My milling experience has been quite different. Of course, your and my technique and tooling may be also be very different. But over the past few years I have milled probably on the order of a hundred straight edges of my own design and manufacture. The majority have been of a prism design and that type of straight edge would immediately reveal any mill-induced warping that might occur as there are three planes intersecting that are milled in succession. My routine is, using carbide-inserted face mills, to first mill the sole, then I mill the 45 degree face and finally the top rail. Of course, these casting are properly thermally stress-relieved prior to milling (or sale) in my oven by me. So, their condition is known.
After milling one face, I often check it for flatness and then go on to the next and then check all three faces when milling is completed. In almost all cases each face checks within 1 thou of flatness after machining on the prism is complete. If the casting were warping them due to either relieved internal stress or induction of new stress through milling, one would expect that perhaps the last surface milled might be straight, but the second face would be warped and the first warped even more. But that is not the case.
Is it possible to induce stress and warping of a casting by milling it? I suspect it is. For instance, I have also noted that milling with high RPM or dull cutters can cause surface glazing and smoking hot blue chips as well as a very very hot part. So, I am careful to avoid those traps. With good inserts and moderately slow SFPM I get a dull grey cross-hatched pattern and I make a light (.005") finish pass after allowing the casting to cool uniformly and to near room temperature. (I have previously written up the methods I use to support the casting to minimize fixturing strain.)
And another but anecdotal report really struck me. I had a guy pretty new to scraping buy an 18 from me a while back. He gave me some updates on his progress (I really do like that) and reported he first mille and then scraped the sole of his 18. THEN he milled and scraped the 45 degree face. I was stunned to hear that his first face blued up essentially the same after milling the second face as it did prior.
For those unfamiliar with my 18" prism design here is a photo.
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And here is a 26" prism of similar design that I also have machined numerous times (not this one obviously) with similar results.
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Denis
