Making Holes

MAKING HOLES:

Rectangular and polygonal holes can be made simply by a series of straight deadend cuts.  This method is simple and well-advised even for screw holes as small as #6 UNF.

Round holes require a blade with an acutely pointed tip used like a saw on both sides of the workpiece.  Even the smallest holes, as for a #4 screw, are best made using this technique.  A twist drill will pull the paper away from the foam, and forcing a punch through will crush the surrounding foam. 

Start a round hole by marking its center, then draw the cut line on one side of the workpiece with a drawing compass or circle template.  Extend the center mark through to the opposite side using a straight pin or the needle tool.  Be careful to align the pin perpendicular to the surface before pushing it through.  Turn the workpiece over and repeat the cut line with the compass. 

Using a sharp pointed model knife aimed nearly perpendicular to the surface, cut just through the paper surface along the compass line using short, saw like strokes.  Do this on both sides of the workpiece, then carefully complete the cut through the foam, halfway from each side.  The plug should push right out.  This process is also handy for making disc-shaped pieces. 

Elliptical holes, as used on the ATV mirror backing plate joint to the servo shaft, are a bit more tedious.  First, mark the center of the hole with a pencil on the foamcore and draw in the major and minor axes; extend the ends of the major axis so it’s visible on the opposite side.  Then cut a cylindrical object of the proper diameter to the desired angle; for the ATV servo, this was the actual mirror drive dowel.  Hold the cut surface down on the foamcore, aligned properly per the axis marks, and trace around it with a sharp pencil.  Then use the needle tool centered on the ellipse center and tilted to the same angle and along the axis to transfer the ellipse center to the opposite side.  Turn the workpiece over and mark the major axis; this line should fall through the pinhole and both major axis end marks made from the other side.  Draw in the minor axis, and trace the ellipse as before. 

The elliptical hole may now be cut using the same technique as for round holes.  When cutting through the foam, bear in mind that the blade angle relative to the surface will vary from perpendicular when crossing the minor axis to the full angle crossing the major axis; allow the blade to self-align by passing it through both sides while cutting the foam.

MAKING BENDS:

The simplest “bend” is just an end-to-side butt joint between two separate pieces of foamcore.  This method leaves an ugly raw end showing, which can also allow moisture penetration.  Its strength depends solely upon the paper peel strength. 

Strong, accurate and stable linear bends can be made by first cutting a V-groove on the inside of the bend with a depth just down to the inside surface of the outer paper.  After the V-shaped strip is removed, apply a bead of hot-melt glue to half-fill the slot and form the final bend.

After the glue sets, the bend is permanent and in most cases doesn’t need reinforcement.  The outside corner is neat in appearance since the paper runs continuously around it; this also contributes to strength and moisture-resistance. 

Steps to making a miter bend, viewed in cross-section

step 1

step 2

step 3

The width W of the V-groove at the inner surface is determined by the depth of the groove, D, and the inside bend angle Θ.  For this purpose, D should be the foamcore thickness T minus the thickness of the outer sheet of paper P.

For a right-angle bend, W = 2D = 2 (T-P).  Smaller bend angles require smaller groove widths, and vice-versa.  For you precision freaks, the groove width W may be calculated for any material:

W = 2(T-P) tan(Θ /2)

To cut the groove, mark the two edges of the groove equidistant on either side of the center.  Plan to cut each side of the groove in two passes.  The first pass may be done with the blade perpendicular to the surface, just to cut the paper.  The second cut should be done with the blade angled so the tip just contacts the inside of the bottom paper at the midpoint of the groove; the blade angle from vertical ideally is one-half the desired bend angle.  The care with which this pass is made on both sides will determine the straightness of the outside joint and the precision of the inside seam. 

After the cuts are made on both sides of the groove, the V-shaped scrap should pull out cleanly.  If not, don’t fret; any gaps in the foam will fill with glue.  At this point, it may be a good idea to form the bend and check it for accuracy before committing it to glue.  If the outside bend is a bit uneven, place the inside of the bend over a square corner of the bench and lightly burnish both sides of the outside corner with a ½” diameter dowel or some such tool. 

