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laminated bending springback

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Forum topic by JonJ posted 2340 days ago 1117 views 0 times favorited 12 replies Add to Favorites Watch
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JonJ

163 posts in 2465 days


2340 days ago

Topic tags/keywords: question clamp bending poplar harpsichord

This is sort of directed at GaryK, as he is the resident harpsichord expert, but someone else may have an answer, too.

I’m starting a new instrument that will use some harpsichord costruction techniques, and will have a German double bent (s-shaped) side about 5 or 6 foot long.

I am building it out of poplar, and will laminate 3 quarter inch pieces together on a form and probably vacuum bag them. Here is my question…the tigtest curve will probably bend about 5” in the length of 1 ft. How much extra bend should I put in my form to compensate for the wood springing back when coming off the mold?

I will probably pre-bend (with heat or steam) all pieces before putting on the mold so it’s not like trying to herd a cat into a sack getting everything pulled down in the vacuum bag.

Any pointers?

-- Jon


12 replies so far

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GaryK

10262 posts in 2613 days


#1 posted 2340 days ago

Good question. With a 1/4” thickness you will have quite a bit of springback. It’s hard to say how much.
With 1/8” you would have a lot less. Closer to zero.

I would be prepared to make more than one to find out what works. Make your mold tight to start so that
you could take material off if it’s too tight.

-- Gary - Never pass up the opportunity to make a mistake look like you planned it that way - Tyler, TX

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JonJ

163 posts in 2465 days


#2 posted 2340 days ago

Thanks Gary…I’ll blog what happens when I try.
I’m hoping by pre-bending I’ll eliminate some springback. I know of a harpsichord builder who bends the full 3/4” as one piece, but I’m not that ambitious.

Looks like I’ll be going back to the mill a couple more times before I get it right, but maybe I’ll get lucky.

-- Jon

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swied

74 posts in 2387 days


#3 posted 2340 days ago

I saw an episode of How It’s Made on the Discovery channel a few weeks ago. They showed clips of the construction process of building a Steinway Piano. The part where they did the lamination bend was fascinating. They applied glue to all the boards, and made a stack of them. Then about eight men lifted the whole dripping mess, and walked it over to the mold. They locked one end in place, and then marched in step around to the other side. When everything was in place they applied all the clamps, and tightened everything up.

I didn’t catch the type of glue that they used, but noticed that no one was wearing a resperator.

-- Scott, San Diego

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JonJ

163 posts in 2465 days


#4 posted 2340 days ago

I saw an episode of How It’s Made on the Discovery channel a few weeks ago

I’ll have to check that out…I’ve got a ton of ‘em recorded on the DVR, but didn’t see that one. I love that show! It’s a good place to get ideas, and some of the processes manufacturers use are unreal.

-- Jon

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swied

74 posts in 2387 days


#5 posted 2340 days ago

I seached Discovery’s websit, but didn’t find it in their episode listing. I did another Google search on “Steinway Piano TV Episode” and got the following link to a show called “Made in America.” I think that this might be the show that I saw.

http://www.tv.com/john-ratzenbergers-made-in-america/steinway/episode/377048/summary.html

I have no idea which channel I was watching.

-- Scott, San Diego

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John Ormsby

1278 posts in 2362 days


#6 posted 2340 days ago

Hello Jon, You will want to use a Urea glue like Unibond 800 for this type of lamination. I also would recommend you use 1/8” thickness. 1/4” will spring back. The more laminations one uses, the more exacting the piece will come out of the mold. If you look at my Projects under the kitchen cabinet file you will see some curves I did that are around 7” to 7 1/2” wide curves sitting on a form. You will see that they have very little spring back. I resawed these 1/16” thick in order to get a virtually no spring finished curve. I used special 1/8” bending ply in the core with two 1/16 thick pieces on each side. I will try to post some photos of how tight this ply bends later.
John

-- Oldworld, Fair Oaks, Ca

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rikkor

11295 posts in 2499 days


#7 posted 2340 days ago

Thinner is better to prevent springback.

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JonJ

163 posts in 2465 days


#8 posted 2339 days ago

Thanks for the tips everyone…I will for sure go thinner. The best I will be able to do is to make my 3/4 inch lamination out of 4 pieces instead of 3. This will not be quite 1/8”, but better than 1/4”. I only have 4 pieces to work with on the bent piece…long story, but can’t make any more right now.

I drew out my full scale pattern tonight, and it’s not that tight of a bend. Hopefuly it won’t be too big of a deal.

I’ve have made quite a few bent parts in the past…did 6 cathedral windows, but I was not worried about 500 lbs of string tension pulling on the windows :)

-- Jon

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JonJ

163 posts in 2465 days


#9 posted 1989 days ago

Thought I’d follow up on this if anyone finds this while researching vacuum bagging. I did mill the 4 pieces down to 3/16 each, used Unibond 800, $20 bucks worth of cheap vinyl, and my antique freon compressor…worked out well…even though it took me a year to get around to it (no point in getting in a hurry)
http://lumberjocks.com/jocks/JonJ/blog/7717

-- Jon

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GaryK

10262 posts in 2613 days


#10 posted 1989 days ago

Did you get much springback?

-- Gary - Never pass up the opportunity to make a mistake look like you planned it that way - Tyler, TX

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JonJ

163 posts in 2465 days


#11 posted 1989 days ago

Only about 3/4 inch on the very end, but it was on the bass end where the window of acceptable string length would have allowed it to be 2 feet longer and it wouldn’t have mattered. Since I’m building from scratch I’ll say it’s just how I wanted it!

-- Jon

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Doug S.

295 posts in 2333 days


#12 posted 1988 days ago

Here’s some math for you
http://www.woodweb.com/knowledge_base/Calculating_Springback.html

But thinner is better.

-- Use the fence Luke

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