andrew b

I was thinking of making a foam cutter bow, nothing complicated or expensive. maybe car battery powered for cutting panels up to say 14" or so.

Anyone else made one?

Tim Wiltse-RCU

Andrew,

I use a 12volt 6 amp car battery charger and stainless steel fishing leader. It worked great for my needs.

LAter,
Tim

DICKEYBIRD

I made several bows of different lengths from 1x2 pine boards and 3/16" music wire. Drill 1 - 3/16" hole in each end of the board angled outwards about 15-20 degrees. Cut 2 pcs. music wire about 12" long, grind a groove on 1 end with a Dremel cutoff wheel; put the music wire in the holes, wrap your cutting wire around 1 of the wires in the groove, bend the 2 wires towards each other 'til they're perpendicular to the board and tie off the other end. You should be able to pluck the wire and it sound like a low note on a guitar....maybe an "A"?

Use alligator clips to connect your power source to the cutting wire. You will need some type of adjustable power source - there are several designs on the web using a light dimmer and a transformer. Measure the voltage across the wire and keep notes on what works best for each bow and write it on the bow. Good luck!

combatpigg

My Feedback: (3)

I use .020" stainless wire [single strand] and a $6 light dimmer off of 110 volt AC. With this set up, you need at least 4 feet of wire in the circuit for it to be controllable. Any wire will lengthen when heated, so you need to set up spring tension to keep the wire taught.

Wazmo

I'm building [link=http://nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm]one of these[/link] for my project.

combatpigg

My Feedback: (3)

If that power supply is dependent on using nichrome wire, I wouldn't go for it. Nichrome is way more prone to breaking than stainless, and it creates way more smoke.

andrew b

I got a heap of 12v 7A/Hr batteries so that I can cut cores mobile, handy down the flying field

Wazmo

From the circuit diagram, it doesn't appear to be particularly dependent on nichrome wire. [link=http://en.wikipedia.org/wiki/Electrical_resistivity]Wikipedia[/link] says that SS has about half the resistance of Nichrome, so if the heat range is too high, I should be able to adjust R3 or perhaps add a switched bias resistor to select wire type.

Larry Driskill

I have these two cutters made from PVC.

5/32" music wire "struts" both hold the cutting wire and provides the tension to keep it tight.

I usually use .015" or .018" control line flying cable for the wire.

mnrcaerobat

My Feedback: (197)

Is that your power source on the right picture? How does it work? WHere did you get it?

What does everyone use for templates? I have heard that formica works well. Do I just cut the formica with my bandsaw and sand it to my lines? Does it need to mount to anything?

Wazmo

That's a variable transformer that reduces the line voltage to level suitable for heating wire and substantially reduces the risk of electric shock.

Templates are often shaped with a flat bottom meant to rest on the work surface along with the foam block. Sometimes they'll have index pins instead that press into the side of the foam block, but you have to align them carefully.

You must sand the template smooth (I've heard working down to at least 600 grit, some use fine steel wool as a last step) so that you can drag a fingernail along the entire edge without feeling any snags or rough spots.

Andrew

I'm using a converted 300W PC power supply to drive a pulse width modulation circuit that I built. I use 0.018" 302 stainless steel spring wire (music wire) -- it stretches less than nichrome and seems to be less brittle, plus it costs about $7.20 for 285' from McMaster-Carr. The PWM circuit allows me to fine tune the temperature. My bow is made from PVC, similar to Larry's, but I've added a coil spring to increase tension.

We've built a couple the PS's that Wazmo referenced for use in the robotics lab and they work well, but are a little coarse in adjustment.

uliner

My Feedback: (2)

Don't use a variac (variable transformer) without some sort of isolation. A transformer of the type shown above can give you a nasty and fatal shock. This type of transformer is an autotransformer, meaning is shares a common from the supply. It may be only set to 15 volts, but it may be 100 volts above your body potential.

I use a variac with a $15 24V transformer obtained from radioshack. As an engineer it is not worth the risk.


Dave

Bipe Flyer

On my old foam cutter I used a an Astoflight charger attached to stainless wire with alligator clips that were moved in or out to adjust temperature. My current foam cutter has an 18V AC power supply with Nichrome wire. The stainless wire doesn't stretch as much as Nichrome, but other than that it doesn't seem to matter much. I use a router speed control, similar to this one, between the outlet and my power supply to adjust the wire temp.
http://www.hartvilletool.com/product/10933

Silent J

My Feedback: (2)

They are coarse in adjustment. What size wire are you using in yours? I've been using a small )unknown diameter SS wire. With a 24" bow, I need it to be on the high setting for a 48" bow in order to cut reasonably well. For a hotter wire, should I go smaller or larger in diameter?

Wazmo

For a hotter wire you want more current. More current means less resistance. Less resistance means shorter wire or more cross sectional area (larger diameter). You'll get a larger kerf too, though.

