Doubts about SPC taken in Machining - Part has +-0.01 Tolerance

[soundhararajan]

I have doubt on spc taking in cnc machine.part have tol +-0.01
when running part on cnc target value was set.after produced 8 or 10 parts insert gets wear out..evalute proceess need to produced 40 nos.
In spc studies offset given in cnc is correct or wrong..if offset given means what action taken in spc

 

[Eredhel]

Did the machinist make any tool offset wear adjustments for insert wear? Or are you saying the insert is worn so bad it has to be indexed/replaced after just 8 or so parts?

 

[soundhararajan]

the machinist make tool offset wear adjustments for insert wear.so how can take a cp,cpk,pp,ppk, for the process.tool wear offset negligible or considerable when taking SPC data.

in machine capabality index(cpm) also tool wear considerable or not

 

[Golfman25]

Do a search here. I think bob has some great stuff on machining and spc.

 

[Stijloor]

Bob Doering has excellent information about machining and SPC.

Here is a link to his blogs: https://elsmar.com/Forums/blogs/bobdoering/correctspc-spc-for-precision-machining/

 

[bobdoering]

the machinist make tool offset wear adjustments for insert wear.so how can take a cp,cpk,pp,ppk, for the process.tool wear offset negligible or considerable when taking SPC data.

in machine capabality index(cpm) also tool wear considerable or not

Cp, Cpk, Pp and Ppk do not apply to tool wear. They only apply to normal distributions with a target in the center of the tolerance. Tool wear is non-normal IF controlled correctly - which is allowing tool wear to a control limit, and THEN adjusting. If you "run to the mean" you are overadjusting! More details are in the blog - including the correct way to control for tool wear and calculate capability.