fprintf

Well I ruined my first blank last night, and the first of many I am sure. I am trying to do it freehand with formica templates and the core came out all wavey.

My problems it seems:

Not enough tension in the wire as it drags badly - the ends of the wire come out of the foam at least 30 seconds to a minute before the middle makes it out.
Not enough heat as it takes quite a bit of pressure to drag the wire through the foam. I was trying to keep the temp low enough so that it took 2 minutes or more to go through a 7 inch chord wing but I was really having to pull the wire through.

So I'll try fixing these things tonight and see if it comes out any better. At least the power supply that you guys helped me build works like a charm!!!!

balsaman

Foam is cheap. Practice, practice, practice. Yes it sounds like you need more heat. It takes me about 30 seconds to do a 6" chord. The middle comes out 2- 3 seconds after the ends. Cut from leading to trailing edge. The trailing edge is flatter so the lag doesnt affect the airfoil geometry as much. Also be sure to include lead in/out sections on your templates to support the wire as it's entering/exiting the foam.

Eric

joebob

My Feedback: (14)

Both assumptions are correct. You need more heat, and more tension. As you heat the wire up more, it will stretch more, so you'll have to play with it to see how much tension you need.

thunderchief

You`re right you need good tension on the bow....also the right heat is necessary. Really the wire , believe it or not ,should not touch the foam. Heat from the wire should melt the foam in front of the wire as the wire moves forward. The wire diameter and the heat that surrounds the wire form the kerf that is the cut. If everything is right speed, heat, and tension the wire will stay straight and come out of the foam straight with no lag. If you have hair the heat needs to be turned up. Lay a piece of foam on the table (small piece) , turn on the heat and move the wire through the foam ....adjusting the heat until you get it so that the foam will not move as the wire passes through it. It takes some practice.
The Thunderchief

Ollie

I like the wire temperature so that the cut produces a maximum number of fine, hair like threads of melted foam. There is a slight drag on the wire at this temperature but it is not serious if you use enough tension. Stainless steel wire expands less than nicrome wire when heated and makes it easier to maintain wire tension. Stainless steel will also take more tension than nicrome. I cut at a rate of about one inch per three seconds.

yakman

I cut at one inch per 8 -10 seconds. don't use much heat and dont let the wire drag. At this rate cutting tapered wings is possible and they come out exellent. It takes trial and error. The fine hairs are a good indication of good speed and heat.

fprintf

Thanks guys! I am using stainless steel .020 aircraft safety wire. I used a lot of heat last night and it cut much better. I still am getting fine hairs, so I think my temp is just about right.
I couldn't crank up the tension enough because my inline spring is too weak so I still got some degree of wire drag. Funnily I cannot budge the spring by hand, so the tension is up there already. I have a guitar right next to the power supply and the twang from the cutting wire is actually pretty low compared to the guitar, so I think I need more tension.

Luckily I have a more heavy duty spring that I can install tonight. More trials to come! At $10 for a 2" x 4' x 10' pink foam sheet this foam cutting is cheap, but not so cheap that each core I throw away doesn't make me think I'll be soon making another bunch of $10 trips to Home Depot.

billf

Hello fprintf....

Here are a couple of more $0.02 worth:

The edges of the template should be "slippery" smooth. Try dragging your fingernail along the template, if it doesn't hang up, the template is smooth enough. I use thin aluminum that has the edges polished with 600 grit sandpaper.

If you do you use aluminum, (lithograph plate works pretty well), it helps to stand off (insulate) the template from the end of the foam with something like 1/16 balsa. The metal template will suck some heat from the wire at the ends of the block ,causing a sag further in where the wire stays warmer.

Bill

Soar Head

My Feedback: (5)

Cut smaller cores, and you will waste less foam. Of course, the current (i.e. wire temperature) should be changed when cutting with different length bows.

I suggest you just make straight cuts through your cores. You should be able to make 2 or 3 cuts through a block before throwing it away. At least until you get the speed and hairs like you want them.

I agree with others, that you should have lots of fine hairs, and the temp is right. If you go hotter/slower than this, then you will get wide varying kerf's with tapered cores.

yakman

I go to a factory where they make polystyrene foam products and they give me large chunks for next to nothing. But make sure it has no recycled polystyrene in the foam, as it not as good quality. Keep practising.

Al Stein

I cut at about one inch per second and, like Ollie, I look for the angel hair to tell me that the speed is right.

Now, here's a look at your future: provided you like using foam, you'll eventually try a foam-cutting machine, after which you'll realize that the difficulties of cutting by hand were entirely unnecessary. If you build your own, you'll really wonder why you didn't invest an evening and five or ten dollars to do it way sooner.

