Cutting foam boards

It depends on the thickness.  Below 2" you can just cut through with a utility knife.  It gets trickier with the 2" foam.  I've tried a jigsaw, hot wire cutter, a bread knife but the easiest way was to get a utility knife with a 1" blade and score the foam sheet to that depth and then just punch down to split it apart.  You'll have to clamp a straight edge when you go to score it as it takes some force but it will break cleanly.   I literally just did that 3 weeks ago with 2" green foam (like the pink and blue) for my around the room layout.

You can punch holes or drill for wires.  Personally I will be using a hot wire 'knife' to punch through:

-Greg

I have found using the side of a thin metal wallboard joint knife works well and reduces the amount of broken off pieces. Depending on the foam board you may need a utility knife to score through the outer layer.

Tim

All good ideas, here’s another alternative.

@TedW posted:

All good ideas, here’s another alternative.

I bought a set of those.  My edges come out a little jagged.  Do you find that to be the case as well?

John

I'm going to have to try those soft materials blades.

John, not my experience, but cutting slower will render better results.  To keep the demo video short, I cut fast and the blade heated which melts the foam.  Note at the start of the video the first half of the cut is smooth.  I have only used blue board, so I can’t speak to other types of foam.

@TedW posted:

John, not my experience, but cutting slower will render better results.  To keep the demo video short, I cut fast and the blade heated which melts the foam.  Note at the start of the video the first half of the cut is smooth.  I have only used blue board, so I can’t speak to other types of foam.

I'll try it at a slower speed.  My jigsaw is variable.

Thanks,

John

John, just to be clear, I mean slow the feed rate down.  Not sure lowering the blade speed will work too, but it might due to less friction, perhaps.

I've had the best luck the way Greg describes, a deep score and then break it.

@Craftech posted:

I'll try it at a slower speed.  My jigsaw is variable.

Thanks,

John

The issue I had with the jigsaw and soft material blade was vibration and keeping it straight even with using a straight-edge as a guide - the blade would bend from the denseness of the foam as I increased the pressure to hold the saw in a straight line.   I found it was much easier to just score and break - especially in trimming an 8' section to a diameter of less than 4' as one side of my layout is only 3.5 ' wide.  Your mileage may vary.

-Greg

A buddy of mine uses an old electric carving knife (remember those?) and is happy with the result. I use a hot knife (Tippie) or a razor knife with the blade out to its max.

I'm keeping a eye out at yard sales for an old electric carving knife

For straight, clean cuts I use a straight razor blade without the holder and a metal straight edge. I use my Xacto for curves. I have not used my electric foam cutter for clean lines. It is too difficult to get a true edge with it for me.

For volume cutting use plywood blade in table saw, hand held jigsaw, and free hand with electric carving knife.

One thing not to do, don't try cutting these with a table saw.   While it's possible to do, they also tend to grab and want to stuff the sheet back at you.

Used all the above. Best I've found is a Harbor Freight electric oscillating tool, using the 4" three quarter round, cardboard and leather cutting blade. Once you try it, you'll never go back to anything else. Great for all types of foam. For straight cuts, clamp a board to the foam on your line and keep the blade against it.  Your edge will look like a factory cut.

I use a jig saw with a knife edge blade or Dremel multi-max. The multi-max is great for carving and shaping the foam too. I won't use a hot wire cutter since the fumes from the foam are toxic.

I agree with John- table saws can be tricky. Having said that, I've watched carpenters and masons use skill saws on construction sites for years with no problems.

@RSJB18 posted:

I agree with John- table saws can be tricky. Having said that, I've watched carpenters and masons use skill saws on construction sites for years with no problems.

Once I realized that the foam was very "grabby" in the table saw, I was able to finish my cutting, but it's certainly not something that I'd want a steady diet of.  There's a lot of "launch power" in that blade, it could get exciting if you slip!

I've worked with all sorts of tools over the years John. Table saws get my undivided attention over anything else.

I know a few guys who have sawn off more than just the wood they were cutting.

I have not figured out a way to laser cut all foam boards without poisoning the air with chorine gas.

EVA foam is a good alternative, as well as polyurethane foam  both can be laser cut as well a foam-core boards.

XPS is extruded polystyrene foam that cuts too. Most often seen as the blue “Styrofoam” or pink “Foamular” rigid insulation foam you can get in building supply stores. EPS is expanded polystyrene foam made of lots of little foam pellets that is most often seen as packaging or as the material in cheap coolers. EPS is what most people think of when they think of “Styrofoam”.

the laser will do very detailed intricate cuts but if too small a flame will destroy it.

