[Robin Hewitt]

I've bought the wire/foam and made a jig to cut a pattern for a cast iron, cannon axle box. Basically a hollow truncated cone 14" long.

The foam turning part gets clamped to the mill bed, the wire gets clamped to the pillar. I plan to drill a hole through the foam so I can cut the inside once the outside is done. Sounds easy, probably isn't because I have never tried hot wire cutting before

You experienced foam cutters have probably already seen where this will go hideously wrong, so please, help a body out before I switch it on tomorrow

My big concern is what happens when it finishes the internal cut and the foam comes free, would simple finger pressure be enough to move it away from the wire so i don't burn a groove up the inside

OTOH, perhaps that is the least of my worries. Why won't it work?

[Robin Hewitt]

The problem is it cuts too deep when I get back to where I started, good enough though

[paulC]

If the core is rotating can you mount the unit in such a way that the wire is central below the cylinder? That way gravity should allow it to fall free then you can stop the current to the wire.
Paul

[Robin Hewitt]

Hi Paul

Not easy without lying the machine down

I don't know if the wire isn't much too hot, I wired it into the suds pump which drives 6-30 volts DC and put a 30W automotive bulb in the circuit to give me something to push against. If I had an optimum temperature in watts I could measure the voltage drop across the wire. I'm using a 30 gauge Nichrome wire at 14 ohms per meter.

Width of cut seems to depend on feed speed, but is that normal or a symptom? If I take a fine cut the foam seems to close the gap, grab the wire and bend it alarmingly. I don't think that's happenning when I go deep.

I don't know if the wire tension is right, just used a convenient spring.

The feed seemed a bit quick at 0.6 inches/second so I put in an extra 3:1 reduction.

I have enough foam for 2 more attempts but experience truly is that thing you get just after you needed it most

best regards

Robin

[paulC]

I think if you are bending the wire you are running too fast or not enough current/heat. If you can't rotate the entire unit the only thing I can think of trying is to keep rotating but stop the wire current. You could also try putting a needle through both layers once past the wire to keep the two forms together and stop the outer dropping. Just so you know the only time I've cut foam was with one of those kids toys, so no experience here.
Paul

[Robin Hewitt]

I have been asking around and it could be wire tension, or lack of it. Perhaps the wire starts straight but ends the cut with a considerable bend in it causing the step where it joins up. I am not entirely convinced because you'd think the excess cut would be more pronounced in the middle and I'm not seeing that. OTOH, people on here are talking about the wire breaking and I am nowhere near that kind of tension

Curiously the rule is in the Oxford Dictionary of Quotations as one of the best instances of accidental verse...

and so no force, however great
can stretch a cord, however fine
in to a horizontal line
that shall be absolutely straight

I am running the wire considerably hotter than is required to cut the foam and, apparently, speeds of up to 1" per second should be feasable. I am way below that.

I've ordered more foam, think I'll try a bigger spring

Another thing I've noticed, if the wire passes too close to the surface the finish on the foam degrades. A cooler/tighter wire might help there.

[vulcom1]

Just reading what you are trying to accomplish. I am building a cnc foamcutter but have for years cut my own with a bow. If the centre of your wire is dragging or bowing you need more heat or slow down. Ideally your wire does not touch the foam as the heat does the cutting. For finishing the cylinder so there is no marks I think you have to start at the bottom and finish at the bottom. This way when the outer shell drops the wire comes free of the outer cylinder and then there is clearance for the wire in the gap at the bottom.Let it cool down before removing the wire.
You are finding out that there is a fine line from having a good finish to garbage. It has to do with speed and heat and once you get it right you will know what I mean. With mine I use a transformer which has a pot for varying the heat and it is marked. When I change foam types I mark down the number for a reset. Giving you the numbers off of mine won't help as
everybody's is different.
Hope this is of some help. Now to just get my HCNCPRO board with the steppers in the mail and I will be doing it all over.
John

[Robin Hewitt]

Ideally your wire does not touch the foam as the heat does the cutting.

Hi John

That's what I thought, but if I place a small piece of foam on the wire and let it drop through it cuts at 0.1 to 0.2 inches per second depending on the wire temperature.

I think the wire gets covered in an insulating layer of sticky molten styrene and some drag is inevitable.

I have added a much stronger spring to tension the wire and plan to try again with the heat turned right down.

The problem is at the coincidental start/end of the cut, Perhaps I should adjust the cut radius just before the end to leave a lump which I can sand out later.

I can't spiral in because the thin side will melt at the end of the cut and make a mess of the surface.

best regards

Robin

[vulcom1]

Robin,
If you leave the nub and sand later would work. Is the centre part scrap? If so you could also do it in 2 stages. Cut half of the centre out with the wire ending in the middle. Remove that piece and then adjust so your next cut the wire ends back in the middle where there is no foam. You would have to support under the outer core so it will not drop down and contact the wire. This may leave some small marks that you can clean up. Not sure but just doing a little thinking to see if there is another way.
For setting my wire heat I like to use a piece of scrap that is the same length as the piece being cut. That way you can tell if it is hot enough for it to go through easy the length of it. I also like to warm my wire up for a few seconds before cutting to let it stabilize. I seem to get a better core if it is warmed up when manually cutting.
Just trying to help and would like to know how you make out.
John

[Robin Hewitt]

Just found another clue.

On my blue foam the motor stalled halfway around the internal cut and it took me a second or so to get it going again.

That put much the same mark as I got at the cut start, a sudden 30 thou indent tapering quickly back to size.

I thought it was a bend in the wire catching up but couldn't see why it wasn't more pronounced in the middle.

Could it be that the wire superheated being so very well insulated?

I did mention I haven't done this before, didn't I?

If I use the CNC feeds to move the wire in and out, I should be able to steer it into fresh uncut foam and avoid all slow downs.

[Robin Hewitt]

That fixed it

Curiously, the wall thickness of my frustrated cone is thinner at the narrow end where it was moving more slowly and the cuts were wider, matters not a lot.

Next foams will be the capstocks and various hooks, but first I shall send this one to the foundary