Steel tube core ?

[JoeCB]

Hello all, I spend more time in the machining forum but have learned a lot from lurking here. I'm in the planning stages for an cast aluminum crankcase housing for a antique outboard motor. The mold will require a core about 7 inches long X 1-1/4 diameter. The cored hole is a through hole that will eventuallly be bored out to size for a bronze bushing. Question, could a thin wall steel tube be used for the core? Assuming that the tube would not release, it will simply be bored out in the process of machining the casting.
Thanks, appreciate your thoughts...
Joe B

[Russ Hanscom]

I am not into casting, but I would be concerned that since aluminum has a higher thermal expansion coefficient than steel, thus shrinking more, that the aluminum will have a tendency to crack while cooling before it develops sufficient strength. You might get by with a really thin walled tube.

[Harold_V]

I have no experience in casting aluminum, but I'd be concerned with chilling, too. If the tube has much cross section, it will prematurely chill the aluminum, possibly resulting in cold shuts. Might be worth a try, but I'd certainly not experiment with a complex casting if success was important.

I'm not sure how I'd feel about a steel insert if it was to be removed by machining. The likelihood that it will end up on center is not good, so it could prove quite troublesome in machining. It definitely wouldn't be a good idea if carbide was being used, as the grades are not the same for steel as they are for aluminum, and the risk of the steel slipping is quite real. All in all, I think I'd avoid the idea. YMMV!

Harold

[JoeCB]

Thanks for your thoughts. Yes a THIN wall steel tube ( .020 perhaps) to minimize chilling. I'm not concerned with the machining out aspect, should be no problem, light cuts with the boring bar will do it, or score it from the inside and collapse it.
I'm hoping someone with casting expierence has tried this and can report on success, or failure.

Joe B

[Pipescs]

Worth a try. If it fails you can always go the sand and sodium silicate route

[Harold_V]

I'm not concerned with the machining out aspect, should be no problem, light cuts with the boring bar will do it,

My comment about not being on center is the one of concern. If you miss center enough that you can't remove the piece in one pass, it's possible that as the tube is removed on one side, the remaining piece will collapse and wedge between the tool and bore. Most likely lead to a broken cutting edge. Light cuts could prove to be troublesome, in other words. If the tube remains solidly in contact with the aluminum, sure, no problem aside from using carbide which may not be compatible with the two metals, as I stated.

Please report on the results you achieve, regardless of your approach. Many of us would benefit.

Harold

[JoeCB]

Thanks again for the comments. Harold, yes good thought on the off-center and jamming issue. If we decide to try this, I think the splitting the tube by scoring from the inside the peeling it out will be the plan.

Joe B

[steamin10]

When it comes to founding, my advice is to stick to tried and true methods. I am an ameteur, but have seen many answers to head scratchy questions.

Why not just 'machine' a core? Any of the sodium silicate, linseed oil or sugar bases, will allow rubbing or shaping of the core form for size and rough detail. Coating the surface with clay or slip material, will give a fine finish, which is not needed on a rough casting. Gas tight and lamination free is. Aluminum with a cool metal core will crystalize to slush and pull as it cools. This will cause an open and pourous casting in the shrink areas, and that is bad news. So is blows from an undry core. Cores must degrade and collapse slightly, as the metal shrinks to final form. It is why plaster of Paris is a poor choiice for a core, as it is too strong, can contain too much water which will steam the metal causing porosity and blows. It will not vent well either. It can however be worked well when curing , and a fine finish applied to make a model of the finished project either for mockup or pattern purposes. Many short run patterns are created with Plaster of Paris, Hydrocol and the like. Common dry wall mud in a bag, with designated batch times are an alternative for making patterns. Example, take a made wheel, add some tread dimension with a few wraps of smooth tape, and press it into damp bank sand. (beach sand) Pour some plaster of paris into the impressions ( making more than one right?) and let it harden. As soon as it sets, pull it up and rough finsh it right there. The resulting copies can be sanded and sealed, and mounted on match boards and you have a wheel pattern board with 4-6-8 wheels. If damaged by rough handling, they are easily repaired, or replaced. Cylinders can be fashioned from wood turnings, or coated with Auto body fillers that can be sculpted, smoothed and painted.

Casting is to be kept simplest for best success. Dont try and redesign the wheel.

[jpfalt]

I've used electrical conduit through the center of aluminum castings and the only issue I had was the opposite of the concern about chilling.
The casting section was thick and the conduit was 3/4 inch size.

The aluminum heated up the conduit and as the casting solidified, the conduit became a hot spot as once the conduit was hot, the heat had no real place to go.

The casting had a lot of porosity around the conduit at the mid point through the casting. The aluminum initially solidified around the conduit, but remelted when the mold was full. That metal was the last to solidify and due to shrinkage was loaded with porosity.

[Tharper]

As others have said don't reinvent the wheel. Making core boxes is not that difficult - especially if you are dealing with low volumn and can use hydrocal plaster.

I use simple wood turnings glued to a MDF backer board as a male master. This is sealed with a couple of layers of shellac.
From these the plaster core boxes are formed. I seal the plaster with shellac. Any pinholes or defects are filled with Durhams water putty than its primed and
painted with automotive enamel. If the plaster core box gets damaged its easy to cast a new one from the male master. Don't forget that the cores require proper draft
too.

Here is a sample with the patterns, core boxes and masters.

Cores halves ready for assembly. We have used both the sodium silca and the molasses recipie with good success. Just make sure the cores are vented.

Cores halves glued and in place. These cores have been powdered with graphite.

And a finished piece

I hope this helps!

[Pipescs]

I have to agree with Tharper


I used plaster of paris and a scrap wood box to make my first core box. As your core is a straight piece you can turn it on a lathe should you have access to one.

Link to making the cores for the Feed Water Heater

http://www.chaski.org/homemachinist/vie ... 3&start=12


This one took less than two hours to make and was done in plastic but cheap plaster of paris works just as well for a one off job

[Tharper]

If you have to turn a core in the lathe glue a piece of brown wrapping paper between the halves. After the piece is turned you can split the pieces apart
easily.

Elbows are not that difficult to do either. Simply glue-up up the blank again using brown paper between the quarters. Turn and bore than cut facets at 45 degrees and
22-1/2. Using a template sand it to a half bagle shape. Match-up the quarters and you have an elbow. For turning I use a MDF mounted on my face plate and a parting tool. (Hey it works!)
I fasten the piece to the MDF from the back with screws.

You can also make cores in multible pieces. This one had a curved neck and had to be made in four parts which are lap jointed.