Pattern Questions

[gwrdriver]

I need to make a pattern for a dome to be cast in bronze . . . the finished part is shown on the left. The machined wall thickness will be 3/16". A preliminary sketch of the pattern (with 1" x 2.5" chucking piece) is on the right. This a section taken through the centerline of a circular pattern.

I'd like to hear pointers for making the pattern, especially what amount of machining allowance I might need in the walls to be sure I get a full 3/16" thickness. It's possible I might make this pattern in wax or foam, but I already know my local brass founder doesn't know about "lost"-anything casting. I know about linear shrink, but this one might shrink in ways I wouldn't suspect so whatever suggestions you have will be welcomed.

[steamin10]

1. Make it heavy, you have less chance of a cold shut or misrun. 1/4 inch comes to mind as being a dependable section if even.

2. using sand mold techniques, use at leat 3 degree relief to pull you pattern from the sand. More if it is real deep. 5 degree is better. that being said, you may have to add more wall thickness, to cut your part out.

2a. Or you can two part split the mold for the deepsection and have no taper at all. This makes it a core plug and two outside forms. That means a split form, that is keyed to match for setup.

Clearer than mud?

[theg8nw]

here is a link that should help you .. the example they show on it is very close to what want to do..

http://www.efunda.com/processes/metal_p ... _intro.cfm

shrinkage table is a link near the bottom of the page.. or here it is also..

http://www.efunda.com/processes/metal_p ... _table.cfm

hope this helps....

[alphawolf45]

To make a perfect pattern and core would take much longer than machining whole thing from barstock ..I would cast it as a rough slug and finish machine it.

[hedley]

In my experience, casting a bell shape in those dimensions in green sand, would be difficult without a well vented core.

If you were to make a solid bell with a core print protruding out the bottom and provide the foundry with the pattern and a few well vented cores that neatly fit the core print (one to use and a couple to stuff up getting the runners and gates right)

The smaller the taper on the outside of the pattern the more accurate and smother the surface must be.
Any undercut or dent in the pattern will restrict its withdrawal from the mould. ( this is the reason 5 degrees has been mentioned in earlier posts) This is less of a problem if the pattern is fully cylindrical because the moulder can rotate the pattern to flatten out the bump in the mould that corresponds to the dent in the pattern.
A drawing of this solution below, shows where you could arrange the part line.
It requires a deep cope and good core venting through the drag to the outside world.
The steam released in a green sand mould tends to leave big holes in the castings when the mould is not adequately vented.

[gwrdriver]

Thanks for everyone's helpful input so far, and yes I assumed coring would be required, I had planned on it. There are other ways of doing this, for instance the Brits often spin and beat their domes from sheet brass, but that's always a stretch in so many ways, and the pun is intended. No matter which way I go, I plan to consult the foundary involved to see what they need and how they want it done.

[steamin10]

A fast way of doing one off is to carve a mold from just setting plaster. It is spun at low speed and carved with appropriate hand tools to the shapes and measure desired, while still soft. When fully set, it is placed in an oven, even a home oven, at 300 degrees, for an hour or so to remove all moisture. Plaster of paris, is very fine and tight.

In use, this plaster mold, can be buried in founding sand and cast as is, or cast while warm to prevent thermal shatters. It can be quite successful. The biggest problem is to get the water out of the plaster, or you can get blows. Using up to 1/3 white silica in the plaster can reduce the mix and lower the hardness, while raising the porosity.

Remember brass and bronze pour hot, and can revert plaster, dirving off the chemically bonded water, so this is not a good method for heavy castings, but will work well enough for lighter pieces.

The site listed show the bell being cast on its side. Most casters will tell you thats a no-no, as the shrink will happen with the heat at the top, distorting the shape. Bells are normall cast mouth down, but thinner sections will have more success casting it top down, like a cup, as long as you can prevent the center from falling in early in the pour.

[patternguy]

gwr, I have made a few bell patterns in my 27 years as a pattern maker. The way I have made them is the way you have it drawn. That way you can take your pattern to the foundry and they can cast it loose if you only need one or two castings. Most foundries like to have 3/16" wall thickness, so you might be able to cast to size and then just polish it with no machining. As for draft, like Dave say's deeper you go the less degree of draft you need. this can be confusing, so I like to tell people to put a 1/32" to a 1/16" draft. I would use a 1/16" if you can. As for shrink, brass and bronze will shrink 3/16" per foot. I build patterns with these neat things called shrink rules. They are rulers that are longer than a standard rule by the shrink per foot.( My bronze shrink rule is 12 3/16 long ) So you don't have to spend $120 on shrink rules, just multiply your dimensions on your drawing by1.015 and use that dimension to give you your shrink. I am going to make a bell pattern in a few weeks, so I will try to take some step by step pic's and post them. Good luck and have some fun. Dave C

[gwrdriver]

Thanks for all the suggestions.