Bridge Cap Thickness

[Farrell]

Ron O. asked:

"What did you end up doing at the bridge dog legs? I read the posts you received. Did you steam and press the vertical laminates before the final glue-up? Wolfenden suggested this technique with regard to bending the rims also."

My first attempt was with Tightbond. I found that I applied so much clamping pressure that I distorted the bridge (distort is really a very mild term to describe the result of this excercise). I have been using small pieces of wood along the bridge at each clamp to spread out the clamping pressure. I did not use the one big caul method that Ron N. provided a picture of. I don't understand how you can be assured of getting proper clamping pressure at all locations. And it seemed like a lot of work to make a caul for each bridge.

So, try number two. Well, first of all I think that I simply was trying to put too much bend into a 32 mm thick pile of maple. Never the less, I tried Del's recommendation of notching part way through each lamination at the hard bends with a saw. After seeing how much wood I removed with the saw, I immediately thought of the wondrous gap filling properties of epoxy. So I wet all the pieces down with unthickened West System epoxy, and filled the saw cuts with a thick mixture of their High Density Filler, assembled and gently clamped to cauls. Just enough clamp pressure to get things into position at most locations, and a lot of clamp pressure at the dogleg. Several laminations still cracked - but no big deal with all the epoxy in there (fortunately the panel covers the bottom of the bridge root and the cap covers the top - this bend area is rather unsightly). With the light clamp pressure (everywhere but the dogleg), I ended up with perfectly smooth big bends in the bridge - as desired. My dogleg ended up only 1 or 2 mm short of the bend I was shooting for - it just got smoothed out a little. There should be more than ample room to just have the section-end bridge pins ride a little high and low there - should work fine.

I bought a big roll of 6 mil plastic from Home Depot (fairly thick stuff). Cut a piece a foot longer than the bridge and maybe six or eight inches wide. Arranged epoxied laminates on this long narrow piece of plastic. Pull up edges of plastic over sides of bridge and let flop over top. At this stage I could take off my gloves and handle my maple-laminate-filled burrito with extra slow setting epoxy sauce with bare hands. The plastic keeps the epoxy off everything - clamps, cauls, you, etc., and keeps in on the bridge laminates - kinda makes a neat little trough that is filled with epoxy. The bridge literally sits in a bath of epoxy while the epoxy cures.

I may try steaming next time. Maybe not. The thing with steaming would be the wait for the maple to dry. Now how long is it going to take to dry with clamps and cauls and everything covering the sides. I suppose you could have the top and bottom exposed to the air. I guess I'd have to take some and figure its moisture content, steam it, and then monitor its weight to determine the moisture content to see how long it takes to dry. Or maybe you need to steam that thing, clamp it into the desired shape, and have a little heat source below and just toast it for a while. Still though, how long?

Terry Farrell
  
----- Original Message ----- 
From: "Ron Overs" <sec@overspianos.com.au>
To: "Pianotech" <pianotech@ptg.org>
Sent: Friday, March 14, 2003 6:48 PM
Subject: Re: Bridge Cap Thickness


> >At 6:20 PM -0500 14/3/03, Farrell wrote:
> >What are the guidelines on how thick a bridge cap should be? And 
> >why. I've got a new vertically laminated long bridge that can be 
> >trimmed down to any height, and I'm trying to figure out how thick 
> >to make the cap. The cap will be horizontally laminated with about 
> >1.6 mm laminations. Laminations will be epoxied together. The 
> >installation should allow the cap to remain at near full thickness. 
> >It's a bit of a pain making the laminated cap, and that process is 
> >only complicated by increasing the number of laminations. I have 
> >five laminations now and it adds up to about 8 mm thick. I would 
> >like to not make it any thicker than optimal, or to the point of 
> >diminishing returns.
> >
> >Terry Farrell
> 
> Terry,
> 
> 8 mm of cap thickness will be more than adequate. The greatest stress 
> on the bridge cap wood will be at very top surface of the cap, 
> diminishing further down into the cap. We are currently building our 
> caps 6 mm thick using 6 x 1mm thick laminations.
> 
> Good on you for getting stuck in at the deep end - all the best. What 
> did you end up doing at the bridge dog legs? I read the posts you 
> received. Did you steam and press the vertical laminates before the 
> final glue-up? Wolfenden suggested this technique with regard to 
> bending the rims also.
> 
> Ron O.
> -- 
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