Foam cutting is a relatively simple process once you have the required tools and an acquired level of experience. So once you are set up, don't be discouraged if you butcher a couple blocks of foam. You'll soon get use to the rhythm, and your cores will turn out nice and smooth.

Tools Required

In general, the main tools, accessories and software that will be required to perform all your foam cutting requirements are listed below:

• A hot wire cutting bow;
  

• A suitable power supply to power the hot wire cutting bow;
  

• In order of preference: a band saw, or a jig saw or a fret saw to cut the various types of templates you will require to make wing cores or fuselage parts;
  

• In order of preference: a pedestal drill or hand drill for drilling holes for 4mm dowels,
  

• Label paper;
  

• A 30cm length of 4mm dowel,
  

• A jig for cutting foam blocks to size (how to make one below); and,
  

• Compufoil airfoil plotting software for designing wing core templates, or the like.

The Hot Wire Cutter

Before you start, you should make the decision on whether to make or buy. CLICK HERE and see this link on prices for a complete unit.

The hot wire cutting bow is a cheap and easy tool to make. There are many variations on how to make them, a sample is listed below.

How to make a Bow
In this case, the bow is made up of an aluminium square tube brace with hardwood arms. This is my preferred bow as it is easy to set up, and importantly, very light to use.

The ni-chrome wire is possibly the more difficult item to get.  However, if you have a good electric trade shop near by, they most likely will have it in the form of a replacement heater element. These are of the radiator style - ceramic tube with the ni-chrome wire tightly wrapped around it, at either end a metal terminal to connect to the heater. An element of at least 40cm will have a surprisingly large amount of wire thereon.

The spring that maintains the tension has a hook at either end and can be found at your hardware store. See example below:

Note this a short version of what you would be after. About 200mm in length would be good.

The spring has to be quite tightly sprung, but not so tight that you cannot partially stretch it by hand. The spring serves two purposes, in addition to maintaining the tension, it also compensates for the movement in the length of ni-chrome wire for when it heats and cools. Should a spring not be used, the ni-chrome wire will expand under heat and become slack. The hook end of the spring that attaches to the eye screw should be cut slightly shorter to make it easier to connect to the eye screw. Take off only 3-5mm. 

The ni-chrome wire is twisted at either end to hold in place. Make at least 8-10 twists.

The wire trace that connects to the spring should be non-stretch. Fishing wire trace works well. After you have the ni-chrome wire in place, attach the spring to the bow and connect wire trace to the opposing arm. Take the wire trace through the hook of the spring and tension. Once the ni-chrome wire feels tight like a guitar string, you've reached the correct tension. Clamp or twist the trace into that position permanently.  IMPORTANTLY, the tension should not be so tight that you cannot remove the spring by hand from the eye screw. As, when the bow is not in use, the spring should be disengaged to prevent stretching of the ni-chrome wire.

Power cables
Use at least 8amp automotive multi strand wire. NOTE: the rating of the wire should always be higher than the maximum power output of the power source you are going to use.

Wiring the bow is very simple. To keep the wiring tidy, run one cable through the length of the brace. Attach wires to the eyes that attach to the ni-chrome wire, and solder in place. Lead the power wires off to one side of the bow so they are out of the way when in use.

Use connection plugs of your choice to connect to the power source.

Length of Bow
The length of the bow is up to you. I have two bows, one 1300mm and the other 300mm. The larger is for cutting wing cores, whilst the other is used for trimming and fuselage work. Below is a picture of my trimming bow:

The Power supply

Here a number of options that can be used, but seek suitable advice if you are unsure about such devices.

Firstly, never use power directly from a house power outlet (AC). Results could be fatal. Use a fuse protected power supply that provides DC. Usually an output of 12-18 volts is good, with a current rating of 3-8amps. The variable power supply that I use is 18volts, with a 5amp rating. It has a gauge the indicates the amount of amps that are being drawn. This is very useful once you know what current works well with what foam, as you can set it straight to the correct value without having to perform tests. In my case, for both white and blue foam, I set my current to only 1.5 to 1.8amps. This provides a nice clean cut without the foam being damaged by an over heated wire.  My cut rate at this temperature is about 1cm every 1-2 seconds. Below is a picture of my power supply that was made by my friend Don:

People with an adequate knowledge of electronics can easily make a suitable supply. The transformer can be easily found at any electronics store for around AUD$40.00. The variable control is simply a 555 timer that varies the pulse width. 

Well my very good friend Don has finally completed the information on my power supply he made for me. He has forwarded circuit diagrams and a text overview for construction. He says it very easy to make by anyone who has a reasonable electronics knowledge and good soldering skills. He does however stress that suitable advice should be sought when connecting to the mains. Click Here, or on the navigation button on the top of this page.

An alternate power supply provided from another friend is shown below. Check out the diagram:

Click on to enlarge

The unit seems to work well, but Don said that some light dimmer switches do not like switching inductive loads, and may fail. So the 555 timer option would still be the most reliable option. If you are still interested in trying this option, I have listed what was used to make the power supply shown to me.

