Cutting foam cores - how to improve? - RC Groups
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Dec 26, 2009, 04:54 PM
Registered User
jaron's Avatar

Cutting foam cores - how to improve?

I've cut my first foam cores and the result isn't so impressive. After reading through several threads I guess the problem with the rippled surface of the outer wing is caused by to much heat (?).
- the foam I used is called XPS (feels like the blue Dow stuff) but I had the same result with ordinary polystyrene foam
- the thickness of the wire is 0.35mm (0.0014"), maybe I should use 0.5mm (0.02") instead (?)
- cutting speed was about 10mm/sec (23.6"/min)

Any advice on how to improve the cutting?

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Dec 26, 2009, 09:50 PM
KE your cub.
Curare's Avatar
I've found that the rippling can be caused by a template that's not perfectly smooth.

The wire catches on a minute lump or divet and stays in that position untill there is enough tension to overcome this little hiccup. the end result is a burnout like you see.

Try finishing your templates to be as SMOOOOOOOOOOTH as you can. I use laminex for mine, and rough them to shape, the go down the grades, from 320, to 800, then a little bit of steel wool for a fine polish.

See how you go with that.
Dec 26, 2009, 11:30 PM
TLAR Aviation
Harry D's Avatar
I agree totally with Curare's advice about getting the templates absolutely smooth.

Also, I found that I could never get decent results trying to pull the wire through by hand. Even though I thought I was cutting smoothly and evenly I wasn't, and little hesitations and uneven speeds (particularly on the narrow end of a tapered wing) always caused the kind of ridges you show. Ater I changed to a setup that automatically pulls the wire through by means of a system of pulleys and weights, I never once had that problem again - and I've cut hundreds of cores since.

Of course, this may not be the cause of your problem, because I suppose some people can get good results by hand, and you may not have been doing it by hand anyway.

You do need to experiment with various cutting speeds and wire temperatures. Expect to waste a fair bit of foam before you get it all figured out. Make notes, and change only one thing at a time.

Finally, be sure you're using virgin foam. The "recycled" stuff is just about impossible to cut smoothly because it is uneven in texture and contains hard lumps that will do all sorts of bad things.

Good luck. Keep trying and you'll get it right pretty quickly.
Dec 27, 2009, 08:04 AM
Red Merle ALES VI
Curtis Suter's Avatar
Thanks for the photos, your cut at the root or the larger end looks good, the tip has the ripples and I agree it's either caused by a rough template and/or too much heat. The wire is going slower through the smaller chord than the larger chord.

I would make sure your template is very, very smooth. You can run your fingernail at an acute angle across the template to give you a good idea.

I also take a felt polishing wheel for my Dremel tool and make one or two passes across the template and it really makes it much smoother.

What type of template material are you using?
Also, if you're hand cutting for very little expense you could build an automated foam cutter, I wrote an extensive article on how to do so, see:

Happy Holidays

PS My family name originates in Kolliken, what a beautiful country!
Dec 27, 2009, 11:05 AM
Registered User
jaron's Avatar
Thanks to all for the valuable information!

I think my templates are ok so far but I'll have to check them again. I've built them from 1.5mm plywood and used graphite from a pencil to make them gliding.
I've got to experiment more (better templates, lower temperature, more tension on the wire, thicker wire...) and the automated cutting approach looks promising.

Curtis, I've read your article before I opened this thread, nice job!
You're right, Switzerland is beautiful but winter gives me a hard time here.
The location in this video is about 20 miles from Koelliken (click HD for better quality):
Aerial Video - Central Switzerland (3 min 39 sec)
Dec 27, 2009, 11:32 AM
Red Merle ALES VI
Curtis Suter's Avatar
Originally Posted by jaron

You're right, Switzerland is beautiful but winter gives me a hard time here.
Thanks for the video, that's awesome!

I lived in Germany for six years and it's not quite as bad as Montana, that I recall! We had -34C a few weeks ago! I guess when it hits -17C or more than it just doesn't really matter anymore.

Last edited by Curtis Suter; Dec 27, 2009 at 11:52 AM.
Dec 27, 2009, 11:55 AM
Crash proof pilot
FlyDave's Avatar
Hi Jaron, i'm from Switzerland too ! :-)

Try to find Styrofoam (the blue foam), it cut's better than the white thing.
I use 0.4mm NiCrome wire.

regards !

