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How do you cut foamboard?

Thorondor

Active member
#1
Simple question, really. I just can't seem to cut foamboard without it ending up jagged and ripped on the edges. It doesn't matter how sharp my knife is or how many strokes I use to go all the way through. Can somebody please give me a flippin' idea of what I am missing? I particularly can't do curves.
 

FDS

Well-known member
#2
Curves need you to use the point of a finer bladed knife. I like Xacto type knives, I change my blade every sheet. You also shouldn’t try to drive the knife through in one go, it’s a series of light strokes, letting the blade do the work. I freehand my curves, holding the knife like a pen.
Everyone has a preference with knives and techniques for cutting. Practice and a quality knife that is the right shape is the thing.
 

skymaster

Well-known member
#3
there is no right or wrong way of cutting foam board. some times the foam board comes with a cheap paper coating that no matter how sharp your blade is, it just drags I use dollar tree box cutter and use fine grade sand paper to resharpen it. if your cuts come out jagged no big deal use sandpaper to smooth it out. on curves i use a board pin to make punktures along the round corners and then dragg the pin over them to cut. you can actually cut all the way to the bottom of the foamboard with the tip of the pin, once cut use the sand paper to smooth it out.
 

Merv

Well-known member
#4
As others have said, a sharp blade is critical to a clean cut. I use utility blades with a handle, which gives me far more control over the blade. Try holding the blade at more of an angle, 50-60 degrees from perpendicular.

In this picture, they are making a 90 degree cut, but notice the angle, it’s about 60 degrees from perpendicular.

1579656583779.jpg
 

Thorondor

Active member
#5
As others have said, a sharp blade is critical to a clean cut. I use utility blades with a handle, which gives me far more control over the blade. Try holding the blade at more of an angle, 50-60 degrees from perpendicular.

In this picture, they are making a 90 degree cut, but notice the angle, it’s about 60 degrees from perpendicular.

View attachment 155388
That is exactly what I do. Maybe I should get blades from Hobby Lobby instead of Dollar Tree?
 

Merv

Well-known member
#6
That is exactly what I do. Maybe I should get blades from Hobby Lobby instead of Dollar Tree?
I get mine from the lumber yard, hardware store or a big box like Lowe’s, 100 for $5-6 or so.

One more thing, I always put something plastic under the FB, it keeps the wooden work bench from dulling the blade as quickly.
 
#7
I use a utility knife. Milwaukee blades work better than the cheaper brands. They claim to have "3x" longer life than other blades. I don't know if that's true, but they do a great cut. They are so cheap that I pitch them as soon as they get dull. I don't bother trying to sharpen them - its not worth my time.

1579665138793.png


One pass for straight cuts, bevel cuts, score cuts, etc. with no tearout using above mentioned blades. For any curved cuts, I use an x-acto number 11 blade, with multiple passess.

As far as cutting surface, I use a "self-healing" mat. Mine is Fiskars brand, but I know there are a few others. A 24x36 mat is a little spendy, but it will last a long time (and it's 2 sided). It will help preserve your blade.

https://www.amazon.com/dp/B000YZASYO/?tag=lstir-20
 
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Hai-Lee

Old and Bold RC PILOT
#10
Try using some sandpaper on the curves and to dress up the torn edges if sharper blades do not do the trick for you!

Have fun!
 

Zetoyoc

Well-known member
#11
this is probably not quite what you are looking for but most of my foam cutting is on an old bench top scroll saw I found on craigslist for $20 makes great clean curves and with practice straight lines too :) for the parts i can not cut on the saw i do use a sharp #11 i think it is hobby blade and take a few cuts. and if a critical part i stay a bit proud of the line and sand to shape.
 
#12
this is probably not quite what you are looking for but most of my foam cutting is on an old bench top scroll saw I found on craigslist for $20 makes great clean curves and with practice straight lines too :) for the parts i can not cut on the saw i do use a sharp #11 i think it is hobby blade and take a few cuts. and if a critical part i stay a bit proud of the line and sand to shape.
I honestly would have never thought of a scroll saw and I will keep it in mind in case I ever lose the use of my laser.
 

FDS

Well-known member
#14
I would try a few different shaped knives. The retractable blade ones are not a good shape for curves. Get a small pack of quality Stanley type blades, you don’t need a holder, just tape the top edge up. Also get a “craft” type knife, try both those options out. Scroll saw is a good one but how do you get straight cuts?
 

Merv

Well-known member
#15
I agree with @Zetoyoc, a scroll saw works great. I also use a bandsaw to cut out planes. With a band saw, I make a stack of foam board and cut many copies of the same part at one time. I use a few pins to keep the layers of FB from sliding around.
 
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Merv

Well-known member
#16
Scroll saw is a good one but how do you get straight cuts?
For me the cuts are never perfectly straight, they don't need to be, just get close. I always finish my stack with a sanding block, A few strokes of 80 grit sandpaper glued to a straight piece of scrap wood will make my OK cut perfectly straight.

I must add, I don't make planes the way FT does, that is cut the whole fuse and fold it together. I cut sides, tops & bottoms separately then jig them up the old school way. The way we used to build balsa a plane. FT uses tabs to make everything fit together squarely. For me, the tabs are not worth the effort and don't use them. I rely on the jig to get everything square.

If I want to make 3 of the same plane, it is faster to make a stack of 6 sides on my bandsaw, then jig them up.
 
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Hondo76251

Well-known member
#17
I've always had the best luck with the fine exacto blades (#11 mostly) I buy good quality ones and change them often. I've never had much luck with the box cutter or box cutter blades. I keep two ready to go on the bench, one with a dulled tip for doing score cuts, the other with a sharp fresh blade.
 

Bricks

Well-known member
#18
For cutting the fold line I use an old piece of foam board glue 2 double edge razor blades to it. Glue the blades to the right depth to not cut thru and a straight edge one cut and done.
 

Gazoo

Well-known member
#19
I like to use 8mm snap blades from Dollar Tree. I also have some Exacto type blades as well.

For straight lines, I cut just the paper on the first swipe. Then, follow the cut with another swipe going through to the bottom paper.

For cutting outside curves, I don't try to cut the curve in one swipe. I will take several swipes and gradually shave it off.

For inside curves, I will cut only the top layer of paper. Then, punch through the cut with a pin to mark the cut location on the other side. Then flip the foam over and cut on the punched line. Only cut the paper. Then the foam will "snap out".

The same goes for cutting tab slots. Cut the paper on one side, mark the corners by punching a pin through, flip the foam and cut the slot at the pin marked locations. Now the paper is cut on both sides at the same location. Push the foam blank out of the slot.

I tried cutting slots all of the way through (from one side) when I first started building. They always ended up skewed.