Somebody you know has a machine you can look at, and how to build it will be very obvious as soon as you see one in action. Mine is a copy of a friend's copy of one of his friends' machines. It very much resembles the Feathercut machine in operation and appearance (except that the frame of mine is luan plywood and the cost was nearly nothing). The biggest difference is that I suspend my bow from above (rather than drag it along on a tailwheel as the Feathercut does), so mine works a little smoother.

Check it out -- you'll love the way they work. I don't even like working with foam, but with my little machine, I still enjoy cutting the cores!

DocYates

My Feedback: (102)

Anyone here interested in posting on how to build one of these machines? I would love to build one myself, and as far as I know, I can't find anyone around who has one.
Tommy

Soar Head

My Feedback: (5)

Look here for a few sets of plans & pictures.

http://www.charlesriverrc.org/articles_foam_vac.htm

Jaymom

Al Stein.... Where's the pic's of yours? Can we see them on here?

fprintf

I just may have to build one of these. the hardest part to get, it looks like, is the aluminum channel that the rollers attach to. The drop arm looks like it can be just a 2x4 or 1x4.

Tom-RCU

Check out: http://www.8020.net/

They deliver.

paperairplanes

My Feedback: (10)

Built the same power supply, same setup, tried different materials for the templates, etc.. etc... Have been investigating the whole procedure for at least a year now. There is alot of good inof out there, all of which has been EXTREMELY helpful. One thing I have noticed with the drop bar method of a cutting machine, is inconsistency. Nothing which you can see with the naked eye, but.... Spent alot of money on foam trying and testing different things. On the average once you have your own system down, it will work great. Howver, if you want to mass produce wings, I think the cad way is THE way. But I am sure this will have its evenings of pullin hair out to get it right. If your planning on really getting into this, at least just look at some of the homebuilt stuff for cad wing cuttin machines. I wished I would have "jumped" to this before... but, thats the way it goes. Currently under construction, is this machine here...http://www.8linx.com/cnc/cnc.htm

If nothing else, this whole endeavor has been a learning process and to say the least, extremely fun!!!!!! Just what I need, is another project...... Can you ever have enough??????

fprintf

The project lives!!!

I took a lot of advice and hung the bow from the ceiling and made a drop arm. On a very short span core it cut perfectly after some experimentation. Now I just need to figure out how to square up the foam block before I begin without spending any more money on this thing. I think I am up to $80 so far (I coulda bought that LilBird2 kit I have had my eye on...)

One thing that I think has affected my cut is the stiffness of the wires connecting to the bow. It is basically a peice of extension cord and I think depending on how it hangs it tends to drag or torque the bow. I found this out when trying to drop the arm straight through the foam - some cuts were perfectly square, others warped. I think I may have to locate some of that flexible lamp cord and see if that makes a difference.

Thanks for all the advice so far. This is coming together!

RAMFlyer

My Feedback: (65)

Whenever you cut foam you must use templates or the cut will be out of your control.

When blocking or squaring a foam block I use metal yardsticks that act as the templates to guide the wire. This gives me one flat surface against which to square all the other surfaces.

I bought three 36" metal yardsticks from Home Depot (Small commercial as I work in the hardware dept). I cut one in half so I have two 18" rules. I "glued" one of the 18" rules to the end (last 1 1/2") of each of the 36" yardsticks with JB Weld. The 18" rules are glued at 90 degrees to the 36" ones with approximately 2" on one side and the rest on the other side. Use a square to make sure they are at 90 degrees. The 2 inch "overhang" ensures that the wire rides straight through the whole cut out of the foam block even if I have a bit of wire drag. The new foam cutting yardsticks end up looking like a T square with one of the cross arms being much shorter than the other.

The yardsticks have holes drilled every 12" or so to accept 8d nails that have been sharpened to go into the foam easily and straight.

Another way to block foam is to suspend the cutting wire a set height above the cutting table and feed the block through so that the wire acts like a planner. I use a hollow core door as my cutting surface and build the blocking frame long enough that the wire is longer than the door is wide. Build a couple of ply mounts to screw to both sides of the door with notches at every half inch. Set the wire into the mount on each side of the door in the notch that gives you the height you want and feed the block through. This again gives you one flat side against which to square all the other sides. Flip the foam block over, drop the wire a half an inch and feed the block through again. You now have two flat opposing surfaces. Pin the T square yardsticks you made against the sides of the foam block, cut the ends and you only have the short sides left to do.

Don't forget a couple of bricks or heavy books on top of the foam block so it doesn't move except when you want it to.

Email me if my descriptions aren't very clear!!!

Flypaper 2

Ramflier:
My setup is somewhat like yours. On the front of my hollow door I have two vertical pieces of 1/2 by 1/2 spruce screwed on with countersunk screws a little wider apart than the foam. Mark the foam on both ends. Put the wire up against the uprights with the power off. Line up the marks on the foam with the wire, turn on the power and take a slice down. Both methods will work well.