# Basic question about hot wire cutters

Discussion in 'Physics' started by Mkie Rumney, Dec 29, 2010.

1. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
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Hi, newbie basics this ...

I'm making a hot wire cutter for a building application that is to have a 600mm+ wire.
Most modellers' cutters are nearer the 150mm mark.
There are obviously various different ways to combine power output, wire length and wire thickness, to achieve different temperatures in the cutting wire.
One example found recently is 9 Volts, 2 Amps, wire diameter 0.25mm and wire length 130mm.
Assuming this set up can deliver the right temperature will I be able to achieve the same heat output in a 650mm long wire (5 x the 130mm) by either using a 1.5mm diameter wire, or forming a thicker wire with five of these 0.25mm wires twisted together ... to change the resistance ... ?

Probably a no-brainer to anyone reading this so please excuse the simplicity of the question!

2. ### Rbeckett Member

Sep 3, 2010
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Mike, have a look at tower hobbies for a foam core wing cutter. Some of those exceed 1 meter in usable length for making very precise NACA airfols and may be what you need. I have several planes with winspans over 2 meters that have foam core vacuum skinned wings on them and they perform flawlessly and are very accurate and strong. Just a thought.
Bob

3. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
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thanks Rbeckett ... that's news to me ... although I'm trying to recycle/DIY/ save money.

Have since realised that it's a question of cross-sectional area, not diameter, when it comes to resistance calculation ... so now the question is slightly different:

would a wire of 0.6mm diameter and length 650mm have about the same resistance as a wire of 0.25mm diameter and length 130mm?

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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We need to know what voltage you are working with. What the wire is made of, and it's circular cross section(area). AND the length of course.

Jul 7, 2009
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You can figure this out yourself with the formula

$R = \rho \frac{L}{A}$

where R is the resistance, rho is the resistivity of the material, L is the length, and A is the cross-sectional area. If you want to compare the ratio of two resistances of different lengths/sizes of the same type of wire, the resistivity divides out and it's purely a matter of geometry. To make accurate predictions of resistance as a function of current, you need to know how the resistivity varies with temperature and how to calculate the heat transfer from the wire to the environment, which is not an elementary task.

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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we always cheated on the math

and used a Variac to drive any length of wire we wanted to use.

7. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
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Hi, and thanks for posting back ...
I put these in the first post (except for the wire material as I don't know that I can definately get a Nichrome as opposed to Copper)
Values are likely to be:
9 Volt 2 Amp supply to 130mm of 0.25mm wire ...
changing the wire to 650mm of (as I now think having found out more) 0.6mm wire.

8. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
10
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Hi, and likewise thanks for posting back ...
I think this formula confirms what I assumed would have to change in order to get near-enough the same heat output from the two wires of similar material but different length and cross sectional area.
When I first posted I was assuming a relationship based on diameter but now see that it's cross sectional area ... some some progress made!
As for the last point about calculating resistivity variation in relation to temperature ...
I will assume that if I can achieve the same resistance, voltage and current as in the example found on line (which has been proven to work because it gives the correct temperature for the job) then my version with the longer fatter wire will do the same job ... (?)

9. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
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Thanks for this reference ... they don't seem to stock anything like that at present but it's given me something different to search for online

10. ### Markd77 Senior Member

Sep 7, 2009
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I think you will need more power than the 0.6mm wire.
Your calculation looks right in that the longer wire will have the same current and voltage as the shorter wire. Problem is that it's a longer wire so you will be getting less heating per inch so it should be cooler. I think you could use the original wire size and increase the voltage by 5 times, then the current and the heating effect should be the same.

<ed> This new voltage may present a shock hazard so it is better to use a lower voltage and higher current, so thicker wire </ed>

Last edited: Dec 29, 2010
11. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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I have one of these, it is about 6 feet long with several pulleys to trace the airfoil between root and tip. The power supply is 24V out, but I didn't notice the current rating on it, haven't used it in a while.

The other "half" of this setup is the vacuum bagging for wings. Wrap fiberglass around the foam core, paint resin on it, lay down your gel-coat color on mylar on top of it, put the whole thing in a bag and hold it under medium vacuum for a day or two and it comes out looking like a car finish and about as stiff as aluminum, but weighs almost nothing.

The inventors of this system were genius, pure genius.

12. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
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Thanks for this ... have already decided not to increase voltage for that very reason ... I'm going to try to find the values set in the larger commercially available cutters that use longer wires, to see how the numbers add up.
Maybe the original idea of five 0.25 strands twisted into one wire (albeit based on the wrong premise of 5 x the diameter instead of 5 x the cross sectional area) would work better.
If this proves too hot I could try 4 x the 0.25 and so on until the optimum temperature is achieved?

13. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
10
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If you could identify the Amp rating, the wire length and the wire diameter that would give me another set of parameters to compare with the original set of: 9V 2A and 130mm of 0.25mm diameter wire ...
Would be interesting to see how far off Ohm's law the real world strays.

14. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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It is 5 Amp capable, though there is a dial to adjust output via PWM (digital supply).

The wire is the "standard nichrome" that you get when you order lengths of it. Make sure you have one side mounted with a spring so the wire stays taught when it heats up. The spring on the wing cutter is about 2" long and about twice the "stout-ness" of a ballpoint pen spring, but "pulls" instead of "pushes".

The real world will cause you to adjust the heat based on several factors, such as material thickness and composition and speed of cutting. Example with foam, some are more sensitive to heat than others and "shrink back" from the heating element. This causes droops and gaps in the foam from melted foam dragging away from the wire. You want to turn the heat (power) down a little bit, OR move a good bit faster.

Some form of isolated variac would be the ideal source, the nichrome doesn't care if it is AC or DC feeding it.

15. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
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Thank you, that adds another piece to the puzzle.
I know what you mean about different foams behaving differently ... but you've given me an idea for how to get some tech data on the one I'll be using ... the manufacturers cut profiles to customers' spec so they should be able to help on that front.

Does the point about Nichrome taking both AC & DC mean I don't necessarily have to use a transformer to DC?
(like a car battery charger, which is what I'm trying to get on "freecycle")
or is DC preferable in some way?

Feb 5, 2010
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17. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
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Hi Kermit2 ... in the UK the equivalent translates to anything from twice to four times the cost depending on how "complete" a product it is ... begins to get to the point where I might as well spend it on a dedicated bit of kit for modellers, which is not an option!

Jan 18, 2008
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19. ### Mkie Rumney Thread Starter New Member

Dec 29, 2010
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John, this sounds interesting.
Apart from anything else he refers to his Volts / Amps set up, which is a part of where this thread started.
I'm going to email them to see if they have a UK contact I can maybe get a sample from.
Many thanks, Mike.