Silent J

My Feedback: (2)

Thank you very much. Now I need to find out what size I have... And the resistance... It's actually wire from a multi-strand picture-hanging wire. It was stainless and cheap, so I thought I'd give it a try. The results are marginal on the nsrca.org power supply, so I'd imagine a supply with more voltage would make it work ok.

bdavison

My Feedback: (1)

Formica works really well for templates. It's unlikely to burn, and you can use it over and over again.

Wazmo

Multi-strand tends not to work as well because it's not smooth and has less surface area in contact with the foam. I think single strand wire is what you want.

Andrew

.020 IIRC.

Thinner wire will give cleaner cuts if you can keep the heat up. I've been satisfied with the .018" on my cutter. I was not sure that the PC power supply would have enough voltage or how the PWM circuit would work on wire, but I prototyped it and it worked fine. It really gives a fine control over temperature and I'm running about a 45% duty cycle when cutting. Maximum cuts are about 30", with most shorter. The PS overload circuit will occasionally kick in at max temperature, but I only dial up that high when I want to clean the wire.

Like Wazmo, I think you will find the smooth single strand wire to give better service and a much nicer cut.

Search [link=http://www.mcmaster.com]McMaster-Carr [/link] for part no. 9495K62 for stainless spring wire (music wire). Their 302/304 stainless wire listings are on catalog page 3513. They have a short description section on wire types on pg. 3512

Silent J

My Feedback: (2)

I don't use the actual multi-strand wire for cutting. Sorry about misleading you. I "unwind" the multi-strand into it's 6 or seven individual strands. That way I have 6-7 individual strands about .02" in diameter. (I'm unsure about the actual diameter, but it's near 0.02") But I don't think the wire gos through much quality control during manufacture. I believe I've seen isolated "hotspots". Something like SS fishing leader or Aircraft Safety wire would be made with higher standards and better quality control.

Brian

DICKEYBIRD

Dave I've heard that before but I don't have the "sparky" knowledge to accomplish it. Could you post a drawing that shows exactly how to wire it up? I have a big fat Variac for my foam cutting (it must weigh 20 - 30 lbs.) and I've always been careful with it but saw what happened when I let the wire touch my table saw top. A big noisy, blue flash that scared the poop out of me![X(]

propbuster

My Feedback: (1)

Wazmo, Regarding the foam cutting power supply from that site, my Dad (a retired electronic engineer) built one with mixed results and ended up changing the design. The following is an excerpt for one of his letters about it.
Tim, I finished the first power supply, the one from nsrca, and found that I did not like the limited control of the output which went from zero to full scale in a very small rotation of the control pot. Knowing that some of the people on the internet used a lighting dimmer I found another power supply using a dimmer. Also knew that you should not use a resistive load dimmer when the load is a transformer or a motor but thought I would give it a chance. Well the dimmer circuit works great! There is little or no transformer buzz and it does not get hot. I had already built the box with a fan so I left it in and adapted the dimmer design into the old box. I added a meter circuit for indication of output VOLTAGE. Since the output voltage of the transformer is not regulated the output voltage varies a little with load. The meter measures 0-25a/c volts rms. I set the meter for full scale at about half load (1 amp) 24.8 volts. It also worked out good. So now that I am happy with the design, I will finish building the final box in a couple of weeks. I sent away and got the best wire that I could find. It costs 50 cents a foot with a minimum of 50'. It is called RENE foam cutting wire , is .014 dia. and has a resistance of about 4.3 ohms a foot. The bow is 36" long. At that length the resistance is about 12 ohms. I calculated that the power supply should be set at about 80 % output. Anything shorter in length must be used in the short/long switch in short position. After making the square block for the wing panel(must use a squared up block of foam),made a jig for that too, I made the wing templetes from a double thickness of formica. That works very well too. Since I have the stationary power tools, it was easy to cut and shape. The bow and supply work fine at 80-85% of output. Cut the blocks square(made four 3" tk. blocks x 30" long) figured I needed some practice and was I right. Second set of panels came close to perfect. Found the secret. Have the foam below your waist, never stop the bow, try and keep the bow moving at the same rate, the templetes MUST be as smooth as possible(were the wire rides) and enter/exit the foam at same time. Must have sufficient templete over size in length to carry bow wire in and out of foam. Also I used roofing nails to hold the templetes plus I added a couple dabs of 5 min. epoxy to hold in place. The wing was longer than needed so I could trim off the epoxy damage.

Andrew

LOL! Big noisy blue flashes do the same to me, too. I usually yell, "Oh s**t!" I suppose just to verify that it indeed scared the poop out of me.[X(]

propbuster

My Feedback: (1)

Regarding RENE wire, see http://www.skykingrcproducts.com/acc.../renewire.html