What am I forgetting? (Sandcasting Aluminum)

Thanks for the thoughts and encouragement Pete.

The Mizzou in the furnace has a darker gray smoother finish, the stuff on the lid is lighter gray and sort of chalky, like the thin edge on the top of the furnace. I think the lid is showing signs of a mixing or application error. Your point about the lid is well taken. If it starts to flake off I may forcibly remove it and/or reverse the wool so I don't get chunks into the crucible.

Started a load of green sand today, 20# sifted sand, 2# sodium bentonite, 1.32# water (6%). Mixing by hand, still have a few 'sticky peas'.

Rammer has 2 coats varnish, riddle has one. The small flask has a nice stacking action, the large flask is too much of a friction fit that sanding should fix. Is varnishing the flasks worthwhile? Planning on using both Petrobond and green sand.

Still need a pattern, still need to do preheats on the furnace and crucible.

 

For those who think 'this thread is useless without pics'
I was surprised to see condensed water inside the plastic sand container this morning.
About 25% loss to pea gravel on top of the screen and spilling in general.

 

I never would have guessed that it'd take so much woodworking to experiment with foundry stuff.
One flask, 2 riddles, one sieve, rammer, moldboard, striker, and repurposed kitchen knife -wood is all from retains/scrap and all through 2nd coat of varnish.
The other two flasks are being reworked to slide together better, issues with the plywood catches curving this way or that.

 

A little updating.

I finished the last 2 flasks through varnish. The small one with the grooves has some issues with the 'guidance system'. It's looking like shims and screws to get it to register.
I decided the Mizzou on the lid was a liability. It cracked off the Kaowool quite easily when prodded then I reversed the blanket in the lid.
I did my gas line assembly and vitreous firing on the furnace and crucible today, got excited after seeing a YouTube video on crucible making 2-3 steps in after today's furnace rebuild link on the Iron Pot thread. There was enough of the a crosswind that the flame would putt-putt-putt unless I laid the slat over the silding air intake. I might see about making something to help with that. Getting use to the 'right is loose' regulator and sliding draft control. I ran the furnace by itself on low maybe 15 minutes, added the crucible, ram low for a while, turned it up, rotated the crucible, and finished between dull red and orange.

I still don't have my pattern yet. Expecting a 2.4 degree/side taper x ball nose bit to be delivered soon. Not sure if I will wait for that or try something else natural of off the 3D printer. Funny that I am now tooled up but still have no idea what's going in the flask first. I guess it doesn't matter, you can always re-melt the bad ones, right?

So, what else am I forgetting?
-Hot metal slag poking tools for aluminum: mild steel OK or go stainless?
-What type of cloth bag for the parting dust?
-What sort of workbench surface for playing with sand?
Thanks again, I appreciate all the tips!

 

Is your crucible elevated on a plinth or is it just sitting on the bottom? It should sit just above where the flames come in so it isn’t being directly hit by the flame if possible. A piece of hard firebrick will work.
They make a very fine mesh bag for the parting dust, but a cotton sock will do in a pinch.

Pete

 

Thanks Pete. Scrap cloth or holed socks are not in short supply at my house.
The crucible is on the Mizzou on the Kaowool on the bottom of the furnace. It looks like I could raise it about 1" or 2.5cm and stiill have 1/2" or 1.25cm clearance to the inside of the lid. It would not be prefect but more towards your suggestion. This is for a '6kg' crucible. Would I be better off with a smaller crucible and better splitting the differences?

 

A smaller crucible would be good for a couple of reasons. Getting it out of the direct flame is one. It’s not the flame, it’s what’s in the flame that causes the damage ie a concentration of oxygen. Your naturally aspirated burner is probably far less damaging than something with forced air. Your present circumstances will be fine for now but I’d rotate the crucible with each use so the same spot isn’t taking a beating. After you use the furnace awhile you’ll see the impact of the flame on the target wall of the bore and I think you’ll get the idea. Anything inside that furnace is going to take a beating anyway. We’re not in Kansas anymore, but best practices will add to longevity.
The other concern about crucible size is retrieval. The one inch or so between the crucible and furnace wall is sufficient for combustion but it limits what you can use to safely lift your crucible without scraping the hotface. Lifting it full from a single point with something like pliers or tongs is bad ju-ju. You’ll see folks doing it on YouTube all the time. It’s a bad idea. With really small crucibles with little weight involved it’s probably not a big deal. But a larger, heavy, heat-softened crucible is a different matter.
I think Tobho Mott (Jeff) is using a type of fireplace tongs for lifting and pouring that might be a good fit for yours. Maybe he’ll chime in with a picture. Or you can make your own. It’s a bit of a project but that’s what most of us do and you’ll find plenty of examples here.
These are ones I’ve made




The one on the right is for both lifting and pouring. Myfordboy has a build demonstration on YouTube.


Pete

 

This site: http://www.metalworker.eu/gieten/htmlen/tl--gereedschap.htm has a good description of the myfordboy style of tongs and goes into a bit of detail regarding the positioning of the handle with respect to the crucible, might be useful for anyone considering building that type of tongs.

