Laying out rectangular cores on foundry pattern

[jlakes85]

I probably should have thought of this before, but didn't..

If i were to lay out valve ports on a cylinder casting using an appropriate shrink rule, would that be ok, or would i need to take into account shrinkage aspects from multiple angles?

-jlakes85

[Harold_V]

I'm assuming you are making a pattern to cast the cylinder.
Use the shrink rule. However, be advised that the shrink of the casting may or may not be linear, or exist at all. Depending on the grade of iron, there may or may not be shrink, so it's a crap shoot.

When graphite precipitates in gray iron, it counteracts the shrink from cooling. Variables in the chemistry of each heat dictate the degree of change, and there may not be any.

H

[FKreider]

...be advised that the shrink of the casting may or may not be linear, or exist at all...

I experienced non-linear shrinkage on some castings myself recently, so I can attest to Harold's comments.

Are you designing a slide valve cylinder or piston valve?

"Square core" kind of sounds like slide valve, if this is the case then I would ask why bother with a core at all? Slide valve cylinder ports are easy enough to mill into your casting. Cores add complication and cost to a casting, when I am designing a pattern I always avoid them if possible.

[optigman]

Iron shrink is 1/8" per foot or .010" per inch. Ductile Iron depends on if mold is green sand or hard sand. Hard sand mold ductile iron shrink is 1/16" per foot or .005" per inch. Green sand mold ductile iron make standard rule / no added shrink. Every dimension on the drawing should have shrink applied to it.

[MsChrissi]

"Every dimension on the drawing should have shrink applied to it."
Can I not design the part as a solid full size then scale the 3D printed pattern to accomplish the same thing?

[K. Browers]

Good day,
If you are using a 3D cad program for pattern making and pouring Cast Iron what I found works is to add a 1% shrink factor and also add more material for machining allowance. I usually add quite a bit for machining allowance as it can compensate for any casting errors. In 1.5 inch scale I would probably add .25 inch to all machined surfaces including the cylinder bore. Its easier to machine the part than making a new pattern and getting a new casting made if the first ends up too small. I agree with Mr. Kreider to machine the valve ports after casting and not fiddle about with cores for that.
Happy building
Karel

[K. Browers]

Hello again,
Oops sorry. It was clear in my mind but unfortunately not in my description.. What I recommend is to take your full size model and do a 1% enlargement in all 3 axis X,Y,Z. Then add your machining allowances. Do not forget to allow plenty of draft so the pattern will easily pull from the mold.
Karel

[jlakes85]

Hi Frank,

The cylinder will be slide valve, however the valve centerline is set back from the cylinder bore and the power and exhaust porting are more complex geometries according to the drawing. I made core plugs for all and then poured the core boxes from them. The plans do have separate drawings for a "valve face" so im assuming he was aware that trying to core in the parts would not be exactly controllable. The valve side will be cast in the drag, ordinary CI

[Mike Walsh]

Also something to take into consideration - the cores will expand and contract when they come in contact by the molten metal.

You will need a core print, and that is another factor to take into consideration. You will experience the expansion/contraction and need some "give" for the core so it can move as needed and not break.

[jlakes85]

Hi Mike,

I should be in good shape on the core prints. They are 2" in height with a 1.5 deg taper down to the valve face and ample on the cylinder bore side

[Mike Walsh]

Hi Mike,

I should be in good shape on the core prints. They are 2" in height with a 1.5 deg taper down to the valve face and ample on the cylinder bore side

Sure, but your cores will expand and contract. So if you are targeting a dimension with your 2" height, I would expect that dimension to be off by a certain amount, unless you take this into consideration.

[FKreider]

It sounds like your pattern/casting may be more complex than some of us are imagining. As mentioned above, I would add shrinkage uniformly to the pattern as a starting point, if everything works out OK you will have a usable casting. That being said; the volume/mass of a part has an effect on how the molten metal cools and in reality it might not shrink uniformly.

Good luck! We would all be interested to see pictures of the pattern and casting when its done!