Using concrete as a fixture

I have a bunch of castings to mill on a horizontal which are tubular in shape but arent straight, they bend around into different angles and elbow shapes. The ends are the only thing needing machined. So i was thinking about making some fixture pads out of concrete to clamp the parts down onto using a part to make the mold with. There are dozens and dozens of different styles and sizes of these parts and dedicated fixturing is about the only way i see to make it happen. Concrete is cheap and easy to make so one fixture for each different family of parts shouldnt be to much $. And quickqrete would dry and set in about as much or less time than we could make a dedicated fixture.

So has anyone here tried using concrete before? Im sure its not a new thing but ive never seen it done around here. Do you think bolting or clamping say a 2" thick pad of concrete to the table would hold up to the vibrations of machining and to the compression of clamping a part down onto it.

Somebody tell me im not crazy.

Would you use them as disposable fixtures or should they be split and used again?
And are the castings high or low precision?
And how precise does the end result need to be?

Maybe casting them in wax is an option too?
Is reusable.
Or casting in ice? Or better and slower: pycrete

All in all I think there are some good options to explore...

The fixtures would need to be permanent and reused again in the future. The castings are sand cast and the machining needs to be precise. If the part only sits deep enough into the fixture to keep it held solid in place, like maybe only 1/4" depth, i think the variances from one casting to the next shouldnt be a major problem. There are lots of different parts so it has to be either a single fixture for each individual part family or an adjustable fixture that can be used on many similar sized parts.

I think it is unlikely that that a casting of concrete around one piece will also fit another.

You can probably try that out fairly quickly. If it does not work, you'd need to find a way to overcome the variations.

I was thinking more along the lines of making a concrete plate say 2" thick, then pushing a part into it maybe 1/4" deep or something to make just enough of an impression to leave a negative mold of the bottom of the part. Once it dries bolt the pad down onto the machine table, clamp a part down onto it and start machining away.

My only real concern was whether or not the concrete would hold up to the vibrations and clamping pressure of the machining process.

Concrete's not an ideal material for doing this, since it takes a long time to cure to full strength (7 years underwater) and it has some shrinkage issues. Look into a plastic material instead, like Jett-set (which jewelers use for fixturing). You submerge it in hot water, and it forms a malleable blob that holds things nicely. When you're done, clean it off, heat it up again, pop your part out and reuse it indefinitely. JETT BALLISTIC FIXTURING COMPOUND - 1LB. - Kingsley North