i'm working out how i'm going to make this part out of aluminum- here is a gallery of 3 images with some dimensional and fabrication info: https://imgur.com/a/WdnYy6w here is the part if you don't want to click the link: the part is 29" long, 5" OD measure from the outside of the flights, 2.5 OD on the shaft and 1.5 ID. the end bit (on the left side in the last photo) is just for workholding, and gets machined off at the end. to create this pattern, i've cobbled together something kind of like a lathe that uses hotwire as the cutting tool. i bent the wire to form a 2d profile of the flight geometry, and then sweep it down the workpiece as it rotates. its pretty satisfying! if there's interest, i can post a bit about that tool once i've worked out all the main kinks. anyway, it actually does a fairly good (if not entirely repeatable) job of creating this pattern. now that im confident that i can create the correct geometry, i was hoping to get some input on the following topics- 1. type of eps: ive been using pretty light eps for test pieces. based on what ive seen, its preferable to have lighter material where possible, but id also like the surface of the flighting to be as smooth as possible / require less post processing. this will be in a prototype compactor and so the flights will see quite a bit of pressure. in fact, the reason i'm making this out of aluminum at all is that the steel augers i've used up to now generate too much heat- hence the aluminum and the hollow shaft for forcing air through to remove heat. anyway, i'm wondering if the overall thickness of the part would allow for denser foam, which i think would yield a better finish and more dimensionally accurate part? also, i'm curious if you can touch up any surface issues with wax or something like that before you dip coat- i think i've seen some examples of that. all that just to say- how heavy of a foam do you think i can get away with? 2. sprue design: from what i can tell, this part is pretty well suited to casting because the walls are fairly thick. im hoping to confirm that, as well as some input on how this might like to be poured. my naive guess is to put the part in shaft side up, vertical, and that way the backside of the flights will fill naturally. i'm curious if the overall length of the part would be an issue though. hopefully these arent stupid questions. i've been considering casting as a fabrication method because machining this part out of a 5" OD billet is very very expensive. the other thought i had was to cast just the flight section (which i think ends up being cheaper than bending aluminum flights, strangely) and tig, or perhaps even just jb weld it on. but i dont like those options as much because i think the heat transfer and overall strength is compromised, and im not sure its any cheaper either. not to mention more labor intensive. im really happy to have found this forum as it seems to be full of people will to help and offer their valuable knowledge! thank you
yes, freecad for life! here is the info from fcinfo macro: that comes out to about 200 inches cubed, and a little over 19 lbs of aluminum
The FreeCAD screens have a look to them for sure. So you'd be looking at needing 3.22 liters for the object let alone gating, etc. if I did the math right at 2.7g/cm^3. That would put you in A20/A25 sized crucibles for a pour.
The shape of the part makes it an ideal candidate for lost foam compared to conventional sand casting. Whether or not aluminum will have the strength and hardness is yet to be seen. A hard anodize would help durability a lot. I doubt you will get much cooling with air unless you force a lot through that shaft. Water would be far more effective. Sounds similar to an ornamental milling machine for making fluted wood spindles. A rotary 4th axis on a CNC router would do a nice jobe too. Post up your method. I presume you have some sort of lead screw timed to the rotary axis to advance the hot wire. How did you produce the hole in the shaft? I'd probably use a round nose router bit and then assemble the two halves for the foam machining blank. In general, Extruded PolyStyrene (XPS or EXPS) will machine and finish better tan Expanded PolyStyrene (EPS, white beaded stuff), and denser foam machines and finishes better than less dense foam, but at the expense of having to evaporate more plastic during the pour. For XPS, I use 1.3lb/ft3 density. This is the pink Owens Corning Foamular 150 insulation board available in the big box stores. It's a good compromise of strength and density and finishes well with light sanding. I have also used 1.5lb/ft3 density which is Foamular 250. There are similar DOW products (blue in color). The problem with XPS is it is hard to find in >2" thick sheet, and glues that laminate don't machine and finish well through the joint, but some do hot wire ok. For this reason you may want to use EPS, but you still have that hole to contend with, however the XPS will finish better. Yes, waxes can be used to touch up flaws and for fillets and appliques. I use it sparingly and wouldn't necessarily recommend trying to coat the entire part. Freeman Manufacturing sells a wax specially formulated for lost foam casting that has the same melt temp as the foam. It works well. I would cast it vertcially and feed from down from the top of the annular shaft. The flights will naturally fill. The challenge will be keeping the long core stable. You can't cover the hole with the sprue. It needs to attch to the rest of the sand mold like a core print. It will be a tall mold and a bit of a challenge to pour due to the pour height. You may need a lifting apparatus for a one man pour and/or a platform to stand on. Here's an example at about the 21 minute mark. It will also be a very heavy mold and that means it will be hard to vibrate and pack it. The one shown in the video is >450lbs. You should search my molding rig. Without good vibration I doubt you will be able to adequately stabilize the cored shaft. I have threads here and YT videos that discuss and cover all of this. Search! Best, Kelly
Add at least 5lbs for sprue and cup, and whatever safety margin for slight spill. As a lost foam caster, there's nothing worse than pouring 1" short and having an otherwise perfect casting. I'd melt 25-30lbs for that. Best, Kelly
so very helpful everyone! i will have another detailed post soon with some information on the hotwire lathe setup and my progress. one question- kelly, im not quite sure i know what you mean when you say "The challenge will be keeping the long core stable", are you referring to the hollow center of the main shaft? i dont think i understand what the concern is there, i figured it would just get filled up with sand. maybe i just dont know what you mean by core print. thanks!
