For a local machinist, forum member 3gger. The nut it is to replace is badly worn. He replaced it with a 3D printed nut temporarily, but that's not cutting it, he needs bronze. A threaded insert might have done the job, but we wound up casting a whole new nut. He came up with the 3D file for the split pattern and corebox. Too many holes! Better. After some back and forth to dial in the 3d model, we each printed out a pattern. Mine was not perfect, some PLA that got gummed up inside the print head oozed out onto ot at some point early on. But, moldable almost right off the printer. His got some paint and sanding. He made the cores himself using an epoxy binder. I used the ones that looked like they had the least glue in them. They were very sturdy, not at all crumbly. They're not really all that round either, but he says that's fine as he'll have to machine out a larger hole anyways. Made 3 molds: 2 nuts (1 each using each of the 2 patterns) and a cylinder ingot he could turn into one of those threaded inserts. Perhaps for repairing the nut on another axis, or as a backup if the nut we cast turns out not to be usable. We ran out of metal before the second nut was quite halfway filled, but that wasn't really unexpected. I made sure to pour his cylinder ingot first, well, second, but I mean before the second nut. The core broke down and fell out just by hammering on the riser a little bit. He brought a bunch of scrap copper and some tin/lead solder alloy he had, and we aimed for an 88 copper / 12 solder mix. He brought me a bunch of scrap iron too. I ran the burner kind of rich to create a reducing furnace. Melting the copper, I noticed it turned more clean and mirror shiny than any other copper I've ever seen just before it melted, so maybe that was enough. But we added a scoop of charcoal granules as well thinking it would shield the melt, which might have been a mistake. I expected it to be polite and float on top waiting to jump into my dross scoop and come back out at the time of my choosing. But it was not so cooperative, some even seemed to hide at the very bottom of the crucible. It wound up dumping into the pouring basin for the last mold after all the metal was gone. (I did ask for phosphor copper shot for degassing coppers last time I was at the supplier, but they were all out). I hope when he goes to machine it, that he won't find any big charcoal inclusions. He doesn't seem worried though, said any such "porosity" would only act as a place to hold some oil and keep things running smooth. But of course I'm hoping it'll be 100% solid clean metal under the casting's outer skin. A couple notable moments: when my oil tank ran dry and a long bubble got trapped in the line. I noticed less fire in the furnace and tried to open up the needle valve a bit before seeing the air bubble and realizing what was up. Refilled the oil tank, and when the oil started dripping into the furnace again, too fast, it was quite dramatic until I got the valve closed back up a bit again and the excess dragon sauce burned off. Fire shooting out of every crack and gap. At least I gave Gene a good show! Then I was probing the melt with my 1/4" steel rod (same one I've been using since 2013) to see how much solid metal there was beneath the surface. A chunk of copper stuck to the rod, I suppose they brazed together? So I left it in there for a minute to melt back off. That worked. When I pulled the rod back out it was glowing brightly of course, and bent at the end. I was about to gently bang it against my dross bucket to straighten it back out as usual, when the 6" or so at the end that was bent just... fell off onto the ground, reminiscent of the way aluminum gets crumbly just before it melts. Wow! 3gger filmed the pour and posted it up on YT: https://youtube.com/shorts/gdqV8hH1_JM?si=gPXaBdsxE1IetXwn The homebrew bronze seemed a bit lumpy as it poured, I do hope the nut will machine well! Jeff
Another one for the books! Thanks for taking us along. I have been using 2:1 and 4:1 epoxy (not 5 minute 1:1) to coat 3D-pinted and wood patterns. It adds about a day to the process but generates a thicker film thickness and tiny fillets in the nooks and crannies of the patterns. Sometimes I add fumed silica to keep it from slumping and glass spheres if it will need more sanding. On white printed patterns I will often tint the epoxy so I can see better. My last attempt at reclaimed bronze was lumpy too... but it froze in the runners so I am attributing most of it to being too cold. I signed up for a local bronze casting cast hoping to glean some more copper alloy melting knowledge.
I noticed a difference between using an A20 and going to an A16 crucible for bronze: we had to do all the skimming in the furnace and get it out and poured fast as the thinner sections of the casting would freeze with the A16, while the A20 retained the heat for longer. We used only a small amount of phos copper shot for fear of making the casting porous. We were using a coagulant (unexpanded perlite) and some sort of bronze specific flux, a brown powder and the phos copper shot.
Lots of porosity, yikes! I don't know whether to blame the copper melting portion of the alloying or the unfamiliar alloy or the uncertain amount of epoxy used in the core (the porosity does appear to be worst right above it), or something else. A bit disappointing. But Gene is happy. He says the porosity will only give a place for oil to hang out. The pattern could have used a little more shrink/machining allowance but the nut appears to be usable at least... Jeff
I don't think it's the alloy, just whatever run of the mill casting issues, you'll probably find it's only surface deep and a bit of extra thickness would have helped. I made a lathe cross slide nut on bronze on a lathe and where the single point tool began to engage the metal was pretty bad but there was enough material to machine off the bad start and still have a good thread. This spindle housing casting looks bad on the surface but was fine when machined, maybe could have used some more coagulant and flux before pouring.
