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Quality Control. Chapter 6- Additional SPC Techniques for Variables. PowerPoint presentation to accompany Besterfield Quality Control, 8e PowerPoints created by Rosida Coowar. Outline. Continuous and Batch Processes Multi-Vari Chart Short-Run SPC Gage Control. Learning Objectives.

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quality control

Quality Control

Chapter 6- Additional SPC Techniques for Variables

PowerPoint presentation to accompany

Besterfield

Quality Control, 8e

PowerPoints created by Rosida Coowar

outline
Outline

Continuous and Batch Processes

Multi-Vari Chart

Short-Run SPC

Gage Control

learning objectives
Learning Objectives

When you have completed this chapter you should be able to:

Explain the difference between discrete, continuous, and batch processes.

Construct and use a group chart.

Construct a multi-vari chart.

Calculate the central line and control limits of a specification chart

learning objectives continued
Learning Objectives-Continued

When you have completed this chapter you should be able to:

Explain how to use precontrol for set up and run activities.

Calculate the central line and central limits for a Zbar,W and Z,W charts

Be able to perform G R&R

continuous processes
Continuous Processes
  • Usually operates 24 hrs a day, 7 days a week, stops for scheduled maintenance.
  • Often involves a conveyor or moving assembly line
  • Associated with product involving hazardous materials
continuous processes6
Continuous Processes
  • Examples
    • Paper-making machines
    • Oil refineries
    • Soft drinks (continuous then discrete)
    • Control charts for each value (multiple stream output)
continuous processes7
Continuous Processes
  • It is extremely important to have knowledge about the process and objectives for the control chart. When it is difficult to obtain samples from a location, sensors may be helpful to collect data, compare to control limits, and automatically control the process.
continuous batch processing
Continuous & Batch Processing

A good example of a continuous process is depicted by the paper-making process. They operate 24 hrs a day, 7 days a week, and stop only for scheduled maintenance or emergencies. Observed values are taken in the machine direction (md) or cross-machine direction (cd).

continuous batch processing9
Continuous & Batch Processing

If the flow of pulp is controlled by 48 valves, then 48 md control charts would be required to control each valve. In this case it is very important for the practitioner to be knowledgeable about the process and have definite objectives for the control chart.

group chart
Group Chart
  • Eliminates the need for a chart for each stream; however, it does not eliminate the need for measurements at each stream
  • Uses the same methodology outlined in Chapter 5, 25 subgroups for each stream.
  • Use lowest and highest averages for Xbar chart and the highest range for the R chart. Each stream has a number.
group chart11
Group Chart
  • Any out-of-control situation would call for corrective actions
  • We have the out-of-control situation when the same value streams gives the highest or lowest value r times in succession
group chart12
Group Chart
  • See Table 6-1 for the r values
    • The technique is applicable to machines, test equipment, operators, or suppliers as long as:
    • Each stream has the same target
    • Same variation
    • Variation is as close to normal as required by conventional Xbar and R charts.
batch processes
Batch Processes
  • Paint, soft drinks, bread, soup, etc
  • SPC of batches has two forms:
    • Within batch variation
      • Only one observed value of a particular quality characteristic can be obtained
      • They need to be obtained at different locations within the batch
    • Between-batch variation
      • Does not always occur
batch chart
Batch Chart
  • It is not a control chart, it might be more appropriately called a run chart
  • Can provide information for effective quality improvement
batch chart15
Batch Chart

Figure 6-4 Batch chart for different batches with different specifications

multi vari chart
Multi-Vari Chart
  • For detecting different types of variation that are found in products and services
  • The chart will lead to a problem solution much faster than other techniques
  • Shows the different types of variation within a single unit or service (within unit variation, unit-to-unit variation, time-to-time variation)
multi vari chart17
Multi-Vari Chart
  • Procedure:
    • Select 3 to 5 consecutive units
    • Plot the highest and lowest observed value of each piece
    • Draw a line between them
    • Repeat the process
short run spc
Short-Run SPC

