Foam cutting


Article published on MODELLISMO , issue Nr. 39 (May-June '99)              Download here the plan.

Use a 1" thick particle board  (or, better, a medium-density one , easier to mill) of approximately 64" by 28". You'll  mill two parallel tracks at respectively 10" and 20" from the pulleys side. A reference grid drawn with a step of 4" of lines parallel to the short side can be useful for wing alignment. There's an L-shaped Al extruded bar (1" x 1") screwed to the pulley side (see Photo 1), to be drilled with a 1" step on both sides, and alternating holes every 1/2" to avoid interference between pulleys. Drill the upper side holes nearer to the bar edge, in order to avoid that traction cables touch the bar edge running from pulley to pulley.

They are four and of plastic type (Photo 1), easily found at the hardware store (2" 1/4 ext. dia., 1" 3/4 int. dia.). They rotate around 4 axis, to be realized with simple screws of alternate length of 2" (2) and 1" 1/4 (the other 2), to avoid traction cable overlap. The traction cables are dacron  ones for fishing use, easier to flow back and forth than  nylon or steel ones, to be avoided 'cause they twist too easily.
The name of the game here is "Avoid friction !"


I used a wood one (1" x 1" x 74") fixed and  free to rotate at  20" from the left. The bar rotation axle is a 3/8" screw surrounded by a piece of fuel line and long enough to pierce through the bar and blocked to your working table. It has to simply sustain the wood bar and the counterweight at the bar right side. You'll statically balance the bar as a prop with a fixed weight on the left (1 lb at 8" from the axle in my case), while there will be a variable traction weight on the right bar tip, which is function of the cutting speed (start with 1/2 lb). It is necessary to be able to horizontally block the bar during the traction cable zeroing procedure, with the cutting bow exactly at 50 % templates advancement. Foresee on the bar a way to block each traction cable with a 1" step, starting at 20" from the axle up to the bar tip; I used simple mammouth  termination for electrical interconnection,  by screwing them to the bar (Photo 2), but you'll surely find a better method. I wait for your suggestions, but a precise and reliable method is a must. Avoid nails and cable tightly turned  around !


Photo 2

Del Brengman suggestion is the best: .02" thick Al sheet . I find .01" Al easily as litho plates surplus, but I'm going to try .01" brass sheet, less soft than Al vs the cutting wire and bow weight. The main advantage of Al is that you can cut it with scissors and will be carefully finished to airfoil profile with 500 grit and more wet paper. After bow and cables various alignments and just before cutting, a spray of Teflon lubricant is a must. Templates shall be prepared  1" longer both aft and forward of profile, in order to allow a smooth ramp for the cutting wire, specially at the leading edge.

Also remember that templates lowest point shall be at least 1/10" higher than the template holder to avoid cutting wire interference with the holder itself or of the bow arms tip with the table. The V-notch at outlet is for wire stop after the cut. The other notches in each template  base are in correspondence with the templates holder blocking bolts (Photo 3). It is useful to mark on templates the advancement along the chord line at 5-10 %  intervals, but it is imperative to mark the foam block reference lines, to pass from lower template cut to the upper one with maximum precision

Photo 3

It is realized with two side by side wood spars (3/8" x 3/8" x 14") bolted together with the template in between. The holder is kept in position in the track by "expanding" a fuel line inside the milled channel with the help of a bolt and two washers. The fuel line is high as the channel depth minus the bolt head (bolt mounted head down) and setup is done by closing  the nut until the line expands between the washers and the holder is kept in place. Forces in the channel direction not existing during the cut, the friction generated by the line is more than enough to keep the holder securely in place.

Lightness is a must. Build more than one, in order to have no more than 4"  free on both wing sides. I prepared one with a 40" x 1" x 2" wood bar, by drilling the bar with 3 holes couples at 18", 14" and 10" from its center for wingspans of, respectively, 32", 24" and 16". The two bow arms are made of 3/8" piano wire (18" long ) and are mounted by drilling diverging holes in the wood bar (Photo 4). They are kept in position with the help of one brass bush with a grain, so to have an effective arm length of 14" at least, to avoid interference during the cut with the foam block and the distributed weights (lead pieces on a glass plate) over it. The wire I actually use is .025" Monel (a copper-nickel based alloy), which you can find in fishing shops.

