bwhite@oucsace.cs.ohiou.edu (Bill White ) (09/27/90)
Since the subject of plastic cases came up, I went rummaging around and realized I hadn't said a word about hot-knife cutting. This is surprising since I made my own hot knife. In any case, I thought I'd share my plans with everyone. This system is probably not optimal in any case, and has some problems, but here goes ... First off, you'll need nichrome wire for the "blade" itself. I would suggest buying it as opposed to unstringing from old hair dryers (which I did initially!) Use 22 gauge or thicker -- the thinner it is, the easier it breaks, but the thicker, the more power it uses. You also need a ballast. I used four 1000W heating elements, connected in series, with switches to bypass each element, or connect them in parallel, offering resistances of 4E, 3E, 2E, 1E, 2.5E, and 1.333E (where E is the element resistance, a little over 12 ohms final) -- to give power of 250, 333, 500, 1000, 400, and 762 watts). Use robust switches (I suggest rating for 20A). Mount the elements on non-flammible board material, and use heavy-gauge wire between elements. If possible, use steel strap conductors instead, as copper oxidizes quickly at elevated temperatures (I just used copper myself). Make sure one element is always in series, it's possible to forget about that and end up switching all the elements out of line, thus shorting your "knife" to the full line power (yes, I did this! Impressive flash of light .... but the white-hot nichrome falling on my hand was not at all fun). Connect a foot switch to an appropriate relay (you can, of course, use a sufficiently high-powered foot switch, but they tend to cost more than the low power foot switch plus relay; besides, this isolates the main power feed and saves wear on the switch). Use a momentary switch, so if you want to shut the knife off, it can be done easily. For the knife itself, I used two wooden dowels, 1" diameter, with hand grips on them. The first dowel has on top of it a bolt, with a screw clamp connector to fit the loose end of the nichrome wire. Use a bolt and not a screw, sufficiently long to insure it won't pull out of the hole you drilled in the end of the dowel (I put in some Liquid Nail (tm) to ensure this). The bolt has two nuts on it, with two washers in between; the nichrome is clamped between the washers. NOTE: I'd suggest rubber hand-grips, but make sure they're insulated. The other dowel has a spool mounted on the top of it. If you use a metal spool, the temperature of the knife will vary with length (which may or may not be desirable -- it's up to you). If not, use a spool made of some heat- and fireproof material (like asbestos, except I don't think you can use that anymore, or at least find it). Your spool need not be elaborate -- just a cylinder, with a hole through the center, and guards on either side. Through the center hole goes a metal rod, which is bent on either side down to go to the dowel rod (picture one of those wire-wrap tools with the spool on top). The connection is made to the metal rod. The end of the nichrome goes to an appropriate contact to the rod (I use an alligator clip, which is a kludge but works -- just make sure it's firmly mounted to the spool so the nichrome isn't the only thing turning it). For clamping the spool (which in my case is metal, I drilled small holes along the perimeter of the spool on one side, then used a bolt and bracket to hold it (the bolt fits into a hole on the spool) ... it looks like this: |----| /_____ Spool | | Metal rod --> ---| |--- | | |_ | | |----| |<------ Bolt and bracket | _/ | \ __/ / \__| |__/ | | <------- Dowel Rod (please excuse the confusion, ASCII graphics aren't that great) Use a three-prong plug. Connect the neutral to the dowel rod with the spool (connect it to the rod itself). Cover the entire spool apparatus in a fireproof, insulating material, with a hole for the wire to come out, and easy access to the spool for changing the length. Connect a wire to the other dowel rod's bolt, and place the top of this one inside a similar insulator, leaving a hole for the nichrome. Connect this wire to the switch and ballast. TO USE Connect the loose end of the nichrome to the first dowel rod (the one with the bolt), and replace the covering. If you have problems with the nichrome wire slipping out, you may wish to connect it permanently, or use a spade lug on the end of the nichrome heating element. In use, one dowel is grasped in each hand, or one is placed in a vise and the other hand-held. Mounting both is not appropriate as the wire expands upon heating, leading to excessive slack when hot. You may add a spring if desired to rectify this problem. DANGER! While the element is, in theory, at low voltage (as it's closest to the neutral), it still is live and represents a shock hazard, especially if your neutral isn't grounded well. So, DON'T SUE ME if you get shocked -- and read the disclaimer at the end of this. Adjust the length to whatever you want, and close the covering on it as well. Set the heat to 250W to start, and close the foot switch. You should notice a faint smoking coming off the element, as any oils are burned off. The element should be warm enough to cut through styrofoam (it depends on the thickness of your knife wire). If not, step up the power (switch the element off first). In cutting softer materials, it is usually possible to simply pull the element through the plastic. Styrofoam is a joy to cut this way, as no beads end up all over the floor. For harder plastics, you may have to use a sawing motion, pulling the element back and forth. With a long enough element (4 feet or so), you can use it lengthwise to cut plastics. Take care when cutting plastics to use enough heat to cut, but not so much the plastic re-fuses shut after cutting. If you have a problem with this, bend the plastic after cutting, back and forth, to prevent re-fusion. Be sure when cutting to have sufficient ventilation, especially with urethanes and cyanoacrilates. Lexan (tm) stinks when cut, but as far as I know nothing toxic comes off of it. PMMA (polymethylmethacrylate, ie Plexiglas (tm) or Lucite (tm)) has a much weaker smell. In any case, try to avoid breathing the fumes -- I wear a gas mask myself, but then I'm a little paranoid about stuff these days. Be careful when you use high powers, because it's possible to set the plastic on fire. If this happens, shut off the power immediately -- the fire will usually go out rapidly. Fire is almost always caused by the gases igniting on contact with the element, and in my experience it has never been self-sustaining. Note: cuts made with hot knives in PMMA and other sheet plastics will not be as neat or precise, as a rule, as cuts made with a table saw. DISCLAIMER: I make no warrantees on how well this will work for you. Nor am I responsible for any injury or damages due to use or construction of this device. This is intended for purely informational purposes. | Bill White Internet: bwhite@oucsace.cs.ohiou.edu | | HILLYER'S SECOND LAW: | | Remember: if you die, you still live in an | | alternate reality. |