Why can't I cut the foam board precisely?

[JBinFla]

So I bought my first FliteTest foamie to build with my nephew. The Mighty Mini Sportster. Went to make my first cuts on the wing (45-degree chamfers) and my cut was HORRIBLE. I made sure it was a new blade but it was not straight, uniform, or without "snags". So, I went to the garage got my "household" box cutter and put a new STANLEY brand blade in there. It's a name brand, so I figured it must be pretty good. I figured those cheap Amazon box cutters I bought must have poor quality blades. Nope, this was a terrible cut too. Even when it cut the foam board, it kind of left the foam jagged, it did cut the paper cleanly but the foam had a few jaggies in it. It wasn't a nice smooth cut and of course I was very wavy, even cutting some of the connecting paper My nephew was devastated, but I told him it'll fly. I do have an X-acto knife as well but I did not try that as the handle is kind of small and the cuts don't seem as safe with the round handle since you can't control it as well.

Anyway, any tips are appreciated. I watched the video and Bixler makes it look smooth as butter and I'm getting anything but. We had to repair the leading edge with tape, it should fly but I'd like the rest of it to go better. At first I thought it was the blade, but even the new blade did that so now I'm wondering if there's some technique that I've missed. Any help is appreciated.

- Joe

 

[Foamforce]

Your brand new blades are dull. For real. If you want a fail safe way you know what you’re missing, go buy an Exacto knife with Exacto brand blades. I think the #2 is what I use. You will cut just like Josh then.

An additional tip is to angle your blade more so that more of the blade is in contact with the material. Particularly when you’re cutting the bevel on the control surfaces, lean that knife over so that most of the blade is cutting. It will be at about a 30 degree angle to the material versus 60.

A couple days ago I tried out one of the cheap snap off blades. It just ripped up the foam. I couldn’t believe it. I felt it with my finger and it was about as sharp as a butter knife. Get the Exacto knife and then you’ll know what a sharp blade is like. Then after that you can play around with sharpening blades, but you’ll know what it’s supposed to feel like.

Good luck!

 

[Merv]

...my first cuts on the wing (45-degree chamfers) and my cut was HORRIBLE....

I use cheap utility blades, $10 or so for 100 blades.

I have found a handle for my utility blade gives me far more control. You control the depth of cut by twisting the knife. As you hold the knife, hang one finger under the FB. You will be feel the change of depth far more accurately than you can see it. Just get it close with the knife and then finish the bevel with a sanding block to make it perfect bevel cut.

Also helps to angle the blade, not talking about the angle of the bevel you are trying to make, but the blade relative to the direction of travel. The more you angle of the blade back from the direction of travel the easier it will cut.

 

[mastermalpass]

Scalpels are my go to - their blades dull after roughly 6 metres of cut, but I can still get a fairly smooth cut out of them even then, with a few important considerations:

Like FoamForce said; lean the blade. Get the top of your foam cut a good 5mm ahead of the bottom of the foam and you'll simply have less foam to snag on. The thing to look out for here is little tearing snags on the underside of the foam - the trade off with angle cuts is, sure you're applying less lateral force on the foam, but you're also creating a thinner piece of foam which is easier to snag, if that makes sense. Also the end of your blade is being dragged on the cutting matt, so it will dull ahead of the rest of the blade.

Next trick or rather rule: are you using a metal ruler? I swear I see Josh Bixler cutting free-hand all the time. In my Graphic Design class, using a sharp blade without a metal ruler was an absolute no-no! I do not understand how Josh's blade doesn't drift off course, mine does more often than not when I cut free-hand.

Another important thing: the hobby knife is not a guillotine; move SLOWLY. When I move fast I nearly always snag, even if my new blade has only cut out one wing so far.

Finally, for some curves and narrow pieces, I actually stop leaning the blade, square up and then gently saw it through the foam. This is probably bad practice though, 'cause I do it in response to feeling resistence and it probably means I need a new blade. But it pretty much means no snagging or tearing, even when your blade is really bad. But in order to do this, you need to lift the foam off the cutting matt and make sure that blade is going all the way through. If the tiniest bit remains uncut, it can start a tear.

 

[CrashRecovery]

The more you lean that blade back to almost parallel to the board surface, the cleaner your cuts will be.

 

[MrClean]

And as always, practice on scrap first whenever you try a new to you process. It's how you get to Carnegie Hall, PRACTICE.

Even on things that seem simple. Before long, you'll be slicin and dicin and trying to remember doing it any other way.

 

[Inq]

I agree with @Merv... the cheap one sided razor blades you get 100 for $6 https://www.harborfreight.com/100-piece-industrial-quality-single-edge-utility-blades-39748.html are sharper than any utility or Exacto blades. You ever try shaving with a utility knife... No, because they're not sharp enough. Also as @Foamforce pointed out, slice at a very shallow angle relative to the direction you're traveling (not talking about the angle for the bevel).