EDIT : Sorry... i've seen now that you already use a better foam... my mistake...
Dec 27, 2009, 07:15 PM
Registered User
Zephyr41's Avatar
I agree with Harry D- I get that ripple effect when the wire speed is not constant. I've never had much success with hand cutting and even with a drop-arm cutter it's possible to wind up with a slack pull-cord at the tip as the wire gets towards the trailing edge.

Dec 29, 2009, 12:34 PM
Prop Buster Extraordinaire
philba's Avatar
I was able to get a usable clark y core by hand on my third try. The trick for me was steady pressure and making sure that the speed at each end was proportional (and, of course super smooth templates). I put 4 proportional marks on my templates and used them to gauge my speed - helped a little. It also helped me to be able to control the wire temperature. I was using .018 SS wire, iirc.

Those ripples don't look super bad in the picture - you could probably sand them out.

Next time I make a core, I'll probably over size the templates by a mm or 2 so I don't need absolutely perfect technique.

To make it perfect, you'd need some sort of machine. I'd look into an NC foam cutter as it has other uses beside wing cores.
Dec 31, 2009, 02:21 PM
Registered User
Come on guys help the man with what he has a cnc is a big deal and its not with out its own problems to get working right putting it together after u get all the parts u need setup etc.I know that someone can help to get that cutter working.
Dec 31, 2009, 03:03 PM
Waiting for the paint to dry.
ctrout's Avatar
Test till you get it right
this is a technique I use and it will work if you heat sync the small end.
take a small peace of the hot wire and rap it around the end of the hotwire cutter used at the shorter end "wing tip end"
for starters I would try 1 wrap per 3/4" of offset from the heal to tip.
from your pic I would guess 4 wraps and do not leave any wire hanging off the sides.
test this and add more if its to fast or remove if its to slow

the small groves of the 3 or 4 turns of the heat synced wire can be filled in with spray foam if it gets out of hand just cut off the high spots when the foam has cured

Hope it helps
See ya in the sky!
Keep the wire tight

If the wing is the same size at each end 2 wraps at each end can work to. just cut off the area with the heat sync
Last edited by ctrout; Dec 31, 2009 at 03:13 PM.
Jan 03, 2010, 07:18 PM
Prop Buster Extraordinaire
philba's Avatar
Originally Posted by DR. DAVE
Come on guys help the man with what he has a cnc is a big deal and its not with out its own problems to get working right putting it together after u get all the parts u need setup etc.I know that someone can help to get that cutter working.
Your criticism is quite misplaced. I saw no posts that weren't helpful. Just because I (and others) mentioned machines doesn't mean that's the only suggestion.

By the way, I don't see anything helpful in yours - unless you count chastising as helpful.
Jan 03, 2010, 07:45 PM
Entropy is happening!
Jim.Thompson's Avatar
[QUOTE=jaron;13913125]Thanks to all for the valuable information!

"I think my templates are ok so far but I'll have to check them again. I've built them from 1.5mm plywood and used graphite from a pencil to make them gliding". (Quote)


It seems that your post above has confirmed the point regarding the templates being not smooth enough. Plywood is not a suitable material in my opinion, due to the difficulty in obtaining a smooth surface such as described in post replies above. As mentioned by "Curare" and others, laminex is good, its time proven, easy to find etc. The very best as per Phil Barnes DVD "Vacuum bagging of foam wings" is paper phenolic, but it costs and is perhaps only justified for multiple uses. This DVD is an excellent investment if you wish to master the art of vac bagging.

Stick with it, it will all come together with some practice and a little research. That day when you finally cut your first accurate, unblemished blank core makes it all worthwhile.

Last edited by Jim.Thompson; Jan 03, 2010 at 07:48 PM. Reason: Grammar correction
Jan 03, 2010, 11:58 PM
Registered User
I just finished cutting some supergee cores with 1/16" plywood laser cut templates I bought a few years ago. I soaked the edges with CA and polished them with 500grit sand paper.
I got good results and the wire didn't burn the templates. So plywood can yield good results if properly prepared. Not the material of choice, but if you already made the templates they should work.
If your not already using a feather cutter like machine you should really make one as suggested earlier. It can make a big difference over pulling the bow by hand.

Keep practising,
Jan 04, 2010, 12:40 AM
Registered User
Sorry u took it that way but im having the same problem he is having I tried the heat sinks and it got better I got my templetes from Justin .

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