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Old Sep 06, 2012, 08:58 AM
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vtdiy's Avatar
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Well it looks like they give a figure for resistivity with your nichrome specs. That probably will work with this spreadsheet. The figure to use would be the 650 one.

Suggestion, when you try out your power supply and bow combination, set your voltage to about, say, 4 volts (and briefly turn it on to check out the wire temp. If it looks okay (not glowing), push a piece of foam against it and see if it cuts, and how fast. Probably it won't. Then gradually increase the voltage until you get a good cutting temp.

Hopefully it will be at about what the calculator predicted for the wire you chose, and the bow length, and you're good to go.

I haven't told you whether .6mm is right for you compared to .52 because I think you need to re-enter data using the explanations above to get a new result. But in general thinner wire will mean you will need a little more voltage to get the same amount of heat. Hopefully you will have enough adjustment range to compensate. However, please recalculate to see what you have first.
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Old Sep 06, 2012, 09:01 AM
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Quote:
Originally Posted by richie967 View Post
Regarding the Setting question you asked...



The Power Supply does not have a single setting 0 -100%. It has 2 dials, one for volts, and one for amps - therefore, would 50% be 7.5v and 7.5a?
EDIT: The first thing to do is re-calculate the spreadsheet. Let's see what it says first.

Use 80% as the "Setting" in the spreadsheet for now

The specs say "current limiting" so that's a nice safety feature here. We'll just set the power supply's current setting a little above the calculated current required by the bow.

(It doesn't look like you have a constant current supply, which is fine.)
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Old Sep 06, 2012, 09:19 AM
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Okay, as an example using the suggestions I gave, here's what I get:
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Old Sep 06, 2012, 09:29 AM
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To explain a question you had earlier, the amperage dial only controls the maximum current you can draw.

The voltage range is what you normally adjust for controlling the heat. And the heat will be proportional to the voltage shown.

Since you have already picked out your wire, that's one thing you can't change.

So your setting will likely be lower than 80% for your desired cutting heat. Probably closer to 60% (9 volts). If you plug 60% into the calculator and recalculate you will see that the wire size is just about exactly what you have. This just means you'll be cutting at a lower setting, normally and you have a fair amount more range above that.

Notice that the max amps shown is 2.5 amps. So you could set the amperage dial of your power supply for 3 amps, which will limit how much current you can draw on the output of the power supply. As a safety feature. You won't be able to draw more than that.

That should do it.

(Since we aren't absolutely sure of the resistivity, or your desired cutting heat, your results may vary. Start at a lower voltage, and gradually increase it until your wire cuts the way you like it. Let us know how it works!)
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Old Sep 06, 2012, 09:39 AM
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Quote:
Originally Posted by vtdiy View Post
Since you have already picked out your wire, that's one thing you can't change.

So your setting will likely be lower than 80% for your desired cutting heat. Probably closer to 60%. If you plug 60% into the calculator and recalculate you will see that the wire size is just about exactly what you have.

Notice that the max amps shown is 2.5 amps. So you could set the dial for 3 amps, which will limit how much current you can draw on the output of the power supply.

That should do it.

(Since we aren't absolutely sure of the resistivity, or your desired cutting heat, your results may vary. Start at a lower voltage, and gradually increase it. Let us know how it works!)
Thanks so much for the help, I just came to a similar conclusion, and was about to post a similar screenshot,

I am looking at electrifying 1.2m (47inch) of wire, even though i will only be cutting 1m,

The Calculator says that i'll need 0.55 thickness wire to give 1152mm hot wire and 2.9 amps.

Therefore I will keep volts at 15 (but maybe start at 4 as you suggested), Amps at 3, use the wire i've purchased, and see what happens! My Main concern was starting a little fire inside the power supply, but hopefully with this approach I should be on the safe side!

I may also purchase some steel wire as others use, as from what others have said, Nichrome wire has a tendancy to snap. Iplan on tensioning the wire using a guitar machine head at one end of the bow, and crocodile clips to attach the power wire to the nichrome wire.

I will most definitely post results...once this stage is complete i will be delving into the world of fiberglass!