Also check out the main cheap resource for your foamboard to make your whole plane for a $1 or two. They're a rare commodity at the stores. I go by Dollar Tree even if I don't have a plane in mind. About 80% of the time, they're out. Last time they had a box of white and box of black. I scored 20 sheets that day!
https://www.dollartree.com/black-foam-boards-20x30-in/25957

 

[Tench745]

An important point that I don't think has been mentioned yet: don't try cutting all the way through in one cut. Make one pass to cut the top paper, another to cut the foam, and a third to cut the bottom paper. Much cleaner cuts this way.

 

[Inq]

An important point that I don't think has been mentioned yet: don't try cutting all the way through in one cut. Make one pass to cut the top paper, another to cut the foam, and a third to cut the bottom paper. Much cleaner cuts this way.

Good advice for cutting the parts out from plain stock, but I think he is using a kit and talking about the bevels at the hinges.

 

[Tench745]

Good advice for cutting the parts out from plain stock, but I think he is using a kit and talking about the bevels at the hinges.

I find that even those go better if you cut through the paper and then come back and cut the foam.

 

[Inq]

I find that even those go better if you cut through the paper and then come back and cut the foam.

I'll have to try that. I've been doing like Josh demonstrates in his videos... he's cutting from the fold up through the foam and cutting the paper on top all in one pass. And he make cutting look SO simple... he doesn't even use rulers to cut long strait sections.

 

[Tench745]

I like to score the paper then cut from the hinge up, using the score in the paper to guide the tip of the knife. I also like to use a wide blade for the cut, something like a razor blade, or large snap-off knife. That way there is a larger side of the blade to ride on the foam and keep the cut stable. Or, sometimes, I'll just score the paper, peel it off, and sand the foam back with a sanding block for a nice even bevel.

 

[Inq]

I like to score the paper then cut from the hinge up, using the score in the paper to guide the tip of the knife. I also like to use a wide blade for the cut, something like a razor blade, or large snap-off knife. That way there is a larger side of the blade to ride on the foam and keep the cut stable. Or, sometimes, I'll just score the paper, peel it off, and sand the foam back with a sanding block for a nice even bevel.

Like @JBinFla... my cuts look like kindergartener trying to color inside the lines. I'll try some of your techniques. Thanks!

 

[Foamforce]

An important point that I don't think has been mentioned yet: don't try cutting all the way through in one cut. Make one pass to cut the top paper, another to cut the foam, and a third to cut the bottom paper. Much cleaner cuts this way.

I’m going to kick this hornets nest. 😂 I used to make multiple passes like this, but I’ve found that if I keep my blade nice and sharp and cut on a cutting mat, that I get cleaner cuts with one pass. The reason is that if you do multiple passes, you end up doing slightly different angles, so you get little slivers cut out on the edges of the cut on the opposite side. Now that I’m working with a sharp enough blade, I only do multiple passes on tight curves.

Seriously, get an Exacto knife. Name brand with the bigger blade. Most of your problems will go away.

 

[Merv]

When cutting out planes, I make two passes, the first cuts the top paper and most of the foam, the second pass cut the rest of the foam and the bottom paper.

On bevel cuts, are done in one pass because there is only one layer of paper.

 

[skymaster]

It might not be the blade. i have at some point bought some foamboard that the paper has some how got moisture in it. this makes the blade feel like it's dull. try passing the blade on some abrasive sand paper so as ti make the blade a little jagged. I have always used the dollar tree blade cutter and use sand paper to bring it back to like like new.

 

[tomlogan1]

I agree with @Merv... the cheap one sided razor blades you get 100 for $6 https://www.harborfreight.com/100-piece-industrial-quality-single-edge-utility-blades-39748.html are sharper than any utility or Exacto blades. You ever try shaving with a utility knife... No, because they're not sharp enough. Also as @Foamforce pointed out, slice at a very shallow angle relative to the direction you're traveling (not talking about the angle for the bevel).

Also check out the main cheap resource for your foamboard to make your whole plane for a $1 or two. They're a rare commodity at the stores. I go by Dollar Tree even if I don't have a plane in mind. About 80% of the time, they're out. Last time they had a box of white and box of black. I scored 20 sheets that day!
https://www.dollartree.com/black-foam-boards-20x30-in/25957

You can buy a box of $ tree foamboard online and have it delivered to the store so you don't pay shipping. Last time I did this it was about ten cents per sheet less than buying at the store. Plus you get a box of pristine foamboard, not what is left after being picked over.

 

[Foamforce]

Here’s more detail than anybody asked for. This method works for me. How does this compare to what others are doing?

 

[dap35]

I find that even those go better if you cut through the paper and then come back and cut the foam.

I measure back from the cut and score the paper facing. Then I go back and cut like Josh does in his build videos.

 

[danskis]

Bottom line is your right - it will fly fine. A cutting Matt helps a lot. Also if your trying to cut bevels for hinges your probably looking at the wrong place. Don't stare at where the blade is coming out of the foam. Stare at the hinge line. When making a wing the first thing I do is put tape on the hinge line fold.