It wasn't until I drilled a 7/8" hole in all the foam disks, an 1/8" off-center, when I came to understand fully what you were saying. At this point I would rather drill and bore a 7/8" hole in aluminum. I think I'll stick a sprue in the hole.
....and when I said "taking care", I plunged a bottom cutting router bit to make the hole because a Forstener bit still tore the foam. Then I applied some tape to a dowel so the foam blank fit very snugly on the dowel and spun the foam pattern on the dowel while I cut the OD and pulley groove to try to keep all the features concentric. This makes very concentric foam patterns. But very concentric is a relative thing depending upon what you are trying to do. When I chucked on the OD of the resulting cast aluminum pulleys, the center hole was still eccentric enough that the drill bit followed the hole and was a little eccentric compared to the OD; probably a few thousandths. If I used a boring bar as a first step before I drilled and reamed, I probably would have trued things up enough but I didn't have a boring bar that small. The next step was to ream for pressing in a bronze bushing. The eccentricity of the drilled hole caused the reamer to cut a little oversize. So on the rest of the pulleys I reamed them on the drill press so they self centered and the bushings were a nice press fit. A couple thousandths run out on the pulley ODs didn't matter much but a thousandth difference in reamed hole size does when you are press fitting bushings. I also tuned up the tracking faces of the pulleys while they were mounted in the lathe. http://forums.thehomefoundry.org/index.php?threads/furnace-cart-and-lift-build.278/#post-5180 The foamy fab and casting is earlier in the thread. Best, Kelly
How did you cut those nice round wheels? Kelly has good results with Forstner bits. for me, lost foam has been deceptively easy. I'm waiting for my first failure (other than the requisite hole melted through the bucket). I did learn to grade the top of the sand into an inverted cone to contain overpour. But I'm just using aluminum foil for a sprue extension with no pouring cup. Correction: I thought Kelly had good results with Forstners. Is the speed the issue?
I drilled a 1/4" hole through the foam with a forstner bit. Then I cut the wheels with the bandsaw This circle cutting jig slides in the miter groove and can tilt with the table.
Compared to other drill bits yes, I'd say Forstener bits give the best results by far using lots of speed and clearing the chips frequently as you go but I still tend to get a little tear here and there especially where it breaks through the bottom. This is getting very picky but when I want a really nice finish in the hole, I use a Forstener bit to drill a hole in MDF pattern and the attached the foam to the pattern and use a guided router bit to cut the hole in the foam. Many times I end up using that hole to mount and fixture the pattern for subsequent machining operations. The pattern surfaces cut with router bits are nice as is and even nicer with just a few swipes of 220 grit or finer. AP, with a 1/4" pin I bet you need to be a bit careful with how much pressure you apply to the foam pattern. I use long pins (usually 1/2" wood dowels if I can stand the hole size) and build them up with a wrap or so of HVAC tape and that makes them fit snugly. The 1/2" dowels have more bearing area so the foam holds up a little better. You can get pretty good results with router or belt sander circle templates. It may seem a little obsessive. If you are just casting machining blanks it's not worth the fuss but my aim is usually to make as near net shape part as possible that looks to have been tooled......just me. Best, Kelly
Kelly, I was able to use a light touch while cutting with the band saw, sorta like cutting air. Since the disks are so thick, I couldn't figure out how to use the router. Whatever, I'm waiting for shake-out of my first lost foam casting.
I painted up the wheels and sprue with two coats of easy sand 45 min. sheetrock mud. Taped some HVAC foil around the sprues, the top 1" of the sprues has no drywall mud. Looks like six lbs. plus one lb of aluminum for the cup, seven lbs it is. Buries the mold in dry sand, shaken but not stirred. The mold tended to cant in one direction. Added a baked core pouring cup. Heated up the aluminum to 1540F/637C. The dross glowed red and the melt was silver making skimming easy. Oh, by the way, my face shield band broke and the whole unit jumped off my face. Sure am glad I was still waiting for the melt to heat up to temperature and I'm glad I had a spare face shield in the box. The moment the melt hit the sprue...instant blackout. Just pour the whole thing and don't stop. Yep, seven pounds did it. Looks good from here.
Turns out the wheel weighs 3 1/2 lbs. and the big mess weighs 6 lbs. View of the top after facing. Close up of the bottom side after facing.
No progress on the wheels due to fast growing grass which required my attention. I've been thinking about the things I need to do in order to cast lost foam readily. For starters, I would need to mess with the furnace to get to 1500F. Right now, with the naturally aspirated burner, it struggles to reach the last 50 degrees. Then there is sand management, sand shaking, sifting, etc. I would rather make a muller. So, here goes. Another muller build. First, detailed plans. Will it work?
Interesting, so the center scraper pivots to redirect the sand into the middle and out the hole instead of out into the wheel's path? Cool idea. I've not seen one before with the dump hole in the middle like that. Will there be a door to keep the sand from falling out the hole before you're ready, while you're loading sand into it, etc.? Jeff
I'm hoping the scraper keeps the sand out of the hole otherwise I'll need a plug. I may want a shield guarding the hole while loading sand.
I don't think it needs to reverse. Should be able to turn one scraper to redirect the flow towards the center for dumping; while mulling the scraper directs the sand to the wheel path.
Never mind the muller build, it will have to wait awhile. I bought one. I'm waiting for the aluminum to melt so I can cast something. The pattern is something I spent some time on but, I don't remember exactly what I was going to do with it. A half year is a long time.
Hope it isn't a part for the muller you're not building anymore! You'll have to post some pix of the one you bought... Good luck with the mystery casting! Jeff
The mystery casting went well enough. I'll take pictures of the muller as I crate it back up to send back. Yep, it leaks oil and the manufacturer wants me to send it back for repair. The original crating was destroyed when I removed it so, I'll have to fab up another crate. I'll have to drag the cherry picker back out to reload it into the pickup to move it from my shop to the road. Guess what I'm doing this weekend instead of casting.
I finally found the cord for the digital camera. Here's a few pictures of the muller. Ready to send back