Specification Chart

  • Used for small lot sizes – common in JIT
  • Gives some measure of control and a method of quality improvement
  • Central line and control limits are established using the specification

σ = (USL-LSL) / 6 Cp

specification chart
Specification Chart

These limits represent what we would like the process to do.

deviation chart
Deviation Chart

Deviation Chart

  • The plotted points are the deviation from the target
  • Even though the target changes, the central line for the X chart is always zero
  • Because the target changes, we require the variance (S2) of the different targets or nominal to be identical. This requirement is verified by ANOVA or by using
deviation chart21
Deviation Chart

Deviation Chart

  • The plotted points are the deviation from the target
  • Even though the target changes, the central line for the X chart is always zero
  • Because the target changes, we require the variance (S2) of the different targets or nominal to be identical. This requirement is verified by ANOVA or by using
deviation chart22
Deviation Chart

Deviation Chart

  • Advantages:
    • Provide the opportunity to convey enhanced information
    • Can plot different quality characteristics on the samechart
deviation chart23
Deviation Chart

Figure 6-8 Deviation chart for individuals (X’s) and moving range (R’s)

zbar and w charts
ZBar and W Charts

Those are very good for short runs

zbar and w charts25
ZBar and W Charts

Figure 6-10 ZBar chart

short run spc26
Short-Run SPC

See Chapter 6 page 256 for more details

zbar and w charts28
ZBar and W Charts
  • Very good for short runs
  • Different quality characteristics such as length, width etc. may be plotted on the same chart
  • Can be used to monitor an operator’s daily performance
  • Can track an entire part history
  • Subgroup size MUST remain constant
  • Calculations are more involved
z and mw charts
Z and MW Charts

MW chart uses the absolute value

precontrol
Precontrol

Precontrol is an algorithm for controlling a process using its tolerances.

  • It does not monitor the stability of the process over time.
  • Unstable process may be within specifications today, but no guarantee that this will be true tomorrow.
precontrol31
Precontrol
  • Specifications may change
  • Difficult to learn which factors affect the process if the process is not repeatable
  • Any deviation from the target leads to a loss
precontrol32
Precontrol
  • Was developed originally with machining operations in mind.
  • Operator faced with the problem of first setting up the machine and then deciding if the machine is ready for full production.
  • Small lot sizes with each piece taking a long time to produce.
precontrol33
Precontrol
  • Well suited for machining operations where one can devise simple feedback algorithms to bring the process back on target.
  • Requires operators who are very knowledgeable about the process.
precontrol chart
Precontrol Chart
  • “Stoplight” control
    • Prevention of defects even at start-up
  • Statistical charting method of control
    • 3 sigma processes
    • Product specification (tolerance) limits
    • Zones
      • Warning (yellow)
      • Stop process (red)
    • Statistically robust
  • Adopted by many companies since 1950s
precontrol35
Precontrol
  • Steps for the construction:
    • Be sure that the process capability is less than the specifications.
    • PC lines are established to divide the tolerance into five zones (Figure 6-12)
  • The PC procedure has two stages:
    • Start-up
    • Run
precontrol36
Precontrol
  • Steps for the construction:
    • Be sure that the process capability is less than the specifications.
    • PC lines are established to divide the tolerance into five zones (Figure 6-12)
  • The PC procedure has two stages:
    • Start-up
    • Run
precontrol37
Precontrol
  • Steps for the construction:
    • Be sure that the process capability is less than the specifications.
    • PC lines are established to divide the tolerance into five zones (Figure 6-12)
  • The PC procedure has two stages:
    • Start-up
    • Run
precontrol38
Precontrol
  • Steps for the construction:
    • Be sure that the process capability is less than the specifications.
    • PC lines are established to divide the tolerance into five zones (Figure 6-12)
  • The PC procedure has two stages:
    • Start-up
    • Run
slide39