Yes Yes, filling it with sand is easy, but even if the core was a selfsupporting chemically bound sand core, a core that is 30" long and ~<1" is fragile. In unbound sand that is only held in place by intereference fit of the packed sand grains, even more so. A core print is a feature of the pattern in a bound sand mold that engages the core and holds the core in place in the mold. If you tried to cast that in a horizontal green sand mold with a bound core suspended in core prints on each end, the buoyancy force would almost certainly break that core. In a vertical mold there are no buoyancy forces working on the side of the core. To have a chance, casting vertically, each end of that sand core that defines the interior of the auger shaft would have to engage the rest of the sand mold, meaning you would need to leave each end open to the sand mold and gate into the side. So in essence, each end is anchored to the mold to the extent that vibratory packing of dry sand allows. I'd make a larger annular gate that surrounds the shaft to try to feed/distribute the molten metal around the shaft so is more likely to progress down the shaft uniformly. Best, Kelly
ok, that makes sense and i didn't really think about how the mold will interact with the aluminum in the molten state like that. since there are no buoyancy forces when its vertical, i guess the concern is that some kind of asymmetric load on the core might knock it loose- is that the intuition behind an annular gate? so that the metal flows evenly around the core. here is a little mockup of a sprue (in green) connecting to the workpiece, curious if this is like what you were suggesting- its a 1.5" square inlet (I think thats what you used in some videos) that lofts to an annular / connical feeder. i realize it kind of looks like a urinal lol for the styrofoam lathe portion- i blew up my wire power supply (i had an ibt2 to adjust the power going to the wire, its just what i had on hand) and needed to rework that component anyway, so its still in progress. ive opted for a 24v dimmer since 24v is what i have coming out of the power supply. but ive included a couple pics of some test cuts ive made, and i think that overall, the design should work fine. to cut the core of the syrofoam piece, im not completely settled on what ill do, but ive had some luck heating up the end of a pipe and dropping it through a styrofoam blank- its pretty straight, probably good enough for me. once i have that, i have some little spacers on a shaft that is directly driven by a stepper motor, so the workpiece sits on the spacers, which sit on the shaft. your suggestion of laminating two pieces together sounds like a much more precise option, but if this is good enough, its less work. so ill see how it goes. thanks for such helpful comments so far!
There is still a buoayancy force but its only working on the diameter of the of the core trying to push it up veritcally. It's relatively small but is why the core needs to contact the mold at the top. Yes, that's it. Now the metal just needs to coooperate and flow downward symmetrically.... No need for an alaborate power supply. It doesn't need to be DC. The resistance wire doesn't care if it's AC or DC so simple step down transformer and dimmer will do. I don't think you'll get a satifcatory result trying to stick a hot pipe down the pattern to make the core unless you can figure out how to just make the end of the tube hot, otherwise initial inches will be overheated and melt away from the tube diameter since it's exposed to the heat for much longer. You'd probably be better off cutting some teeth into the end and making a long hole saw out of the tube, but keeping that cut straight over ~30" will be no small feat. My first go would probably be cut the hole with a round nose bit, laminate halves, and use a tube as a mandrel to mount it on your apparatus for cutting the augger. Best, Kelly
