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Old Jan 05, 2015, 05:35 PM
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plasoneal's Avatar
USA, OH, Hamilton
Joined Oct 2003
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Question
Problems setting up Hot Wire circuit

I'm setting up a new CNC cutting machine (actually re-working a used one I bought).

I have a 24v/ 15 amp power supply, but when I connect to the 32 AWG "resistance wire", I can only get the wire hot when using less than 15 inches or so of wire. I'd like to be able to cut up to a 48" sheet of foam. I don't know if I've just bought the wrong stuff or if there is a simple fix.

Here's the power supply:
http://www.amazon.com/gp/product/B00...0ie=UTF8&psc=1

And the wire...
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1

It seems things work fine with 12-15 inches of wire, and it pulls about 1 amp or so. In these shorter distances, I have to "turn down" the current by running through this Speed Controller:

http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1

But as soon as string up more wire for a longer cut, the amperage drops down to 0.1 or 0.2 amps, and I've got no heat.

Since this is a brand new 15 amp power supply, what's going on? What do I have to do to get this to pull more amps (in a manner that I can also fine tune it with my speed controller)?

I don't really think its actually a problem with the power supply, because I had exactly the same issue when hooking directly to a car battery or car battery charger.

Is there a simple solution or have I just bought the wrong kind of supplies?

Thanks in advance (obviously electronics isn't my strong suit),

Paul
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Old Jan 05, 2015, 07:01 PM
Duane
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32 AWG is very thin, about 0.008" diameter. 28 or 26 AWG is much more common for hot wire cutting. I think you may need 50V (or more) to cut 48" with 32 AWG wire. Heat comes from wire current, which is determined by the supply voltage and wire resistance, not by the supply current capacity. Thinner wire has higher resistance and requires higher voltage to get the same current. Can you try using thicker wire?
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Last edited by Wazmo; Jan 05, 2015 at 07:19 PM.
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Old Jan 05, 2015, 07:41 PM
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USA, OH, Hamilton
Joined Oct 2003
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Sounds like an 'informed' response. I need to do some very thin cuts, but at shorter distances which is why I selected this wire. But perhaps a I need the heavier wire for doing the wider cuts. Sounds like my logic was backwards. ��.

I have some heavier gauge I will give a try for the wide cut.

Thanks a bunch, will let you know what I find out.

Paul
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Old Jan 05, 2015, 09:07 PM
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United States, OH, London
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Think about it like water lines
Pressure=voltage
Flow=current
Diameter=resistance
Increasing the pressure(voltage) through an orifice(resistance) will increase the flow(current) and current equals heat. Here is a calc to determine hot wire gage....your running around 300F with your setup....
http://www.jacobs-online.biz/nichrome/NichromeCalc.html
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Old Jan 06, 2015, 05:43 AM
Scorpian 60'' 2012
BMtech's Avatar
Kurrajong Australia
Joined Jul 2005
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A few questions
What cutting software &speed are you cutting &foam type
I have 1200mm between my clips with .016 ss single strand fishing leader
26volts @ 2.5 amps is good for 3kg epp & this is the hottest setting
As above more volts & wire size
Rick
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Old Jan 06, 2015, 10:11 AM
The Junk Man
Jacksonville, Florida
Joined Jul 2006
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Resistance wire is used for building coils and I have never heard of it being used in a hot wire foam cutter. It has very high resistance per foot compared to "normal" wire.

I have used .016 fishing leader for years and years, no problem.

Go here: http://www.jannsnetcraft.com/ss-lure...re/311665.aspx

30 feet for under 2 bucks.

Tom
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Old Jan 06, 2015, 12:28 PM
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Old Jan 06, 2015, 12:57 PM
Duane
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Are you sure about that wire size, Kevin? According to the NiCr resistivity numbers I can find online, that size wire at 20" length should yield a bit less than 400 ohms, allowing 30 mA at 12V. You would need around 0.054" diameter wire for those numbers to work out. Perhaps my calculations are way off.
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Old Jan 06, 2015, 03:21 PM
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plasoneal's Avatar
USA, OH, Hamilton
Joined Oct 2003
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Quote:
Originally Posted by BMtech View Post
A few questions
What cutting software &speed are you cutting &foam type
I have 1200mm between my clips with .016 ss single strand fishing leader
26volts @ 2.5 amps is good for 3kg epp & this is the hottest setting
As above more volts & wire size
Rick
I'm just getting started. This is my first CNC machine, and I bought it used.
I currently have Foam Works/Cad Works on the machine that I bought. I did the attached "first project" with the 32 AWG wire, but only at about 12 inch length. I used a 12v tractor battery, but had to step it "down" with the speed controller above, in order to get a "fine" cut. The photo attached was cut at 3 inches per minute, with the 32 AWG resitance wire. Its not perfect, but for a first project attempt, I think it came out pretty good. (for scale, the total project is about 12 inches wide).

