Aluminum sand casting for Chenery Gnome Rotary

[albd5a]

That is a very nice casting !

Your riser located on the left has a very small gate into it, and so that gate solidified before the main casting fully solidified, and the remaining molten casting acted as the riser, and caused the shrinkage.

Perhaps use two risers at a 45 degree angle, and make the gate into the bottom of them much larger.
The gate could be perhaps six times larger than what you used.
You will machine that area anyway, so the size of the gate does not matter.

That is a really good casting for a first-time aluminum melt.
I tended to overheat my aluminum when I first started to cast metal, and that caused a very rough surface finish.
Now I use a pyrometer, and pour at approximately 1,350 F, which gives a good surface finish like you have.

Your mold alignment is very good.
This is a most excellent casting effort for sure.
.

Thank you for your kind words Green Twin. My miscast defects could probably be repaired but I’ll just likely use this one for machining practice. I won’t stop until I get a couple of good ones. I will definitely use your suggestions. THANK YOU. Norman

 

[Roger Taylor]

I would use a single, large center sprue with three large gates at 120-deg intervals, so the sprue acts as a riser.

 

[GreenTwin]

Another option, especially if you get into using bound sand, is to turn the mold on edge, and then you could use a single larger riser at the top of one cylinder, with the sprue feeding a runner into the bottom of one of the cylinders.
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[Richard Hed]

I decided to split my wood pattern on the bandsaw; I am glad I did. Obtain perfect alignment of the two halves with steel pins references. The difficult part was getting the inside of the crank case hollow rather than one chunk of metal to be bored out later. ( did not use a sand-water glass Core. However, I was able to get enough draw on each half to get a casting. I never melted aluminum before so at least I figured out how to do that now. Sure enough , unfortunately I have a slight miscast - View attachment 161436is the same as a big miscast if you can’t use it. Not sure why it’s deficient on this flat circumferential surface. Any thoughts and why I have the deficiency at the top surface of the pattern? The bottom surface is fine.

That is such a good casting that I would consider filling in the hole with a bit of tigging.

 

[GreenTwin]

That is the beauty of making your own castings.
If it does not turn out exactly like you want, just ram up another mold and pour again.
.

 

[albd5a]

That is the beauty of making your own castings.
If it does not turn out exactly like you want, just ram up another mold and pour again.
.

So far my miscast to usable, casting ratio is not very good! I have a feeling maybe it’s a difficult pattern… I do have one crisp completely usable one so far out of about four attempts; a few of my other attempts where porosities or that same shrinkage problem area as before, but they are all very small defects, Really small. I was hoping that somehow they could be just repaired- add on. I don’t have a TIG or a welder but I’m able to braze with MAPP gas if I knew of a solder rod with flux or without flux that would work. Also, this thing is a pretty big heat sink . Would love to repair a couple of them.

 

[albd5a]

So far my miscast to usable, casting ratio is not very good! I have a feeling maybe it’s a difficult pattern… I do have one crisp completely usable one so far out of about four attempts; a few of my other attempts where porosities or that same shrinkage problem area as before, but they are all very small defects, Really small. I was hoping that somehow they could be just repaired- add on. I don’t have a TIG or a welder but I’m able to braze with MAPP gas if I knew of a solder rod with flux or without flux that would work. Also, this thing is a pretty big heat sink . Would love to repair a couple of them.

These photos are in a rather random order, taken during a couple of different casting attempts just thought that some of them were cool looking.

 

[albd5a]

I would use a single, large center sprue with three large gates at 120-deg intervals, so the sprue acts as a riser.

I have a mold, ready to go made up with a large central riser with some spokes going out to the perimeter. However, I kept the other sprue and Riser idea . will try to cast this tomorrow although it’s supposed to rain in the morning. I’m looking for a pattern, sprue riser gate system with a high success rate . right now it’s rather hit or miss. Although, green twins, advice of really enlarging my gates did work. That is my best casting so far; it’s perfect.

 

[Richard Hed]

I have a mold, ready to go made up with a large central riser with some spokes going out to the perimeter. However, I kept the other sprue and Riser idea . will try to cast this tomorrow although it’s supposed to rain in the morning. I’m looking for a pattern, sprue riser gate system with a high success rate . right now it’s rather hit or miss. Although, green twins, advice of really enlarging my gates did work. That is my best casting so far; it’s perfect.

You're int the USA, your friends or neighbors will have a TIG machine.

 

[GreenTwin]

I had all sorts of problems when I first attempted to cast aluminum.
I did not understand the process, and so it was trial and error, with lots of error.
I had flask halves separate because I did not use weights on top, aluminum too hot, too cold, sand and slag inclusions, porosity.
You name it, I had problems with it.

Once I figured out what was happening, mainly by listening to Bob Puhakka's videos, and following John Campbell's 10 rules for good castings, my success rate went from perhaps 10% to basically 100%.

Using bound sand with a ceramic mold coat is a game changer too, especially with iron castings, but bound sand is not really necessary with aluminum castings.
Petrobond makes some great castings with aluminum, and it gives an excellent surface finish if you pour at around 1,350 F.
Petrobond can be used with iron, but is subject to mold failures due to the extreme heat, and I don't recommend it.

Your setup is good; you just need to tweek your method, and then I think you will have a very high success rate.

There is nothing quite as fun as pouring molten metal, in my opinion.
It makes my wife a bit nervous, but she has gotten use to the fire show, and she knows I can do it safely.

Edit:
I use a sheet metal heat shield on my pouring shank, and that allows one hand to be within a few inches of the crucible.
Having one hand close to the center of mass of the crucible/molten metal makes it much easier to handle the crucible, and gives a much better fine control over pouring.