With the groove lying open, apply a bead of hot-melt glue with a diameter about ½ the groove width.  Then form the bend and hold it at the proper angle until the glue sets.  A couple of square-cornered blocks of wood or metal also serve nicely in this role if you are compelled to a higher calling.  If a clean inside corner is required, excess glue expelled from the joint can be removed while it’s hot using a scrap piece of foamcore.

Key points regarding bends:

• Make sure the groove is cut on the INSIDE of the bend; I blew about an hour’s work on the ATV module walls with this stupid trick.
• Dimension the location of the bend on the INSIDE surface to the EDGE of the V-groove, not its center.  When the bend is made, the two edges will come together to form a sharp inside corner. 
• Cut the grooves for all of the bends in one piece before you glue the first one.  A flat workpiece is easier to manipulate.

UNFOLDED JOINTS:

All balloon payload packages eventually must completely enclose a volume.  It’s not possible to form such a structure from a single sheet with all joints folded and no exposed seams; at least I haven’t figured out how to do it.  Even if it were possible, it would still be very desirable to be able to open the structure in order to put something useful inside.  The bottom line is that unfolded joints, such as butts, laps and miters are unavoidable.  They can be made quite serviceable, however. 

If you’re making a closed form where two cut ends must come together, make the final joint as a miter at one corner.  Rough out your workpiece to allow at least ½” extra at both ends; its very hard to cut a clean miter right at the end of a square cut, since the foam tends to crush easily there.  Glue the mitered ends together and add a ¼” square stringer inside for strength.  A piece of space tape over the outside corner will finish the joint. 

If a butt joint is unavoidable, reinforce it with a ½” or wider strip glued to lap the joint.  Glue the reinforcement strip to one side first, then apply a bead to the exposed foam edge and the open surface of the reinforcement before closing the joint. 

The strength of any joint can be significantly improved by adding triangular reinforcement webs on the inside corner.  The price is added weight and lost interior volume.

“CURVED” SHAPES:

Foamcore doesn’t bend without crimping, since paper doesn’t stretch.  So don’t plan on forming a truly curved wall.  A curve can be approximated by a series of parallel, close-spaced small-angle bends to form a polygonal cylinder.  It should be possible even to form a near-spherical polyhedron, such as a dodecahedron (geodesic dome), from a single sheet. 

The width W of each side of an N-sided polygon circumscribed around a circle with radius R is:

W = 2R tan(Pi/N)    computed in radians

Example: an octagon which will fit snugly over a 5” radius (10” diameter) cylinder has INSIDE panel widths of:

2 * 8 * 5 * tan (3.1416/8) = 4.142” (4 9/64”)

When laying out the cuts on your workpiece, remember to add one groove width to the inside width of each panel. 

CONICAL SHAPES:

Again, a true cone can only be approximated.  But a steeple-like conical extension can easily be added to polygonal cylinder.  When the workpiece is laid out flat with cylinder grooves already cut, make a cut parallel to the base of the cone at a distance equal to the side of the cone above the intended cone base.  For a cone height H, and cone base B across opposite outside flats, compute the side length S as:

S = √ (H² + (B/2)²)

Extend the MIDPOINT of each side from the base of the cone up to the cut edge.  Draw light reference lines from midpoint marks at top cut to the centers of each adjacent cylinder groove.  You should end up with a sawtooth-looking pattern when you’re done.  Then draw two cut lines parallel to each reference line from the ends of each groove to the top.  Cut V-grooves on these cut lines.  Then cut through the bottom paper of each of these new grooves and pull away the triangular scrap pieces.  You will end up with a crown-shaped workpiece. 

Form and close the cylinder in as above, then pull the steeple pieces together and hot-melt the butt joints together.  The seams may be sealed with space tape. 

It is also possible to form the same structure with closed outer seams by making a series of grooves radiating outward from a common center point corresponding to the tip of the cone.  In this case, the cylinder at the bottom will have butt-joined seams.  This method may be preferable where the strength of the cone is deemed more critical than that of the cylinder.  Details of such a layout are left as the traditional exercise for the reader. 

A closing observation on cones: if the height of the cone were to approach zero, one would end up with a flat cap closing the end of the polygonal cylinder.  This end might be more easily achieved with a separate piece glued in place, however.