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Help! Bending a broken cherry harpsichord bridge

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Forum topic by RKD posted 07-02-2016 02:35 AM 485 views 0 times favorited 4 replies Add to Favorites Watch
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RKD

2 posts in 160 days


07-02-2016 02:35 AM

Topic tags/keywords: cherry wood old broken bent bending

Hi Everyone

This might seem a little random, but I’ve inherited a 40-year-old kit for building a harpsichord (sort of a precursor to the piano). One of the steps in the construction process involves bending a long, thin piece of cherry (3/8” x 7/16” x 63 5/8”) along a very precise curve outlined in the plans. I was attempting this task today when the piece snapped cleanly in two before even coming close to matching the curve. The break was along the grain of the wood, so fortunately there was a substantial surface that I was able to glue and clamp down.

My question is what’s next? I’m guessing the wood was dry and brittle due to its age. Should I attempt to soften it before bending it again? What’s the best way to do that? Bringing it into the bathroom and turning on the shower? Or is it likely the wood will just break again along the seam? It might be useful to know that once the wood is bent, it glues down to a flat hard surface along its entire length (clamped with padded nails). The piece is precision cut and would be exceedingly difficult to replace, so if at all possible, I’d like to try and salvage it. Thanks for any and all help!!


4 replies so far

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Rick M

7932 posts in 1846 days


#1 posted 07-02-2016 03:09 AM

Heat is the enemy of adhesives. Lignin is the natural ‘glue’ that holds wood together. Heat will relax the lignin allowing the cell walls to slide against one another (bend). You can use heat directly but many people use steam to help carry the heat into the wood. The best woods for bending tend to be open grained like hickory or oak, the worst are tight grained woods like maple and cherry. But it still may be possible to bend cherry if the grain doesn’t runout which is when the grain meets the edge at an angle instead of being parallel. If the wood has runout it will break instead of bend.

-- http://thewoodknack.blogspot.com/

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jumbojack

1667 posts in 2090 days


#2 posted 07-02-2016 03:43 AM

Cherry actually bends pretty easy, when it is thin and steamed properly. Did you use a form or just try bending it? I usually don’t try anything over 1/8” but that thin it can take a pretty severe bend. I would consider a steam box and a form to bend your material.

-- Made in America, with American made tools....Shopsmith

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runswithscissors

2192 posts in 1491 days


#3 posted 07-03-2016 12:34 AM

I would tackle this with dry heat, such as a heat gun. But as RickM mentioned, that glue joint is going to be the problem area. I can’t imagine that steam would be a good thing to subject a precision cut piece of wood to. I did just have a thought about the heat vs. glue issue: pre bend the 2 pieces to the curve you want. With heat bending, you over bend a little, as there will be spring back, and then let the wood cool for a minute or two. It will now hold its shape. In fact, if you need to remove some of the bend, you will need to heat it again. After you have the 2 pieces bent, then glue them together. In any case, it’s going to be a tough job to get a smooth, even curve.

I haven’t bent cherry this way, but have bent (with dry heat) both red and white oaks, ash, yellow cedar, black locust, and others. Softwoods such as fir and hemlock don’t respond to heat. I haven’t had much success with luan (mahogany) either.

If the kit is only 40 years old, surely it’s still under warranty?

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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RKD

2 posts in 160 days


#4 posted 07-05-2016 04:35 PM

Thanks for your help, folks! I ended up following runswithscissors’ advice, and hijacked my wife’s hairdryer to ease the piece into the curve. The glued joint ended up failing in the process, but I was able to curve the two pieces into the shape and have the two broken ends meet (more-or-less) in the middle. A little Plastic Wood to fill in the crack, and we’re back on our feet. Thanks again for the advice!!

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