Foam Cutting Tools for Accurate Repeatable Shapes

There are a lot of Instructables for hot wire foam cutters. But what if that small foam cutter table isn't big enough? What if you have to cut accurate circles or giant rectangles? Using a scroll saw hot wire cutter by hand is fun but not accurate when you have to make something perfect or repeatable. So this is my attempt at making some tools or set ups to go with my DIY hotwire cutter as I had to break down 20 inch thick 4'x8' sheets of foam for a Halloween build which is currently in progress. I highlight some of the failures along the way as well so you can learn from my mistakes and get better cuts with less wasted materials.

Hot wire cutter

**** Bench top power supply Amazon Link This is what I used but there are many available

**** 10' Wire Leads Amazon Link - The short leads that come with it were not long enough.

**** Nichrome wire or Tig welding wire. I used 20AWG wire

Random items used to make up some accurate cutters

**** Wood clamps

**** 12" lazy susan Amazon Link

**** Plywood scraps

**** Scrap lumber

**** shelf brackets - for stability

**** Screws and bolts

**** some casters regular and locking only because that is what I had

****Cinder blocks or anything for weight / blocking

**** Safety****

Hot wires are well hot, hence the name. They will burn your skin just as easy as they melt foam

When you cut something, there is a good chance you end up with liquid foam on the cutter. It will drip and it will be hundreds of degrees. Do NOT put your arms or legs under it as it hurts a lot! Ask me how I know......

In the past, the largest sheet of foam I ever broke down was 8" thick. I made up a crude bow cutter and was able to basically cut out what I needed. I may have also used a saw zall, and a chainsaw and well, I don't want to ever do that again. This year for a Halloween build, I had to cut sheets of foam down that were 20 inches thick. These are BIG 4x8 sheets of foam I found on marketplace left over from a construction project. Well how do you do that? How do you cut something that size?

The easiest way is a non accurate cut where you have two people hold then ends of a long hot wire with a block of wood or handles from a something like a grout float and try to follow a line. You get so so results, but you can cut the blocks in half.

I wanted a way to do it by myself as I didn't have a helper and I wanted more accurate cuts. I made a giant bow cutter on wheels basically. I used some scrap MDF left over from a prior project. I added some wheels, put some wood on the back with some shelf braces and made a long arm out front. This arm got a piece of nichrome wire added to the end and a little eye hook at the bottom.

When pulled under tension so the wire held straight when it expanded slightly when heated, I had a vertical cutter with 40" of depth and 36" height to play with.

Well.... This was my first attempt you know.

I found that the square design base wasn't great. If I angled the cutter at all when moving it, the corner would catch my supports underneath and mess up the cut.

So change 1 - Cut the corner off and make it sort of like a triangle

Issue 2 - My vertical cutter wasn't exactly vertical. I had to move the wire on the end a few times to line it up exactly vertically.

Change 3 - Vertical one direction can be angled another. Yeah, I had to move the back too as it didn't line up square to the hook below and I still had an angle in one direction. See there is forward and back, but also right and left. Who knew?

Where to push or pull. Well.... if you push on the top, it wasn't sturdy enough to not flex the wire just a little bit. So all movement had to be done from the base. This made following a line on the top hard to see.

So.... What to do?

Square cuts are hard unless you have something to cut against. See what I thought would work was draw lines on the foam and I used the cutter to roll into those lines and cut, but it wasn't accurate or square. And as stated above, hard to see the line when you are pushing on the base. The biggest issue is the wire pulls through the foam and doesn't instantly cut. It is a slow process so when you get ahead of what the wire can cut the center is still pulling and it makes an arc not a square line. So how to fix these issues? Some long boards and clamps

I used a 6 foot level, a few boards, and some wood clamps. I drew lines on the top and sides, added the level and board to the lines and clamped in place. With a hard surface top and bottom, I was able to push the hot wire against both surfaces and it kept a nice straight line as it cut across. This gave repeatable accuracy to the cuts and made nice square blocks to cut down further later.

