I really think the best answer is "It Depends". The machined part configuration can affect this decision, along with if you have the ability to probe internal to cycles. All the parts I machine are fully machined from billet plate, and I do not have probing capabilities. This lends to my preference of using the same datums outlined on the fixture drawings. The fixture was built using those datums, so to me it makes the most sense to use those datums when programming to ensure you know where the fixture features / part locations are. By using the fixture datums, I do not have to deal with locating the fixture to an exact point on the table or maintaining fixture offsets either. I use a G92 coordinate offset which makes my program zero the same as the fixture zero. Many times my fixture zeros are reamed holes / flat surfaces, and that works fine for me. They are easy to locate via an edge-finder or dial indicator, and are easily repeatable each time I set the job up. |