Page 38 - MetalForming June 2016

The Science of Forming By Stuart Keeler
Identifying the Cause of Rejected Parts
Analysis of Available Data
Production Problem: four-percent rejections over five operating shifts
Some observations from the time logs:
1) Most rejections pointed to one coil of steel.
2) Rejections occurred immediately after lunch break, shift change or a long
downtime.
3) All rejections took place during one midnight shift.
4) Rejections started occurring after the die was installed in the press.
5) Most rejections occurred after the die was tweaked.
6) Small reject groups appeared throughout the entire run.
Press shops continually search for better methods to reduce the number of rejected parts. As a first step, they determine the number of rejects per day, per run or some other interval. A common practice for deter- mining the percent (or ppm) defect rate is to count the number of parts in the reject bin and divide by the total number of parts produced. Another method: count the number of vertical marks (one mark for each rejected part) on a tally board located near the press. Divide the total number of marks counted at the prescribed interval by the total parts produced to obtain the percentage of rejects.
A run with four-percent rejects can be used as a quantitative measure of the saleable parts versus another run with 12-percent rejects. However, these percentages give no clues as to the cause of the rejects—the real informa- tion needed to improve quality and productivity.
I recommend one simple change to the counting process, to provide the important process information: Rather than counting parts or making tally marks, record the time of day that each defect occurred. Then count all of the rejected parts that occur within a nar-
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementation of other press-shop analysis
tools. Keeler’s metalform- ing experience includes 24 years at National
Steel Corporation and 12 years at The Budd Com- pany Technical Center, enabling him to bring a very diverse background to this column and to the sem- inars he teaches for PMA.
Keeler Technologies LLC P.O. Box 283
Grosse Ile, MI 48138 keeltech@comcast.net
Fig. 1—Time logs show when peak number of rejects took place relative to process events.
Logical Root Causes of the Rejects
____ A) Ground down a draw bead
____ B) Substitute press operator
____ C) Did not have the “Bernie Book”
__1__ D) Spot buy of steel—great low price
____ E) Die/lubricant temperature dropped
____ F) System on edge of forming-limit curve
Fig. 2—List of press-shop events that can cause increased part rejects.
that almost all of the reject times were within a 45- min. duration on one shift (Fig. 1). During those 45 min. only one coil was being run, and
we recorded that coil as the source of the rejects. With the
row time band. Also, record the active process, unexpected behavior or other events noted.
For example, consider a case where 80 percent of rejected parts were formed while running coil number SR- 55299. Therefore, we know that the source of the rejects is the coil of mate- rial, and not a flawed process. This search can go one step further, by investigating the problem coil, so that additional coils with the same problem can be presorted and rejected.
To further illustrate this search pro- cedure, consider the following prob- lem. The production run has a total of four-percent rejects during five oper- ating shifts. Each of the six entries indi- cate the process that contains the max- imum number of time marks for that run.
Example 1: At the end of the five operating shifts, the tally board shows
process source identified, let’s find out what was different about the coil that caused the rejects.
Fig. 2 depitcs a logical group of root causes, with the fourth item noted as the most probable cause. We test the coil for composition, mechanical prop- erties, surface roughness, lubrication and inclusion count, and pledge to not purchase a coil like the problem coil again.
Try the other five examples; answers are at the end of the column.
What does a “Bernie Book” have to do with metalforming? The company studied for this project had a third shift supervisor named Bernie. All die and process changes were made on the third shift, and Bernie’s book contained the data for each die to prove that the die was correctly installed in the press. The changeover was not accepted until the test pieces exhibited the correct
36 MetalForming/June 2016
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