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bridge clamping caul
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=42571		 Page 1 of 1
Author:  Ken Mitchell [ Tue Jan 28, 2014 5:00 pm ]
Post subject:  bridge clamping caul
I have a materials question that maybe someone can give me some direction on.

I'm making my bridges with a good deal of contouring these days, and I would like to make a custom clamping caul that fits the center section of the bridge more accurately, to apply even pressure front-to-back, and side to side. I'm thinking that I could make a caul by putting some sort of epoxy putty material in a small frame, and then push the bridge into it, upside down, to give the impression of the top surface of the bridge. I could then use the Stew-Mac aluminum clamping caul, with the adjustable wings, and get an even distribution of clamping pressure.

Does anyone know of what I might imagine to be 'epoxy putty', something that would be maleable and take an impression until it sets up, and then resilient enough that it won't break during clamping?

TIA for any suggestions,
Ken
Author:  Clay S. [ Tue Jan 28, 2014 5:16 pm ]
Post subject:  Re: bridge clamping caul
Bondo should work. Put a piece of plastic wrap between it and the bridge to keep it from sticking. if you find the bondo doesn't hold up to clamping pressure (it should) you could use "Tiger Hair" - fiberglass reinforced body filler.
Author:  TimAllen [ Tue Jan 28, 2014 8:25 pm ]
Post subject:  Re: bridge clamping caul
I'm sure Bondo would work. I've never worked with it, so if I were to do this I'd use-- epoxy putty. You can buy it at most places that sell glue. For more info see http://en.wikipedia.org/wiki/Epoxy_putty, Lowes, Amazon, Youtube, etc.
Author:  RusRob [ Tue Jan 28, 2014 9:39 pm ]
Post subject:  Re: bridge clamping caul
I am afraid bondo would break under the stress of a clamp. Bondo is essentially talc and although it has a fair amount of flex when first set it will get pretty ridged and crack after it has completely cured. As Clay S. suggested Tiger Hair or similar would work better. That is like Bondo except it has fiberglass threads in it and it is closer to fiberglass resin so it gets very hard and does not crack.
You might have a look at this:

http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1208/index.html

This is a Semi Rigid Urethane Casting Resin. It is used for making very tough prototype parts and castings that will hold up to abuse.
It isn't cheap but it is designed for this type of thing and will last a long long time. I have used their products to do some castings and molds and they have a wide variety of products and are very good quality.

Cheers,
Bob
Author:  meddlingfool [ Tue Jan 28, 2014 10:53 pm ]
Post subject:  Re: bridge clamping caul
How about stewmac (or LMI?) friendly plastic? Little pellets that you can hear in hot water, then mold to your desired shape. Feels like cutting board plastic when it's cool...
Author:  Pmaj7 [ Wed Jan 29, 2014 12:51 am ]
Post subject:  Re: bridge clamping caul
+1 for FriendlyP
Author:  Tom West [ Wed Jan 29, 2014 5:18 am ]
Post subject:  Re: bridge clamping caul
I think Meddling nailed it..........!!
Tom
Author:  Joe Sallis [ Wed Jan 29, 2014 7:48 am ]
Post subject:  Re: bridge clamping caul
I wonder if a little bean-bag with wood backing would work just as well and conform to different shaped bridges?
Author:  RusRob [ Wed Jan 29, 2014 10:15 am ]
Post subject:  Re: bridge clamping caul
That is a sweet setup Filippo. As many bridges as I have replaced in my time I should have one of those...

But then what would I do with my bridge clamps... idunno
Author:  SteveSmith [ Wed Jan 29, 2014 11:58 am ]
Post subject:  Re: bridge clamping caul
RusRob wrote:

But then what would I do with my bridge clamps... idunno


I really like my vacuum clamp for new work but be sure to keep the bridge clamps for repairs.
Author:  RustySP [ Wed Jan 29, 2014 12:31 pm ]
Post subject:  Re: bridge clamping caul
+1 on vacuum. My little rig is ugly but hey it works.Image


Sent from my iPhone using Tapatalk 2
Author:  Imbler [ Wed Jan 29, 2014 3:48 pm ]
Post subject:  Re: bridge clamping caul
Steve, why would you use clamps rather than vacuum for repairs? Probably is obvious, but I'm missing it!
thanks, Mike
Author:  SteveSmith [ Wed Jan 29, 2014 4:41 pm ]
Post subject:  Re: bridge clamping caul
Imbler wrote:
Steve, why would you use clamps rather than vacuum for repairs? Probably is obvious, but I'm missing it!
thanks, Mike


I haven't done as many bridge repairs as some others on this forum but just have found that an uneven old top coupled with an old bridge doesn't work well with the vacuum setup for me. Maybe others have a better technique.
Author:  Imbler [ Wed Jan 29, 2014 5:21 pm ]
Post subject:  Re: bridge clamping caul
Oh, that makes sense. Uneven top with cracked finish probably doesn't seal too well,
thanks, Mike
Author:  Nick Royle [ Wed Jan 29, 2014 5:57 pm ]
Post subject:  Re: bridge clamping caul
Can you just push the bridge down into the friendly plastic and let it set? Or do you build it around the bridge?


meddlingfool wrote:
How about stewmac (or LMI?) friendly plastic? Little pellets that you can hear in hot water, then mold to your desired shape. Feels like cutting board plastic when it's cool...
Author:  unkabob [ Wed Jan 29, 2014 6:53 pm ]
Post subject:  Re: bridge clamping caul
You might consider a rubbery surface on the caul to conform to minor variations from bridge to bridge.

Bob :ugeek:
Author:  Ken Mitchell [ Thu Jan 30, 2014 9:48 am ]
Post subject:  Re: bridge clamping caul
Thanks, everyone, for your suggestions. Friendly plastic ordered and on the way!
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