Which casting technique to use?

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Hello,
I'm new to this stuff but have been pondering and reading about casting for some time now. I rebuild saxophones as a hobby. My desire is to fabricate brass levers for the neck octave of certain types of saxophones that would be very time intensive doing it by hand forming out of brass plate. My question is which method to use? I am presuming that those who read this don't know what the octave lever on a Sax neck looks like. The lever is bent in an arc, probably that of a 12" dia. circle, it has a loop in it, much like an elongated noose. It is hinged close to the center where it pivots to close and open a vent in the neck. It is only about 1/4 - 5/16" of an inch thick and has various degrees of shaping to the looped end. I can't really be more precise other than the shaping is contoured in an arcing fashion so instead of just square sides to the piece of brass it is shaped to an edge on the loop end. The other end is the "rope" part of the noose analogy and has a little cup at the end of it where a leather pad would be inserted and glued. If I were to flatten the arced original piece it would be approximately 8" long. I am assuming I'd have to make a flat copy out of some material to use as a form? I would like to get as close to the original confuguration as possible as the contouring at the noose end as described above would be quite difficult to match and make consistent by hand filing/sanding.
Because it is rather flat and of small dimension I can't see splitting a cope and drag and getting the desired contouring of the piece as described above. Lost wax seems the ticket.It would be great to use the original piece as the form but I would not want to flatten it and possibly ruin it.
Any suggestions would be greatly appreciated.


Gene Olson's picture

one thing comes to mind

one thing comes to mind foremost here.

you have multiple curves to match in 3 dimensions as well as absolute point to point requirements and a large spread out part. Lost wax casting will throw the same monkey wrench twice.
Shrinkage, and the shrinkage will vary with the material poured, and the temp it is poured at. Both the type and temp of the wax and the metal.
When you pour the wax into the mold, it's going to shrink, and again when you pour the metal into the mold, it's going to shrink. This WILL make trouble for any attempt at accuracy.
The mold will have to be oversized to account for the shrinkage and the process would need to be controlled, the wax poured would have to be the same temp every time, the mold temp would have to be the same every time, them metal pouring temp would have to be the same every time. This might be difficult in a small shop.

You might do better casting in a couple pieces and soldering the components together. The keys and vent caps sound like easy lost wax pieces and the arching section might go either sand or wax. The length and shape of that section sounds like it could be fairly easily tuned to spec and then the ends added on.

Sculptor
Elk River, MN


visitor's picture

Splitting a cope and drag

Thanks Gene,
I appreciate the time taken to give me some ideas.
Have you ever sand cast something as thin as what I have mentioned in my initial inquiry? Something flat and about 5/16 at the thickest and tapering down to an edge? Because I have never done any casting only studied in theory at this point I have a hard time seeing the splitting of a cope and drag that would maintain such small dimensions, do you know what I mean? I know the casting sand has a "sticking" component to hold it's shape but how well for such fine detail. I realize I'd have to do some oversizing of the mold,and some dressing of the part. I've got no problem with that. I do fabricate brass parts for horns and have used silver solder techniques to fill voids etc.. If I could just get close to a consistently contoured part I'd be a happy man.


visitor's picture

sandcasting of brass component

I am new to this site but find it most interesting. Living and working in India (I have a lighting Company) it seems that most labour intensive processes which are now lost to the west are still available to us. So the brass component you require may bepossible for me to cast and finish for you if I can have a sample of the finished item. The areas where the dimensions are crtical should be clearly idicated.
For the record, we have recently supplied reproductions of doorknobs and latches to a castle project in Denmark.They were very well received.
We have recently started a range of lighting fixtures made of re rolled brass scrap which have been flamed and patinated. Rather a spectacular result much to my own surprise.
I would welcome an exchage of ideas and assistance from everybody


visitor's picture

Thanks for the offer

I appreciate the offer but I am getting to the point in my repair and refurbishing that in order to save time and become more efficient at what I am doing I have to become familiar with casting techniques. There are many other parts that would be much easier to make by casting. I enjoy the thought of actually doing it myself and want to persue it on my own. thanks again


visitor's picture

Brass Casting

Hi there Madahorn
Apart from the already mentioned shrinkage issues you should also consider the problems of porosity and inclusions of slag etc. in your castings, which can cause weakening or even failure in service of the finished product, and an inability to obtain a perfect surface finish (polish).
Have you cosidered using brass rod and wrought metal techniques with perhaps lathe turning to fabricate these items? maybe easier and a lot more greener when you consider the amount heat required to melt brass.
best wishes Dave P.


