Yes I think we all wreck a lot of foam first time out. Trailing edges were my big problem. Then I started making the airfoil patterns a half inch longer than the finished chord desired and it worked better for me because I could cut off the ragged web. This was with expanded bead foam, btw.

I'm sure others don't need to do this, and it's probably a heat and speed adjustment thing.

Later I found that gluing about a 2" piece of .032" (or thicker) wire to the TE and LE of the patterns as lead outs helped quite a lot. Hard to explain in words how that is oriented -- basically if you imagine a pitot tube, or gun barrel at the LE, that's how the wire is oriented. It protrudes out from the airfoil along the chord line at TE and LE. The hot wire rides along it to exit from the cut. It leaves a slightly thicker TE than a feather edge, which is usually a good thing. In fact you can thicken that lead out to any degree desired. When the panel is freee of the blank, you can always cut back the TE with a ruler and sharp xacto knife to a nice clean line.

For a first time cut, I suggest just working with scrap packaging foam pieces, rather than spoiling a good blank. If you don't have any, then the next best thing would be cutting out thin sheets of foam rather than an airfoil. You can then get practice with many cuts out of a single blank, and you can probably use the sheets in building models.

Trial and error is sometimes the best way to go, safety being the first and most important step. I also spent a couple hours cutting up bits of foam before finding the best technique and settings. As for the pattern it also took some experimentation to find what worked best.
Watching a lot of youtube vids on the subject also helped a fair bit.

The Bow is built, just waiting for the power supply to arrive...

Richie, shorter bows have another advantage -- they don't need as much tension to reduce sag. The bow materials I mentioned worked for a shorter bow. I'm guessing you will need to keep the arms short to get enough tension. But I'm sure you'll work out what works best for you.

Another option for a light bow. Made from 1/2 in ABS hot water pipe bent around literally like a bow when it's armed.I drilled and cut a notch in each end of the bow to install coathangar wire fixtures to hold the .020 piano wire. You can see the wire taped to the bow that goes to the other end of the bow so that both alligator clips are on one end. I unhook one ens so the pipe won't take a set being permanently bowed. This is big enough to cut 24 in. panels. Nice to work with because of the light weight. Another thing I do is to mark each end of the panel to be cut, marked out in inches from front to back or vice versa, numbering each one. This way I can watch each end of the wire as it goes over the numbers so as not to get one end of the wire ahead of the other. As you can tell, I don't have any help to cut cores either

Gord.

I have been using a 1x2 with fiberglass "driveway markers" available at lowes or home depot - cut them in half and use them where the dowels go - I put them in at an angle and they keep the tension on the wire great!

Originally Posted by asat

I have been using a 1x2 with fiberglass "driveway markers" available at lowes or home depot - cut them in half and use them where the dowels go - I put them in at an angle and they keep the tension on the wire great!

Nice. Cheap, simple, insulated, and I am sure I missed something else. Wish I would have thought of that before I bought steel dowels for mine. Thanks!

Fred

Originally Posted by caymin last

Trial and error is sometimes the best way to go, safety being the first and most important step. I also spent a couple hours cutting up bits of foam before finding the best technique and settings. As for the pattern it also took some experimentation to find what worked best.
Watching a lot of youtube vids on the subject also helped a fair bit.

Nice wing cuts,

I have noticed your templates are inside templates, I have cut the wing profile ribs each end, and cut on the outside of them, is there an advantage to using negative templates?

Originally Posted by vtdiy

Richie, shorter bows have another advantage -- they don't need as much tension to reduce sag. The bow materials I mentioned worked for a shorter bow. I'm guessing you will need to keep the arms short to get enough tension. But I'm sure you'll work out what works best for you.

i am trying to cut wing panels of 900mm length, so thought 150mm either side of the wing ends would mean a bow of 1200mm length.

I am going to do what you suggested, and fix the bow to the desk, and move the foam around it, fingers crossed the power supply will turn up tomorrow....

Cheers for all the help all....much appreciated :-)

Guys ,I have a little foam to slice ... was planning on a slicing table/jig , with a wire stretched across the guides , and tightened by a small spring (not heated) .. Was planning on using my old Variac , but it may not have enough amps ... Only 24” wide ... Shorter wire so I might need less amps , right ???

It will work fine GPW. Just start at zero and gradually increase until you have cutting heat. You won't draw more current at cutting heat on a shorter bow if you are using a variable supply and you adjust up to the same temp. Your voltage setting will be lower than the longer bow, and the current will be about the same. All this assumes you are using the same wire on both. If not you should re-calculate.


BTW I don't like Variacs for bows for two reasons: because their output isn't isolated from the power line, and can be set at lethal voltage by mistake. Unlesss there is something limiting your adjustment knob.

If you put an isolation transformer somewhere in your circuit, it would isolate it, and if you make up a physical limit to the knob it will prevent a bad setting. You don't need anything more than 24 volts on a bow.

And to achieve both things at the same time, you could use a 24 volt transformer as an isolation transformer before the Variac. This will isolate it and limit its voltage to 24 V. Just make sure the 24 V transformer has an amperage rating above what you draw with your setup.

Funny!! I had a couple of dock billots to cut and made a Rube Goldburg system to cut them. I put the foam on a piece of plywood with the same setup that you have. I tilted it up on about a 45 degree angle and looped a piece of clothsline around the back of the billot, looped it over a chair with a pair of vicegrips for weight and turned on the Variac. By the way I do have an isolation trans. with the variac. Think I had to add a wrench to the vicegrips to get the feed speed right. I used #10 on the variac for the right heat. Sat back with a beer and watched it cut.

Gord.

VT, thanks for that !!! Will do !!!

Gord, sounds like a good plan !!! Auto feed ...

Originally Posted by richie967

Nice wing cuts,

I have noticed your templates are inside templates, I have cut the wing profile ribs each end, and cut on the outside of them, is there an advantage to using negative templates?

Well, for me yes, I tried the outside type (positive) template and found it a little awkward to start, and get a smooth cut around the curved sections. With the inside (negative) template, the slot that can be placed with a little bit (2-3mm) sticking out beyond the edge of the foam as a perfect guide for starting the cut, the TE of the core is left a tad longer, so it can be trimmed or shaped where needed.
I did have an issue with the T-pins at first, just make sure that they are angled away from the core so the wire can't snag them.

Originally Posted by gpw

VT, thanks for that !!! Will do !!!

Gord, sounds like a good plan !!! Auto feed ...

Sure, GPW, glad to help! btw -- The same way people made their own foam sheets of just about any thickness, who couldn't find blue fanfold, or wanted to slice the blue stuff thinner:

http://www.rcgroups.com/forums/showthread.php?t=243271
http://www.rcgroups.com/forums/showthread.php?t=1221543