SPC charts with Cp and Cpk
Draw a control chart from the measurements your inspectors already record. Every value shows in date order with its control limits and specification limits. Points out of control are marked, a moving range chart sits below, and Cp and Cpk are worked out for you.
PLANSScale only
The problem it solves
- Measurements are written down at every check and never looked at again.
- A bore size drifts towards its limit for a week, and you find out when parts start failing.
- A customer asks for the Cpk of a critical dimension, and someone spends a day in a spreadsheet.
- Every lot passes, yet nobody can say whether the process is steady or just lucky.
How it works
- Put a Measurement line, with its min and max, on the check sheet of a control point. Inspectors record the value at every check, on the web or in the app.
- Open Quality > Reports > SPC Chart. Pick the control point and the measurement (the first one is chosen for you), and if you like one item and a period.
- Press Draw. Every measured value of finished checks in that range is taken in date order; at least 5 are needed.
- The individuals chart shows each value with the centre line, the upper and lower control limits and the specification limits. Points outside the control limits, and runs of 7 on one side of the centre, are marked in red.
- Below it, the moving range chart shows the change from one value to the next. The form shows the count, mean, sigma, the control limits, the points out of control, Cp and Cpk.
Everything it does
Choosing the data
- Control point and one of its Measurement lines
- Item: one item, or every item checked on that point
- From and To dates, on the date the check was done
- Values of passed and failed checks; checks still to do are left out
- At least 5 measured values
What is worked out
- Individuals and moving range (I-MR) charts
- Centre line: the mean; control limits: mean plus or minus 2.66 times the average moving range
- Sigma: the average moving range divided by 1.128
- Cp: (max minus min) divided by 6 sigma; Cpk: the nearer limit's distance from the mean divided by 3 sigma
- Specification limits are the line's min and max
- Points out of control: beyond a control limit, or in a run of 7 or more on one side of the centre
- Moving range chart with its average and upper limit (3.267 times the average)
How it is drawn
- Drawn on the page in your database; no data is sent outside
- Recomputed each time you press Draw, from the latest checks
What it does not do
- Charts for subgroups (X-bar and R) and for counts of defects (p, np, c, u) are not built
- Cp and Cpk need both a min and a max on the line
- The chart is not saved or sent on a schedule, and it is not in the app
- No alert when a point goes out of control: open the chart to see it
Who uses it
Quality InspectorRecords the measurements at each check and draws the chart.
Quality ManagerReads the chart and the Cpk, and raises a CAPA when the process drifts.
Production ManagerUses Cp and Cpk to decide which machine or setting is capable of a tolerance.
Where you use it
- Web: Quality > Reports > SPC Chart
- The values come from checks done on the web or in the SmartMy Manufacturing app
- Not in the app
What you control
Settings
- The specification limits are the min and max of the measurement line on the check sheet
- Item and period: chosen each time you draw
Questions
Do inspectors need to enter data twice for SPC?
No. The chart reads the measurements recorded in the normal checks, on the web or in the app.Which chart is it?
An individuals chart with a moving range chart (I-MR). Subgroup charts such as X-bar and R, and charts of defect counts, are not built.How are Cp and Cpk worked out?
Sigma is the average moving range divided by 1.128. Cp is the tolerance width over 6 sigma; Cpk is the distance from the mean to the nearer limit over 3 sigma. Both need a min and a max on the line.Can we see one item only, or one month?
Yes. Pick an item and the From and To dates before you press Draw.Which plan has SPC?
Scale. Measurements on check sheets start in Growth, so the values are there when you move up.Related features
PLANNING, QUALITY AND COSTQuality checks with sampling (AQL), held lots and non-conformanceCheck material as it comes in, each step on the floor, the finished product and goods going out. Each check has a sheet and a sample worked out from the lot. A failed check holds the lot, raises a non-conformance whose decision is approved, and a Rework decision makes the rework order.PLANNING, QUALITY AND COSTCAPA: corrective and preventive actions with five whysWhen a problem keeps coming back, raise a CAPA. Ask why five times, write the root cause, and plan corrective and preventive actions with owners and dates. Have the plan approved, and close it only when you have checked that it worked.PLANNING, QUALITY AND COSTGauge calibration, and checks that refuse an overdue gaugeKeep every vernier, micrometer, weighing scale and thermometer with its calibration due date, history and certificates. Quality managers get a to-do before a gauge falls due. A check cannot be finished with a gauge that is overdue or out of service.PLANNING, QUALITY AND COSTProduction reports: plan against actual, material against BOM, scrap by reasonThree ready reports for the production manager. What was planned and what was made, by item and month. The material planned and used on orders. What was scrapped, by reason. Each opens as a pivot you can turn, filter and export.LINES AND SCHEDULINGOEE per machine and day, and the machine board on a TVOEE is worked out every hour for each machine and day from its running time and the pieces counted at the terminal. It is split into availability, performance and quality. A machine board on a TV shows every machine as working, stopped or idle.
See your shop floor on it.
Tell us what you make, how many machines and people you run, and how production is recorded today. We will set up a line with you and run a job on the terminal.