Did you know that if you put the level on the side with the cut out for the bubble level that the hot wire will get stuck and try to eat into the plastic of the bubble level? Well now you do. There were two ways to mount the level and I did it backwards and didn't have the smooth side. Such is life. I got it right every other time though but man was I confused on that first cut when it got stuck.

I had a need to cut 40 inch circles accurately. I definitely could not free hand this. I purchased a cheap 12" lazy susan base off Amazon. I used some scraps of lumber from other projects to make a base that held the foam off the ground higher than the mobile cutter base. I attached the lazy susan with some screws sticking up through to hold the foam steady. I cut a hole in the center of the lazy susan plate so I could line up the center of the giant foam block. Putting a screw as a marker in the block, I set it down so the screw would go into the center hole and that got me fairly close to center where I could then turn the circle.

I used my mobile jig and slid it forward until it was just starting to cut the foam. I turned off the hot wire, stabilized the mobile cutter with some cinder blocks and the locking wheels, and then turned on the cutter and made the cut slowly. You have to go slow here or it will pull into that arc shape and you lose your nice finish edge.

Something to note, if the hot wire stays in one place for a few seconds, it heats up enough to melt the foam close to it so it will ruin the circle finish by having a giant indent. To help stop this when the cutter was finished cutting, I would pull the wire away above the lead where it wasn't hot and disconnect the lead to the wire. It cools down in a few seconds so I could let go then and turn off the power and pull the cutter away. You can see in more than one photo throughout this instructable where the foam is eaten away from stopped movement when cutting and the wire melting it.

Now I had to cut 2 of these for my project but currently I had a perfectly centered circle on the jig. Thinking ahead a bit I spent the time to shape and sand the top as I could turn the jig by hand and make basically a lathe to cut away evenly the surface to round it. More on finishing nice and smooth later.

If you already own a table saw, why not use the fence to cut foam nice a square? Well that was my epiphany. I bought this old cabinet saw and outfeed table this past year. It has a 50" capacity for width and length about 7 feet. There is a cut out in the center for the Riving knife and blade guard. I used this cut out to mount the hot wire to. I added a few wood clamps to the table and made an arm that over hangs the table.

I aligned the clamps about 6 times until I got a nice vertical hot wire that was perpendicular to the table surface.

This allowed for use of the fence and table to make nice square cuts. I was able to take those giant blocks I had cut into 20x20ish rectangles and cut them down into smaller shapes like a 50x7x20 block. I know weirdly accurate measurements but hey, that is the size I needed.

Using a nice square surface and fence helped to make some really accurate smooth cuts to get all the different thicknesses I needed for my current build. I had everything from 5" to 15" to 19" to 21" to well you name it.

Most table saws also have a miter slot or two. Using this feature helps you push the blocks of foam evenly through the wire. I added a short piece of wood and two wood screws to help "hold" the foam not letting it slide back and forth but keep it against the fence.

Having a flat surface to work from is important for accurate cuts. I had 2 ways I used the table saw surface, vertical free hand cuts and a horizontal sliver cuts

Vertical free hand cuts to be covered in the next step. But basically any cut I wanted to make against the vertical hot wire line I could pull or push into the wire and make and it would be square to the surface. You are basically using the hot wire like a giant scroll saw.

The Horizontal cuts were a bit more finicky. I had to cut off 2 inches of thickness of the giant circle I made earlier. I needed 18" thick, not 20". Doing that against the fence that was only 2.5" tall isn't ideal or probably feasible. You just can't hold it square. So I made a hot wire that was parallel to the surface using my fence with a clamp on one end and a piece of wood with a clamp on the other.

I pulled it nice and tight and turned on the hot wire but the length at 48" or so was enough that the wire became slack and not straight. So I loosened the fence and pulled it tight with the hot wire on so it was already stretched as much as it was going to be. I test cut with a smaller cylinder I had made to make sure I got the tension right- See picture in this step

I slid the giant circle across the hot wire slowly, remember slow but always moving is key. This cut a nice even circle slice off the bottom of my block and got it to the right height.

