Let's talk Kerfs (curving your foam)

[eaglesdare]

you do have some curves going on there! but that is a really cool looking tear!

[chartle]

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

That's a pretty ambitious compound curve. You would have to curve the short edges first and then curve the main part and as you go cut out wedges of what you already curved. The wedges will have to be compound cuts themselves since it will have to go all the way up to the flat surface.

Have plenty of extra foam available you are going to need it you may waste a few just figuring out what those pie shaped cuts look like. Also I would expect it not to be self supporting. You are going to end up with more glue and cuts than you think.

I repasted your pic.

[KCStudly]

You had [/album] at the front of your url, and [/img] after.

It needs to be either [ album](with only your picture number here)[/album] (and the space in [ album] left out, I just put that space in so that you can see it here)

Or [ url](with your url here w/o "_page" in it)[/url] (again, w/ the space in [ url] left out).

[GPW]

What about cutting the foam into strips and glue them on like a canoe ??? . Going to be skinned like one eh ?

[chartle]

What about cutting the foam into strips and glue them on like a canoe ??? . Going to be skinned like one eh ?

They would have be to be very thin strips. Think about how they would curve as you approach the side walls. Also you now have a lot of cuts, not grooves, and glue.

What makes foam stiff is the skin. Once you cut through it now you are hoping that the covering and the glue that holds it to the foam replaces that stiffness. Also to make that curve, the skin would have to have the same slits and cuts or pleats to make it fit.

[Treeview]

This:

http://www.oneoceankayaks.com/smallwin/stripcalc.htm

http://www.woodworkingspecialops.com/page2.htm

KERF-CUT FORMULA
1. Take the Outside Perimeter of the
radius and subtract the Inside
Perimeter. This gives the amount of
frame to be removed.
2. Divide this amount by the thickness
of the saw blade. This gives the
number of saw cuts.
3. Last, divide the Outside Perimeter of
the radius by the number of saw cuts.
This gives the distance between
saw cuts.

[wagondude]

The skin issue could be solved with stretch fleece like the audio guys use to build those cool looking subwoofer boxes. Then you wouldn't need cuts or pleats.

Bill

[KCStudly]

The stretched fleece would only look truly rounded with a whole lot more ribs, so I disagree.

Kerfing in two directions might work, but seems doubtful and problematic to me.

I would build up oversized wedge shaped sections of foam, similar in profile to your ribs (try to get the inside nice and even or cut the inside out with a hot wire knife, but let the outside run large) and then just carve the outer surface freeform using some guidelines traced on as reference points as you go. Gnarly rasp, Sureform tool, coarse sand paper, long board, work your way down until smooth and round like baby's butt.

[cpinetree]

That sure looks like a great place to use spray foam, like used in attics.

Likely not very cheap, but it sure would come out perfect.

Just staple some plastic on the inside, spray the foam, let it expand and cure, and file/sand/rasp it smooth.

[chartle]

The skin issue could be solved with stretch fleece like the audio guys use to build those cool looking subwoofer boxes. Then you wouldn't need cuts or pleats.

Bill

But the stretchyness of fleece makes it a bad skin. I guess though if you soaked with multiple cots of epoxy it might work.

[chartle]

This:

http://www.oneoceankayaks.com/smallwin/stripcalc.htm

http://www.woodworkingspecialops.com/page2.htm

KERF-CUT FORMULA
1. Take the Outside Perimeter of the
radius and subtract the Inside
Perimeter. This gives the amount of
frame to be removed.
2. Divide this amount by the thickness
of the saw blade. This gives the
number of saw cuts.
3. Last, divide the Outside Perimeter of
the radius by the number of saw cuts.
This gives the distance between
saw cuts.

The canoe thing really would work because the curve shown is not like any canoe shape I have seen.

[chartle]

That sure looks like a great place to use spray foam, like used in attics.

Likely not very cheap, but it sure would come out perfect.

Just staple some plastic on the inside, spray the foam, let it expand and cure, and file/sand/rasp it smooth.

Thats Kind of what I thought but building it up with foam blocks. The spay foam is very soft closer to something that you would make a mattress out of.

If you asked a set designer to make it they would build up the shape with solid foam. Attack it with saws and rasps and other medieval tools and then glass the whole thing. Of course they wouldn't expect it to be pulled down a road at 60 mph and sit out side all year.

[wagondude]

KC, maybe more ribs would be required, but with the supporting foam I don't see where there would be a problem.

Chartle, I agree that the fleece would require more layers and maybe different material over it for extra strenghth.

Bill

[KCStudly]

Oh, I see now. Your were talking about using the fleece over the foam in lieu of the canvas. I thought you were talking about using the fleece over the ribs to get the rounded shape.

Another method would be to lay thin narrow strips of of wood in a criss-cross grid pattern stapled on to the outer edges of the ribs...if the strips can be made to flex around that tighter outside corner rib radius...then skin it with the fleece and epoxy, or even paper mache then epoxy. Use a soft insulation on the inside and skin that somehow. Maybe? Well, maybe this will at least foster a better idea.

[Martinup]

Hi guys:

Still pondering and researching the various types of stryrene and your thoughts and suggestions. I will post more on my build later as I have time and more done.



Martin