Sunday, April 7, 2013

Cutting Foam Blocks

This is the way that I process the foam getting it ready to be hot wired. First things first - the foam.  Foam is typically advertised by its weight  likely pounds per square foot. Historically the best we could do was 1 lbs foam and in the off chance you could get some that was just under you thought that you would have the lightest planes on the planet. I currently have a source that is advertising 3/4 lbs foam and that is what I am building with. I have a small sheet of foam that was in packaging from overseas that weighs .46 lbs with small beads. Side note - it is generally thought that the smaller the beads the stronger the foam, but that also meant heavier foam...

When ordering you will need to be very specific with the foam shop, they have a tendency to substitute foams base on what the industry they serve is using the most. More than once I got home and weighed the foam "just to be sure" and found the foam that was supposed to be 1 lbs or 3/4 lbs was actually 1.5 lbs or more. When asked how they got the order wrong they told me that they "upgraded my order" at no charge. Upgraded to what? An upgrade to me would be 1/2 lbs foam... but they just assumed that I was making moldings or insulation as the other 99.9% of their clients are. They other switch they will often pull if they think you are outside of their typical industry is to use foam that is made of recycled materials from their own production processes, they call this "re-grind." Re-grind will likely be higher weight and will have different densities throughout the block which will cause you cuts not come out uniform. Make sure you order 100% virgin - 3/4 lbs (or 1 lbs if that is what they have) - white EPS foam, then when you pick it up CONFIRM that is what you are getting at the dock. I order 2' x 4' x 2" sheets and block them out myself because 1/8" is a common tolerance and that isn't going to work for competition F2D planes... 

I set up the table saw with an abrasive cut off wheel from a chop saw. Thanks to Bob Mears on that little tip. You can make the same cuts with a thin kerf metal blade but it will produce enough foam on the ground to make your neighbors think you are building a winter wonderland!!! With the cut off wheel there is barely any mess. 

First I cut 1/4" off one side of the sheet to make sure the face is square. Then I cut the sheets to the width of the LE taking into account the short rib and tip are 1/8". 

Second process is to cut the 1/8" by 1/2" slots in that will be on the ends of the finished blocks, the slots key in on the hot wire jigs and once at assembly receive the wing tip. Make sure you put the same face of the foam panel to the fence when cutting both ends to ensure the spacing is the same.

Now they are cut in the band saw to 2 1/2" tall blocks. I have a fine blade in my band saw but I have used a course one before with the same results  I like to hook up a shop vac to the saw to help cut down on the mess.

Re stack the blocks and mark one end of them with a marker. This is critical because from this point on the processes are not symmetrical and you will have trouble keeping track of which end is which if you don't. I use the marked end as the root (closest to the center of the plane).

With the Streamer Shuttle the ribs seat under the spars and into the LE foam so we need to cut those slots. I make each slot only the depth of the spar at the spot of the slot. Yes that is 3 slots per block and yes it takes a bit of time but it is TOTALLY worth it later when you are building!!

That's it. Now the blocks are ready for the hot wire and on to construction.


  1. Anyhow. The Pink foam crunches. It is way too easy to break, or to dent. It is very bad about glue; most of the glues I tried never seemed to dry and didn't hold well when they dried, either. The cells are large and open; gesso goes right into it. The only thing that seems to be filling the pores is spackle, and I'm using a ton of it. Which is kinda hurting the whole intent of using styrofoam (aka, the light weight).

  2. Try White Gorilla Glue on the foam. Remember the glue joint only needs to be as strong as the bead to bead strength of the foam. Spackle? its not a stunt plane that needs great looks and finish its a combat wing that is only going to live 1-4 matches on average.

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