What kind of chart to detect WHEN a in-process NC is about to occur?

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#1
It must be possible to detect when a machine is due wear and tear that causes burrs, draglines, tool changes. What kind of chart should i be using to keep track of these things? I have about 3 months of data that i jotted down as time went by and now with downtime id like to get the ball rolling on something. Any help?
 
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Bev D

Heretical Statistician
Leader
Super Moderator
#2
What does your data look like? if it is tool wear you will typically have a 'sloping' trend - slowly up or down. Here are 3 references for these charts:

Sarkar, Ashok; Pal, Surajit, “Process Control and Evaluation in the Presence of Systematic Assignable Cause”, Quality Engineering, Volume 10(2), 1997-1998.
Wheeler, Donald, “Can I Have Sloping Limits?”, Quality Magazine, May 1999
Selden, Paul H., “Using SPC to Cure Sales Heartburn”, SPC INK, 1999 #1
Also see Bob Doering's blog regarding tool wear charting in the resources section. You will need to click on the orange download button to get the blogs...essentially he places the control limits at the inner 75% of your tolerance limits. Easy to calculate and it works.

If your data is 'flat' and then catastrophe occurs - you need to calculate the number of runs before a bad run. you would do this over ~20 sets of runs. I would set the limits just before the 'quickest' number of runs to failure. Alternatively you could calculate the mean and standard deviation of the number of runs to first failure and use the mean - 3SD to set the limit of number of runs until you change out the tool. (you should monitor the charts and adjust the limits as needed) There are other more sophisticated (complicated) methods available for this based on 'reliability' and 'maintenance' statistics. However I have found that in most cases the simpler ways are better as they are quick and easy to understand.
 

Steve Prevette

Deming Disciple
Leader
Super Moderator
#3
Good post by Bev. In addition, consider there are three phases to the generic "life" of an item - burn-in, steady failure rate, and burn-out. AKA the bathtub curve. Bathtub Curve - an overview | ScienceDirect Topics

A control chart of failure rates (1 over time between failures) is useful. Time to Repair, and Reliability Rate (Up time divided by Unplanned down time) are also useful. There are many tools for assessing reliability and repairability of equipment out there, in addition to Bob's tool wear calculations, depending on what model fits your situation.
 
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