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Hot-wire cutters for plastic foam disks and balls.

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klugesmith

Mon Dec 18 2017, 08:59AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

The disk making project came first, as an unexpected branch of another project. While thinking about easy & inexpensive ways to make a round form, I saw a bundle of new foam insulation boards, being discarded by a neighbor. Each large sheet had been broken roughly in half, to make the bale less unwieldy for the garbage collector.

Some pieces came home and got turned (literally) into 28 inch (70 cm) disks. Cutting tool is a stationary, vertical hot wire. Workpiece is rotated by hand; it takes a minute or two to go all the way around.

Initially I tried a 1/4" diameter pivot pin passing all the way through the workpiece, but that was too floppy. Version 2 requires no hole in the work. A scrap of cat-food-can metal, taped to the bottom of foam board, bears a stubby metal pin. That engages one of several holes near the edge of the plywood.

Radius is adjusted coarsely by using different holes in the plywood, and finely by moving the wire holder.

It was easy to cut foam with some steel wire from a bread bag twist-tie, after removing the paper. But that needed almost 5 amperes of current, at much less than 1 volt. I found some more resistive wire of about the same diameter in a broken quartz-tube radiant heating element. That starts doing the job at only 2 amps, and cuts nice and fast at 4 amps.

By the way, two 28" circles barely fit in a 48" square. When cutting four circles from a 48 x 96" sheet, they can be a little bigger than 29". For nominally metric material sizes, I bet the ratios would be the same.

klugesmith

Fri Dec 22 2017, 08:47AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Now a few words about cutting foam _balls_ with a hot wire. In this case, the material came in ball shape from the craft store, and was cut into non-ball shapes as a favor for my wife. Diameter is 10 cm.

Description of the work holder, wire holder, and their relative motion will wait until I can present the pictures.
Wire is from the same broken quartz heating element, but is more than 8 inches long. My power source, a rewound UPS transformer bigger than a MOT, almost ran out of voltage! With variac at 140 V, I measured 3.86 VAC at transformer secondary terminals and 3.74 V between the ends of the hot wire. For about 2.1 amps, at which the wire cut slowly but left a nice kerf surface.

klugesmith

Fri Dec 29 2017, 06:01PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

The ball was cut using some resistance wire soldered in-line between two stranded, insulated copper wires.
With one hand on each end, the wire is held tight and guided along some wooden sticks.

In picture there's a stick on top of the ball, which had another brick on top to hold the work firmly enough. Low-V high-I rewound transformer is visible in the background. It's much bigger than necessary for this job, since 3 amps x 5 volts would be plenty of power for that size wire.

First cut was at the mold-line equator. The first 40% of cut surface is smooth. Then, being impatient with the slow progress, I started a sawing motion with the hot wire. That made an interesting pattern because of residual curliness in the wire. Next time, try annealing the mostly-straightened wire before rolling it between flat metal plates.

Conundrum

Fri Jan 19 2018, 07:43AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

You could use it for removing screens on OLED panels with cracked glass.
Heat + Moly would work very well!

I had a similar idea for making custom foam cases once when working at a charity shop but LVCR put paid to that idea.

Signification

Sat Aug 03 2019, 08:24AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

QUICK POST BLURT: I have no experience with hot wire cutters (yet), but just got two 50 foot reels of 20 & 18 AWG TEMco Nichrome round resistance wire from Ebay (Never Knew It Was Soo Cheap!). I haven't tried then yet, but as soon as I get time.
Just my 2 cents... I need to study your post a bit more...bet I should have finished reading your posts.

Patrick

Wed Aug 07 2019, 07:10PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I too purchased nichrome wire, Ive been make 10-30 watt resistors that dont hardly heat up. some are used as current sensors, 0.068 ohms. You couldn't but a high what resistor but for huge price, and probably not the value you wanted. I use 5 or 10 or however many strands in parallel and length needed to get my value.

I have a ultra low ohm meter that helps me, 0.000000 digits. its cheap and Chinese, and I only consider the first 3 place digits to the right of the decimal as credible. It uses kelvin measuring with 4 wires to a 2 terminal resistor.

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