Another trick is to use a piece of steel pipe, perhaps 2.5" or 3" diameter, and 1" tall, and place that at the top of the sprue, so that you don't have overflows that can run over and burn the flask.

.

 

[Jasonb]

Before you spend much more time with this part can I suggest taking a look at your pattern. From what I can see the two edges are almost flush with the bases of the cylinder mounting bosses. As these two edges need to be machined it is usual to include a machining allowance. If you were to machine your casting as is then the width of the main body will be less than the diameter of your cylinder bases. If it is not that then your cylinder mounting bosses are too large a diameter which th elack of space for webs seems to suggest..

The bonus of having that extra machining allowance is that your areas of shrinkage may well be limited to what has to be machined off so may not even be a problem with a correctly sized pattern.

And As Pat said earlier those thin gates are cooling fast and not allowing the riser to feed metal back into the part as it cools which is the likely reason for shrinkage.

 

[albd5a]

Before you spend much more time with this part can I suggest taking a look at your pattern. From what I can see the two edges are almost flush with the bases of the cylinder mounting bosses. As these two edges need to be machined it is usual to include a machining allowance. If you were to machine your casting as is then the width of the main body will be less than the diameter of your cylinder bases. If it is not that then your cylinder mounting bosses are too large a diameter which th elack of space for webs seems to suggest..

The bonus of having that extra machining allowance is that your areas of shrinkage may well be limited to what has to be machined off so may not even be a problem with a correctly sized pattern.

And As Pat said earlier those thin gates are cooling fast and not allowing the riser to feed metal back into the part as it cools which is the likely reason for shrinkage.

Jasonb, you are right: My cylinder bosses are a little larger in diameter than drawn on plans. I figured that the centers of the cylinders won’t ‘know’ that. With the shrinkage, maybe that’s a good thing. I’ve considered remaking the whole pattern (larger and with better drafts); or even adding to this existing pattern.

 

[albd5a]

Jasonb, you are right: My cylinder bosses are a little larger in diameter than drawn on plans. I figured that the centers of the cylinders won’t ‘know’ that. With the shrinkage, maybe that’s a good thing. I’ve considered remaking the whole pattern (larger and with better drafts); or even adding to this existing pattern.

Question: How many times can Petra bond be used over again?

 

[GreenTwin]

The local art-iron folks reuse their Petrobond a lot.
They have a powered muller, but it can be hand-mulled too.

I made the mistake of adding non-detergent motor oil to mine, and it lost its green strength.
I should have added a bit of alcohol.
I am hoping I can salvage my petrobond, and perhaps burn some of the oil out of it somehow.

I think Petrobond could be used for years if a little alcohol is added from time to time, and it is mulled.

Edit:
There is one school of thought online that will tell you that you must remove all the burned petrobond from the sand after each pour.
The local art-foundry does not remove the burned petrobond; they just mull it all together after a pour, with no problems.
The petrobond turns black over time, but the green strength is not affected by color.
.

 

[Fidlstyks]

First thing, as said, the gates were way too small. I also like taller sprues. One can make a sprue box like a mold setting on top of his mold, if a taller flask is not available.
As stated also, I place a sheet of white paper in the center line of pattern glue ups. That makes them split right apart.
Also I did not note you fluxed.
Google Hydrogen Embrittlement.
Molten aluminum picks up hydrogen from the air while molten. I think the same thing happens when electric welding witg stainless steel rods. One is to never chip a weld until cool.
I toss Beeswax on molten aluminum for flux. Others buy a flux that is Chloride-Fluoride like pill shape. Otherwise, your machining will show bad spots.
Watching you learn.

 

[GreenTwin]

A taller sprue increases the velocity of the metal, which is not what you want.
When I started backyard casting, I used very tall sprues, and one of our members (luckygen) illustrated that you don't need a tall sprue extending above the top of the cope at all.
These days, the top of my sprue is at the top of the cope (mold), and I use a 1" tall piece of pipe as a catch ring at the sprue.

I keep the lip of the crucible as close to the top of the spure as possible when pouring, even touching the top of the spure.
You don't want a waterfall effect, since that churns air and slag into the melt.

I heat my aluminum as fast as possible, being careful not to overheat the metal, and also pouring immediately when pour temperature is reached.
If you let the alumimum linger at or over pour temperature, it will absorb a lot of hydrogen.
I don't use any flux with aluminum.
The flux that I have heard that works with aluminum is pool shock, which creates some extremely toxic gas (nerve gas I think), and I would not recommend using that.
I have also see some on ytube recommend using washing soda, but that has no effect on aluminum, and several folks including myself have proven that. If you back the washing soda in the oven and drive off the moisture, then you will see that it just sits on top of molten aluminum and does absolutely nothing. It will not even mix with molten aluminum.

Aluminum castings will not be brittle just because they have some porosity from hydrogen.
Hydrogen porosity shows up as pinholes in the metal surface.
If the casting will be painted, you will not see the pinholes.
Even if the casting is not painted, the pinholes are not very noticeable.

You can use an inert gas bubbled through the molten aluminum to degas it, such as argon I think.
Best thing to do is just heat the aluminum as fast as possible, and pour immediately at 1,350 F.
This is what I do, and I don't use any flux, and I don't have porosity.

I will start a thread to show my aluminum casting gaffes, when I started trying backyard casting in 2011.
There is no need for anyone to repeat my mistakes.
I became an expert and finding every conceivable casting mistake that could be made, before I got it figured out.
.

 

[Jasonb]

When I have used Petrobond we threw away the burnt hard sand that was closest to the metal so you can't reuse that percentage.

Practice when ramming up a new pattern was to use fresh petrobond through a sieve to cover the pattern and give the best possible surface, you then used the recycles petrobond to fill the remaining part of the flask as that is not so critical.

So depending on the area in contact with the metal you might be able to reuse 90%