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How to adjust for springback

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Forum topic by Dmitriy_S posted 11-12-2018 03:19 AM 348 views 0 times favorited 4 replies Add to Favorites Watch
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Dmitriy_S

12 posts in 408 days


11-12-2018 03:19 AM

Topic tags/keywords: question

Hello everybody. I’m very new to woodworking. I want to finish arch similar to the picture “arch_example” attached. My arch is 24 1/2 inch wide with 14 inch radius. Depth of the arch is 7 inch. I decide before I use real walnut, I’ll do a test run with plywood. I got 5mm plywood from HD and cut it to 7 inch strips. I decided to do a “bent lamination” for this strips. Once I removed clumps, I found that I didn’t know about spring back. My plywood arch is 1 3/4 wide then it should. Before I build a new form with shorter radius, I want to ask a question: is there an available information, how to adjust for spring back for different wood species . I think that springback for plywood is different from springback for walnut. What radius for walnut should I build a form ?
This is an example of what I want to achieve:

This is my arch

Form:

This is a test:


4 replies so far

View Aj2's profile

Aj2

2321 posts in 2217 days


#1 posted 11-12-2018 04:02 AM

3/32 is a very good thickness for walnut laminations.
If you really want to limit any spring back use plastic resin glue. But beware it’s very hard on tooling for that reason I hardly ever go that route. I use TB3 for walnut and hope for the best.
Good luck

-- Aj

View Mike_D_S's profile

Mike_D_S

595 posts in 2634 days


#2 posted 11-12-2018 04:14 AM

Well the WAG method would say do a test run on the form you have and see how much spring back you get then make a new form based on that.

But assuming you are going to case the door frames like in the pictures, then you can probably tolerate a bit of compression in the arch as those forces will press down on the tops of the verticals on the sides.

-- No honey, that's not new, I've had that forever......

View shipwright's profile

shipwright

8320 posts in 3217 days


#3 posted 11-12-2018 04:18 AM

+1 Aj
Use a hard drying glue like urea formaldehyde (plastic resin) or hide glue. Urea formaldehyde’s require much higher clamping pressure but both will resist springback much better than PVA’s.
Experiment of course but you should get virtually none with these glues if you use reasonably thin layers.

-- Paul M ..............the early bird may get the worm but it’s the second mouse that gets the cheese! http://thecanadianschooloffrenchmarquetry.com/

View John_'s profile

John_

207 posts in 2125 days


#4 posted 11-12-2018 04:18 AM

I think I would try one of the ‘flexible’ products and see if it would look good enough once finished

I have used flexible crown from ResinArt for some 8” wide crown molding on a curved wall (paint grade) and was very pleased with the results. Let me warn you though, it is not cheap. I used a 12’ long piece and it was $32 per foot!

http://west.resinart.com/index.php?option=com_content&view=article&id=44&Itemid=126

For the ‘face’ of the arch, like in your test, I would look into some ‘bender ply’ of the appropriate species. Here is a pic I took of some of it stacked up at my local lumberyard.

I think they call this stuff ‘flexi mdf’ (I have not used it yet)

https://www.google.com/search?q=flexi+mdf&newwindow=1&rlz=1C1GGRV_enUS751US752&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiC0dOjg87eAhXSHzQIHWDlDuIQ_AUIFCgC&biw=834&bih=436

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