Let's talk Kerfs (curving your foam)

[swampjeep]

Thoght I'd throw in some thoghts on bending foam sheets with kerfs. With a little math you can figure out the number and size of kerfs to cut in foam to get a desired radius.

Example:



Lets say you have a sheet of foam that is 1 inch thick and you want to make a 90 degree bend with a 6.4 inch radius. This is a very tight bend for this kind of foam, but with some kerfs cut in it, you can do it.



First some math. Illustrated above you see a 90 degree bend in 1 inch foam with a radius of 6.4 inches. We need to find out the arc length of the outside and inside curves of the foam. This will help use figure out how wide the kerfs need to be.

The formula for finding the arc length is:

arc length = (angle of bend / 360) x 2 ( pie x radius)

so we have:

10 inches = (90 degrees / 360) x 2 (3.14 x 6.4 inches)

The arc length of the outside curve is 10 inches. Then with the same formula we find that the arc length of the inside curve is 8.4 inches.

When we subtract the two we have a difference of 1.6 inches. This is how much material we need to remove from the inside curve with kerfs.



So, if we decide to make 5 kerfs on the inside of the foam, they will have a spacing of about 2 inches apart and 1 inch from the end.



Divide the 1.6 inches by 5 and you get 0.32 inches. This is how wide each kerf needs to be. The kerfs should be cut to a depth of about 75% of the 1 inch foam thickness in a triangular shape. the widest part of the kerf will be 0.32 inches. This can be done with a straight edge and a sharp utility knife.

OR if you want to make the kerfs with a circular saw and more kerfs, then you would divide the 1.6 inches by the thickness of the saw blade (also called the kerf of the blade.) With a 1/8 inch thick blade you would need to make about 13 kerf cuts spaced evenly apart. ( 1.6 / 0.125 = 12.8 )



Apply some glue inside the kerf cuts and bend the foam to the needed radius. It should hold its shape fairly well on its own after the glue dries.

Okay, I found a scrap piece of 1.5" foam. Its 15"x21" and I'm going to put a 12" radius in it to 90 degrees.



I calculated a differece of about 2.3". So, using a table saw, I cut 18 kerfs spaced 1" apart. Tomorrow I will glue it up with Gorilla glue and tape it in place until it dries. Just bending it seems to make a perfect 12" radius to 90 degrees. The glue will fill the voids at the bottom of the kerf cuts also.

[swampjeep]

Okay, I glued up the 1.5" foam. You can see that it is slightly past 90 degrees from the taping. I'm curious to see what kind of spring back will occur; not much I expect. Then I'm going to skin it with cloth and glue and do a stress test of some kind (the "standing on it while drinking a Coke test").

GPW, I was also thinking about this for a hatch and a cloth hinge. Another great benifit to this method.



Makes a very clean bend, anyway.

Share my pain:



The tape came off while the glue was setting. Half of the curve set and the other half sprung out. The half that set seems very strong and, with skin, would hold up as a hatch with no additional support, I think.

GPW, yes, compound bending. I thought about that yesterday when I was cutting the kerfs. I could have rotated it 90 degrees and cut more kerfs. I'd like to try that.

Put on an exterior coat of TiteBondII over the fabric and then a coat of Kilz Latex. Fabric is on inside and outside curves. I stood on it for a couple seconds before I chickened out. Curve is very sturdy with very little flex. I am convinced this method would work well for a galley hatch with no additional supports, BUT I think some additional supports would be good to have. Not sure how weather extremes would affect it.

Now to look into a fabric (or rubber) hinge.

[swampjeep]

That's a very impressive display of strength!! Combined with a couple durable tough layers of canvas, it should be every bit as strong as a wooden teardrop with a thin layer of wood. Maybe even stronger!!! Can't wait to try it!

Mike...

Put on an exterior coat of TiteBondII over the fabric and then a coat of Kilz Latex. Fabric is on inside and outside curves. I stood on it for a couple seconds before I chickened out. Curve is very sturdy with very little flex. I am convinced this method would work well for a galley hatch with no additional supports, BUT I think some additional supports would be good to have. Not sure how weather extremes would affect it.

