I'm painfully aware that Xbar and R charting doesn't work in a machining environment that generates "sawtooth" datasets. I've seen charting that will handle this type of data (most noteworthy is the control limits were angled with respect to the X axis) but cannot remember where. Can anyone point me in the right direction? Also, in a machining center with numerous tools using replaceable inserts (which have an average working tool life of 15-20 minutes), what type of sampling scheme can be devised to account for the constantly changing process. The inserts on the tools are routinely replaced on a schedule programmed directly in the CNC machine or when a tool alert is generated because an insert is exhibiting too much vibration. Depending on the amount of work an individual insert is doing (depends on amount of stock being removed and length of time for the cutting path), all of the tools are replaced at varying intervals of time. Combine this with the sawtooth effects (from tool wear) and logical sub-grouping eludes me. Any advice in this arena would also be greatly appreciated.