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12" foam NC

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David Schwantz

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Trying to turn a NC out of pink foam. 12 " dia. Ain't workin so far. Any tips from those that have done it? Thanks.
 

heada

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Hot wire the rough shape and then sand. Most lathe tools wont work too well on foam and it'll tear more than cut or shear. On odd shapes, I cut slices of the shape from thin plywood, 1/8" so I know what shape to sand to.
 

dhbarr

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What are you gluing it together with, and what are you supporting it with ?
 

David Schwantz

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Glued the disks with epoxy. PVC down the center. Yup, tearing the foam and lots of chatter. Tried chucking it up in my drill press. On the slowest speedit has.
 

David Schwantz

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Sent email to python. I'll let you know what they say Bat. Thanks.
 

heada

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I do it on my wood lathe at it's slowest speed which is ~500rpm

I cut the foam into discs rough shape and glue plywood discs between the layers of foam. The plywood has been cut on the bandsaw and sanded to the exact shape I want. I then spin the blank on the lathe and use 120 grit to get it close to the plywood. Once within 1/4" of the plywood, I step it up to 220 grit, and go to 320 when its within 1/16" of the ply. It'll make LOTS of staticly charged dust so be ready for that.
 

neil_w

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It'll make LOTS of staticly charged dust so be ready for that.
Genuinely curious, how exactly do you get ready for that? If there's a smart way to manage statically charged foam dust I'd love to hear about it.
 

kalsow

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I hacked together a low quality version of the setup described by python rocketry Mine has a 5/8" steel axle, wooden end-stands, a windshield wiper motor and speed control from monsterguts.com and a laminate router on a wooden stand. You're welcome to borrow it -- I'm also in MN.

I've turned a few biggie NCs on it.
 

heada

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Genuinely curious, how exactly do you get ready for that? If there's a smart way to manage statically charged foam dust I'd love to hear about it.
I use my dust collector with a collection hood directly over my lathe and make sure that not only the dust collector is fully grounded but also the 4" line from the hood to the collector. I change out the collector bag before hand too. I don't want to mix foam and wood dust but also a new bag gives the best dust collection. I use a full face shield and spray it with fabric softener before I start. No helping the clothes, they're gonna get foam dust all over them.
 

David Schwantz

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kalsow, thanks. I am in the south metro just past Lakeville. Do you by chance fly in North Branch?
 

David Schwantz

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I did not glue ply discs between the foam. Thought I might try my belt sander as it spins. Think about all THAT dust:)
 

Kelly

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Too late for the project you've already started, but... cut the foam to disks, as close as possible to final size, before gluing together. This allows you to do most of the work with sandpaper. I start with super-rough grit (like, 40 or 60) and go from there. I also avoid using epoxy - it is so much harder than the foam, that the foam tends to sand away faster, and you get tell-tale ridges wherever there is epoxy. The best thing I've found so far for glue is the expanding foam that you use to fill gaps around the house.
 

kalsow

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I also avoid using epoxy - it is so much harder than the foam, that the foam tends to sand away faster, and you get tell-tale ridges wherever there is epoxy. The best thing I've found so far for glue is the expanding foam that you use to fill gaps around the house.
Yep! Epoxy is too hard. I use gorilla glue (urethane). It expands so you need to clamp it while it sets. Still, gorilla glue is harder than foam. If the discs are big enough, I leave a couple inches near the circumference unglued.
 

ThirstyBarbarian

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I built a foam rocket by hand, which I really do NOT recommend, but it was a fun project and produced a rocket about the size of a LOC Warlock that was light enough to fly at low power launches on G motors. Very fun! Later I glassed it by hand, and now it’s too heavy (4 lbs) for a class 1 rocket, but it’s still very fun to fly.

I swore I would never build one the same tedious way again and did some research on better techniques and equipment.

I think these guys kind of pioneered big foam rockets, and they used a hot wire setup guided by a template. This site is stored in a web archive, so it’s kind of clunky, but still useful. https://web.archive.org/web/20150910054712/http://www.vatsaas.org/rtv/

Here‘s the kind of setup I want to build. This builds on the Vatsass brothers’ template technique. It’s a lathe type setup, and he uses a hot wire cutter that is guided over the foam slug using a 2-part template mounted on opposite side of the lathe shaft. After the rough shaping, he smoothes it using a sanding board on the same template. Then he glasses the part right the on the lathe. Then smoothies the glass using a sanding board and the template. This seems very slick to me. http://speedmotionrockets.com/Foam Lathe.html

This place sells a lot of foam cutting tools that I think will be handy. https://hotwirefoamfactory.com/

I’m looking forward to seeing what you come up with.
 

prfesser

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The foam will flex some under rotation. Get the entire block down to a reasonably cylindrical shape, then work on the tip. Less mass at the tip means it flexes/vibrates less under rotation. I just made a balsa nose 3"x12" for a WAC Corporal scratchbuild.
 

David Schwantz

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I cut the foam into round disc. The base being just oversize of what I need. Thought I would add the shoulder after.
 

jbr

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I cut the basic shapes from 4" foam on the bandsaw
put a 3/4" hole in each middle
glue them together on my 3/4" threaded rod
put that on sawhorses with conduit straps to hold it in place
I turned down the ends of the rod and use a variable speed drill to power the rotisserie
use 60 grit sandpaper to get the basic shape
use 120 to get the fine details
the hard part is cutting and gluing on fiberglass
 

David Schwantz

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Using another chunk of sonotube for shoulder.
Will epoxy another foam disc on NC and shoulder around that. Also will have the PVC pipe down the middle of NC, so might add a dowel to go through it all.
 

3stoogesrocketry

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Im going to be that guy and say don't waste your time glueing rings together. The last big cone 13 inches diameter I built used foam and ply. First I got a full scale 2D printout. I then cut two 2D scale cones out of 1/4 inch baltic birch ( luan works as well ) and glued them together in half lap joints to form a + . Then you can simply lay square blocks of foam together and cut those to shape . Overall it's alot less work.
 

TonyL

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My friend once made me a 12 inch ogive from pink foam by building up a lathe set up, and he said his garage was pink for a very long time. That was my motivation for the stacked disk method. Controlling dust on the bandsaw has been much more manageable.

I cut the disks with correct tangent bevel on each, and use a standard body tube size hole in the center. The center hole can be cut with a hole saw or come in at one spot with the bandsaw. The kerf of the bandsaw seems to be inconsequential as long as all the joints don't line up. I print out the templates and glue them to the foam, makes cutting fairly easy to follow.

3M sells a styrofoam spray adhesive that seems to work well enough. I find epoxy does not really stick to styrene and body work materials like polyester fiberglass resin and body filler both dissolve styrene quickly [as do some paints].

Gluing the disk stack on a body tube also provides for a usable space inside the nose, and a place for a deployable/changeable tip.

I made a sanding board by gluing a piece of 60 grit to a piece of wood. I find a spiral [like a barber pole] sanding pattern seems to get the most uniform results. With tangent bevels on each disk one only has to level the joints and it is done. Sanding seems to minimize tear outs compared to cutting, and hot wire requires its own particular skills.

Filler directly on the styrofoam I find challenging and nothing has worked great. Lightweight spackling compound has worked ok for big divots. If one is fiberglassing it, one can address smoothness in the laminate.

Sometimes I just cut the disks, glue them together and fly it like that, looks fine at 1000 ft. I also add a plywood layer to the stack wherever I want a hard point.

hope this helps,

Tony
 
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