Now to look into a fabric (or rubber) hinge.

Alrighty, The key is waiting long enough for the TB2 to cure. I stood on the test curve this morning (200lbs) and it would make a great step stool. It needs a full 24 hours to cure.

[GPW]

Maybe a conical router bit to more precisely cut those kerfs??? http://www.cremonatools.com/product_inf ... ts_id=2185[/url]

[eaglesdare]

i wonder how my kerfs were cut. i am working on a wall in the back part where the hatch is. i need my foam to bend. i can not remember (i wasn't paying attention) when mine were done for the curved roof. so i don't know how the cuts were made. i did not realize that the cut is like a V shape or actual foam is cut out.

so what can i use to make those nice cuts? i think i have a router, well i know i have a router, but not sure what bits (if i can find any) that i have. i have a jigsaw, but i don't think i can resize that so it won't cut thru.
we have a circular saw, but again, not sure about how small i can make that saw for those cuts.

i could do it by hand, but that is a pita. i just tried.

[Ratkity]

It works by a straight cut with a circular saw set to the correct depth. You don't need a wedge unless you are making a major bend.

I think they did cuts every 1 inch?

Hugs,
Ratkity

[GPW]

Eagle , what thickness are you bending ... ???

[eaglesdare]

this is the 1" stuff. i was thinking about the heat method. just thought this would be easier. not sure how i am going to heat it. i suppose i could use the blow dryer out there. i might attempt this later when its a bit cooler outside. its too hot right now, especially if i am going to add more heat in the mix.

i am not trying for a major curve, just a slight bend. i'll get hubby out there later and try the blow dryer. maybe he can do the bend as i heat it up. i am pretty sure i will not beable to do this on my own.

[GPW]

You might try bending a test strip with a damp towel and an iron ... The damp towel keeps the iron from melting the foam ... Weight down one edge , prop something in the middle underneath of where you want your curve , then press down on the other end while ironing the bend area ... Worth a try , and it will stay in that shape ...no kerfing necessary ...

[swampjeep]

maybe even try this for a slight bend with 1" foam...

lay it out in the sun for a while, maybe 1 hour then see if it will bend from the heat of the sun

not sure if that would do it, but it sure wouldnt' surprize me if it would

[GPW]

SJ, tried it , the foam just sat there, even painted black ... Nada !! 190F seems to be the magic bend temp.

[GPW]

Here’s a little tool we’ve made and used in the past to cut cavities in foam (for airplanes ) .. Easy to make and use , and by using Swampjeep’s calculations we should be able to shape the cutting wire so as the foam is bent the kerfs will close entirely forming a tight gap free surface ...
This can be powered by anything from a car battery , train transformer(the easiest to use ) , or variac transformer (used carefully) ... You just have to get the wire hot enough to melt through the foam and NOT glowing .. Run along a good straightedge , this should produce a clean smooth slot with no foam dust , and the rounded tip of the wire would eliminate any stress points in the bent area of the foam reducing any chance of cracking ...

Works great for planes !!!

Having to kerf 10 sheets of foam , I needed a quick , easy and dustless way of cutting precise kerf slots ... and yes, this should be done outside or with good ventilation ...

[swampjeep]

Here’s a little tool we’ve made and used in the past to cut cavities in foam (for airplanes ) .. Easy to make and use , and by using Swampjeep’s calculations <snip>

just want to point out, credit for these calculations needs to go elsewhere, I just pulled them out of the BIG thread LOL

thank's for the added info though

[GPW]

SJ, wherever it came from , Thanks!!! ... It’ll come in Handy !!!

[Martinup]

Hi Swampjeep:

Thank you so much for the tutorial on foam cutting. I have some compound curves to deal with and this bit you posted is exactly what I need. See the pic below. But what I really found the most interesting is from you basics on forming arcs out of foam I can expand on this to create a compound rear hatch detail.

[/album]http://www.tnttt.com/gallery/image.php?album_id=2412&image_id=88722[/img]

I am still getting the hang of posting pics into my posts . . . so if it doesn't come through you can see my album to see my build pics and all the compund curves I am dealing with.

Cheers,
Martin