Request for Help - Hot wire foam cutter for the Hat Curator from the Art Museum we met on Tuesday

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Dave Velzy

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Nov 9, 2017, 6:29:38 PM11/9/17
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Years ago, I made money with snowmen decorations cut from foam with a hot-wire cutter. 
Being that I was a teen and fearless... a transformer with a rheostat didn't cause any problems. Duh. 

Flash forward to now.- Today I used my 0-15 V at 50 Watt hobby transformer to heat old nichrome wire from a dryer. 
I could dial in temperatures that cut a variety of foamed plastic neatly without too much stink or deformation. It was a short wire and an short test. 

Googling found the folks in RC aircraft have been doing advanced hot-wire cutting of polypropelene foam airfoils for years. I think that is what she is cutting,  
The most advanced process recommendations are cutting foam with stainless steel fishing leader wire and a 0-30v  3A  lab power supply. 

There are lots of 0-30v 3A kits. on eBay. 
Should we suggest a cheap kit we build <$10 - 
TANSTAFL? Aren't there somethings like a box and transformer that are missing from the kit? 

or a cheapo power supply like this? There must be 50 similar on eBay. 

A power supply and a couple of wire holders could greatly speed up the creation of forms. I'll bet that a total output race between hot-wire and laser cutting would lean toward the low tech. 
Yeah - You can buy hot wire cutters on eBay, but the RC guys have a great point, which is. You cannot regulate the heat on the wire with the store bought kits. 
You can also build something with a 24V transformer and a light dimmer, but that has limits and added costs too. 
Plus if a really good Power Supply is around $40 the why not save the work of building something. I'm lazy I guess. 

Anyone have an opinion to contribute?

Dave





Lorin Edwin Parker

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Nov 9, 2017, 9:08:40 PM11/9/17
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Hot wire cutters are great, and a must for a lot of foams. I've improvised a few of them over the years, as needed using batteries, or power supply, or variac. Essentially, it's just a resistor, so Ohm's law and stuff we already have around hive would be fine for making a good one. No need to purchase a power supply.

You can do it with DC or AC between 9-30V. So, an extra transformer or a laptop power supply, a battery charger, etc would be fine.

Stainless steel wire? Really? 30AWG stainless steel wire is 7.2x10^-5 ohms/cm. If you applied 30V, as you suggest, a one meter wire on a 30V supply would draw 428.57 Amps. That's almost 13kW of power there... Or, really, a short circuit and a dead power supply. Are they using an isolating current transformer or something?

30AWG nichrome 60 wire is .223 ohms/cm , so a meter of that would be 22.3 ohms; hence, a meter at 30V would draw 1.3A. That's 39 watts, which seems more reasonable. Thinner wire would be more resistive (exponentially), but 30AWG seems pretty thin to me. Almost all of that power is dissipated as heat in this case.

If you want to make something safe, durable, and that has heat control, what about the following:

An AC foam cutter. Isolating stepdown transformer 120 to 28V, 2-3 amps. An old trick for current isolation is using 2 transformers back to back. I have more than enough transformers in the stash at hive. Adding a dimmer switch or thyristor to this would allow one to vary the ac voltage and, thus the wattage. It would be a lot more solid than a cheap DC power supply. Transformers are heavy, durable, reliable, safer, and more expensive. The expensive part is a non-issue since we have lots of usable transformers at hive.

Maybe I'm a throwback, but a switching power supply with 10-100 parts with many failure modes just doesn't make as much sense for something so simple as, well, something simple. Dimmer switch, transformers, wire. Maybe a diode for a half-wave. 4 parts we already have.

Does this make sense?

Lorin


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Chris Davis

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Nov 9, 2017, 9:33:08 PM11/9/17
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Many years ago, I plugged a large rheostat / transformer inline with the door buzzer to at least slow down the rate at which we all go deaf. Anybody have an idea of where that has gone? Could be useful for this project.

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Lorin Parker

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Nov 9, 2017, 9:46:29 PM11/9/17
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That variac is on the electronics bench now. Turns out that the buzzer actually has a volume control built in. 

Honestly, I think a variac is more valuable as test equipment. This could easily be done with lesser tech, but, as a bench test / safety device, only a variac is a variac....

Lorin

Chris Davis

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Nov 9, 2017, 11:11:02 PM11/9/17
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K

Lorin Edwin Parker

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Nov 10, 2017, 11:05:24 AM11/10/17
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Sorry, grumpy after my surgery. I didn't mean to say that we shouldn't use the variac for this to start. It would be the ideal way to find out exactly what voltage = what tempurature. In fact, I agree that's how I would start. I just meant to say that it's not a permanent solution. Plus, the possibility of someone, later on, turning it up to 1:1 is just dangerous...

So, I revise my suggestion.

In the electronics area, there is:

1. isolation stransformer -- it looks like this
Inline image 1
2. Variac -- it looks like this:

Inline image 2

Plug the variac into the isolation transformer (you need the isolation transformer, as variacs are autotransformers and don't prevent huge amounts of current from tripping the breaker, or worse -- isolation xformer limits the current). 

Turn variac all the way down (and offf). Connect wire via insulated clips to the output of variac between. Hot on one side, neutral on other. 

Clip a volt meter on in the same place, at either end of wire, (polarity unimportant). Set it to AC volts.  

*optional -- put a temp probe on the wire (k thhermocouple, but put something non electrically conductive between wire and probe {kapton tape works -- it's in the top drawer of white drawers}).

turn on variac, slowly turn up the dial until voltage reaches 12-20 volts and experiment. 

Use ohm's law to get the current at the best settings. 

