re: Go-bar trouble |
SnowManSnow wrote: For the back strip I don’t use my deck. Instead I simply use a strip of oak that is reasonably flat over the strip... then lay 2 35 lb dumbbells along it to clamp. Sent from my iPhone using Tapatalk I like that idea! |
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| Author: | Colin North [ Mon May 06, 2019 5:01 pm ] |
| Post subject: | Re: Go-bar trouble |
2/3 thicknesses masking tape each side keeps it in line and makes cleanup easy. Hold it down with gobars and a 1/4" sapelle strip faced with cork and packing tape to prevent any glue sticking. |
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| Author: | banjopicks [ Mon May 06, 2019 7:38 pm ] |
| Post subject: | Re: Go-bar trouble |
I was just double checking the plan for grain orientation and I noticed it calls for cross grain. I see this as meaning I will need to glue up some end cuts to get this. Is that right? |
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| Author: | SteveSmith [ Mon May 06, 2019 7:55 pm ] |
| Post subject: | Re: Go-bar trouble |
I usually just cut a strip off of the end of the top. |
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| Author: | banjopicks [ Tue May 07, 2019 7:32 am ] |
| Post subject: | Re: Go-bar trouble |
SteveSmith wrote: I usually just cut a strip off of the end of the top. Ah ha! Thanks for that tip Steve. |
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| Author: | Woodie G [ Tue May 07, 2019 7:59 am ] |
| Post subject: | Re: Go-bar trouble |
Keep in mind that the pressure exerted by a go-bar or spring-bar is applied along the axis of the tips...even with a bar bent so that the tips are nearly flat on the work, the line of action will be on the line between those tips. With Titebond (which has low initial tack and tends to allow components to slither about with abandon until the glue line is well-thinned), keeping the tips of the bar aligned such that there is no side load will help with keeping things in one spot. |
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| Author: | sdsollod [ Tue May 07, 2019 8:39 am ] |
| Post subject: | Re: Go-bar trouble |
Rather than a go-bar deck, I made a simple vacuum clamp. The frame is covered in plastic. There is weather stripping under the frame. I sealed my radius dishes in epoxy so the weather stripping seals well. The pump is a medical grade vacuum pump from ebay and the fittings came from Joe Woodworker. I works well and doesn't take up a lot of room. |
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| Author: | Tim L [ Tue May 07, 2019 11:38 am ] |
| Post subject: | Re: Go-bar trouble |
Are you using a radiused dish for gluing up the center strip ????? I ask because the in picture it looks like you are. With the cross grain in the strip and the removal of portions of that strip for your back braces that doesn't help trying to keep them in place much. I use a flat board with a straight edge to align the pieces for the back strip and I don't worry about leaving a small gap where my braces will go. |
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| Author: | SnowManSnow [ Tue May 07, 2019 1:08 pm ] |
| Post subject: | Re: Go-bar trouble |
banjopicks wrote: SnowManSnow wrote: For the back strip I don’t use my deck. Instead I simply use a strip of oak that is reasonably flat over the strip... then lay 2 35 lb dumbbells along it to clamp. Sent from my iPhone using Tapatalk I like that idea! A length of railroad tie also works great. Or at least a few shorter ones so it is manageable Sent from my iPhone using Tapatalk |
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| Author: | banjopicks [ Tue May 07, 2019 2:29 pm ] |
| Post subject: | Re: Go-bar trouble |
I don't have any ties or dumb bells. I'll find something. |
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| Author: | banjopicks [ Tue May 07, 2019 2:34 pm ] |
| Post subject: | Re: Go-bar trouble |
Tim L wrote: Are you using a radiused dish for gluing up the center strip ????? I ask because the in picture it looks like you are. With the cross grain in the strip and the removal of portions of that strip for your back braces that doesn't help trying to keep them in place much. I use a flat board with a straight edge to align the pieces for the back strip and I don't worry about leaving a small gap where my braces will go. Yes I was but I won't again. |
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| Author: | banjopicks [ Thu May 09, 2019 8:12 am ] |
| Post subject: | Re: Go-bar trouble |
I think I did better this morning. I used end cuts from a top I thinned too much. I also used a metal straight edge to align the pieces. After the HHG set up a little I moved the bars that were holding the straight edge onto the caul. I was going to use weights but this seems so easy. Attachment: Gluing the backstrip.jpg
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| Author: | CarlD [ Thu May 09, 2019 1:52 pm ] |
| Post subject: | Re: Go-bar trouble |
Here's some pics of my go-bar deck. Don's point about torsion box describes what it has (think box beams in house construction). They are sized to an F clamp end so the deck can be used for other clamping operations and go-bar storage. I'm using 24" kite spars, 26" spacing between decks and cork on bottom surface of top deck to keep top end of go-bar from slipping. I block the radius dish up for gluing braces and have a 4" tall platform for flat gluing operations (should have made it 3"). Tops and backs are glued onto the sides in their mold (depending on the depth of the body, the go-bars can have quite an arc to them). |
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