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Springback Formula For Arched Jambs4/22
Does anyone suggest a spring back formula that has been accurate ? Mr. B.H. Davis ?
No BH here but in the bit we've done there is no formula other than experience (we don't have) and doing a mock up and adjusting from there once, twice, or 42 times.
Use thin enough material and there isn't a spring back. 1/16" works very well, 3/32" you get a bit of springback.
Plus it depends on the glue too. Using a stiff epoxy and you get much less. Use a PVA glue and you get more. If you use PVA let it dry for 12 hours.
If you use plastic resin glue there will be very little spring back
No formula here,We use 1/8'' layers of MDF with PVA glue for the top piece, very little spring back, but to help with the situation, we mount the casing to one side of the Casing opening, that corrects any little spring.
On low flat arches for exterior doors we use 1/4" laminations with PUR glue.
You will be chasing the elusive formula for some time, since there is none. Even if you have one that works, change the radius, or the species, or any variable, and it is no longer predictive.
We solved this one 30 years ago when faced with building 40 interior doors with arched head jambs - in about 7 or 8 different radii. Bent lamination was too variable, so we came up with something else.
Brick laid and lined head jambs is the better way. Saw (or pattern shape) out a bunch of scrap on radius, clip the ends so they butt on radius, and stack them up, staggering the joints (brick laid). Once that is glued to the correct over width and chord length, the face can be cleaned up with scrapers and sanders. Then a 1/8" (or thinner, or thicker) piece of nice wood is clamped to the assembly with cauls and allowed to set up.
We made a jig (named the "Angel" jig after its inventor) to facilitate clamping up these brick laid heads. We also made a "swingometer" to bandsaw radii reliably and accurately. This not used much now, as I prefer to just use the shaper to size the inside radius on radius perfectly. One can also make the curved elements over size and glue them all up, then saw out on radius.
The photo below shows the jig, tho that is an unusual arched head assembly, and not a very good representation of what I am talking about.....
This makes a rigid head, even at 3/4" thick. Do not try to force thicker wood liners onto thinner brick laid arches. This can distort your otherwise rigid head. Keep it all easy.
Some might call it an "engineered" arched head.
I apologize for the delay in seeing this thread. Now that I'm retired I don't check in on Woodweb every day any more. It seems I'm busier in retirement than I ever was when working !!
Giving advice on radius work is now awkward for me because as is typically the case I have a non-compete agreement with the new owner of my company. So where in the past I could measure what I wanted to share against my own business interests now it is against someone else's interests so I'm a bit more restricted.
However I will send you a PM and see if I can give some guidance on the spring back issue. Publicly I'll just say that for years we kept a record of the spring back we got with different materials, radii and adhesives. Since curved jambs were made on a near daily basis you can imagine that list what quite extensive.
So I completed my arched jamb and by a hunch I over-bent it by 3/16". After taking it off the mold the spring back was a little less than an 1/8" which will be fine. I added the data to my list of laminations and will be using it for reference. Thank you all for your input !