In this situation you will need:

• Transformer with output of approximately 25V 2AMPs
  
  In this case the transformer had multiple connection options:9V 6.6A, 12V 5A, 15V 4A, 18V 3.3A, 21V 2.9A, 24V 2.5A. The last being the one to solder up to.

• Indicator light rated for AC
  

• Fast-acting glass 2amp fuse (x 2, one for back up)
  

• Panel mount fuse holder for the above
   

• 3amp rated banana plugs (x 2), male & female (for panel mount)
  

• 2-3amp rated flex cable

• Terminal blocks (Insulator)
  
  Cut away number required from strip (try cutting down to a minimum of two,
  so you can use the centre hole to screw down to case)

• A standard home light dimmer, one that also serves as an on/off switch 
  In this case you would discard the large panel for the small. 
  The switch will be used to turn on/off the unit.

Make or buy a metal box to house the electric. Use the banana plugs as shown on my unit above to connect the flex cable to the bow. Use the diagram above to assemble the unit. As said above, seek suitable advice if you are even slightly unsure about putting it all together.

Other options...
I know of people who use car battery chargers as their power supply, and they have good results. The only problem with these chargers is that you cannot adjust the current, as the resistance of the ni-chrome wire does this. If using a car charger, the types most suitable have a switch for 6 or 12 volt operation and a current rating of 4-8amps. Lower rated chargers at 2amps may require you to shorten the length of the bow to achieve the correct temperature.

Another power source can be your electronic speed controller (ESC) attached to a high mah rated battery pack. Be sure the ESC is 20amp+. Adjust the throttle to increase/decrease temperature. Unfortunately, your time will always be limited under this method. 

Whichever way you decide to go, always avoid short circuits (positive negative terminals touch). They damage equipment and can cause fires. Polarity on these setups do not matter.

Template Cutting

Below is a typical template produced by CompuFoil:

Fig 1.

The solid lines represent the template. The dotted lines outside the wing profile indicate the surface thickness after a balsa sheet laminate has been added.

Once you have played with the (CompuFoil) program, and have your desired profiles,  you have to make a decision on what type of material you are going to use for the template - I can give you a couple of ideas.

What is preferable is a thin, heat resistant, and easily cut material. Some people use metal sheet for its heat resistant qualities, but it is difficult to shape. Others use thin ply or craftwood about 4mm, but these are easily damaged by the heat of the hot wire. Others use laminating material for table tops. This sounds like a very suitable material, as it is easily shaped and heat resistant. But not always that easy to get. The most simple template material I have used is 10mm craftwood, coated in epoxy or fibre glass resin to provide some protection against the heat of the hot wire cutter.

When using the 10mm craftwood there is a trade off. The extra width provides greater protection against the hot wire, as the heat dissipates faster across the wider surface. However, if cutting delta shapes some accuracy is lost. But, as the method is used on both left and right wing panels, the error created in the profile by the extra width will be the same for both wings.

How to cut out the template

A mini sized band saw is ideal for this application. Their bands are usually fine and take little in their cut. Using the 10mm craftwood, I like to make a two piece template. The first piece being the shape of the wing core, and the second being the template base. The two pieces are joined by two small dowel plugs (say 4mm diameter). This is an excellent method for accuracy, and you'll see why later.

Once you have printed out your template onto label paper, the label paper should be cut along the bottom line of the template base, so you can match up this straight section to a straight section of your craft wood. When cutting around the sides and across the top, leave plenty of excess. If you cut too closely along the top of the wing core, you will have difficulties in seeing the cutting line. 

Once you have applied the label to the craftwood, it is then time to make the first cuts. Here you should only cut the left and right side, and the top of the wing core. Do not cut the wing core template away from the base of the template at this stage! Once you have finished, you should have the wing core and the base template cut out as one piece. 

Next a set of two 4mm holes will be drilled from the bottom of the template. Line up the two templates from the bottom so they are parallel. If you are cutting a delta, then you must line up the templates from the rear. On the template above, the numbers on the side represent cutting stations. The rear is identified by the end with the higher number. For the template above, I would make the holes at cutting stations 2 and 6. The holes being drilled from the bottom should have a depth that will come close to the surface of the top of the wing core. But do not break to surface. If cutting deltas, still try to drill the holes in parallel. This will be quite important to achieve. See below:

In this case the pair of templates are for a delta wing. This smaller being the tip profile, and the larger being the root/ centre profile.

Drill the holes your preferred way, and you will be ready to cut the wing core away from the base. When cutting this section, make sure to cut beneath the wing core template line so not to lose any of the core's shape. This cut must be precise! As the cut also forms the surface of the bottom of the wing.

Once this is complete, your template is now ready for finishing. To achieve a nice clean and smooth surface on your foam core, your template  MUST have smooth surfaces to glide the hot wire cutter across. The finishing process requires the top surface of the wing core and that of the bottom template to be made as smooth as possible. Importantly, don't go sanding like mad to achieve this, as you will damage the intended profiles.  Use car body filler as an option to repair cut marks. Otherwise, plaster board filler combined with fibreglass resin also works well. DO NOT fill the drill holes on the surface of the bottom template.

