Thread: casting tolerances

If I am wanting to have something cast, say something with a hose type end (barbed end left as cast), out of aluminum, what kind of tolerances can be held? Not looking for gnat's ass or anything, just what can be done.

Originally Posted by PHDesigns

If I am wanting to have something cast, say something with a hose type end (barbed end left as cast), out of aluminum, what kind of tolerances can be held? Not looking for gnat's ass or anything, just what can be done.

Investment castings are the cats meow for details, but can be pricey!

Originally Posted by PHDesigns

If I am wanting to have something cast, say something with a hose type end (barbed end left as cast), out of aluminum, what kind of tolerances can be held? Not looking for gnat's ass or anything, just what can be done.

It depends on the casting process. your biggest problem is going to be the parting line that will be on the barb... without some dressing, it may prevent a seal. With too much dressing, you won't get a seal either!

What makes it un-feasible to chuck them up in a lathe and turn the barb smooth?

Take your pick from sand cast, investment/shell cast, die cast. The tolerances can go from 1/16" for small stuff to a few thou. But there may be some proprietary software or experimentation involved in calculating the exact dimensions of the moulds/patterns for the highest accuracy.

As already suggested, the casting technique matters hugely. For loose pattern sand castings, my patternmaker recommends a 1/8" machining allowance all over. Die casting is both more accurate and more precise by several orders of magnitude!

Search for NADCA die casting tolerances for die-casting (if that is your intent).


EDIT: Pace have got an extract from the Nadca book up on their website:
http://www.paceind.com/NADCA-Tolerances-2009.pdf

There can be a wide range of shrink in aluminum alloys. For the least shrink and superior mechanical strength, I buy new, recycled diesel piston ingots from an L.A. smelter but I've also melted a lot of scrap pistons.

I recently cast a 16" diameter, hollow part from scrap pistons, to which I applied just 1/8" shrink to the pattern, which equates to 3/16" per foot and nailed the desired as-cast dimention. I pleases me that it rings like a bell and I know from experience that it's strong and hard.

Piston metal has less than half the shrink rate of say aluminum electrical conductor scrap, which is close to pure aluminum and very soft. Alloys like 356 shrink at a rate midway between the pure and pistons. High shrink rates stress the cooling part more, as it squeezes against the hard core while it's "short-hot".

Below, turning that part on a 17-1/2" faceplate. I'm not soliciting business, I only cast my own parts, just suggesting a range of alloy that works best for me mechanically. Most foundries will have an equivilent sand casting alloy available, whereas my choice includes both die-cast and sand-cast alloys, because commercial forged pistons are made from both. Some pundits caution aganist using die-casting alloys in sand*, not sure why, works well for me.
*Petrobond.

Bob

I like your approach Bob, any idea what the alloy is in those pistons, or something equivalent? Do you need to age it for strength?

Also, is that a Regal roundhead in the photo?

We do permanent mold and sand castings here.

Generally, we leave 3mm for machining stock. Generally we will run a 3mm profile tolerance or permanent mold parts. Sand is 3 to 4mm. High pressure die casting can have accuracy of .25mm or better, but you need a LOT of volume to pay for the tooling (more than 20,000 EAU).

We routinely cast hose attachments for water and air hoses on sand castings. This is not really a barb, but a lip that prevents the hose from popping off. Usually diameters for this kind of feature start around 50mm and go up from there.

True, it is not really a barb, but that shape more or less. Not sure what they are after. Current parts look sand cast, and have the part line. Sometimes they work, sometimes I end up machining them down. Depends on the hose. Thanks for the inputs.

You don't say anything about quantity and size of casting. It doesn't matter who's casting tolerance that you are using, they are all related to part size and weight.

For aluminum, diecasting would be the most accurate, then investment, shell, and sand casting.

There are some low volume, investment type castings being done now where a rubber mold is made from a "master" part then the mold is used for casting the wax. Fairly labor intensive, but the initial cost is low.
JR

Originally Posted by PHDesigns

If I am wanting to have something cast, say something with a hose type end (barbed end left as cast), out of aluminum, what kind of tolerances can be held? Not looking for gnat's ass or anything, just what can be done.

You seem happy with the replies but things like size/diameter, wall thickness, why alu (anodized or not) and what it'd be used for and of course quantity would have gotten you more precise suggestions.

You can (I hope) see what you mean but I doubt if the rest of us can.

Gordon

Originally Posted by richard newman

I like your approach Bob, any idea what the alloy is in those pistons, or something equivalent? Do you need to age it for strength?

Also, is that a Regal roundhead in the photo?

Hi Richard, yes, a LeBlond Round Head Regal, swing is obvious in that pic.

Pistons alloys tend toward higher concentrations of silicon, as well as descending amounts of other allowing elements, such as copper, tin and zinc. The high silicon content, (generally between 7-9% and occasionally approaching 12% or full natural solution, not smart enough to talk about or do Eutectic) adds fluidity to the molten metal, provides higher definition of detail, reduces the coefficient of thermal expansion, and other good qualities. Those qualities are not only good for the rigors that pistons are required to preform under but makes it very compatible with the permanent mold foundry, the most common method of producing pistons.

Silicon is also lighter than aluminum so the more, the lighter.

As far as "aging" goes, choose one.... or more of the following.....: Way too much agin' goin' on personally to wish it on something else, or, I'm not sophisticated to know or care, 'cause cast/machine/use-as-is works fine for me and finally, as far as aluminum alloys go, artificially treating them to one of the T #'s is quick and consistant.

I do know that some guys toss them out back for a while, before machining and putting to work, while others put pins in voodoo dolls... naw, kidding about that last bit but to give a better answer, I'd have to google...... Now iron.....

Bob

Originally Posted by JRIowa

You don't say anything about quantity and size of casting. It doesn't matter who's casting tolerance that you are using, they are all related to part size and weight.

For aluminum, diecasting would be the most accurate, then investment, shell, and sand casting.

There are some low volume, investment type castings being done now where a rubber mold is made from a "master" part then the mold is used for casting the wax. Fairly labor intensive, but the initial cost is low.
JR

Sorry We must have posted around the same time and I agree with you.

I know Seacast in Mt is doing some investment castings with #6 machine screw threads, which are usable without finishing. This is in pistol frames for a major firearms manufacturer... Needless to say Investment casting is getting pretty amazing on the dimensional tolerances. They will also recieve casting wax parts, and cast those, if you wanted to machine wax (way faster on big parts) and have them cast those, or do a urethane mold, etc...

Just another idea.