**************************************************

EDIT:
Just got this back form the retailers of the power supply...

1. Is the power supply a constant current supply?
2. Is the amperage figure just a circuit protection max figure?

Yes, it is a constant current supply. The amperage figure is a rough figure of what current is being drawn.

Not sure if that changes anything,
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Old Sep 06, 2012, 06:32 PM
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You really can't fry the power supply internally as it is rated at 15 amps, and current limited to 15 amps. The most you can put through it is what it is rated for.

The voltage will be whatever you find is a convenient cutting heat. Since you have a dial Instead of a digital input, just start at zero and gradually increase until you are cutting freely. I thought you had to program the voltage.



EDIT just looked more closely at the face of that power supply in your first photo. It's a little weird. Both adjustments aren't calibrated except very crudely, and the legends say "current limit" and "Voltage Limit". I'd have expected it to say Voltage Output. And if it is a constant current supply (I doubt it) why would it say "Current Limit?"

Also the digital display in the photo shows voltage at the top and current at the bottom. But the painted legend to the left has these two reversed.

Kind of a cheap imported unit from the looks of it. Probably will work okay, but not what I think of when I think of a variable bench power supply. This is more of an adjustable fixed supply, with a digital readout.

I'm sure it's fine for cutting foam.
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Old Sep 06, 2012, 09:59 PM
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Quote:
Originally Posted by richie967 View Post
.I may also purchase some steel wire as others use, as from what others have said, Nichrome wire has a tendancy to snap. I plan on tensioning the wire using a guitar machine head at one end of the bow, and crocodile clips to attach the power wire to the nichrome wire.
I'm not sure about the Nichrome wire, but steel and stainless wire tends to stretch a good bit when heated, so the set-up with a guitar machine head will require tensioning once the wire has been heated (power on), and then adding some slack before turning off the power, to prevent too much tension which could snap the wire as it cools down. Some type of spring tensioner would be way better.
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Old Sep 07, 2012, 08:12 AM
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Thought it was too good to be true!

Quote:
Kind of a cheap imported unit from the looks of it. Probably will work okay, but not what I think of when I think of a variable bench power supply. This is more of an adjustable fixed supply, with a digital readout.
You maybe right, I can confirm it is probably a cheap import (I thought it was made by a UK based company, but upon closer investigation, it seems it could quite possibly be a cheap import)

It arrived yesterday, the voltage dial was not connected to anything on the inside, so have sent it back for a full refund!

I am instead going to go for a proper bench power supply, they seem to start at double the price of this one (~80)!

Quote:
I'm not sure about the Nichrome wire, but steel and stainless wire tends to stretch a good bit when heated, so the set-up with a guitar machine head will require tensioning once the wire has been heated (power on), and then adding some slack before turning off the power, to prevent too much tension which could snap the wire as it cools down. Some type of spring tensioner would be way better.
Point taken, I thought the guitar machine head was quite a neat idea to get the right amount of tension, as i wouldnt need to use the bungee and pivot method. I have attached the Bow design I am going to build below. 2" x 1" wood should be strong enough shouldnt it - how do people attach the electrical cable to the hot wire? croc clips?

A friend of mine has a Halfords 12v 8A Car battery charger, so i am going to build the bow and try that out! not sure how power supplies work, but ive put these figures into the calculator, and it says that for 12v and 8a, 650 resistance wire, you need 0.66mm wire, spot on!
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Old Sep 07, 2012, 08:24 AM
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vtdiy's Avatar
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Quote:
Originally Posted by richie967 View Post
It arrived yesterday, the voltage dial was not connected to anything on the inside, so have sent it back for a full refund!
Yikes!

re. bows:

I've used a bow for 5 years that wasn't even as sophisticated as the one you show. That one will work fine, but I prefer the lightest bow possible because I work alone.

I just drilled a piece of 1x2 edgewise to accept two half inch dowels angled away from each other. Those are the bow arms, and it is a true simple bow, as in "bow and arrow". I just used the bent tension of the arms as the spring.

Once a year I might have to re-tension with a n additional wrap of the wire. The bow has about 28" of free length and 25 inces of cutting length.

