Ball Hopper Monitor - Casting Project

More Rob Wilson photos.

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More Rob Wilson photos.

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GreenTwin said:

I force-dried the coating with a heater, and sanded it with 400 grit sandpaper.
It is doing what I want it to do.
The black spots are air bubbles, apparently, but don't seem to make a big difference.
There will be a top coat of shellac.

I will probaby use a finer sandpaper, more like 600 or 800 grit, if there is such a thing.
This is a good method, so I will start spraying the hopper parts.
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Click to expand...

The foam backed fine sanding pads available from model suppliers, hobby shops, and amazon are wonderful for smoothing complex shapes, They can be washed after use, but do not require water in use unless you want to wet sand. Up to 5000 grit in case you want a mirror finish in mud

A search for "softback sanding sponge" on Amazon will take you down a small rabbit hole for a while.

Have fun!

Photo pc240091 in thread #580 (above) gave me a bit of a start. That huge radial (centrifugal?) blower on the furnace looks like it could shoot flames thru the roof.

The blower you refer to is shown in this photo.
Some folks use a VFD on their blower.
This may be a low rpm motor.




Here is what looks like an analogous commercial unit made by Mifco, and the combustion air blower is quite large.



A Mifco "B-301" is about the size of my first furnace, which is probably on the large side for hobby work.

I experimented for several years with different burner types and blower configurations.

Member "ArtB" is the individual who finally explained combustion engineering to me as relates to a foundry furnace.
Per ArtB, there is a fixed surface area inside of a furnace, say "X" square inches, and so there is some maximum gallons per hour of oil that can be forced into the furnace with an oil burner "Y" that can be completely combusted inside the furnace.

Once you get beyond Y gallons per hour, then the furnace can no longer fully combust all of the fuel that is being injected into the furnace, and any fuel added after "Y" does not burn until it exits the furnace via the opening in the lid.

When I started building a foundry, I assumed that with burners, bigger was better, and I also assumed that a higher fuel flow rate would burn hotter than a lower fuel flow rate.
As with many assumptions I made in the beginning, I was wrong about the fuel flow vs temperature.

I made a series of valves (a valve tree), so I could quickly test various fuel flow rates.
Each valve was calibrated with its own needle valve.
I could turn on 1, 2, 3, 4, 5, 6 etc. gallons per hour, and I experimented with flow rates up to 10 gallons per hour.



After much experimentation, I found that for a 13" diameter, 14" tall furnace interior, a diesel fuel flow rate of about 2.7 gallons per hour produced the hottest furnace interior, and produced the fastest iron melt.
I ended up calibrating my siphon nozzle (cold) to 2.7 gallons per hour, and then I adjusted my combustion air blower (which is a Toro variable speed leaf blower) to give about 4" of flame out the lid when operating (operating rich or reducing, to minimize oxidation of the iron).
This happens to be the air produced at the lowest speed of a Toro leaf blower.

Many large blowers have an intake damper, to allow adjustment of cfm into the furnace.
A large blower will outlast a smaller blower, due to the commercial motor.
I initially used the output of a shop vacuum, but shop vacs will wear out over time.
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I purchased a Grizzly dust collector motor, and made a dual siphon nozzle burner from it, assuming if one nozzle was good, then two were better.
This burner produced some serious flames, but it forced so much air into the furnace that it actually cooled the furnace.

At some point I am going to install this blower in my shop, and run a duct out to my furnace, so that I don't have the noise from a leaf blower right next to my furnace.
This blower will outlast my leaf blower.
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This was a dual 180 degree siphon nozzle burner arrangement that I tried.

A single burner tends to have a high velocity (the same velocity as what comes out of a leaf blower), and so that fuel/air flow tends to climb up the back of the furnace, instead of swirling around inside the furnace.
You can counteract this somewhat by angling your single burner tube down a bit.

Two burners reduce the combustion air flow to 1/2 of what a single tube burner sees for the same fuel flow, and so you get a much more evenly distributed ring of flame low in the furnace when using two burners at 180 degrees.

This burner would have worked well, had I known how to tune it correctly.
I was forcing too much air into the furnace, and that caused this arrangement to run too cool.
At some point I will go back to using a dual burner system, since they do work very well, and produce a faster melt.

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This is the Mifco data for their furnaces, with blower motor horsepower and cfm listed.

Mifco lists melt capacities for aluminum and brass/bronze, but you can also melt iron in a Mifco furnace.
The key to having a furnace last when operating at iron temperatures is to use a high quality refractory, such as Mizzou, which works well with iron.
There are some plastic refractories that are rated for 3,800 F, and those work better as a furnace hot face than Mizzou.

I am not sure what continuous temperature the interior refractory of a Mifco is rated, but brass/bronze melting temperatures are not much different than iron melting temperatures.
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The black spots in the sprayed-on wall joint compound are air bubbles.
I don't think they will be a problem, since they are small relative to the lines in the 3D print.
Sanding minimizes them.
I think the final coat of shellac will close these holes.

The good part about spraying on the wall joint compound is that it produces a very even coating on the surface of the 3D print that is thick enough to bridge over the print lines, but thin enough to not obscure the fine details of the print.
This is exactly what I was trying to achieve.

The problem I have had with using paint and shellac to try and fill the print lines is that these materials are so fluid that they just follow the print lines, and don't bridge them, so you end up getting taller peaks, and still have valleys and visible lines
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Hi Pat, I know I have suggested this before and had the idea dismissed, but seeing the large amount of work you are doing to smooth these prints I thought I would try again.
Please spend a few minutes and watch this video- one of many on the subject, some much more complicated than they need to be-



It may save you many many hours of sanding.

Ken.