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Oct 10, 2018, 08:10 PM
Registered User
Help!

Show me your bow!


I love building with foam - it's cheap, widely available, you don't have to mail order it when you discover you estimated what you needed wrong, a quick trip to Home Depot or the Dollar Store and you're back in business. And it's not just for cheap looking builds either, some of the best scale models on this site are foam and not carbon fiber or glass and it didn't take a month to build a plug, make molds etc.
Admittedly, at the higher end the techniques are more refined but a first timer can build a perfectly functional plane with nothing but Dollar Tree foam board, hot glue and packing tape.

I'm trying to attempt some of the more refined foam techniques - namely cutting foam core wings with a hot wire and need some tips on what makes a good hot wire bow. Mine haven't worked out to well - I'm using wood and am having trouble making one that will hold a constant tension on the wire. They work well for a cut or 3 and then seem to lose tension.

I bought a good power supply on Ebay - it's the same one they use on the Feather Cut System. I got it for a song from a non modeler who didn't really know what he had.
Since I got the power supply on Ebay I looked for bows, and they are ridiculously expensive for what they are.

I am hoping people will post some pictures of their setups so I can get a better idea of how to go about building my own. I could also use some tips on what you're using to make airfoil templates and how to scale a particular airfoil to a set size. Thanks!
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Oct 10, 2018, 09:04 PM
Registered User
sdbruder's Avatar
Im about to build mine (never did one) but I’ve read a lot about it:

the bow per se is not that important, the important part is that when your hot wire gets hot it looses tension, so there is no fixed way to maintain tension.

So a good bow will incorporate it in the design: I plan to use a spring to maintain tension of the nickel-chrome wire.

EDIT: and Im eager to see what other more experienced people are building as foam cutting bows. I already saw ones made with wood and PVC pipes. What more?
Last edited by sdbruder; Oct 10, 2018 at 09:07 PM. Reason: clarifications
Oct 11, 2018, 02:28 AM
serial scratchbuilder
Granted's Avatar
Some of mine:
Oct 11, 2018, 04:43 AM
Entropy is happening!
Jim.Thompson's Avatar

Wire size and type.


I have several bows, I can post pictures tomorrow. However, the ones in the post above by "Granted" look a perfectly good sample of bows.
What may be affecting your wire loosening (or at least making it worse), is an unsuitable size and type of wire.
What wire are you using?
I am now using .016 inch diameter stainless fishing leader from the local tackle shop.
A bit of wire slackness can be tolerated, as long as the wire is driven with the right speed and the right current.
I usually arrive at a current of around 3.5 amps for the above wire size.
Smaller wire will break more often. Larger works, but will require more current and creates a larger kerf.

Jim.
Oct 11, 2018, 11:02 AM
Registered User
Clever assortment of tools.
Envious, but storage is Always an issue.
I've found that 0.008 Braided Stainless Steel Control Line wire (Brodaks' .. SIG or?)

http://brodak.com/control-line-parts...-x-2-x-42.html

Often free if one attends a 1/2A CL Combat event For the 0.008 ..Fast combat for 0.012 lines .
But those can be kinda overkill thickness for Cutting wire purposes
My 0.008 has proven extremely durable as in my current /in use wire is 15 yrs old
Happily the stuff sags V little at all and being Thin makes a V clean cut.
Last edited by Bare; Oct 12, 2018 at 09:31 PM.
Oct 12, 2018, 12:34 AM
Registered User
sdbruder's Avatar
Quote:
Originally Posted by Jim.Thompson
(…) What may be affecting your wire loosening (or at least making it worse), is an unsuitable size and type of wire.
What wire are you using? I am now using .016 inch diameter stainless fishing leader from the local tackle shop. (…)
Jim.
Just bought some nickel-chrome wire, 0.20mm (0.0078 in) and 0.25mm (0.010 in), that’s ok?

I think they are AWG33 and AWG30.
Last edited by sdbruder; Oct 12, 2018 at 12:40 AM.
Oct 12, 2018, 09:00 AM
The Junk Man
Quote:
Originally Posted by sdbruder
Just bought some nickel-chrome wire, 0.20mm (0.0078 in) and 0.25mm (0.010 in), that’s ok?

I think they are AWG33 and AWG30.
Ni-Chrome wire stretches BADLY when heated. Great for toaster elements, bad for cutting wings and other foam.

Get some cheap 0.016 to 0.014 stainless steel wire leader.

Tom
Last edited by T_om; Oct 13, 2018 at 11:49 AM.
Oct 12, 2018, 12:14 PM
Registered User
Wow T_om, you have obviously thought out the whole hot wire cutting subject from top to bottom - I'm very impressed with your set up.
I think I am going to Home Depot to find some stiff and light material to make the bow. I've seen the ones made from pvc pipe and if I can't find anything else I will go with that but I was thinking that aluminum angle stock would well so hopefully I can find something like that.
Oct 12, 2018, 01:15 PM
Registered User
I was watching a foam cutting video somewhere and it said that the wire only gets hot between the clips. Is is true?
Oct 12, 2018, 02:40 PM
The Junk Man
Quote:
Originally Posted by joe_crash
Wow T_om, you have obviously thought out the whole hot wire cutting subject from top to bottom - I'm very impressed with your set up.
I think I am going to Home Depot to find some stiff and light material to make the bow. I've seen the ones made from pvc pipe and if I can't find anything else I will go with that but I was thinking that aluminum angle stock would well so hopefully I can find something like that.
PVC pipe is too flexible to use as a bow IMHO. Look at the last photo I posted. That is a 1x2 wood beam with 3/8 inch music wire legs. Drill the holes in the wood at an angle to "spread" the wire legs so as to make considerable tension on the wire stretched between.

Tom
Oct 12, 2018, 02:40 PM
The Junk Man
Quote:
Originally Posted by NightTrain0128
I was watching a foam cutting video somewhere and it said that the wire only gets hot between the clips. Is is true?
Yes.

Tom
Oct 12, 2018, 03:17 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by sdbruder
Just bought some nickel-chrome wire, 0.20mm (0.0078 in) and 0.25mm (0.010 in), thatís ok?
I think they are AWG33 and AWG30.
I have already given my recommendation. See Tom's further explanation below.

Quote:
Originally Posted by T_om
Ni-Chrome wire stretches BADLY when heated. Great for toaster elements, bad for cutting wings and other foam.
Get some cheap 0.16 to 0.14 stainless steel wire leader.

Tom
Jim.
Oct 12, 2018, 09:33 PM
Registered User
regardless what Tom claims... Thick wire is a poor choice.
Trying it is all the proof req'd.
Oct 12, 2018, 10:02 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by T_om
Get some cheap 0.16 to 0.14 stainless steel wire leader.

Tom
I think Tom meant to write .016 to .014 (inch) wire. I missed this at first reading and when I quoted him.
Correct me if I am wrong.


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