Cutting Foam Core

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Foam core is a stiff, planar material which crimps when overstressed, so curved shapes canít be formed.  But it can be cut by hand with near-machine precision, and strong, straight bends of practically any angle can be formed quite easily.  With a few simple tools, some patience and a fertile imagination, one can quickly fabricate some pretty elaborate shapes. 

Tools required:

         Modeling knife with a good supply of sharp blades.  Single-edged razor blades will also work in a pinch.

         Machinistís square.

         Metal straightedge.  The scale on the machinistís square works fine for most work.  Longer cuts and bends may need a     metal yardstick clamped in place at one end.

         A large piece of cardboard for a cutting surface.

         A flat work table.

Other common hand tools, such as needle-nose pliers, may be helpful for handling smaller pieces.  A sewing needle pressed into the end of dowel is handy for holding small pieces and for marking the centers of holes on both sides of a sheet.  Straight pins serve well to help align less manageable joints prior to gluing. 


Simple Cuts:

The keys to making clean, precise cuts are a sharp blade and metal straightedge.  Mark the cut line with a pencil or pen directly on the paper surface.  Then place your work piece on a cutting surface which extends past both ends of the cut.  Align the straightedge directly over the cut line and plan on holding it steadily in place until the cut is complete.  A C-clamp is handy for long cuts, but in most cases, the edge can be held in place fine with one hand while you cut with the other.  Be careful not to crush the foam; itís especially susceptible to crushing at cut edges. 

The cut should be made in at least three end-to-end passes.  The first pass should just penetrate through the upper paper surface and only slightly into the core.  For accuracy, the blade should be aimed slightly into the straightedge so that it wonít drift away; this will also minimize the gap between the cutting edge and the straightedge.   

Start the cut by poking the blade point squarely into the surface through the top paper layer.  Then reduce the angle between the material surface and the blade edge to no more than about 30 degrees to avoid tearing the surface.  Using a steady motion, pull the blade through to the end of the cut.  Remember, the first pass should only cut the upper layer.  A dead end cut may be terminated precisely with a near-vertical poke of the blade. 

Keeping the straightedge in place, make the second pass like the first, except this time, cut through the foam and slightly into the surface of the bottom paper layer.  This pass establishes the angle of the cut edge.  If you want a simple square cut, then be careful to hold the blade perpendicular to the foam core surface through this pass.  Square up any dead ends with vertical pokes completely through the lower paper. 

The third pass should cut completely through the lower paper.  You may dispense with the straightedge this time, but itís still possible to let the blade drift off at this phase if youíre not careful.  Keep the blade angle steady from end to end, and use enough force to cut completely through the lower paper.  If the lower paper is not cut through end to end, turn the work piece over.  Incomplete parts of the cut line should at least be visible as a distinct ridge; if so, insert the tip of the blade into a cut portion and carefully pull it through the ridge, allowing the blade to self-align on the opposite side.  If a ridge isnít visible, then get a new blade and repeat the third pass from the first side. 

A sure sign that your blade is getting dull is ragged cut edges in the foam or tearing of the paper.  A fresh blade is good for about 3 - 5 lineal feet of cutting.  Blade life is definitely extended by use of a clean cardboard cutting surface.  A brand-new blade is a pleasure to use; make sure you have enough on hand before you start your project. 


Making Holes:

Rectangular and polygonal holes can be made simply by a series of straight dead-end 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 work piece.  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 work piece 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 work piece 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 work piece, 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 foam core and draw in the major and minor axes; extend the ends of the major axis so itís 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 foam core, 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 work piece 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.