Cant find Depron? Want lighter Polystyrene sheets?
Then this is for you.
Depron (XPS) is great but hard to get and it costs valuable $$ here in NZ, while Fish bins are free and made of lighter weight but good quality EPS.
This toy is used to fabricate sheetng from EPS, I have had success in cutting sheets as thin as 3mm.
EPS sheeting will allow you to build planes designed for balsa from equivalent section of styrene.
Step 1: The Skimmer Board Construction.
Pretty simple this one, grab a FLAT board (mine is malamine for slipperyness), a guitar string, couple of screws, a spring and some tapered bits of wood.
Make notch at the half way point on each side of the baord for the wire to pass through.
On the back side use the screws to anchor the wire.
I have a spring for tension which may not be a good thing as it also allows the cutting wire to bow. This was done to ease thickness adjustment and the bow in the wire does not really effect performance as cuts are linear and horizontal. As you can see the back of the board is a bit of a mess on this build.
Tapered blocks of wood, carefully marked and notched are used for setting the gauge of The cut.
Mine are at notched for 6mm and 10mm while the others are for doing tapers (see later).
If you want to get "rocket science" on this the a feed tray powered by a geared DC or stepper motor will make more uniform cuts, find an old photocopier for parts.
Step 2: Skimming in Use.
Once ou have the board made and blocks in place, add some power to the cutting line (about 2-5A at 12v depending on the string gauge used) and crefully slide the polystyrene through the wire.
All going well you will end up with a nice uniform sheet of poly.
The sheet can be finished with 80grit wet and dry (black) sandpaper, if you find the sheet is bowed it is likely due to skin tension from the melted layer which the string makes, after sanding it should be ok and flat.
I use the van batteries to run the wire with a basic 555 timer based dimmer control (fairly standard circuit above) but the fet has been upgraded to something that can handle 16A (STP16NF06 60V 16A)
The Van batteries are charged off solar pannels.
Step 3: What Can You Do?
I used this to cut 10mm sheets for tailplanes and fins for the above models, a scaled Chris Foss WOT4 and the ME109.
If you set the board up with one block only it is posibble to cut each side of the fuselage taper, its easier and more accurate than sanding them. Second pic shows the ME109 fuse with taper. (yes the profile is wrong, I used the wrong thrust line in making templates so these will go in the blender to make filling for epoxy, back to the drawing board)
The WOT4 fuse was made with two pieces of 30mm EPS from fish box lids, after the side profile was cut and taper in place I skimmed the sides off the block and gutted the remaining piece to make room for radio gear. and Epoxy was used to glue it back together.
I still hand sand the wings, inital blanks are sut to shape with a sharp blade, centerlines carefully marked and using templates of root and tip foils made from printed foils glued to cardboard, the profiles are drawn on. The rasp make inital shaping fairly quick, they are finished with 80grit.
Step 4: The Ole Tiger.
Latest creation is an RC 200% scaled version (800mm span) of the Ole Tiger C/L model from Aeromodeller magazine, I used to drool over this plan when I was a kid..
I have intentionally made this profile as it is a scale version of a model plane not a scale model of a real plane.
You can see I have a sence of humour...
This model is in contrast to the skimmer instructable fully made with razor blade and sandpaper, no hot wire cutting at all.
Wing root template is shown in the pic and is a symetrical 20% NASA 63 section from here:
I used LibreCAD on Puppy linux to import and scale the image taken from a screen capture.
Using the fish bins the one owes me cost of glue (Gorilla Grip mixed with a drop of water to aid foaming) or about $10NZ. Ply sides are from the bottom of a box I found and wing spars (there is one in the rear supporting th TP also) are 6mm fiberglass tent poles, no money spared as you can see..
All up weight at present with radio servos motor etc and 3x18650 li-ion cells is under 800gm, at 1kg wing loading is ideal at 19oz/sqft.
She will be painted yellow, and named "Pork Pie", after the 1981 NZ movie "Goodbye Pork Pie".