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Discussion
Foam Block Support for 4-Axis Hot Wire
How are you securing the foam blocks for cutting with a 4-axis hot wire? I'm especially interested in how to accurately position secure the block so it is square and the part precisely located within the block. FWIW, I'm using the devFoam/devCAD software system.
A tapered wing may require a carriage to go below or past the sides of the block, so it needs to be elevated and away from the edge of the table. This precludes lining it up along the edge of the table. Parts cut from a sheet of foam require the sheet to be supported vertically and square, but fixtures may interfere with the movement of the wire. So far, the best technique I've found is to tack the block to the table using hot glue. The glue can be removed by spraying it with isopropyl alcohol. For an elevated wing block, I glued a larger block to the table and used the hot wire to slice off the front and top of the block. That gave me an edge that was square to the wire at a constant distance from the origin, and a level surface to mount the block to be cut. The base block can be sacrificial if the wire needs to dip below the level surface. The thickness of the base block subtracts from the height of the sheet that can be used to cut 2D parts, so it may need to be removed for different jobs. That may mean discarding it because it would be hard to reproduce the original alignment. Or it could be recut when reinstalled. What are some other methods of supporting the block to be cut? |
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i use this you can 3D print it uses uses 37mm x 22mm sash windows weights and 3mm x 15mm pins
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Just use some pold nicads to weigh down the block..and carefull placing on an elevated table with alignment lines on it.. I use end stops to repreatedly zero the tower positions
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I have been tempted to glue a steel sheet to the table, so I can use magnetic supports, ala Airfield Models' building boards. But the hot glue has proven to be a good substitute for magnetic holders. I have some plywood fuselage jig parts that I don't use much because the magnets I put on them aren't strong enough. Maybe they can be repurposed as foam holders, with the addition of heavy weights and pins. Or perhaps I should try using hot glue to fix them in place. |
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I'm not confident my end stops are accurate for zeroing the tower positions. Instead, I align the wire with the aluminum edge of the table and set that as my origin. I wish there was a way to calibrate my system to make sure the two sides are parallel and the towers vertical. Unfortunately, my hardware doesn't have any means of adjusting or shimming the towers to correct for any off-vertical alignment. I can conceive of something like a metal cylinder that one would place in the middle of the table to use as a calibration fixture. When the wire touched the fixture, it would close a circuit to indicate contact. By touching the cylinder at different heights and from different angles, it should be possible to thoroughly calibrate the motion of the towers throughout their range of motion. But I've never heard of anyone doing something like this and the devFoam software doesn't have any provisions for entering calibration constants even if I had them. |
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After a lot of experimentation, I've found my answer: hot glue.
Hot glue is wizard for tacking foam blocks to the table or to other foam blocks. If necessary to support a sheet vertically, I use a fuselage jig glued to the table and glued to the back edge of the sheet. That supports the sheet from behind where it can't interfere with the wire. Hot glue can be removed with isopropyl alcohol, and it's great for cleaning dabs of glue off the table. But most of the time I'm gluing foam to foam with small dabs, and they just break loose with a bit of pressure. Then they can be peeled off of the mating surface. When cutting tapered wing cores, the wing block needs to be supported off the table in order to give the ends of the wire room to roam. It's possible the wire at the big end will need to go below the block, and that makes supporting it problematic. The technique that is working for me is to hot glue a long block of foam across the middle of the table. I have two gcode files that shape the block. One cuts a deep notch out of the front face to establish a square edge at a known distance from the origin. The other cuts across the top of block to establish a smooth, level surface at a known height. The block to be cut is tack glued to this surface. It can be lined up with the end of the surface and the square edge for precise positioning. I also use hot glue to tack a framing square to the supporting surface. This lets me line up the wing block square with the known edge for truing up a block or slicing it to the proper length. The square just pops off the support block once the wing block is tacked down. The block glued to the table is sacrificial. If the wire comes in at or below the bottom of the wing block, it just cuts through the bottom block instead of running into a hard or soft stop. When the bottom block is damaged, I can cut off 10 mm or so and glue another sheet on top. Then I run my two gcode files and I have a newly restored precision surface at exactly the same location as before. That way I don't have to go back and modify any of my wing gcode just because the support has changed. Although the bottom block is sacrificial, I'd prefer not to have to repair it after every cut, so I try to avoid damaging it. That means putting the wing block as far to the side as possible with the longer wing chord at that end. The default devWing cutting strategy is to slice off the sides of the wing block parallel to the leading and trailing edges. This causes the wire to come in at the level of the supporting surface, damaging the bottom block. So I've learned to turn off that feature. |
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My two towers are movable, which is great b/c you can adjust them to best fit any block width, but lousy b/c it means I have to align them anew each time.
To square the towers, I have some plywood panels with parallel edges in different widths. (They are in the scrap bin, honestly.) This is quick and easy. But I haven't found a good solution yet to get up to height while retaining square and level. So far, I just use a bunch of bricks as support, and square off the foam block against the cutting wire. I'd be interested to hear/see other people's solutions. |
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I have thought of a four-piece table top between the towers, like the folding leaf on a dining room table. The center part would lift up and overlap the side portions when collapsed. At full extension, it would be suitable for cutting wing cores. When contracted, it would allow a greater angle to the wire and be more suited to cutting fuselage sections. The hinges would keep things aligned, or at least make calibrations repeatable.
But so far, the angle I can create with my wire has been adequate, so I'm not motivated to disturb my setup. |
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