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Old 15th July 2005, 11:33 PM   #1
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Default Electric hot wire knife?

Does anybody here know of either a site or have information how to build one of those electric heated wire knives?
I was thinking about building a hot wire knife with a difference. Instead of just one tensioned wire I thought that a simple jig could be built to string a series of parallel wires like a comb. Say as many as 12-20 wires per inch and about a foot wide. This could then be used to melt a row of shallow notches into the end edge of a piece of light louvre. The notches could then be used as a guide to string a wire stator (30 guage) without the need for a jig. Further once strung ot would be easy to solder the wires together at the ends or segment as desired. This is not so unlike the idea of useing holes at the end of a panel to thread the wire through but much less work.
Any input would be most welcome. Thanks in advance. Best regards Moray James.
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Old 16th July 2005, 12:22 AM   #2
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It's do-able (I use hot wire cutters to shape foam), but I think that a step-and-repeat jig would be a whole lot easier. After all, you'll have to construct the hot-wire assembly anyway...

Think nichrome and a Lionel train transformer.
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Old 16th July 2005, 02:53 AM   #3
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Default OK tell me what a step&repeat jig is?

Thaanks Sy I had thought about useing one of those Weller soldering guns as a power supply but I do not think that I've seen one that has a variable heat control, though some have two temperature settings on the trigger. Maybe a small variac could be used in conjuction to set a working wire temperature? Train set power supply... now who do I know that has kids with a train set? Any other ideas? Regards Moray James.
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Old 16th July 2005, 03:10 AM   #4
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I think you can put together a reasonable jig with a board and a fine brad. The idea is to fix the hotwire, have the brad spaced by the step distance, then sequentially move the plastic bit by putting the brad into the last melted "cut". This isn't written very clearly, I'm afraid, but a bit of sketching should make it clear.

Toy train transformers are variable, cheap, and effective.
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Old 16th July 2005, 03:21 AM   #5
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Default OK

Ok I get it as far as the reference brad to line the wire up. Are you suggesting just a single wire for the tool and to move the piece for each cut? Would a string of spaced wires not work? IE dificulty in generating consistant wire temperature along the string? Given the number of cuts to be made 12 to 20 per inch I figured a long tool would be more consistant and easier to do time wise. Regards Moray James.
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Old 16th July 2005, 03:24 AM   #6
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Realistically, how long will it take you to make the "time-saving" tool?

Not to mention the other complications you mentioned. Figure each cut will take a couple of seconds, so you can do it all in an hour or so.
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Old 16th July 2005, 04:13 AM   #7
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Default Alternate idea

Wonder if I just couldnt' heat up a length of threaded rod and use the thread of the rod to "score the plastic grid all at one go? the cuts don't need to be very deep to hold a piece of 30 guage wire. So how do I control the heat of a piece of 1/4 or 1/8 32 threaded rod that is a foot long? Moght be just as easy to set the thing down on a old iron set to the right temp. Any other good ideas? (I kinda like that last one myself) Regards Moray James.
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Old 16th July 2005, 01:32 PM   #8
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Here's another method of building "light grid" stators:

Jig is made from: 2pcs 1"x12" electrical conduit, 2pcs 1/16"x3"x18" sheet metal, 2pcs 1/16x1"x18" sheet metal.

Drill four 1/8" mounting holes about 1/4" from one of the long edges of each piece of sheet metal.

Mount the 3x18 sheet metal on the end of the conduit pieces so that the edge with the holes extends 1" or so beyond the conduit. (Forming an elongated "T" about 13" tall.)

Bolt the "Ts" to 2 front-front light grids (wire will be on the outside) using the outer rows of cells in the grid. (The sandwich is sheet metal, 2 grids, sheet metal.) This gives you a big "single blade paddlewheel".

Suspend the "paddlewheel" between a couple of workbenches so that it is free to rotate but not otherwise move.

Attach the end of your wire to one of the grids.

If you get the "right" amount of tension on the wire, it will pretty much stay where you wrap it. Rotate the paddlewheel while feeding the wire and you end up with a big rectangular "coil" about 1" thick by whatever length you are making the cell.

Adjust wire spacing if required, superglue the wires to the grids, cut the wire at the end of the grids, and presto...two stators! (If you want more than 1/2" of wire at the ends, add a stick of wood or some other spacer at the ends of the paddlewheel.)

Hope that was clear enough!
Paul


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Old 17th July 2005, 03:49 PM   #9
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Moray, for a hot knife, you want nichrome wire and low voltage. A nice high current transformer (easy to find these days) of 5 - 28vac with a variac in front of it to control temp works ok...

The problem with your scenario will be controlling the "burn" - the nichrome likes to start out hot, and cool as it meets the work. Getting a consistent depth of burn will be the most difficult part. Not sure how you could do that.

Nichrome also does NOT stay tensioned when it heats up - you'd have a shot at going width wise on 8-9" wide or less cells. Not the long way at 4 ft...

The fumes are TOXIC!

Threaded rod will not yield parallel lines, dude!


I think I'd opt for a different method.

I'd set up a jig with a very fine saw blade - they are sold in precise thicknesses, btw. You could easily stack a number of these blades (typically 3-5" diameter) on the arbor of a table saw with precise spacers and cut a number of slots in one pass.

Incra sells a "step and repeat" jig for table saws - or you can make one for this sort of thing very simply, given that you only need to go the width of a louvre, (or 8-9" like Acoustat?) or less.

This will give you repeatable and accurate depth of cut and width.

If you use this, please give credit!

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Old 18th July 2005, 09:38 PM   #10
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Default cutting slots

Thanks Bear for the input. The more that I think about the wire cutter device the more work it seems. I had long ago thought about multiple rotary cutter disks on a single mandrill but did not want to spent that much money or time.
Yesterday I did some simple tests with some machine screws and also with some wood screws. The machine screw (1/4-20) worked well enough simply heated on the stove but the thread is not deep enough to form a nice deep slot. The wood screw which has a deeper thread was almost good enough to get away with. By the way both screws formed very nice evenly spaced slots on the end edge of a piece of light louvre.
I will have to take a piece of threaded rod to a machine shop and have the thread made deeper. Then I will spot weld the rod to the edge of a piece of steel bar stock and then brase the bar stock to the bottom of an old iron. That way I will have a convienent heat source that is fully adjustable. With a little lubricant on the threaded rod and just enough heat to melt but not burn I should be able to get the job done. I have found that the end of the light louvre has to be smoothed down after cutting (melting) as the plastic that was where the slots now are pushes out. This was easy to clean up with a few strokes of a good file. Hope that this information sparks some interest and experimentation. Best regards Moray James.
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