Why do Gage R&Rs fail?

Transcription

1 Why do Gage R&Rs fail? Common reasons a gage fails a gage R&R Consider these in all gage system designs. 1) The part itself is awkward or encumbering Maybe the part needs a holding fixture to free up your hands Maybe there needs to be a reference mark or a collar to ensure measurements are done in the same way 2) The gage itself is poorly designed Maybe you need centering pins or you need to look at clamping Maybe you can t read analog readouts directly (parallax) Maybe components are loose or other sources of vibration are throwing things off 3) The selected parts are too close together You really want to utilize as much of the tolerance range as possible There is nothing wrong with including out of spec parts in the R&R. It s actually preferred. (Save those setup parts!!) 4) Blind trust in a purchased gage, especially a CMM or a machine When purchasing an expensive gage or a machine, always have completion of a Gage R&R as part of the terms and conditions. And it is to be done on actual parts, not masters. (Think about a micrometer and rubber bands). A CMM calibration is NOT a Gage R&R. It is a calibration. No conclusions as to an R&R can be inferred. It let s you know how repeatable the machine is inside it s measuring space. That s it. 1

2 Take Care of Manufacturing 101 Think about these before doing the R&R Study. Consider gage design from a human factors standpoint Is it easy to use? Is it easy to hold? Is it easy to read? Is it designed for consistent loading? Are the pickups correct for the application? (Right probing types, thermocouples, metering, etc) Environmental Does it need vibration isolation? Does it need Normal Temperature and Pressure (NTP)? Operator Are the intended operators going to conduct the R&R? (They d better be ) Are they trained in its operation? Pet Peeve: The term is Normal Temperature and Pressure, NTP (20 C/68 F and kpa/1 atm). Standard Temperature and Pressure, STP is the same pressure, but 0 C/32 F. 2

3 Gather the Parts Carefully Use you Capability Study Hypothetically, we have some dimension we need to Gage R&R. Odds are, we will have to do a capability study first. So we do a 300 piece sampling run (blue) and pull 6 different subgroups of 5 throughout the run (shown pink). The parts in the yellow zone are a few setup parts, before the process gets in control. You will be numbering (in order) your capability study parts. Go ahead and measure them. You will have 30+ parts. Once they are measured you can select parts that have noise for your Gage R&R. But also GRAB THOSE SETUP PARTS!!! It is totally valid to cherry pick parts for a Gage R&R. While statistics are involved, it is NOT a random study, it is a designed experiment. Also your capability study measurements can be used as you Pass 1 measurements in your R&R, minimizing the total measurements you have to do. What s the risk? If the R&R fails, your capability study is also invalid that means measuring your capability parts again. 3

4 Measure the Parts Carefully And Record the Results 4 Then it s just the standard drill. 3 different operators measure 10 parts 3 times each, however There s a phrase in statistical studies that says block what you can t randomize, and randomize what you can t block ALWAYS scramble the order of the parts. This removes effects like gage drift over time and/or operator fatigue. It s best to jump between operators on runs to mix it up. This helps get a better picture of bias. It s ALSO best if the operators do NOT have the matrix, a fresh sheet for recording each run. If operators can see their previous results, it may skew their new measurements. It s ALSO best if you are using actual operators. The operators are part of the gage system, the quality engineer is NOT.

5 A brief overview of what the results are in the ANOVA table. But we don t eyeball this and say looks good, we use statistics. An ANOVA diagram which performs an F-test SS is the total noise. Remember the cube and errors from the presentation. df: Part, Op, and Total are all one less than sample. O2P is product of Part and Op Repeatability is Total sum of everything else MS: is SS/df. It basically scales the error to the same units. Two Factor ANOVA Table with Interaction Interaction Significant? FALSE Source of Variation SS df MS F p-value F crit Signif? Part Yes Operator No Operator to Part No Repeatability Total Two Factor ANOVA Table without Interaction Source of Variation SS df MS F p-value F crit Signif? Part Yes Operator No Repeatability Once we have scaled the error, we then calculate the F statistics. These are basically the proportions of the different error components to the total error. F-crit is the hurdle from the F distribution. If F > Fcrit, that component is a significant contributor to the error. We also calculate a p- value, which is. Often misinterpreted. It is the probability that what you observed occurred due to chance alone. The upper table is an ANOVA WITH interaction. If the Operator to Part is not significant, you default to the lower table (eliminating this effect). Compare the values between the tables. Does it make sense logically? 5

6 A brief overview of the typical R&R Results. Variance Component % Cont Std Dev (SD) Study Variation (5.15*SD) % Study Variation % Tolerance Uncertainty Analysis (±) Part to Part % % 166.5% via 95% Operator to Operator % % 0.00% via NDC REPEATABILITY % % 6.38% Worst Case Total Variation % % 167% Part to Operator % % 0.00% Uncertainty effect on Toleranc REPRODUCABILITY % % 0.00% Spec Reduced Total Gage R&R % % 6.38% Upper NDC 36 <-Gage discrimination acceptable Lower % Study Variation: Gage system can tell the difference between the parts. - ACCEPT. % Tolerance: Gage system can monitor parts to required tolerance. - ACCEPT. Overall Conclusion: Gage system acceptable. Total Variation Part to Part Total GR&R Reproducability Part to Operator Repeatability GAGE R&R RESULTS Operator to Operator Linearity (R 2 ) % Cont is the percent of each variance divided by the total system variance. % Study Variation is the same, but the standard deviations are used. (Which is why they are proportionally different). Key Points: You can use repeatability and reproducability results to determine what to go after. % Study Variation answers Can I use this gage in an experiment? % Tolerance answers Can the gage effectively check to the tolerance? (PPAP) NDC is the number of buckets that you could divide the min to max into and successfully sort the parts. Uncertainty sets your guard bands if you want to guarantee there will be no false pass you reduce your check limits by your uncertainty. Linearity means the gage error is uniform (linear), not that the gage itself is linear. Should be 0.95 or better. 6