I made a couple of patterns for some angle plates to be used on my lathe last week. They're simple pine patterns intended to be cast in aluminum to keep the swing weight down, but substantial at 3/4" thickness They require no stiffening webs. One leg is radiused to match the 9" swing of my homemade bench lathe.
It's been quite awhile since I've cast any aluminum -- I've been casting iron at least once a week since re-lining my oil furnace in late winter. This pour was really relaxing by comparison, and the first time I've melted aluminum in my re-lined oil furnace. Not ideal, it's overkill for that, and I have another smaller propane fired melter that would have made more sense to use. But that one also needs re-lining. So oil furnace it was. The melt probably took all of ten minutes to go from cold furnace to pour, and I could have melted another charge of aluminum in little more than the residual furnace heat from the first, but dinner beckoned. I started molding while the furnace was heating just to save time before dinner, intending to do both plates, but only had time for one. Still, it was pure enjoyment. I forgot how quickly aluminum melts and cools, how nice it pours and -- it was just a pleasure to do that on a clear summer evening.
Thanks for the post and pictures SR, reads like we were right there with you. I cannot even imagine a 10 minute melt. Do you have rules-of-thumb for your gating?
Thanks Tops! I don't really have rules of thumb other than remembering failures. Of which I've had plenty, and will continue. In the case of a thick angle plate, I know from experience that the place where it's going to create problems is at the center inside of the angle. And the thicker the plate the worse that will be. The failure will take the form of a central deep shrink cavity and hot tears. So, wanting to avoid that, I figure out that metal is leaving that area to flow to the ends of the angle plate, where it is cooling and solidifying sooner than at the center. In a way, a failed casting like that is a 3D graphical representation of cooling rates, metal shrinkage, etc. It is a frozen moment in time. This is nothing new, this kind of thing has been observed for as long as humans have been melting metal into useful objects. It is discussed in every book on casting ever written, is the subject of discussions, recommendations, theories, etc. And yet, there is no teacher better than failing at casting yourself, and then trying to figure out what to do about it. It becomes real, instead of abstract. You hold the casting in your hand and see the flow and fracture lines, you imagine the metal tightening up, ripping apart, and freezing into a solid. You are holding a fossil of molten metal activity, and you are there to see it. So, you can read about "what to do" about sharp angles in this case, and get a perfectly legitimate answer. Or you can imagine for yourself what might be done about it, and probably come up with your own answer. Probably 95% of the time they will be the same answer. But once in awhile yours will be different. And if you pursue your own line of thinking and your experience, those ideas will refine towards a personal style that works extremely well. For you. I'm getting long winded here, I know, I'm sorry. My hope is that we not just get caught up in rules, but leave room for experience and failure, because out of those,essentials, new insights arise. And we get an intuitive sense of how to handle new situations, new shapes and configurations, new metals, different sands, etc. OK, horribly off topic here, back to your question and this single simple pour. I needed more metal in the center of the angle, so I put a cavity that was thicker than the thickest cross section of my angle plate directly behind the spot where I knew the problems would arise. The hope is that it will solidify slower and the casting will parasitize it of liquid metal and remain full itself. That bank of metal is often referred to as a riser, and if it doesn't go up to the top of the mold, I call it a blind riser. (Apologies here to avid students of casting nomenclature if I am seeming to cause corruption to your own favored terminology.) A riser that goes to the surface will generally cool faster from contact with the air than one that doesn't. A riser that is closer in shape to a sphere will cool slower than the same volume that formed into any other shape, all things being equal, since a sphere has the least surface area of any shape of equal volume. My blind riser isn't a sphere, more of an upside down muffin shape, but it is still compact compared to a long cylinder that goes to the surface, and so has less surface area. Having less surface area, it can be smaller and do the same job. Being smaller it takes less metal to fill. Taking less metal to fill means that I need to melt less metal. Melting less metal means I have a shorter melt time, less excess metal and degrade less re-usable metal, and use less fuel. The upside down muffin shape is easy to mold and make as a small wooden pattern than a sphere. How do you get that blind riser pattern out of the cope? Well if it's heavy enough, it might just release itself onto the drag when you lift the cope off. If of light pine, like mine, maybe not. But if you push say a large nail through the cope right over the riser pattern, it will give you something to rap it with before you open. If that doesn't work (happens, in fact did happen the second time I molded this angle plate) and you open the mold with the damn riser pattern still stuck in there, you can reinsert the nail and push it out. That might break off some of the sand the the top of the riser, but no problem, just dump it out. You can see the effect on the riser of that having happened in the above photo. You can also see the cast shaft of the nail. As for gating, well I use a thin metal sheet to cut them, bent into a U shape, but with hard corners. Same thing for runners. Runner in the cope, gate in the drag, in this case. A right angle into the pattern cavity, and a bit of an appendix straight on from the riser was hoped would trap any gunk, but who knows? It felt good to do it that way, and feeling good is very important in anything you do. Like casting.
And the second plate after lifting the cope, showing a better looking riser. This time I decided to try using the smaller propane furnace, even though its 3 year old plaster and sand lining had suffered rain damage and erosion. But it still worked, and melted 4 pounds of aluminum in a sedate 25 minutes, using my small atmospheric propane burner. I could have molded in that time, but I'd already done it before hand, so not very time efficient of me. The propane burner and small furnace are mild mannered enough to enjoy that kind of thing. The bigger oil burner demands full attention, and particularly with a pot of iron in the hold.
The alternative.....(from twenty years ago). No riser, gating into the wrong place. This tells you where to put them. It isn't a failure. It's a recipe.
After both angle plates had their long side trued, one was bolted to the faceplate through the short side and shims inserted until the trued side was square to the faceplate, as checked with a machinist square.
The second angle plate was then bolted to the first with both trued faces together, and both therefore square to the faceplate.
Then the short face of the outer angle plate is faced off, making it true square with its long face. It's now an angle plate. The combo can be unbolted from the faceplate, turned end for end, and bolted in again, this time without shims. And the short face of the other angle plate made square. Here's the first plate squared up:
And then I faced the second one. There you have it, two useful and accurate angle plates turned on the lathe, and cast from scrap. (note: no end mills were harmed in the making of this project.)