At that point, we could make a nice cutter. I can donate some nice connectors for wire and such. But if you want to rig it up with this bench gear for use now, go for it. 

Needless to say, it's electricity running through a bare wire. The current will be limited by iso xformer, and the voltage will be limited by variac, but, still... It's electricity and it'll be hot too.

Lorin
  
P.S. Sorry Chris. Thank you for leaving the Variac with us. I was grumpy about it used as a high-end doorbell volume control when the doorbell actually had a built in volume control (face palm) -- but we didn't know that until years later. I have now become protective of it, as it is really useful all kinds of thngs. That said, hive should probably buy a bigger variac current metering and higher rating for bench use. 


K

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Dave Velzy

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Nov 10, 2017, 5:39:55 PM11/10/17
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Thanks for the inputs. 
I made a hot-wire hand tool with a PVC frame and 12 inches of hot wire. Some wing nuts clamp the wire.

My home bench tests with a 15V 2.5A AC variable transformer on salvaged 20ga nichrome wire from a dryer element took about 3.5 v to melt polypropelene foam cleanly. 
I've got yards of both 20ga and 16ga nichrome wire (think it's nichrome as it was a dryer heater coil.) 

The RC aircraft guys making airfoils with a hot wire use very fine gauge ss fish-line leader or old E4 guitar string. My wife donated an old E4 from her dead guitar string collection. 
When strung on the frame the e string pulled from 5 to 8 volts. I did break an E string cutting a thick piece of foam, but it was an old string and was being stressed. When I restrung the unbroken wire survived the rest of tests. Perhaps the fine ss fish leader needs a test. 

The cutter got through a variety of foam up to 6 inches thick with little problem. The thicker foam cuts needed more volts/amps (power) or a lot of patience. 
Cutting foam does need to be done in a well ventilated place. Yupper.

On the power supply, the folks at the museum need simple, safe and reliable. Being intrinsically lazy I googled and found the following two links. 

Any donations of parts for the Power Supply are appreciated: Housing, transformer, dimmer, power switch, pilot light, fuse or breaker, line cord and connector to tools. 0 to 24v or 30v AC output. ~3A max 
Depending on what parts we all have around or could donate this could be a Tuesday night huddle project. I'll dig through junk piles and go shopping sometime next week.  
My home collection of power bricks and transformers top out at 15V and 2.5A and got pretty warm in the tests. The RC guys recommend at least 24V and 3A. 

Another item to build will look like a jig saw, with PVC frame and masonite base. About 12 inches from base to top of arm. Wing nuts and washers to hold the wire to the frame.
Accessory = Masonite oval templates of head cross sections to hot wire out ovals. Carpet tacks driven through the masonite template hold to the foam while cutting like a carpet tack strip. . 

I'll put the two tools on a cord and plug setup to connect with the power supply - one tool at a time. 

Another request: Whoever has the Hat Curators email address, Please contact her to get some more foam to do trial cuts.

Thanks, 
Dave Velzy

Lorin Edwin Parker

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Nov 11, 2017, 11:57:40 AM11/11/17
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I can scrounge up a lot of that for you, but may not be around till next week. The hot-wire PS design you cite is a good one.

Dave Velzy

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Nov 11, 2017, 7:37:51 PM11/11/17
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Thanks Lorin, 
Scrounging around in the Hive in the box marked AC PSU Transformer, I found this transformer which outputs 14V 31V 45V 77V and 91 V depending on the pin out. 
Since it weighs quite a lot it probably will pull more than enough wattage. Is this xfmr available for use? or is it held for something?

Will bring a dimmer along to test the variability on Tuesday.

Again being lazy, I did find this PS on line for Proxxon-Transformer 5A at $60 It has a weird plug for the tools, but it's all the stuff a museum would like. Simple, safe, reliable. (CE rated = European UL)

It hooks to this hand tool Proxxon hand tool $54

and the same folks have a jig saw variation Proxxon hot wire cutter $136



I think the real questions are:
1) Is hot wire the best thing we could to recommend, versus laser, or CNC router. 
PRO - simple, cheap, reliable. CON - Not sexy. Just simple. 
Needs a test, but prototypes are working all over the world.

2) If hot wire is the answer (probably) then it's a make / buy decision. 
Were it myself, I'd make it, just for the satisfaction. It might even cost me more in time, but I'd own it. 
On the other hand since this is going to a curator in a museum, (not necissarily a maker) who came to us for a solution, I'd go with a product that could be purchased. It doesn't seem like the cost of the solution is a problem and it's a safety tested product with a warranty that can be duplicated if required. 

If I was the Museum director, I'd go with the store bought tool. 

Discussion? 
I'll email Obie on Monday for her input.

Never guess I spent 40 years in supply chain doing make buy analysis, huh?
Best Regards, 
Dave

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Lorin Edwin Parker

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Nov 11, 2017, 8:03:48 PM11/11/17
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Can't really say what the optimal solution is. In conservation field there may be other factors at play too (does scorching of material lead to discoloration, degredation, etc). Who knows.

All I can say is that all the sorted xfrmers in the box above electronics are up for grabs. Go for it.

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Raven Abird

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Nov 12, 2017, 10:10:04 PM11/12/17
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Hey Dave?  Storm stopped in here at the Hive,  and he had a thought for cutting out the oval disks...
What about a giant 'cookie cutter' for each sized oval?    no scorching or melted edges... and you could stack several sheets of foam and punch out multiples at a time.    
Could this be done with a levered cutter?  if the metal was sharp enough, it should cut pretty easy, right?

Nancy

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