Why am i not getting a smooth finish (styrofoam) Iam using 28g nichrome fed by 2.25A 12 v ac and wire length is 10 inches. This is just a trial to check if my setup works and what kind of finish i would get. I will go on to cut bigger wings once i overcome all the finishing issues. Any tips and comments by seniors are welcome.

Your wire is way too hot. At the correct temperature, the wire should move through the foam at about 1/8" per second and there should be some very fine "whiskers" left attached to the foam. A variable amperage power source is most desirable. Get a 5 amp 120 volt to 24 volt-AC transformer to isolate you from touching a live wire and ground at the same time and accidently "offing" yourself. Then connect a common 120 volt light dimmer switch in series with the 24 volt circuit to adjust the heat. Then gradually turn up the heat until the wire just starts to cut the foam. Assuming you will be hand cutting the foam cores, you will need two people to cut the cores (unless you build a fancy geometric jig to exactly position the wire). Mark the cord line on each template. You will need to do this to accurately align the templates to avoid (or add) wash-out or wash-in. Both templates should be divided into the same number of increments and each increment numbered. For a constant cord wing where the two templates are identical, just mark 1/4" increments on both. If the wing cord tapers, then measure the cord and divide it by 2, then divide those sections again by 2 and again and again until the longer template has about 1/4" spacings. Then do the same for the shorter cord template so as to have the same number of increments. You run the wire for your end of the wing with the longer cord template and call out the numbers of the increments as the hot wire passes them. The second person guides the foam block along their template, feeding it at a rate to match wire location to the increment numbers you call out. This alignment/indexing process assures the wire cuts at a constant speed which is essential to getting a usable core. Also, make sure the templates have absolutely smooth edges, If the hot wire snags at any point. it will melt out more foam. Know that the wire needs to be tight to minimize the middle of the wire lagging behind during the cur. Start the cut at the leading edge so the wire has not had a chance to lag behind here, otherwise you will get a dished out leading edge. Also know that it is very difficult to cut a trailing edge to a feather thickness because the lag of the wire will end up cutting a concave scallop at the trailing edge. Cut the trailing edge of the core to match the thickness of the balsa piece to be attached to it. Good luck!

Thx for the detailed explanation. I will make sure i follow these vsluable tips just one question though i have seen in mamy videos just one person cutting the cores so wondering why do i need a second person unless its a veey big wing. Thx anyways and i wiill soon post sn update.

Videos tend to present an ideal situation. Not to say they are being deceptive, just that they have the luxury of showing their best takes. That said, there are lots of things that you can do by yourself with enough practice. Doing it with two people can allow for keeping both ends of the cut synchronized. Marking the templates from 1 to 10 along the chord of the rib allows both people to move at the same relative pace across the wing as whoever is on the longer chord counts off the numbers.

Aviater,

The reason for some of the deep "gouges" running the span of your core is due primarily to a non-uniform wire travel speed. To slow (or stopped momentarily) and it melts out a groove. When you try to control the speed of the cut at both ends by yourself, you will invariably get the result seen in your photo, especially when the wire is too hot. When the wire is at the correct temperature, there will be very fine whiskers attached to the core and the kerf width will be no wider than the wire. If the melted out area is any thicker than the wire, then it is too hot. It is nearly impossible of one person to properly control the speed of both ends at the same time. Best to get a helper. If you are in a model club, I bet there are several people who would be willing to assist and learn along side you.

Assuming that you will eventually cut symmetrical or semi-symmetrical wing cores, be sure to mark the ends of the foam block with a cord line reference line to align the templates. Be sure these lines have no relative "twist" between them or you will get a twisted core. When cutting the cores, always start at the front of the foam block on the cord line and cut back to the leading edge of the template. It is important to cut in slowly to the leading edge to give the wire time to "catch up" in the middle of the span. Otherwise, the leading edge will not be straight. Don't wait more than a second or two at the leading edge before starting cutting the upper surface so as to avoid melting a groove along the leading edge. Be sure the wire is stretched tight. It will get longer as it heats up and loosen some, so the cutting bow needs to have some form of spring tension on the wire to keep it taught as it heats and cools. If the wire starts to faintly glow at any time, then the temperature is dangerously too high... by about 500 degrees F or more. Be sure to do all cutting either outdoors or in a well ventilated and exhausted area, as the styrene fumes from the cutting process are very bad for you.

Thx and i really appreciate your kind feedback.

Hi guys
this is my second attempt after following the advice by seniors to reduce the amps so this time its 300 mA on 12 v, result is way better than before albeit some imperfections. Comments and more suggestions r welcome.

Aviater,

The second set of photos you just posted would indicate the temperate is about right as the tiny "whiskers" are clearly visible. There will usually be some small imperfections that need to be easily sanded out to a smooth surface. I suggest making sure the edges of the templates are absolutely smooth to keep the hot wire from snagging (however briefly), as this will also cause unwanted grooves in the foam core. Do not press the hot wire against the template while cutting the core because this causes friction that can snag the hot wire. Instead just keep the hot wire barely touching the template using the least possible pressure needed to maintain contact.

Know that when you make a longer cutting bow, the voltage needed to maintain the right amperage across the longer wire will increase and you will have to adjust the temperature again. This is why I recommend using a light dimmer switch in the circuit to allow adjustment.

There are several options available when it comes time to skin the core. Southern's Sorghum water based contact adhesive by Dave Brown Products has been considered the bench mark for gluing on wing skins, but has been discontinued. I found this close alternative at https://aerocompositesrc.com/product...-s-sorghum-ii/ Another alternative is 3M's formula M77 or M88 spray can contact adhesive which also works well. Make sure the glue you use is compatible with the foam core. Spray the core and the skin and let them air dry to the touch. Then lay the wing core back in the block of foam from which it was cut and set it on a flat surface. This will keep the core from warping while applying the pressure needed to attach the skin. Make sure the core and skin are aligned exactly before letting them touch because the bond will be instant when they touch and no further adjustment will be possible. There are a variety of skin materials available ranging from 1/16" balsa to 1/64" plywood to my favorite which is plain old poster paper (cheap-cheap-cheap) from Dollar Tree or Walmart. The poster paper has a "grain" to it that will allow it to be bent around a leading edge, but only if the "grain" is running parallel to the leading edge. You can bend the poster paper some and immediately know which way the "grain" runs, It will bend easily in one direction, but buckle and crease if bent in the other direction. Using this technique, I can usually start at the trailing edge and apply the skin forward to and around the leading edge and then continue back to the trailing edge to cover the wing top and bottom in one piece. You will know why I use poster paper once you calculate the cost of balsa or the very expensive 1/64" plywood. The poster paper is only slightly heavier, is still plenty strong and can be spray painted or covered with Ultracote.

Thx, i will keep you posted about further developments.

Not trying to be a hard ***** here but this thread is in the for sale section of the forum.