Chilling with Pb
by Jake on Feb.11, 2007, under One Meter
I worked quite a bit with the one meter this weekend. I had been contacted by someone who was looking to purchase a raw keel bulb for a similar r/c model so I sold him one although I hadn’t planned to get into this part of the project until much later. If you remember from a long time back, I drew up a common bulb NACA foil shape in 3D on the computer (using Solidworks) and calculated it’s volume / size to achieve a 4lb bulb. With some printed templates transfered to a piece of sheet aluminum, Neville turned a solid PVC plug of the bulb on his lathe. We then built a plywood box, greased everything with Vaseline, screwed the plug to one side, and filled it half-way with Rock Hard Putty (similar to plaster of paris). Once that set, we greased the top of the hard Rock Hard Putty and poured the other half of the mold. Everything separated perfectly and we left things to sit in Neville’s garage. The mold had quite a bit of moisture in it from the curing process and during the next several weeks, the mold started to grow mold (he he). Since I was planning to pour molten lead into the mold, it was important that the mold be dry since any water could flash boil and cause the mold to crack or explode which would likely put molten lead in and on places I would rather not have it (including myself). In order to dry the mold I placed it in the oven set at 150 for about two hours, then at 185 for another two hours. Then I moved it up to 225 for an hour (above the boiling point of water), and then at 350 for about 20 minutes before molding. I did this while I prepared the lead.
The lead was prepared using a propane camp stove and a coffee brew pot with a lid. Using a skimmer / ladel, I would hold used tire wheel weights above the pot and melt them with a map gas torch. Pretty soon I had about 12 pounds of molten lead cooking in the pot. WARNING: if you try this, note that some very toxic gases come out of this mixture and it is imperative that you work in a well ventilated area and wear adequate breathing apparatus. I kept a fan blowing from behind me to help move the fumes away from me as well. Once the mold was ready, I heated the spout of the pot with the torch (to make sure the lead didn’t cool and clog) and gently poured it into the mold. After letting things cool down for 20 minutes, the mold separated easily revealing a pretty clean bulb. I was concerned about the 1st half of the mold that was poured because it had numerous bubble ‘pocks’ left from when the liquid putty made contact with the bulb. The concern turned out to be pretty founded as there was some air trapped just below the surface of the mold that expanded and cracked big flakes on the inside of the mold when the lead got things really hot. In general, there are several hair line cracks in the mold that showed up after the drying process so it will need to be remade – and when we do so, we’ll use fiberglass reinforcement and use one of these half molds to form the 1st bottom so any air bubbles will end up on the side opposite the plug. That, or I may just make the investment in a machined aluminum mold complete with fittings for the keel blade slot. Oh, BTW, the bulb weighed in at exactly 4 pounds.
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Team Seacats - Catamaran Sailboat Racing Team » I Can Taste It
July 5th, 2007 on 8:34 AM[...] getting the putty mold repaired to cast another bulb or two. If you remember, this mold was poured in two parts using a PVC plug in the middle suspended in a box with a screw. The first half that was [...]
February 14th, 2007 on 12:45 PM
Jake
I don’t pretend to be an expert, but in my industrial design class, when we did foundry type stuff, we put some powder into the crucible that supposedly helps with the bubbles.
February 14th, 2007 on 1:38 PM
that’s not a bad idea….I’m planning to apply a vacuum for a minute or two to the gypsum goo before we pour the mold next time. Any air inside the mold will cause a lot of stress with the quick temperature cycles it is subjected too. Adding fiberglass to the plaster mix will make it stronger to the hair line cracks but will make getting air bubbles out during the pour of the mold even more difficult – so I think the vacuum is the only real solution to making a mold this way. It will be interesting to see how many cycles we can get out of it. The gypsum in the plaster is supposed to start breaking down becoming brittle at temperatures over 375 degrees or so. The lead is probably around 420 to 500 degrees during the pour.
July 5th, 2007 on 3:07 PM
A few US1M and IOM sailors up in Massachusetts…we’re are admiring your work.
our work:
http://www.ourwalden.net/mmyc/body/body3-1-1.html
regards,
John W.
July 5th, 2007 on 3:51 PM
Hey John, thanks for the comments. I have been trying to find a plug-in for the website that will organize the threads a little better for researching in the archives (I go back every now-n-then to reference some measurements or mixture ratios). Did you find the page link below? If you look at the bottom of each page, there is a “previous entries” link and if you keep scrolling back you’ll find the beginning. One day I’ll be able to offer the option of viewing them in chronological order and perhaps with summaries.
This is a US One Meter project and in the very beginning we started with a much different hull shape before we trashed that one and I started over by combining features of several different existing designs that I liked resulting in something that I think looks good and fast.
http://www.teamseacats.com/category/other-projects/one-meter/page/5/
July 5th, 2007 on 3:57 PM
Ok…obvious question…what are you planning to use for a keel fin on this beastie?
July 5th, 2007 on 4:10 PM
That’s coming next week!…but I’ll give you some high points; the center strut is a piece of 3/16″ X 3/4″ 6061 Aluminum bar stock. It will be molded into the bulb if possible and the keel trunk in the hull is already fitted for it (vacuum bagged around a same piece of aluminum bar stock). For the prototype, I’m going to bend two pieces of carbon sheet around the strut in a jig designed to bend them very close to the designed NACA foil shape and fill between the sheets and the aluminum spar with epoxy / microballoons. The leading edge will have to be trimmed quite a bit with this method.
I will admit that the aluminum is cutting it close structurally. The aluminum at the hull to keel joint will have a safety factor of 2.2 against permanent bending with a 4lb bulb (fully horizontal) but because it is supported with the carbon skins, flexing should be minimal.