As a former owner of a high precision contract machining company, I understand and agree with bob's premise.
In most modern CNC machining centers (turning or multi-axis) the repeatability is so good that in most cases, the primary variable is tool wear (which can be affected by purity or impurity level of the material being machined as well as speeds and feeds and tool design.)
Unless there is a probe (visual [infra red, laser, etc.] or touch) checking the tool bit continuously for wear, breakage, etc., all the algorithms in the world cannot overcome the machinist's favorite phrase: S--- happens!
In the 90s, when I was still running our shop, our turning centers and multi-axis machining centers routinely ran Cpk of 2.0+
We did our SPC in a relatively straightforward way, with the exception that instrument readings were directly input into a computer from the measuring instrument and the calculations performed by software instead of a guy working a calculator from hand written data entries.
Our SPC findings triggered whether we would enter into a Design of Experiments to alter feeds, speeds, tool design, or material supplier. I recall writing here in the Cove some time back that we ultimately settled on Carpenter Steel as a sole supplier of some specialty alloys because other suppliers did not provide the consistency of grain which is an important factor in consistency of machining, despite the chemical analysis being the same - physical properties of the material determine chip size and production as much as tool design coupled with feeds and speeds.
In most modern CNC machining centers (turning or multi-axis) the repeatability is so good that in most cases, the primary variable is tool wear (which can be affected by purity or impurity level of the material being machined as well as speeds and feeds and tool design.)
Unless there is a probe (visual [infra red, laser, etc.] or touch) checking the tool bit continuously for wear, breakage, etc., all the algorithms in the world cannot overcome the machinist's favorite phrase: S--- happens!
In the 90s, when I was still running our shop, our turning centers and multi-axis machining centers routinely ran Cpk of 2.0+
We did our SPC in a relatively straightforward way, with the exception that instrument readings were directly input into a computer from the measuring instrument and the calculations performed by software instead of a guy working a calculator from hand written data entries.
Our SPC findings triggered whether we would enter into a Design of Experiments to alter feeds, speeds, tool design, or material supplier. I recall writing here in the Cove some time back that we ultimately settled on Carpenter Steel as a sole supplier of some specialty alloys because other suppliers did not provide the consistency of grain which is an important factor in consistency of machining, despite the chemical analysis being the same - physical properties of the material determine chip size and production as much as tool design coupled with feeds and speeds.