Photo 4

Do not use piano wire because it will get brittle after a few thermal cycles and most probably in between a cut. The cutting wire has to pass around the bar (Photo 5) in a small cave prepared with a file the nearest as possible to the bar tip (less than 1/8"), in order to avoid that the bar tip touches the cutting table. Last important pieces are the lead weights to be put on both bow arms. They shall be movable on them in order to move the bow Center of Gravity, depending on the cut of either the upper or the lower profile. At profile leading edge, which is the max slope point, in order for the wire to exactly follow the lower template, both weights shall be advanced to the arms tips, while for the upper template, being the leading edge an almost vertical wall, both weights shall be moved back, in order for the wire to more easily ride the profile. You can use about 10 oz of lead sheet (as plumbers use) per arm, to wrap around them (watch out always for possible interferences). Weights must be blocked in position during the cut (I simply use tape). The bow shall be hanging in the most neutral way as possible; I use two rubber bands connecting both bow tips to a rope laid over the table and exactly over the foam block center line (mid root to mid tip).

Photo 5

You'll easily understand from above the importance of a light and simple bow. Pay attention that rubber bands be vertical to the working table and to the foam block, to avoid lateral forces on the bow.
Keeping always well in mind to avoid friction, avoid also that supply cables could brake the bow. Both traction cables shall be hooked to the cutting wire tips always in the same positions, to allow the speed ratio between root and tip to remain constant. I use simple piano wire hooks (or fishing snap ones)connected via mammouth to the traction cables and hooked to the cutting wire where it wraps around itself and the bow arm (Photo 5). Monel wire shall be drawn around each arms cave, then around itself 3-4 times and finally back around the arm again, in order to create a loop where to hang the hook. Most important is that both traction cables are perpendicular to the cutting wire during whole cut.

Both sides cutting speed setup (corresponding to the lever ratio on traction bar) will not take into account the root/tip chords ratio, but the ratio of the effective paths travelled by the hooks, so this ratio is rather different than expected at a first glance. Really the path at root is longer than the root chord, while at tip it is shorter ( for a trapezoidal planform, of course). Seeing is believing !
Blank tests shall be done firstly by blocking the traction bar in a horizontal position, the bow exactly over the mid-root /mid--tip center line, and pulling both traction cables until you feel exactly the same tension. You can use two pieces of wood against the Al extruded bar to stop the bow, while I use a highly technical system to hold the traction bar : a string hanging from the table !
After this it's now time to test if the bow advances contemporaneously and equally on both templates for every advancement percentage (previously drawn on templates), starting from L.E. and sensing with your fingers whether the tension on both tractor cables is the same when the bow crosses both 100 % points (i. e. at T.E.). If necessary, fine tune the lever arm ratio on tractor bar and the tension on tractor cables, but again with horizontal bar and bow at 50 % advancement on both templates.
Believe me, your fingers sensitivity is more than enough to judge on equal tension of tractor cables: push with both inches on bow arms tips and fine tune until you feel the same "weight". When satisfied (typically never !), fine sand again both templates with 500 grit, because blank tests always damage a bit the template surface and the wire could stop there. Personally I even spray Teflon on templates just before the cut.
At last it's time to position the foam block (to be wider than chords of about 1/4" on both L.E. and T.E. for better cutting wire's temperature stabilization and because it is better later during vac-bagging) and cover it with a thick glass sheet with narrow weights over it, in order to avoid even the smallest movement of the block during the cut. Put now the bow in position at L.E. and switch on the power. You'll have adjusted before on some scrap piece of foam the right cutting temperature, until you get the famous "angel's hair" on the freshly cut foam surface.

I prefer to cut a bit colder (30-40 sec to travel 8"), in order to minimize grooves on foam in case of speed unevenness.
For the upper template is always necessary to help the bow with your fingers at the start to ramp up, because, even with the bow C. of G. full aft (with the help of both weights near the bow bar), this help is in practice not enough for the almost vertical wall at the nose.
I always start with the lower template (both weights fully down) and, after this cut, you'll never touch anything to see the result (won't you, O.K. ?), because you won't be able to position the foam block exactly in the same position anymore and you'll kiss your so much beloved accuracy good bye.
Change at this point the templates with the upper cut ones, without  forgetting to move the bow arm's weights fully aft towards the bow bar.
Finally, (Photo 6) if the result won't be up to your expectations, I authorize you to sigh and cry desperately for the time and foam lost and to curse Del Brengman (sorry, Del !), me and my dear friend Beppe Ghisleri, who pushed me to write down this terribly masochist experience.

Photo 6

                              Tullio Bonfiglio