Now for the dowels. Four dowel plugs will be needed. Two will be placed in the bottom of the wing core template so you can plug the two templates together. The second two are inserted into the bottom template so you can plug the template onto a flat board.

Now to check your templates' surfaces. Check all four surfaces (two on each template) with your hot wire cutter by running the wire across the surfaces. The wire should run nice and smoothly without any twangs.

Once this is done your templates are finished. See below for examples:

The top template is for the tip profile, the below for the centre. Note the dowels protruding from the pieces. This allows the wing core section to plugged into the base, and the dowels in the base allow for the whole template to be plug into a table top or board.

Above the templates are plugged into the base board, and ready to try the first cut. This cut will define the top profile of the wing.

The second cut requires the wing core template sections to be removed. Now the wing core will be cut away from the remaining foam.
Note: once the left panel is completed, the templates are swapped around so the right panel can then be cut. This being the reason for aligning the dowel positions as shown above.

Preparing the foam block

The next step is preparing the foam block. Here you shape the foam to your desired shape: rectangle or delta. A jig is used to ensure nice and clean 90^o cuts using your hot wire cuter. See below:

Not the best drawing, but you will get the idea from it.

By placing the foam block in the jig, you slide the hot wire cutter in using the guides to make a vertical cut. If you can, use a saw to cut a line in the base board from one template guide end to the other. This will allow the hot wire to move a small, but safe distance away from the foam after you have made your cut, giving you time to turn of the power without damage to the foam.

Have a look back at the template at Fig 1. At either end of the template are two dashed vertical lines. The inside dashed vertical lines are the indicators for where you should cut you foam block to size.

The Finale: Cutting out the foam core

Now you are close to making your first foam wing core. You should now have your templates and your foam block ready. Next, you will need a suitable surface to plug your templates into. Your can use your work bench, or preferably a nice and straight plank of timber. Drill your holes into the timber to accommodate your templates. Make sure they are parallel:

The top of the wings above are the leading edges

Use double-sided tape to hold the foam in place. Others I know just use a heavy weight from above - seems to work OK.

Now, don't expect a perfect cut core the first time. This is very important to know. I still damage cores every now and then by using the incorrect heat setting, or by not pulling the wire evenly across the template. Test the cutting temperature of the wire. The wire, when moving through the foam, will slightly hiss, and the cutting rate should be about 1-2cm per second (or less). You will have to test the cutting rate to see what gives you the best result. Cutting slower with less heat will give better results, but you can damage the template by not moving fast enough! A catch 22 situation.

Now you are satisfied with the temperature, get ready to make the first cut. The first cut will be from the leading edge to the trailing edge on the top of the wing core template. The first section of the template is known as the "ramp".  This is where you rest the wire prior making the cut. Take note of the station markers. On a rectangle wing, you pull evenly across the length from front to end, ensuring the wire emerges from the foam in one instance. On a delta, you must ensure the wire passes over the correct stations at the same time. Having two people for cutting deltas is a good practice. I have also found that using a sawing action works well whilst pulling. Or, slicing from the small template to the larger as you pull back.

Turn on the power to the hot wire cutter. Place the wire on the ramp and pull the wire across the surface of the template. Ensure to pull smoothly, holding down the bow to keep contact with the template at all times. BANG! The top cut is done! Remove the wing core templates from either end. You are now ready to cut the bottom surface. Starting at the ramp again, cut the bottom surface. Once finished, remove the core and have a look! Job Done.

More for Delta's

For those cutting rectangle wing cores, the job should be done. But for delta's, you're only half way through cutting. Now you have to cut the second panel. Now you will find out why it was so important to make the dowels the same distance apart for both templates. Exchange the inner and outer templates position. To make the opposing panel, the templates need to be in opposing positions:

Before beginning the second set of cuts, check the surfaces of the templates. Lightly sand the surfaces of the templates if they have been damaged by the heat. Repeat the cutting process.

Tight Delta's

Cutting tight delta's is not easy. Lots of damage can occur to the outer (smaller) template as the speed of the wire across it will be much less than the inner panel. Try using the method below as a helper in cutting tight delta's:

Practice makes perfect! Well almost perfect. Remember the core will be covered in either brown paper, balsa or epoxy. In the case of brown paper and balsa, most imperfections will be hidden. The smoother your finish, the stronger the wing will be from the greater area the glue has contact, and the airfoil more accurate. You should lightly sand the surfaces of the cores with sand paper to finish.

One additional important note.
If you intend to balsa skin your foam cores, you will need to complete your cuts without damaging the upper and lower foam off cuts. These excess parts of the foam block will be extremely important when compressing the balsa sheet against the foam. So do not discard or damage them.

I hope the above has been useful as an introduction to foam core cutting.

This page was last updated on Tuesday June 26, 2001.