*****A trick to note here is that the wire coming off the spool won't be totally straight. Likely it has some bend to it or a kink somewhere that makes it not pull straight. Use a heavy duty Hex wrench or even a round screw driver to pull up against the wire back and forth on the bottom, sides, etc, to straighten the wire out. *****

In general free hand moving foam through a cutter is really hard to do accurately. If you push too quickly the wire pulls or arcs slightly and then when you slow down it catches up and moves where you didn't want it to. If you have to make something repeatable that is a complicated shape, make a pattern out of a piece of wood first. You can then attach the wood pattern to the surface of the foam and use it as an edge guide for the hot wire.

I found that even with a wood pattern, it was easy to pull too hard into the wire and get it just slightly off kilter. When that happens, the wire is pulled against the pattern at an angle and the bottom of the foam is no longer square. As such, I tried to basically free hand around the pattern and leave about 1/8th of an inch gap. I can sand that down easily and if I pushed too much and the wire hit the pattern, it couldn't go through into the foam too far except maybe the bit of flex at the bottom. This allowed for fairly accurate free handing with guide rails, you know like when you bowl as a kid (ok or as an adult) and put up the gutter guards.

I also had to free hand cut out the centers of a large piece that had holes through it. I didn't want to cut into the shape and glue it back so I fed the wire through a hole I drilled through the 20" thick foam and then free hand moved the mobile cutter to cut the shapes. Not near as accurate as I wanted but I will make it work and sand all the extra away.

Again, free hand movement in foam is not ideal.

When I needed to cut smaller circles, 15" and 19" if you consider that small, my giant circle cutter base was too large. I took the lazy susan off the base and left the short screws stick through one side. This helps to hold the foam in place while doing minimal surface damage. I also had to cut down the excess plywood to make an octogon type shape that was smaller than 15 inches for the cuts I needed. I then set the lazy susan on my table saw surface, mounted the foam centered as evenly as I could, and moved it into the hot wire. Slowly turning by hand with some consistent down pressure allowed for accurate circle cuts without moving the jig. It also allowed me to move the foam in and out for micro adjustments if the circle wasn't cutting quite right. I had a few times where I started the cut too far out and had to move it in just a little bit.

This was 3/4 PVC, some end caps, and 2 eye hooks. This is for rough cuts, not high accuracy, but just removing large sections of foam you don't want to scrape and sand off.

I have made a 2 foot version and a 4 foot version at different times over the years. Again both work ok but are not highly accurate. If you put straight edges on either side you can get more accurate results by holding pressure against them.

PVC has some friction or resistance when you put the elbows together. They hold just well enough that I could take them back apart as needed. I was also able to use that resistance to my advantage when I had to tension the wire more. I would just twist one side and it would tighten the wire up. I never glued any of the connection points.

I also made just a 90 corner and put some custom shapes into the tig wire. It works ok for removing large chunks of foam but it isn't accurate. Due to the inaccuracy I gave myself lots of room to shape it down to the final finish surface by cutting outside of the line I drew not on it.

Sometimes you have to chunk out large amounts of foam. A Curry Comb works great for this. It is accurate enough to get the chunked out areas like rounding over an edge without pulling too large of chunks you can't fix. It makes a mess with tiny foam beads everywhere sticking to everything. They are definitely static charged.

But when you want to get a nice smooth finished surface you have to sand. I have used sand paper rolled up, attached with glue to boards etc. My favorite and what seems to work the best is a round drywall sander with an 80 grit pad. I personally use a radius360 9" round sanding tool. It was the original design and now there are a hundred copies of it on amazon. Find one you like but the foam pad underneath with the 80 grit smooths foam amazingly well.

While I think professional hot knives have their place (I actually bought one from the Hot wire foam factory this year), having a stationary hot wire to cut accurate repeatable shapes is still going to be important on most projects. Hot knives and free forming on an open table without guides can make amazing things. But there are also times where you need accuracy but can't afford a CNC. With a little ingenuity, you can sort out what you have already, and get professional looking results.

Feel free to comment on what hot wire accessories or tools you have made. I always like finding ideas that make prop making easier.

Hopefully in about a month from publishing this Instructable, you will see another from my Halloween build this year. Can you sort out what it is from the background shapes in the pictures, drawings I am working from, and notes throughout on the foam?