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CNC Hotwire Attachment

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Hawkeye

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#1
It would seem logical to use CNC to cut foam patterns for lost foam casting. Rather than make a CNC hotwire machine, I opted for a hotwire attachment for the X2 mill conversion. It consists of a frame to hold the wire and a pair of arms attached to the table to move the foam workpiece relative to the wire.
P4100877a.jpg

A few years ago, my mother asked me to remove all the electrical parts from an old crock pot she wanted to use as a planter. Lots of good nichrome to last quite a while. The wire is supported by plastic block with brass screws to allow squaring the wire to the table, with a spring to hold some tension on the wire.
P4100880a.jpg
P1010005a.jpg

I powered the wire with a 12 VAC, 50 VA transformer controlled by a dimmer switch. It cuts quickly and cleanly through a piece of 1" blue foam. I'll think up a shape to program in G-code and give it a try when I have some time.

P4100877a.jpg P4100880a.jpg P1010005a.jpg
 

Rbeckett

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#2
If you need additional foam core cutting wire check out Tower Hobbies. They have been doing foam cored wings covered with obechi and fiberglass foro a long time. The process is pretty simple. Cut the core to shape, laminate the obechi and cover with resin impregnated cloth. Cover that with a wicking layer and put it all in a vacuum bag till the resin hardens at 25-28 inches of vacuum. Stronger than steel or aluminum and carbon fiber can be added to stress points like landing gear and wing roots to toughen the airframe. Also will make a great lost foam casting block too. Mold it in sand add sprues and vents and fill er up!!! Please dont forget the pics of your projects as you go, it is a very interesting process and pretty neat if you have never seen it done before.

Thanks
Bob
 

Hawkeye

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#3
Bob, I have two coils of nichrome around 10' long each. So far, no red glow when it's heated, so I should be good for a while. Tower Hobbies is a logical source. Good to know.

I worked up some G-code to try out the cutter. It's really cool to watch. I tried 15 inches/ minute and that worked well. I forgot the camera until I'd pushed the start button, so all the pictures have the finished cut.
P4130889a.jpg

I just clamped the foam to the two arms. It worked easily.
P4130891a.jpg

The pieces pop out easily. I had wondered if the heat would cause them to weld in, but there was no sign of that.
P4130892a.jpg

For casting patterns with compound shapes, such as the tower for a vertical steam engine, you could cut one profile and then rotate the piece and cut the other sides.

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Rbeckett

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#4
My sailplane wings and pattern plane wings were all foam cored with carbon fiber reinforcement, obechi and finishing resi skins. Super light and pound for pound 3X stronger than metal. The pattern plane would do a Lomchevak manuever with great snap and authority with no fear of over stressing the spar or airframe. That plane ultimately got sgot down by another hobbyist turning on his radio on the same channel. By the time he realised it, it was too late to prevent a straight in full throttle dive. It did make some very pretty confetti out in the middle of the corn feild. I still harrass the fellow everytime I see him, but I got over the plane crash pretty quick when I bought a better design plane to replace it. This one has pneumatic retracts and a smoke generator on the exhaust so I was learning to sky write when I had to quit because of my eyes...A freind of mine still flies that plane regularly and says it is as stable as the day I built it almost 7 years ago.

Bob
 

Hawkeye

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#5
After my house fire a couple of years ago, I thought I had the little CNC running again, with a treadmill motor as a replacement for the seized motor and dead controller. Then it started getting really inaccurate and unreliable. I tried all sorts of things to no avail, then finally gave up.

More recently, I built a 3D printer, which has yet to become useful. In the early going, the Y-axis stepper was really jerky and useless. That turned out to be a loose connection on the stepper cable.

Today, I finally got around to applying that info to the CNC mill. I found a loose wire on the X-axis cable. My mill is back! Right now, I'm setting it up to cut a foam plug for a cast aluminum cover for the oil filter on my Aspencade. After the fire they threw away the hot-wire attachment I had made before, so I had made another one.

So, if you've got jittery steppers, it could be settings, but it might be as simple as a bad connection. Go over them like you expect to find one.
 

Boswell

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#6
Hawkeye, glad to hear you got your mill working again!
 

Hawkeye

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#7
Thanks. I tried running the hot-wire today. Almost got all the way through before the power supply went weak on me. Interesting what the wire does when it's only half hot enough.

Making a new power supply - more like the one they threw away after the fire.
 
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