Brad Gover
Involved In Discussions
Hi all, my company mills high purity graphite. Our customer has a tolerance window of +/- 0.0002". I read an article on this site "Statistical Process Control for Precision Machining Part 1", dated 12/2/08, by Bob Doering. In the article he describes how precision machining does not follow a normal distribution but due to tool wear more resembles a uniform distribution. Bob recommends calculating the control limits
[nominal +/- 0.75(tolerance/2)]. When I plot the data on a histogram, the data looks more like a normal distribution than a uniform. It does fail the anderson darling test for normality though. When I put it on a RbarS chart the control limits are 0.000009" wide. The operators do make adjustments as follows. When a part is on the mill, they will machine 9 pockets and then measure those nine pockets to determine if they need to make an adjustment. If they are on target they will machine the rest of the pockets (54 total pockets). When that part is finished, we inspect all pockets on that part. If they are on target, we will machine another part.
If not, we make an adjustment and do the 9 pocket inspection again. Question, what would normally be a uniform distribution without operator adjustment be a normal distribution with so much operator adjustment. We go through one tool for about every 2 parts. Graphite is very abrasive. We know our process is not capable to the customer specs at a 1.33 Cpk. Run charts arn't working as I am unable to determine stability due to tightly compressed control limits. Artificial control limits arn't accurate in determing stability as we change tooling often (one tool per 2 parts). Any suggustions on how to moniter process?
Thanks for the help!
Brad
[nominal +/- 0.75(tolerance/2)]. When I plot the data on a histogram, the data looks more like a normal distribution than a uniform. It does fail the anderson darling test for normality though. When I put it on a RbarS chart the control limits are 0.000009" wide. The operators do make adjustments as follows. When a part is on the mill, they will machine 9 pockets and then measure those nine pockets to determine if they need to make an adjustment. If they are on target they will machine the rest of the pockets (54 total pockets). When that part is finished, we inspect all pockets on that part. If they are on target, we will machine another part.
If not, we make an adjustment and do the 9 pocket inspection again. Question, what would normally be a uniform distribution without operator adjustment be a normal distribution with so much operator adjustment. We go through one tool for about every 2 parts. Graphite is very abrasive. We know our process is not capable to the customer specs at a 1.33 Cpk. Run charts arn't working as I am unable to determine stability due to tightly compressed control limits. Artificial control limits arn't accurate in determing stability as we change tooling often (one tool per 2 parts). Any suggustions on how to moniter process?
Thanks for the help!
Brad