Precontrol

  • Assumes
    • Normal Distribution
    • USL – LSL = 6 Sigma
    • Any shift in mean -> increase in defects
  • False positives:
    • 2% chance of two consecutive products in same yellow zone
    • Nearly 0% false positives for any product in red zone
precontrol40
Precontrol
  • PRE-Control Rules:
  • Qualifying Process
      • 5 consecutive units in green zone –o.k. to run
      • 1 yellow, restart counting
      • 2 yellow in a row, adjust the process
      • 1 red, adjust the process
      • < 5 signifies:
        • Process capability << 1
        • PRE-control is not appropriate
precontr ol
Precontrol

Precontrol Rules cont’d:

  • Sampling
      • n = 2 units
      • Sample six pairs between adjustments. See Table 6.2 stoppages
      • 1 unit in red zone
      • 2 consecutive units in opposite yellow zone
      • 2 consecutive yellow zones , process adjusted and procedure goes back to start up
precontrol42
Precontrol
  • Precontrol can be used for single specifications
  • Precontrol can be used for attributes
  • Precontrol is also used for visual characteristics by assigning visual standards for the Precontrol lines
tolerance precontrol chart
% Tolerance Precontrol Chart
  • Z Charts: Ability to accommodate more than one quality characteristic
  • We can combine Z chart into one technique by the use of percent tolerance precontrol chart (PTPCC)
  • Target or nominal
  • X*= (X – nominal) / [(USL-LSL)/2]
  • A negative value indicates that the observed value is below the nominal
gage repeatability and reproducibility
Gage Repeatability and Reproducibility

Gage Repeatability and Reproducibility (GR&R) studies provide information on measurement system performance by analyzing measurement error from various sources.

Typically the sources of variation are divided into three categories: part-to-part, operator or appraiser, and gage or equipment. In some instances another category, interaction between parts and operators, can provide additional information about the gaging process.

gage repeatability and reproducibility45
Gage Repeatability and Reproducibility

Not all calculation methods provide the same information or the same result, so it is important to pick an appropriate calculation method.

gage control
Gage Control
  • All data have measurement errors
  • An observed value has two components

Observed value = True value + Measurement error

  • Total Variation = Production Variation (process) + Measurement Variation
gage control47
Gage Control

Measurement Variation = Repeatability (equipment variation) + Reproducibility (inspector or appraiser variation)

gage r r
Gage R&R

METHODS:

In most GR&R studies, multiple operators measure the same parts in random order to collect data. Most studies use two or three operators and five to 10 parts. In instances such as automated measuring processes, the GR&R studies are not affected by operator influence. These data then are analyzed using a calculation method.

gage r r49
Gage R&R
  • Gage has to be calibrated using standards
  • Data is collected
    • 2 or 3 appraisers
    • 2 or 3 trials
    • 10 parts
    • Part characteristic is measured in a random order
gage r r50
Gage R&R
  • Calculations
    • Average and range for each part is calculated by an appraiser
    • Values are averaged to obtain:
    • Data is collected
    • 2 or 3 appraisers
    • 2 or 3 trials
    • 10 parts
    • Part characteristic is measured in a random order
gage control52
Gage Control

Analysis of Results:

gage control53
Gage Control

Analysis of the results cont’d:

  • The percent of the total variation

% EV = 100 (EV/TV)

% AV = 100 (AV/TV)

%R&R = 100 (R&R/TV)

% PV = 100 (PV/TV)

gage control54
Gage Control

Evaluation:

If repeatability is large compared to reproducibility:

  • The gage needs maintenance
  • The gage should be redesigned to be more rigid
  • The clamping or location for gaging needs to be improved
  • There is excessive within-part variation
evaluation
Evaluation

If reproducibility is large compared to repeatability:

  • The operator needs to be better trained in how to use and read the gage
  • Calibrations on the gage are not legible
  • A fixture may be needed to help the operator use the gage consistently
gage control56
Gage Control

Guidelines for acceptance of GR&R (%R&R):

Under 10% error

  • Gage system is satisfactory

10% to 30% error

  • May be acceptable based upon importance of application, cost of gage, cost of repairs, etc.

Over 30% error

  • Gage system is not satisfactory. Identify the causes and take corrective action