Question on Machining tolerance on Cast Iron casting. 2

[Zorbu]

I was recently told by our machining department that we could not machine castings to any predictable tolerance because the casting will warp during the machining process. My understanding is that they machine it, then measure it to determine the size of the piston to go inside. Is this true for castings? What can be done to improve this either with different materials or casting design. The part is a roughly 6.75" outside diameter cylinder with a 5.75" ID.

I realize that this is not a lot of detail but I'm just looking for general information and guidelines.

Thanks in advance....

 

[Nate101]

I machined thousands of cast iron fuel pumps and other than orange hands it held tolerances fine.

If you're dealing with thin walled parts, there maybe an issue, but it would relate to any other material that's thin.

I don't know of any heat treat cycles cast iron reacts favoratable to, but this may help also.

 

[griffengm]

Zorbu,
In some respects, your machinists are correct. This was the reason so many machine manufacturers allowed large castings to "age" or "weather" before starting machining on them.
That said, I made many thinwall(.060-.090" wall) engine cylinder liners at one point in my career and we were able to hold tolerances within a few thousandths of an inch(some tighter). Key issues were defining work holding and process. You might need to make some cuts on the outside then inside or vice versa. The wall thickness you give was about the starting point for us. Allowing the castings to sit for a time between operations may also allow them to relax a bit as the internal stresses equalize.
If I recall correctly, the casting process itself also had a lot to do with uniformity. I believe spun cast was the most desirable.
Hope this helps.

Griffy