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Episode 23 - Building with foam.


Crazy Engineer
Hi guys,
1/2 way through this one. I'm slowly working my way through your podcasts.

Good to hear Nick's building, taping, cutting techniques. Found it interesting that the three of you have very different styles. I have my own, kind of a mixture. I used to tape the plans together, cut the perimeter leaving about 2 cm. Then I would cut a few holes internal to the part and stick tape across the hole to attach the plans to the foam. Cut perimeter, then all internal marks. Now days I just lay the plans on the foam and poke holes at the corners and at the placements of marks. For curved edges I just use a hole punch every 1 to 2 cm and cut from dot to dot. When I am peeling the plans off the foam I usually grab a pencil and mark the holes with lines indicating the general direction to the holes preceding and following. Then cut dot to dot. I only print one wing and do what Kevin suggested, flip the plan over and punch the holes. One set of plans for two parts makes them more similar. I tried cutting two sheets thick, but I found that to be pretty difficult.

To tape plans together, I print on 11x17 paper. Well, even before that, I take the plans and reformat them in Illustrator. The reformatting allows me to orient the pieces together in a way that I'm only taping 2 to 3 edges together. I found it hard to get the 20 some pages square, but I can do 3 to 4 pretty easily. So, I tape 4 together and then use the second set to make the next set of 4 tiles. By punching holes through at corners and marks makes the plans reusable.

Good information guys!
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I can't remember which episode, but I think this was the one when someone said something about making a single wing spar out of 1" x 1" square dowel? Laminates are always stronger than single pieces because of mixing the wood grain (same applies to carbon fiber, fiberglass etc.). So it would be my recommendation that if you need a 1" x 1" spar, make it by gluing four 1/4" x 1" pieces of wood together. The common wood glue would do very fine as would epoxy. By gluing several sticks together, the wood grain will not traverse the width of the wood.

You guys also mentioned skewers. Handy little things. One suggestion for the 1/4" size is Walmart sells bamboo marshmallow roasting sticks that are 36" long and 1/4" diameter. I got a pack of 24 for a few bucks. These can also be glued in a triangle for a really strong but pliable support.
I liked this episode lots. When I saw the title was a little worried that Steve was going to tell us how to build a helicopter with foam.. that would have been a good trick!
Gluing Sheets of Foam Together

I've found that the best way to glue two sheets of foam together is using white Gorilla Glue. Remove the paper from the foam, if you don't, the weakest part of the joint is the paper delaminating. After the paper is removed, smear Gorilla glue on the sides of the foam you want to glue. I use a playing card to squeegee off the excess. I have done this by putting glue on on of the two or both sheets. Because the glue expands, you will definitely want to hold the sheets in two ways, you want to hold them together and you want to hold them so the they do not slide relative to each other. I use a couple of straight pins and stab them trough the foam and then weight the foam. Let cure...

I have also used flooring adhesive. This also works really well. Dilute it about 50/50 with water that had a touch of ammonia added. Lay the pieces of foam on top of one another. Tape one edge with packing tape or masking tape. Open the two pieces of foam using the tape as a hinge and lay flat. With this method you need to have a comb that allows a very small amount of the glue behind as it is squeegeed off. I don't have a good tool for this, but I was thinking about a playing card that had a bunch of slits cut into it with a knife. Smear glue on both facing sides. Let dry until it gets tacky. Fold the hinge shut and it immediately bonds. It will dry over the course of a week or year, but it always stays pliable like rubber cement. Very good for parts that need to flex.
I liked this episode lots. When I saw the title was a little worried that Steve was going to tell us how to build a helicopter with foam.. that would have been a good trick!
I'm sure if it could be done Steve would try it! Thanks so much for all the comments. I really found them informative and well try out a few techniques next time!