Projects like this (along with my RC stuff) is why I think I could benefit from such a small gauge wire. I may just have to get that 50v+ power supply for the longer run.

Thanks for all the interest. I hope to confirm my setup with larger wire later today, and will confirm the results.

Paul
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Old Jan 06, 2015, 05:19 PM
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plasoneal's Avatar
USA, OH, Hamilton
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Quote:
Originally Posted by Wazmo View Post
32 AWG is very thin, about 0.008" diameter. 28 or 26 AWG is much more common for hot wire cutting. I think you may need 50V (or more) to cut 48" with 32 AWG wire. Heat comes from wire current, which is determined by the supply voltage and wire resistance, not by the supply current capacity. Thinner wire has higher resistance and requires higher voltage to get the same current. Can you try using thicker wire?
Confirmed the above, which should be no surprise. I had some ~ 21 gauge wire (0.7 mm) which came with my unit. Although this is much heavier than I'd plan to use, I confirmed I could get it "cherry red" at 48 inches with 24v, and could turn it way down with my speed controller. It was pulling max about 6.6 amps, and I could drop it way down below 0.2 amps or so. Looks like ~ 2 amps to get a good cut on my "packing" foam for a test.

Thank you so much for your help! I may need to play around with things a bit to get the overall right balance for what I plan to do. But in the end, I might need various wire sizes for different spans/tasks.

Paul
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Old Jan 06, 2015, 05:22 PM
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plasoneal's Avatar
USA, OH, Hamilton
Joined Oct 2003
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Quote:
Originally Posted by poormansairforce View Post
Think about it like water lines
Pressure=voltage
Flow=current
Diameter=resistance
Increasing the pressure(voltage) through an orifice(resistance) will increase the flow(current) and current equals heat. Here is a calc to determine hot wire gage....your running around 300F with your setup....
http://www.jacobs-online.biz/nichrome/NichromeCalc.html
Thanks for the link to the calculator!
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Old Jan 07, 2015, 12:14 AM
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United States, OH, London
Joined Mar 2013
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Quote:
Originally Posted by plasoneal View Post
Thanks for the link to the calculator!
No problem. I use it all the time.

Quote:
Originally Posted by T_om View Post
Resistance wire is used for building coils and I have never heard of it being used in a hot wire foam cutter. It has very high resistance per foot compared to "normal" wire.

I have used .016 fishing leader for years and years, no problem.

Go here: http://www.jannsnetcraft.com/ss-lure...re/311665.aspx

30 feet for under 2 bucks.

Tom
Tom,
resistance wire has been used as hotwire for forever.

SS wire is a resistance wire as well. It has to be to produce heat. I plugged Rick's numbers into the calc I posted above and his numbers agree perfectly which means that .016 SS wire has virtually the same resistance as .016 NiCr at 2.67 ohms/ft for a total of 10.5 ohms for his setup and he is running at 740F. I've used SS as well as guitar string.

FWIW, did you know you can run a current thru a CF tube and get it hot enough to bury into a foam wing? If I recall correctly it had a similar resistance to my NiCr wire.
Dan
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Old Jan 07, 2015, 12:58 AM
The Junk Man
Jacksonville, Florida
Joined Jul 2006
1,818 Posts
Quote:
Originally Posted by poormansairforce View Post
SS wire is a resistance wire as well. It has to be to produce heat.
Sigh. Who, exactly, said that stainless steel leader wire had no resistance?

Feel free to use what you want. I know what has worked for me for a very long time now.

Tom
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Old Jan 07, 2015, 06:05 AM
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Italy
Joined Jan 2003
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Quote:
Originally Posted by BMtech View Post
A few questions
What cutting software &speed are you cutting &foam type
.....
About the software You can see this: http://www.foamcuttingsoftware.com/eng/default.asp
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Old Jan 07, 2015, 06:11 AM
Scorpian 60'' 2012
BMtech's Avatar
Kurrajong Australia
Joined Jul 2005
1,460 Posts
Good to see that your making head way with the cutting, we have cut hundreds of wings in all types of foam epp,eps closed cell polyurethane & have used gmfc as you can save the heat setting &foam types for them
Rick
Off the edge gliders
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