FAI for Heat Treated Details - Inco Parts Shrink

[Joelbear5]

I am having issues trying to figure out a way to cover all First Article Inspections (FAIs) of detail inconel parts made that shrink after weldment and heat-treat.

The detail parts are made of Inco 718. After welding and after heat treat, they exhibit a shrinkage. Some features are shrinking out of tolerance because they were machine to final dimensions.

I'm thinking about getting the engineers to put a note that reads "All dimensions applied post heat-treat".

How do other people control for the dimensions on detail parts that aren't heat treated until the final assembly? I'm assuming we need to do some research on the shrink-back ratio.

-Joel

 

[Stijloor]

A Quick Bump!

Can someone help?

Thank you very much!!

 

[andygr]

Lets start with the engineering call out for when dimensions apply.
This is an acceptable design note that would be stated on the print or in some systems you will find it stated in a spec. This is similar to when you have a coating process and the dimensions of the machined part are usually defined as being measured prior to coating. Not uncommon at all.
BUT
Before you get coverage for parts that do not meet design with such a note I would recommend that you first look getting engineering to determine and define which dimension are the ones impacting fit form and function. If it is the final dimensions then the work and change needs to be to the detail parts not the final parts.

Net forming parts is a very unforgiving process and you need to make sure you take a verrrry close look at tolerance stack when net forming starts at the detail part level.
I have been told in the past by more than one engineering type "I am not changing my tolerance You need to just make the parts on the high side of the tolerance to ensure that they will work"