Hotwire Foam Cutting – Mostly Miata

Cutting foam with a hotwire requires several tools and a few skills. Some of the skills used are intuitive and some are more complex. The procedures discussed below will be for construction and use of a hotwire foam cutter.

Foam can be cut with a band saw, table saw or other type saw. However, that creates foam gnats and dust. Using a hotwire cutter produces fumes that can be harmful but at the same time there is very little residue created. Since I can ventilate my workspace I choose the hotwire cutter which keeps the mess to a minimum.

There are several ways to use the hotwire to cut foam. Using a table and cutting guides. Using templates and pocket cutters are the most common ways.

Tools and materials needed can be obtained fairly inexpensively. The most costly tool needed is a power supply that is adjustable in voltage and current. Some wood to make bows, aluminum angle of different types and sizes and some stainless steel or inconel wire to make the cutter.

The wire used for the cutter can be any wire with the proper resistance and strength while hot. Stainless steel leader wire will work. It is available from saltwater fishing stores.

Different types of wire used have different resistance depending on size and length. The wire I am using is similar to a stainless steel wire, but I do not know its makeup. I think it is an inconel type wire.

The wire is caused to heat up to cutting temperature by the amount of current pushed through it. 3.2 amps is perfect on the wire I use to maintain that temperature. The power supply I use is a surplus Hewlett Packard 6438B DC rack mountable unit. It is old but works well.

It is mounted on a shelf above the cutting table.

It has the capability for adjusting the voltage and current. Most of the time, I only use the voltage control. Each bow I made is a different length so slight adjustment is necessary to keep the current at 3.2 amps. The most voltage needed on my wire is about 26 to 28 volts.

There are adjustable power supplies available online for reasonable cost. My friend Mike uses a LONGWEI variable DC power supply. It is available from Amazon at

https://www.amazon.com/dp/B07ML2MP9Q/ref=cm_sw_r_sms_awdb_imm_t1_t9IbGbMYED3GD?_encoding=UTF8.

The bows can be made of 3/4″ to 1″ PVC pipe or wood. I chose wood. I have three different size bows. They are 25″, 38″ and 49″ cutting widths. They are made from wood sawn to size on the table saw. Choose strong pieces of wood without knots. The wire should be tensioned to minimize stretching when hot.

These are my bows:

The center piece on the two larger bows is a piece of treated 2×2. The arms are made of some spruce I use for spars and cut to size on the table saw.

The tension wire is 1/8″ cable crimped onto the arms with either a spring or turnbuckle for tensioners. The spring on the top one does not work as well to tension the wire. So use turnbuckles instead. Again, the cutting wire must be kept tight while cutting.

The 16 gauge power wires and the cutting wire are attached to screw eyes at the business end of the arms. The connection must be good or the wire will not heat. Wrap or tape the leads around the arms to keep them out of the way.

The hotwire is attached to the screw eye by running through it and twisting the tag end around to secure it.

Tighten the arm pivot bolts to hold the arms securely after the cutting wire is tensioned. This keeps the arms from moving while in use.

Use bullet connectors on each bow to connect to the wire from the power supply. This makes changing bows “Super Easy”. Adjust the voltage to keep the current at 3.2 amps each time you swap bows.

Larger blocks of foam sometimes need to be cut into thinner and smaller blocks. To accomplish this, a flat table and some aluminum angle is needed.

A couple of 4″ pieces of 1/8″ thick x 1″ to 2″ aluminum angle is attached to the bottom of the table flush to the table edge.

These will support the 1/16″ x 2″ x 48″ aluminum angles used as cutting guides.

Smooth and polish the guide edges of the 1/16″ angle so the wire will slide easily. Attach the cutting guides to the angle under the table with clamps. Try to keep them perpendicular to the table. Check that the guide edges are square to the table to make nice square cuts.

The guides above were the first ones I tried. They work well for short cuts but flex too much when extended up for taller cuts.