This cuts wings of 48", since most wings are made up of two panels -- either for reasons of dihedral or taper. Of course even larger wings can be made of more pieces.

But I find that 24" panels are much easier for me to cut alone than longer ones, and a light small bow is easier to use (and store!)

Just personal preference.
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Old Sep 07, 2012, 08:29 AM
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By the way I use lead split shot (small round fishermen's sinkers with a split in the middle, here in the US) not alligator clips, to attach the power leads to the cutting wire. I've found that alligator clips make occasionally intermittent contact, leaving ridges in the foam.
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Old Sep 07, 2012, 08:34 AM
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Bows...

Quote:
Originally Posted by vtdiy View Post
Yikes!

re. bows:

I've used a bow for 5 years that wasn't even as sophisticated as the one you show. That one will work fine, but I prefer the lightest bow possible because I work alone.

I just drilled a piece of 1x2 edgewise to accept two half inch dowels angled away from each other. Those are the bow arms, and it is a true simple bow, as in "bow and arrow". I just used the bent tension of the arms as the spring.

Once a year I might have to re-tension with a n additional wrap of the wire. The bow has about 28" of free length and 25 inces of cutting length.

This cuts wings of 48", since most wings are made up of two panels -- either for reasons of dihedral or taper. Of course even larger wings can be made of more pieces.

But I find that 24" panels are much easier for me to cut alone than longer ones, and a light small bow is easier to use (and store!)

Just personal preference.
I would also prefer a much lighter bow, The Aircraft is going to be 2.5m wingspan. It comprises of a center 50cm wing, and 2 outer 1m wing panels.

I will use a piece of 1" x 2" for the main piece, and then do what you have suggested with dowels. I guess you can bend 1/2" dowel enough to take up the slack. This is an excellent idea! I will do this!

Hopefully the Jablite foam will be good enough to cut and cover with fiberglass, and not melt!
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Old Sep 07, 2012, 08:35 AM
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Down to the fishing shop then!

Quote:
Originally Posted by vtdiy View Post
By the way I use lead split shot (small round fishermen's sinkers with a split in the middle, here in the US) not alligator clips, to attach the power leads to the cutting wire. I've found that alligator clips make occasionally intermittent contact, leaving ridges in the foam.
Good idea, that sounds a lot neater than Crocodile clips...
Cheers
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Old Sep 07, 2012, 08:45 AM
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Working alone, I eventually found that clamping the bow to a sawhorse, wire side up, worked better for me than moving the bow over the foam.

I lay the foam down on the wire at about mid-chord, letting it sink down until the pattern contacts, and then draw it towards me to make the cut. Then reverse the panel and cut the other half. Then turn the foam over and repeat. In all it takes 4 cuts.

I don't press down at all. I let the weight of the panel bring it down and keep it down on the wire, and against the airfoil patterns. I use very light pulling pressure while cutting. Heat needs to be adjusted to make this work well for you.

I'm always cutting outwards towards the TE or LE from the middle. Different than most people, but works for me.
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Old Sep 07, 2012, 08:53 AM
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Cutting technique

Quote:
Originally Posted by vtdiy View Post
Working alone, I eventually found that clamping the bow to a sawhorse, wire side up, worked better for me than moving the bow over the foam.

I lay the foam down on the wire at about mid-chord, letting it sink down until the pattern contacts, and then draw it towards me to make the cut. Then reverse the panel and cut the other half. Then turn the foam over and repeat. In all it takes 4 cuts.

I don't press down at all. I let the weight of the panel bring it down and keep it down on the wire, and against the airfoil patterns, and use very light pulling pressure while cutting.

I'm always cutting outwards towards the TE or LE from the middle. Different than most people, but works for me.
That makes sense, I guess pressing down and dragging the wire through the foam also gives a untidy cut. Gentle prssure to push the foam through the wire is also a good tip.. cheers
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Old Sep 07, 2012, 09:47 AM
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I agree with vtdiy it sure makes a differance if you can achieve a very light 'feel' fo the cutting. I spent 8 hours one day destroyong foam to figure out that trying to get the bow to follow my templates from above was pointless.

Fred
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