I’m sure someone has either thought of or done this already, but I can’t find it in forum history, so I thought I’d check it out this way…
Lets assume for the sake of simpliclity that I’m going to make a stringerless 1# eps blank for a balsa sandwich. The standard process, as I understand it, is to lay up a few sheets of eps foam with some glue that you can hot-wire through, and then use a rocker template and hotwire to cut out the rocker.
Has anyone laid up their eps sheets on a chunk of bottom rocker cut-off, then vacuumed them down so that the rocker is glued into place? I tried once to make a blank out of three sheets of eps, bent into rocker during glue-up… But I didn’t use a vacuum, so there were some catastrophic voids between the sheets where they flexed through the curves. But if you could pull them down into a rocker table, then you’d have a lot less waste, and a lot less work. You could also use the same rocker table to lay up your lams, so as not to lose any rocker during the assembly process.
So, who has done this already, and how did it work out?
Something very much like that indeed! When I tried that the first time, I didn’t have it nearly as well-clamped, nor as accurately rockered. I was actually using the rocker I had already cut into the stringer to guide my shaping, and the air voids between the sheets really screwed the finished board up.
Anyway, how did that method work for you? Any problem with voids? Did the rocker stay completely glued in place after you removed the clamping, or was there a little spring-back? I’m liking the idea of vacuum clamping it, and I think I know where I can get some cut-offs to use as guides. How did you decide upon and measure/set-up your rocker?
There was a little bit of spring back but it has held up very good. In fact, I’m going to use it on a tandem I’m just getting started on. There were no voids at all but I really weighed the thig down. I think I put on more weight after I took the picture. I also used that polyurithane glue so it would have foamed to fill any voids. As you can see, I left no room for glue voids
mine’s alittle different but I’ve done in in a bag as well.
I saran wrap my floppy pieces of EPS usually 2" top and 1" bottom to the bottom of a board whose rocker I wish to grab. When the epoxy/glue dries the bottom rocker is set you juts need to shave down the top to thickness.
I help the situation out by putting a piece of the bamboo veneer in between to act as a horizontal stringer but you don’t have to. The key is that when you lam the bottom on you’ll lock in the rocker but until then you can alter the rocker any way you want while you are bagging on the bottom wood.
If you preshape your bottom piece on a flat tabletop before doing this you could set glue the pieces togethor, set the rocker and bag on the bottom all at the same time.
Most of my rockers have been set just with the industrial saran wrap and some strapping tape here and there like this… No need for a rocker table.
The light bulb just went on, Oneula! I remember you talking about that before, but it hadn’t clicked. That’s actually a damned ingenious way to grab the rocker you want off an existing board! I will certainly be trying that, at least on the boards I have that have removable fins… the bonzers with the glassons are going to be another problem. But at least this way I can borrow the rocker from a board I already have, instead of having to source a cut-off from the nearest EPS blank supplier (houston) or trying to create a good rocker from scratch, which I don’t trust myself to do!
I live around houston and have been doing some research. You can get some #1 EPS from these guys (http://www.houstonfoam.com) for a lot cheaper than sfoam and not have to worry about doing the glue ups. I haven’t asked them about cutting rocker to get a blank and table like Bert does. When I do, I will just take them an sfoam catalog and ask how much for the cuts. I know you can get enough EPS sheets to do 5 boards for $50 (minimum order). Composites one also has a sales office here. I hope this helps.
I’ve been doing a little research myself and feel it’s only fair to point out that you may not be comparing apples to apples…
S-Foam blanks are of a different density (approx. 2 lb/cu ft) and have tiny beads. It shapes more like Clark foam than standard 1 lb/cu ft Home Depot stuff. I have samples and it seems like good stuff.
They've done plenty of homework in developing their rocker and thickness profiles... is it fair to take that info to another (cheaper) foam dealer?
They have a lot of educational material available for free on their site.
They are all geared up for stringer application.
They will do custom work if you ask.
Check their site... there really is a lot on there if you dig. http://www.sfoam.com/
Sorry, the idea wasn’t to steal their hard work any more than coping a rocker from an existing board. I wouldn’t think you would want their exact specifications, just using it as a visiable example of the type of cut they would need to make. I haven’t contacted sfoam but I didn’t see where they were selling #1 or #3/4 density that these other guys are using in their super light perimeter stringer sandwich constructions (slpssc).
Here is a cut/paste form their site. I don’t see any mention of foam other than the 2 lb cu/ft density.
“This foam is custom blown to a density of 2 # per cubic foot ( 1.9 to 2.4 nominal ) by our vendor from virgin material. It consists of Polystyrene “beads” that have been expanded, compressed and adhered to our specifications; and give sFoam™ blanks the right combination of strength and workability. Bead diameters are 2.00 to 2.75 mm nominal.”