Table saw without a doubt is the best way to get a straight cut.  Take your time and have blade one inch above the foam board.  This said and having been a Industrial Arts teacher teaching wood shop, for over 30 years and yes I still have my fingers is in my opinion to cutting straight edges.  But you can do what’s best for you.  Good Luck.

I used a fat max razor knife, the one with the long  and wide, ( 1inch or so) blade in a retractable handle. I found the blades could be sharpened with a fine sharpening stone to improve cutting. I used a straight edge and freehand cut radius turns. I was able to cut up to 3 inch thick pink board, (2 + 1 inch) . Score and repeat then again if necessary then snap.

There are several types, I used the type with a locking thumb screw on the blade. It also doesn’t produce much pink dust.

It became a go to tool and worked well.

@laz1957 posted:

Table saw without a doubt is the best way to get a straight cut.  Take your time and have blade one inch above the foam board.  This said and having been a Industrial Arts teacher teaching wood shop, for over 30 years and yes I still have my fingers is in my opinion to cutting straight edges.  But you can do what’s best for you.  Good Luck.

That may have been my mistake, I don't think I extended the blade much above the foam, so it was probably cutting toward me more than down.  Good point for the future.

The table saw does make a LOT of blue (or pink) dust!

Use these cheap box cutters.  You can extend the blade out over 3 inches to cut the thick stuff.  Less than $10 for a box of 12.

Another trick is to put a smaller blade in your table saw. If you have a 10" saw use a 7" blade. The smaller blade will turn at a slower speed and reduce melting and chance of kick back.

As @laz1957 stated, get the blade high enough to cut down, not into the board.

Slower speed?  It's going to rotate at exactly the same speed, at least in my saw!

At a smaller diameter the cutting  teeth will move slower even at the same rpm because at a smaller diameter the circumference is smaller so the teeth have less distance to travel at a given rpm

Geez, it’s foam board, you’re not splitting an atom, just cut it with a butcher knife and chainsaw if that’s what you have.

How does a 7” saw blade spin slower than a 10” on a table saw.  That I’d like to know.

Circumference = 2 x pi x Radius

where pi = about 3.14.

Because Diameter = 2 x Radius,

Circumference = pi x Diameter.

For a 7” diameter blade, circumference = 3.14 x 7 = 21.98”.

For a 10” diameter blade, circumference = 3.14 x 10” = 31.4”.

The ratio of the circumferences for the 10” to 7” blades is about 1.5 (31.4” / 20.98” = 1.50).

So for a given rpm, the teeth of the 10” blade cover 1.5 times the distance of the 7” blade.  The teeth on the 10” blade travel 1.5 times as fast as the teeth on the 7” blade.

This is the same concept as why your 10-speed bicycle travels faster in 10th gear than in 6th gear.  

Hope this clarifies

I know about circumference, I was simply pointing out that the blade spins at the same speed.  As to the difference in kickback for cutting foam, I'd have to try it to know if it would make a difference.

In any case, this is a moot point.  I just cranked the blade up fully on my DeWalt DW745 Jobsite saw, and the 10" blade protrudes 3.25" above the table.  Given that fact, a 7" blade would not fully cut 2" foam as it would only protrude 1.75" above the table.

I question the veracity that a 7” blade spins slower than a 10”.  Aren’t they both powered by the same motor at the same speed.  Either way, I don’t think it’s going to make much difference.  I wouldn’t cut sheet goods on a table saw anyway. Too hard to maneuver and overkill.  Put it on a set of horses and use a skilsaw.  Or use whatever you have.  I prob wouldn’t cut foam board with a saw anyway, too much dust for no reason.  I’d cut one side with a utility knife, snap it at the seam and cut the back half if necessary.  Like a sheet of drywall.   Don’t over analyze, just get it built.  Cheers.

It doesn't spin slower, their point was the teeth are traveling slow since the circumference is smaller.  However, in the case of cutting 2" foam, it's a moot point as you can't extend the 7" blade sufficiently to even cut the foam completely.

I can attest to the good result with the Bosch soft materials jigsaw blade. I tried a hot knife first and it was a mess with lots of fumes. The Bosch blade worked perfectly with no dust. The trick is using a variable speed jigsaw and setting the speed slow enough that the blade doesn’t burn the foam but fast enough to get a clean edge and not have the blade edge catch and bounce the jigsaw all over the place.  I used a Hitachi jigsaw and the speed can be dialed down quite a bit.

Many options to choose from here...I confiscated, from the kitchen, a decade's old electric carving knife after wife and I received a new one as a present from the children.  Quality and weight wise, there's no comparison between the old and new.  I successfully cut and shaped dense pink polystyrene used for mountains on the layout...

Also , I use a band saw to cut foam boards.