Rich Waugh's picture

Madahorn, Sorry I'm so

Madahorn,

Sorry I'm so late in getting this one answered, but I've been a bit under the weather and feeling uncommunicative, to say the least.

I agree with Gene that shrinkage will be something of an issue on this one, but I do think you can overcome that with a bit of experimentation.  The lost wax method would be fine, particularly if you use either vacuum-assisted or centrifugal casting and jewelers' or dental investment.  Using centrifugal casting and Kerr SatinCast investment, I have successfully cast a number of small parts such as you describe without any problems - once the shrinkage issue was compensated for.

Industrial casters use a "shrink rule" to calculate the model size for different metals.  Basically, a shrink rule is just a metal ruler with the graduations "adjusted" for later shrinkage.   So, if you need to end up with a 1" part using metal with a 2% shrinkage factor, the appropriate rule would actually have a 1"+.02" space between each pair of marks labeled as one inch.  That works on big stuff, but for little stuff like yours you need to do it differently.  A cheap digital readout caliper and a calculator will accomplish the same thing at a smaller scale.   As Gene mentioned in a previous post elsewhere, you can set up a simple spreadsheet in Excel to do the calcs automatically for you, thus saving all your work for later replication or modification if you change metals.

To produce the waxes in quantity, you 'll have to make an RTV silicone mold of the original model, then inject wax into it.  Once hardened, the wax can be removed, sprued, treed and invested for burnout and casting.  Making the mold for the wax will take some time and care, but it somewhat forgiving of minor errors in part-lines and such since the wax is pretty durable and the RTV silicone is pretty flexible.  Small undercuts and such aren't a problem, in other words.  You'll figure it out quickly, I'm quite sure.

One issue not mentioned so far is the loss of alloying metals when melting brasses.  Typically, these alloy metals are zinc, tin, aluminum or lead, and all can boil out in the melting process.  You need to keep the molten metal covered with flux and not overheat it unnecessarily, and you definitely want to wear a respirator approved for heavy metal fumes.  Some alloys work better than others and some are a better color match than others, though I imagine you're plating your work after final polishing.  You can contact the folks at Atlas Metals in Denver for good info on the various alloys available.

I hope this helps some. 


Maddahorn's picture

Thanks to all

Thanks everyone who responded. Lots of good info here. Now I have to figure out a game plan and get to it.
Has anyone made their own furnace? I have a number of good books and it seems simple enough. Besides the mechanical aspects of making the furnace I have gleaned from the info I have that it is important to "season" the inside refractory cement b4 using and pay attention to not shocking the "system" to keep stuff from cracking. I do plan on constructing my own furnace. The commercially available furnaces are much bigger than I need.
Any other insights would be appreciated.
thanks again.


Frank Castiglione's picture

Refractiry Cement

Hi Maddahorn,
In another life I ran a steam plant. When the masons repaired the refractory, two elements needed curing; the cement that was basically used to wet the bricks, soaps and tiles and the plastic refractory. Both the wetting cement and the plastic refractory contained water.We would use a salamander to cure the new work for at least twenty four hours before firing the boiler. Then we would keep a small fire for another day. These were wood fired boilers but easily reached a 1600 degree fire.
The floors were made using fire brick suspended on cast iron rails. A space between the bricks allowed the forced draft to come from under the fire. These bricks sustained the punishment of rakes scraping the sand, rocks, slow dogs, what ever several times a day and yet lasted for around a year.
Frank


Rich Waugh's picture

Maddahorn, For the work

Maddahorn,

For the work you're going to be doing, you'll need both a burnout kiln and a melting furnace. Two definitely different critters.

