Let's talk Kerfs (curving your foam)

[Toby]

Thank you OP! I saw that but swore it couldn't have been that obvious. So, how does one come up with the angle of the bend?

[KCStudly]

While I have been a staunch advocate of unit construction and doing as much work on the bench as possible, you also have to recognize the places where there is no substitute for fitting to suit.

It would be very difficult to be able to preform your roof and expect it to fit the wall profile with a high degree of accuracy later. You would almost have to build a mold the size of the camper in order to form the roof over... and isn't that what the walls are anyway?

For example, even just wrapping my hatch inner skin over the hatch frame, I had fit issues due to the skin winging up along the edges and general inconsistencies. It took a lot of hand work to get the hatch fitting the wall edge again.

Now that being said, if you were to make the roof section first, lay it on edge on top of the walls (walls laying flat on bench or floor) then trace the profile of the roof onto the walls, I could see that working... but you would need a work space large enough that you could build the camper in anyway.

Is the issue one of a "ship in a bottle"? Do you have a large enough work area to build but too small of a door, or is it a small work area?

[Toby]

Thanks for your input KCS; and lol, yes it is a 'ship in a bottle'. A basement big enough to work in but too small of doors and a garage with the larger door but no space to work. That, and the weather plays a contributing factor too. I have no outside enclosure to work in either. I would rather work this winter inside now and save some great camping weather later. Call me 'impatient'.....

[GPW]

Sometimes we think it'd be easier to work in segments , with edges cut to proper angle and assembled , rather than trying to make a smooth curve by kerfing and bending ... Joining angles could be easily cut with a 4+fool long hot wire bow , gravity cut , materials jigged to proper angle ... All quite easy and nowhere as messy as kerfing ...

I still feel the best way to curve foam smoothly is with heat although we've not found an easy (cheap) way to do that yet ... ( Still working on that for a thrifty Home application.. ) That way , no material is eremoved and the overall strength is magnified , not compromised by cutting ... Remember on a kerfed foam structure the actual continuous skin thickness holding the segments together may be as little as 1/4" ...

[ghcoe]

I did my rear radius in segments. I had to do it because the radius was too tight for a kerf bend. The segment method is a pain over the kerf. Sanding the edges down and making even across the radius was difficult.

Segments

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I found using 1" foam and kerfing the inside of the exterior piece and kerfing the outside of the interior piece and then gluing them together was the easiest. No sanding required except along the edges

Inside kerfs

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[GPW]

I was thinking of larger segments and leaving the segmented look , which would not be a problem ...

Ps. whilst cutting , DO NOT touch the bow .... let gravity do the work ...

[OP827]

Thanks for your input KCS; and lol, yes it is a 'ship in a bottle'. A basement big enough to work in but too small of doors and a garage with the larger door but no space to work. That, and the weather plays a contributing factor too. I have no outside enclosure to work in either. I would rather work this winter inside now and save some great camping weather later. Call me 'impatient'.....

Even large steel structures can be fabricated in pieces and shipped and assembled elsewhere, why not small trailers on a small scale then? If you design your trailer in panels to be assembled that are small enough to fit through your "logistics envelope" which in this case doors and corridors of a house, you should be successful. I would fabricate flat panels and curved panels to logistically suitable size, fit them dry together in a basement, then disassemble and "ship" to final assembly place, sounds like fun and you get to work in a warm climate

The question is can you dry fit your trailer pieces together in your basement?

[GPW]

“ The question is can you dry fit your trailer pieces together in your basement? “ ... no basements here .. but a good point !!! Some things are just so much easier to do on a nice flat table ...especially for we older folks , unaccustomed to crawling on the ground or working on shaky ladders ...

[GPW]

Op, taking the modular idea further ... if we were mass producing Trailers of one design , we could fabricate a big bending jig and produce large heat curved parts with not much trouble ... For just one build , that’s not really practical... I still believe the heat bending method produces the smoothest and strongest parts of any method so far explored here... and parts that are STABLE, self-retaining their new shape after being bent .

We’ve heat bent plane parts for a long time with great success... No reason we can’t do it for trailers ... other than cost .. Still exploring that ...

[rowerwet]

When first designing Mercury, I wanted a raindrop front. I was going to make it of two layers of 1" foam, instead of kerfs I just planned on gluing bed sheets to the outside of each bend when the panels were flat. Then bending the foam around a frame. Then using GS to adhere the second layer of foam to the fabric of the first.
This was shown quite a while ago, by another builder. Time making the frame to support the bends, killed the idea.

[GPW]

Yes , it’s the form that takes all the time and materials , and then what do you do with it ??? ( except bend more foam ... ) Too small for a skateboard ramp ...
I’ve been thinking of those INDUSTRIAL electric heat blankets ( they get HOT as you want ...we only need 190F ) You wouldn’t need a really Big one , since you’re not going to bend the whole sheet at once ...or are we ?? ( few bucks for those , again to use once ... ) Anybody want to make trailers and need a whole bunch of curved parts ... ???

[OP827]

Op, taking the modular idea further ... if we were mass producing Trailers of one design , we could fabricate a big bending jig and produce large heat curved parts with not much trouble ... For just one build , that’s not really practical... I still believe the heat bending method produces the smoothest and strongest parts of any method so far explored here... and parts that are STABLE, self-retaining their new shape after being bent .

We’ve heat bent plane parts for a long time with great success... No reason we can’t do it for trailers ... other than cost .. Still exploring that ...

Like this in a hot water bath - https://youtu.be/bVpC7TOIlH0?
Seems like possibly the easiest way to control temperature, because its water. Just need a big enough tray with water for foam piece and bending jig in it, right?

[pchast]

Hmm. If you only need 109 deg couldn't a regular electric blanket be
re-engineered to produce enough heat?

[GPW]

Pete, sorry , my bad , I typed it wrong .... it’s 190 F degrees

OP, That seems like another thing to try .. but anything we do to bend large pieces of foam is going to take something bigger than the average trailer builder is willing to commit ...  I’m still thinking about the easiest method so far , a small bending jig and a couple cheap harbor freight heat guns ... Same thing we’ve used for years to form the model foam (1/4” - 1/2” Blucor ) ... just Bigger ... The only thing is the curve is made by bending small areas at a time (gradually curved) rather than all at once ...

[rustytoolss]

Had I known more about doing kerfs. I would have bought some 1" xps for one section of my roof (2pcs stacked). But not knowing I bought all my foam in 2" thick. I will need to make a 18" inner radius 90 degree curve. I'm going to try this with a piece of scrap 2" foam first. To see how this go. If I don't feel like it's going to work out well. I may have to make my radius in long small sections. Can anyone tell me the spacing for kerfs / and depth of cuts for 2' foam / and number of cuts needed?
Another question I have is. I bought a fine tooth blade for my 10" table saw. Thinking it would give me better cuts in the foam. But I have problems when I cut the foam. The saw will grab the foam , almost like a kick back thing. So I have been using my circ saw instead . I don't get any kick backs with the circ saw.The circ saw has an old general use blade.
Could the fine tooth blade on the table saw be creating my problems ? I actually cleaned off all the foam residew off the side surfaces of the table saw blade thinking that would help prevent the grabbing problem. But it made no difference.