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Old 02-17-2013, 11:05 AM   #25
Brevis illi vita est
Joined: Jun 2010
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Originally Posted by VxZeroKnots View Post
damn good info, I was planning on prototyping with cardboard, but I guess i can go high zoot and use foam baord.
Cardboard will work. It just bends differently depending on whether you are bending with, across or at an angle to the corrugations. Again you do a crush on the inside of the bend line. A super dulled pizza wheel works pretty well as does some hammering on a narrow strip of metal (wide putty knife). More often with cardboard I just cut pieces and tape them with wide masking tape. I made some metal brackets for a radar detector setup on a guy bike last summer and I modeled in cardboard for peices that were eventually bent steel. I just didn't have any foam core handy (I usually scrounge it used).

As an aside, Acrylic (plexiglas) comes in bending grade and glazing grade. The difference is one is extruded sheet and the other is cast sheet. they look the same but if you try to hot bend glazing grade it don't work well---to much internal stress in the sheet. I dunno about lexan but I would mention what you are doing to whomever you get your plastic from and get their advice.

Parts of all sorts often have features on them that have nothing to do with the final use of the part. they are there just to make the part easier to manufacture. You see them all the time but are seldom aware of what they are or why they are there. On molded plastic parts there may be little posts on the inside. These are zero draft pins and serve to pull the part out of the mold. On cast metal parts there may be special machined surfaces somewhere that serve to register the part in a machining fixture. On all sorts of parts there may be holes or slots that are used to bolt the part into a machining or finishing fixture. You can play this too. if some part is being a bitch to clamp (like over a bending fixture) you drill a couple little holes and screw it to the fixture.. You might make one or more sides larger so you can screw up the edge with clamps or holes and then trim those edges off later. Holes near the center of a part can be filled or just ignored.

Once you have a prototype you like you cut it apart to get you flat patterns. Then put it back together and lube the inside with vaseline. Stuff it with bondo (buy it by the gallon). You will get a little shrink but not much. Peel off the now trashed prototype and you have a male plug to fabricate your finished part over. Stick on you pieces of plastic (Hot glue, DF tape, screws) and everything is held in position for welding. You can also make a female plug. just make a little cardboard "box" (more like a collar) to go around your prototype, lube the outside, etc. make it plenty thick.
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