Originally Posted by
metalmayhem
I've run plenty of large aluminum plate and holding up parallelism and flatness isn't much of a problem if proper procedures are taken . #1 thing , don't over tighten , Most bowing is due to user error and not stress relief . When in doubt put a dial to the part while clamping , it's surprising how little clamping force it takes to bow a plate .
6061 is a pretty stable material and stress relief isn't much of a problem unless there is a lot of material being hogged out of it . If thats the case then the proper thing to do is hog it out leaving some material , then re-clamp and run finish passes .
As mentioned mic6 or other cast aluminum is soft , it is also brittle . If you plan to have threads that are repeatedly being tightened and un-tightened then helicoils or other thread inserts are a must . Otherwise you face stripping out the threads . If your excessively tightening then you may also face fracturing a chunk out of the plate even with inserts .
If a cast fixture plate is meant for the long haul then sending it out for hard anodizing is wise . The stuff is always a pleasure to machine and cuts like butter and it really does work great for making fixture plates .
Overall , 6061 will out perform cast plate , is cheaper and more readily available . Only thing to remember is that not all 6061 is the same , be sure to use quality materials vs offshore crap