Boeing 727-200 scratch build

[shoyn]

Hi alex, where did you get the wire for your hot wire cutter? I have tried different kinds of wire but it keeps breaking on me. Thanks in advance.

[viper1990]

Hi there shoyn

I got my hotwire from Diaso where they sell nichrome wire, 0.27mm x 4meters. But i bought that 3years ago. i'm not too sure whether they still have them in stock though. Not only i use nichrome but also normal steel wires, or guitar strings (ask for the small E string if you go to any musical stores such as Yamaha at plaza sing). They work pretty well for me!

Note that even though nichrome wires can still break if its glowing red hot. The rule of thumb for voltage supply to the length of wire will be: 14volts/100cm. If you do not have a variable power supply,Any power adapter lying at home will do fine and using the rule of thumb,i get a ratio of 12volts/85cm. Anyway you don't really have to follow the rule of thumb,its just a rough guide, just agar-agar will do

[shoyn]

I have tried guitar strings and stainless steel wire but all broke on me. Didn't know there was a formula! I have a variable power supply and used about 16v on a 50-70cm wire. I think that could be part of the reason why it broke. But if I were to lower the voltage any further, it becomes pretty hard to cut. Another question, how do you prevent the wire from sagging? Every I pull the bow, the wire sags and makes an uneven cut.

[viper1990]

Probably its due to the high voltage you use per length. 16v for your 50cm-70cm wire that you've mentioned is really really high! perhaps thats the reason why it heats up and breaks. I'm always using 12V for my 80cm wire. I prefer to have slow and gentle cuts ,reason is because its easier to control and it "eats/burns" up less layer of foam. Eg, i want to cut a 5cmx5cmx5cm block, it'll probably end up with a 4.95x4.95x4.95 block but with higher temps, it'll be a 4.8x4.8x4.8 block which is slightly smaller. But you've to be really patience while cutting slowly. Let the wire "glides" through the foam instead of pulling and forcing wire through.

The method i tension the wire is pretty simple. I'll try to explain in words.
1. My PVC hot wire bow is 80cm from end to end.
2. I'll cut the nichrome wire 75cm end to end (if unsure,good to cut excess incase you need the extra length)
3. I'll press and flex my bow inwards so that the tip to tip of the bow now becomes 75cm
4. attach nichrome wire to bow while holding the bow in a flexed position. There are many methods, tie a knot, clamp, solder etc...
5. release the bow with the wire attached. Now the wire is tensioned nice and tight.

*Because my bow is made of pvc pipe,its easier to bend, if yours is made of wood and metal, cut your nichrome wire slightly longer.

[shoyn]

Mine is also made of PVC pipes. Tried coiling the wire around screws but it seems to lose its tension when the wire is heated up. What method did you use to secure your nichrome wire? Yet another question, what is the brand of the filler you are using to fill up gaps and holes on the foam? Thank you.

[viper1990]

I also did the same way you did, by coiling the nichome wire around the screw about 3rounds and end it with a knot at both ends. Yes the wire does loosen up when it's heated up ,but it won't be that loose till it sags, mine still have tension in it while its hot. Perhaps you can try tensioning it even tighter.

The putty filler is use is xtraseal putty filler.

[shoyn]

I see. I think I have to correct what I said earlier. The wire does not sag but it loses a bit of tension. Is that ordinary wall putty? Wouldn't that be slightly heavy for plane applications? Have thought of mixing wall putty and micro balloons, wonder whether that will make the wall putty lighter.

[viper1990]

Yes those are normal wall putty which hardens when dried up. They are heavy due to the water contents but once dry they are much lighter. Microballoons are defiantly lighter. I never personally mixed both of them together so I don't know what will happen. But I assume that it won't work well cause wall putty is not an adhesive which won't hold the microballoons together. Epoxy will be a better choice .

[viper1990]

Yes those are normal wall putty which hardens when dried up. They are heavy due to the water contents but once dry they are much lighter. Microballoons are defiantly lighter. I never personally mixed both of them together so I don't know what will happen. But I assume that it won't work well cause wall putty is not an adhesive which won't hold the microballoons together. Epoxy will be a better choice .

[viper1990]

Yes those are normal wall putty which hardens when dried up. They are heavy due to the water contents but once dry they are much lighter. Microballoons are defiantly lighter. I never personally mixed both of them together so I don't know what will happen. But I assume that it won't work well cause wall putty is not an adhesive which won't hold the microballoons together. Epoxy will be a better choice .

[shoyn]

Tried mixing them together, putty and micro balloons mixed pretty well into a thick paste. Should have did these long ago to glass my planes.

[viper1990]

Tried mixing them together, putty and micro balloons mixed pretty well into a thick paste. Should have did these long ago to glass my planes.

oh dear, i triple posted. Was using my phone to reply, something might have went wrong with the internet connection. Anyway... hows the result like? Did it solidify into something hard and sand-able? Or it is brittle and cracks if you apply too much pressure on it? Might try it out myself one day..

[shoyn]

Hand and sand-able. No cracks when I apply pressure on it. But I'm not sure whether it made the putty lighter or not.

[shoyn]

Hard, not hand.

[viper1990]

Great. Thanks for sharing shoyn. Will want to try it out.