Foam Cutting

[piker]

O.K., Alan has been asking about foam cores for wings and how to cut them.  I promised to show my simple process for his reference, and figured it would be useful for others. Starting this thread is also an invitation for questions and input from others who have experience and different ways of cutting foam.

My approach is as simple as I can make it as I don't cut wings very often, so I'm not interested at this point in making a more intricate process.  This one gets the job done well enough for me.  In the past, I have created slightly more "production" type set-ups for multiple copies of the same wings, and then there's the process I used for creating my Sandringham fuselage from foam... but that was really using the same style of cutters and templates, but with a different approach to the sectioning.

I'll try to keep this as brief as possible     The pictures should do most of the talking...

To cut a wing:

Cut the foam blank to the plan form of the wing.  I used right angle guides, hot glued to the table, to create the vertical slices.  The wood guides are covered in aluminum duct tap to help the wire slide.

Build a hot wire cutting bow in an "H" shape with the wire tensioned on one side and a means of maintaining tension on the other.  One of the bolts is loose to allow pivoting.  Power supply wires are attached at the ends of the cutting wire. 

Create cutting templates to guide the wire through the foam.  In the case of a wing, these are the root and tip section minus the sheeting thickness.  I usually cut my templates from 1/8" aircraft play, sanded very smooth along the edges.  I've also used cardboard edged with aluminum tape, but this creates a rougher surface (fire for the sandringham fuselage as that was to then be sanded to final shape), and I've also used aluminum for templates, but it's harder the cut and file to shape.

Draw a reference line on the end of the foam for aligning the template.  Hold the template with pins.  When locating the templates you can choose to introduce twist for washout, and/or Dihedral if desired.  keep in mind that the "beds" (the left over foam) will be used to clamp the sheeting on while gluing, and if gluing the full span at one time as I did with my racer wings, the dihedral has to be considered carefully.  In my case I set it up so the top of my wings are flat, and cut the cures so I could cradle while gluing the sheeting upside down on the flat bench.

I usually cut wings with a root and tip template and require an assistant for guiding one of the ends.  While doing this, mark the templates along the top and bottom surfaces with the same number of marks for the root and the tip.  When cutting, one person will call out the marks as the wire moves along so the assistant can keep at the same pace.  This is required to get the proper shape of the wing.  It's critical.

Always cut on top of the template and use gravity to help hold the wire on the template.  It's VERY difficult to cut while lifting.  The lifting and feeding forces are hard to coordinate.

Cut with a smooth, steady hand.

The templates need a surface for lead-in and trail-out of the foam.  Never stop while in the foam.

In the pictures, I show one template, with the marks, that I've used in the past.  The other templates are a similar wing section but with a unique template for the top cut and another for the bottom.  This offers the advantage of more template area for stable pinning, but the drawback is that the aligning the templates is critical to create proper airfoil section.

For my most recent wings (the ones in the pictures) I used a different technique that eliminates the need for a tip template and a helper.  Because the wing is tampered, I could secure one end of the wire at a point where the wing LE and TE meet (in this case about 45" from the root), and simply follow the root template.  The plan form taper and thickness tapper are created automatically.  But, the location of the point is different for when the wing is right side up or up side down, AND the height off the table has to be carefully determined.  I drew the plan form on the table and used that for locating the foam and the fixed point, and I used spacing blocks to lift the wire off the table the appropriate amount.

When cutting wings, don't forget to make a left and right.

I'm using Ben's Astro Digital charger as a power source, from 12 volts.  This charger has a "test" feature that disables the "peak" function thereby providing a constant, adjustable current.  You enable the "test" function by holding down the red button while hooking the charger to the supply.  Then you press again to "charge" "Slow", then again for "high".  Then adjust the currently.

I usually use Nichrome wire, available at hobby shops, but this time they didn't have any so I used very thin music wire.  The music wire required about 3.7A for a good temperature, whereas Nichrome usually only requires and 2.5A

I used pink foam this time but blue and white are also good options.  Blue and pink are about twice as heavy as white, but more dense so stronger.  The weight penalty was insignificant for my small racer wing.

When cutting blue or pink, you'll end up with "hair" on the foam.  This is actually a sign that the temperature was good.  It sands off easily with a very light sanding...it's not even really attached.

I usually cut my blanks with the LE of the wing chopped off the foam so I have a good LE reference for the templates and space for a good lead-on with the wire.

To give an idea of cutting speed, my 5-1/5" root cut took almost 60 seconds, but let the heat of the wire cut the foam at the pace it wants to.  Too much heat will create a low spot in the foam, and too little... well as long as it's cutting, there's no such thing as too little, but really slow can be tedious.  Still, don't rush the cut. 

Ummm.....  Any questions?

[piker]

More Pictures:

[sihinch]

Awesome write-up, thanks! 


Can you cut me some cores? 

[Bigstik]

Thanks Rob. It's greatly appreciated!

Bigstik