 

Thanks Everyone!
I ordered 2 more crucibles, covering 1 and 2 steps smaller from current.

 

I think I found tongs similar to the ones Jeff mentioned,
http://forums.thehomefoundry.org/index.php?threads/lifting-tongues-and-pouring-rings.898/#post-20840
bought today at the Menards store:

Wondering if I should try to make them fit 'better' or leave as-is.

 

Get a close approximation of the volume of the crucible in cubic inches (3.14 x radius x depth). Aluminum is approximately 0.1lbs per cubic inch. Figure out the maximum weight capacity and load the crucible with whatever you have up to that weight and test it. Lift it, shake it, etc.
Make darned sure to do it over something padded for when the crucible slips out!

Pete

 

Should be 3.14 x (radius)² x depth

 

Thanks Guys.
My 'monster' crucible, weighing 3# or 1.4 kg, will take 4# or 1.8 kg aluminum according to the website. The second smaller one arrived and am expecting a third. I should probably figure out a safe working volume for each once they are here and fired.

PS- worked up a drafted pattern 3x6x.375 inches or 75x150x9.5mm. How does this look for a pouring arrangement? Details down as poured in the drag, parting line at flat back, small ears to removed after machining.

 

I sewed a couple dusting bags. The plaid is working the best so far, the threadbare pillow case one not as good and is off-camera.

I worked my sand over by hand as I am muller-challenged. I knew I had some 'peas' of mostly clay in there and there was a little condensation on the sides (container has a seal). I riddled through 23# / 51 kg with the 'medium' sized one and got 3# / 6.6 kg of stuff that would not pass. I grabbed about the same amount of riddled mix and smashed and shook it in a plastic ice cream pail. Riddled that down to 1.3#, repeated the mix and mash to 0.9# and then spread that out and left that to dry overnight. I ground that up with the rammer and got it down to 2 tablespoons that would not pass which were ceremoniously thrown out the door. The mix 'feels' better, not having any lumps- but I still do not have firm grasp what a firm grasp should look like. I was considering adding another 1% of the bentonite but maybe it would be better just to let it breath a little as my initial mix was 6% water added to sand with 10% sodium bentonite.

Still waiting on the 'medium' sized crucible. I will fire both new ones and then get to pounding sand. I also need to remember to take an inventory on propane so I don't run out mid-melt. I also added wedges to the one flask that was wiggly when closed. My pattern-routing cutter (2.4 degree tapered ball nose) came last weekend but I have not loaded it into the mill nor ran a pattern with it.

 

Your percentages sound right. I wouldn’t change that. Once the water is mixed in, letting it sit for a day or two gives the water a chance to really soak into the clay and it will feel a little different. Your hand should be cleaner than that. Search YouTube for casting sand squeeze test.

Pete

 

I tried ramming one up today. Discovered some errors in the 3D print going back to the CAD such that the letters were not drafted and pinching in the corners.
I tried machining the same file with the new tapered cutter and that had issues too, mostly where I designed a fillet that it tried to cut using the ball end, making it curve the wrong way. It also dug deeper on the cleanup pass and tabs for some unknown reason. I think I can get rid of most of the defects by redoing the CAD and CAM files, make them simpler and a bit more forgiving. Maybe I should have just used letters and dowels glued to a board...
By the way, what do people use to finish wooden patterns? This one is aspen if it makes a difference.

 

I use varnish to seal the wood followed by a paste wax just prior to ramming. The type of wood probably doesn't
matter much so long as it's dry before sealing.

 

For milled patterns use whatever wood gives you the best finish with the minimum of mill marks.
I have a 24 x24 CNC router with a Dewalt611 minirouter.
I use Baltic birch plywood sometimes, poplar, yellow pine, or 6mm PVC sheet for small stuff where warping won’t be an issue. Whatever the pattern looks like is exactly what the casting will look like, and it’s a lot easier to remove wood than metal, so you want your pattern to be as “right” as possible. Even the tool marks that you can see but can’t feel on the surface (like from a facing operation) can show up on a casting.
I sand to at least 220 and paint with automotive filler primer, then sand at 400 or more or else steel wool. I also finish with paste wax. Although I always use parting dust or graphite on the pattern, I try to prepare the pattern’s finish to separate from the sand without it. That’s a bit of a stretch, but that’s my mindset.
Ive fallen into a couple of traps with lettering: not enough draft and trying to make them too deep. I think you mentioned above that you were getting a tapered bit with a 5 deg included angle? Don’t discount a 30 deg, or even a 60 degree. The depth of the letters almost never has to be as deep as I think it does to do the job. Generally I try to figure for more draft as the letters get deeper. And I’ll run a separate shallower pass to cut “capture” areas in numbers and letters like the circle in a 6 or triangle in an A. You can always add wood filler to problem areas. Check out other castings like automotive parts or memorial plaques that have numbers or letters. You’ll see that they don’t exactly try to over-achieve. Melterskelter has done some excellent work in this area.
Lastly, you need a means of rapping and lifting. I’m guessing you intend to cut those tabs off? You could increase the thickness of a couple of them so you can drill them to take wood screws so you have something to rap and lift the pattern with.

Pete