Foam Cutting:

Cutting foam is “Super Easy” now that you have the tools. But there are some things to keep in mind whenever cutting foam with a hotwire.

The wire will BURN you if you touch it! Be careful.
The power supply can cause a SHOCK! Do not touch the connections with the power on. If the hotwire breaks, immediately turn off the power supply and do not touch the ends of the broken wire until the power is off.
The guides and templates should always extend past the end of the foam being cut to avoid uneven cuts at the edges of the foam. The hotwire should enter the foam and exit the foam at both ends of the hotwire at the same time. Again this helps avoid uneven edge cuts.
Before making a cut, turn on the power supply and be sure the current is correct and the hotwire is up to temperature before cutting.
Test cut several waste pieces to see how the hotwire is cutting. There should be hairs in the cut area if the wire is cutting well. If not adjust the current/temperature. Too hot and the wire will melt the foam too much.
Use the hotwire to slide down the guides through the foam in a slow but consistant speed. Hold the cut off piece to prevent it falling on the hotwire and cutting gouges into it. Sometimes an assistant is needed to help with this.
Do not stop during a cut or the cut will be very uneven and possibly gouged by the excess heat.
Keep the hotwire firmly pressed against the guide or template for smoother cuts.
After making each cut use a rag and wipe the excess melted foam off the cutting wire. A paper towel will work but it burns through very easily. BE CAREFUL not to burn yourself! This will reduce the smoke and keep the wire clean for the next cut.
Heat from the wire can be wicked away by the guides and templates. This can cause uneven cutting since the foam near the cooler sections cuts at a different rate than sections where the wire is hotter. Just be aware of this and adjust the wire temperature accordingly.

Foam blocks are often cut to shorter lengths. To cut foam blocks to length, extend the block over the edge of the table between the cutting guides. Use a straight edge touching both guides to set where the cut will be made. Turn on the power supply and make the cut.

Cutting foam blocks to thinner pieces can be done several ways. If the block is large, it will need to be done laying on the flat table. If the block is smaller and thinner pieces are needed, a cutting jig can be made.

To cut foam blocks into thinner blocks, lay the block on the table and use weights to hold it down. Lay the 1/16″ x 2″ x 48″ guide angles beside the block. Use shims to raise or lower the foam block and/or the two guides to set the cut height. Clamp the guide angles down securely. Use the appropriate bow to make the cut.

Thinner pieces can be cut from foam blocks with the use of a jig. It helps hold the block so the thin pieces can be cut. Plywood or MDF is used to make the jig. Size the jig to meet your needs.

This jig works well to cut thin pieces. The foam block is placed on the face of the jig.

The jig is positioned with a straight edge between the table cutting guides and weighted to keep it in place. An assistant is very useful while making the cut. They can keep the foam block against the jig face while you make the cut with the bow. It can be done without an assistant, but the bow has to be held by only one hand making it a little more difficult.

Cutting foam cores for wings and other parts often use templates. The templates are made from aluminum sheets with the edges smoothed and polished for smooth cutting. This is best demonstrated by a video my friend Mike made recently. Watch it several times if you plan to do this the first time. Mike tells several important tips in the video.

Here are some pictures of the wing core cutting process using templates.

One tip worth mentioning again is attaching templates to the foam. Templates are often held to the foam with finishing nails that have been sharpened. Be sure to use enough nails in each template, or the template will shift causing the part to be miss-shaped. (Ask me how I know)

Making pockets in foam for servos and other parts can be done with a hotwire bent it to different shapes. Pocket cutters are easy to make. Use a tiny drill to make a holes in a spruce stick to insert the hotwire into. The hole should be snug enough to hold the wire tightly. Bend the shape first then mark the holes and drill them. I have used a piece of the hotwire to punch this hole.

The 16 gauge wire is connected to the hotwire with some compression connectors made out of brass tubing and crimped onto the hotwire. Again bullet connectors to attach to the power supply lead.