The burnout kiln for flasks the size you'll probably be using could easily be made by stacking up soft firebrick and developing the heat using salvaged burners from an old electric stove. One or two of the top burners would develop sufficient heat, and the controls for them will work fine for burnouts. It's crude, but effective and dirt cheap. I've built several of them this way, then switched to "rolling my own" coiled nichrome wire heating elements, but still using the stove controls. Why not, when they're free?

For a melting furnace for brasses, you need to be able to develop and sustain temps around 2500 degrees Fahrenheit, so you need to use a high quality rammable or castable refractory. A simple blown gas burner will do fine, shouldn't take more than 50cfm blower and a 20# propane picninc cylinder will supply all the gas you need. Check around the web for designs for blown burners on places like the backyard caster's forums, or on the blacksmiths' forums.

I've used a simple blown burner set into the side of a 5 gallon oil drum lined with castable refractory very successfully. You can also use refractory fiber insulation such as Kaowool or Inswool, but it needs to be top coated with something such as Plistix 900 or ITC-100. The fiber is too fragile, otherwise. Such a furnace will handle a crucible capable of melting ten or more pounds of brass, and can easily be scaled down to fit your needs.

The starting point is to determine how much metal you need to melt, then double that figure for your crucible size. From there, you take the dimensions of the crucible and size the furnace to it. For the small amounts you're probably going to be wanting, you might very well be able to simply melt the brass in a crucible mounted on a centrifugal casting machine. The one I have will take a crucible that will melt 4 ounces of silver easily, and I believe larger ones may be available. Again, determine what you need and see what is available at Rio Grande or someone similar.

Hope this helps. Ask as many questions as you need to get where you want to go. We'll try to help.

Rich


Maddahorn's picture

burnout kiln?

Thanks Rich, but what is a burnout kiln? used for?

For a furnace I was thinking not much bigger than a coffee can size for the work I'll be doing and amount of material needed for the parts. I have a few choices relative to plans from books I've bought.
Can you recommend a favourite source to order yellow brass?
thanks again
fred


Rich Waugh's picture

In the cire perdue, or "lost

In the cire perdue, or "lost wax" casting process, the model is made of a wax designed for the purpose.  After the wax model is made, it is sprued, gated and risered to a sprue cup form and surrounded by investment plaster.  This makes up what is known as the "flask", or the mold.  Once the investment flask has set, it is then put into the burnout kiln and carefully brought up to a temperature at which the wax is completely burned out of the plaster, leaving a cavity that will be filled with molten metal.

It is necessary to use a controllable kiln for the burnout process inordeer to properly calcine the investment plaster, ensure total removal of the wax, and have the mold at a suitable temperature for the actual casting process so that there is less chance of getting incomplete fill in the mold.  The temperature required for to calcine the plaster is about 550F and the burnout requires about 1200F for best results.  1200F is also about the right temp for the flask to be at when the molten metal is introduced. 

The burnout process doesn't require a terribly high degree of control, just reasonable.  Proportional cycling switches for the elements will be fine, (the kind on the stovetop burners), and a bit of attention during the process.  You can, of course, use PLCs and a computer to automate the process down to a gnat's whisker, but that's overkill for what you're doing, I think.

 

Rich


visitor's picture

brass ingots- source?

Thanks Rich for all the info. You've been a big help.
I know what you are talking about re: burnout proceedure. I'm still not "up" on my casting jargon.

Can anyone recommend a source for yellow brass ingots for casting purposes? There are a few places I googled and not sure if it matters who or where you get them from.


Rich Waugh's picture

What really matters on the

What really matters on the casting ingots is which alloy you get. There are hundreds of different copper alloys, and each one has different characteristics. You can find a lot of background information on the various alloys at: http://www.copper.org/  

I get my copper alloys from Atlas Metals in Denver: http://www.atlasmetal.com/   I've found their people to be knowledgeable and helpful whenever I've had questions. 

 

Rich