Before anyone adjusts a drifting machine, prove the gage. Check it is in calibration, check it reads right on a known size, and check its Gage R&R against the tolerance. Here the gage uses 7.5 percent of the tolerance, so the drift is real, and only then are the suspect parts sorted with it.
With the lathe stopped, the first instinct is to touch it. The shift lead reaches for the wear offset and wants to bring the diameter back by 0.008. The operator asks a better question: is the micrometer telling the truth?
If the gage reads high, you will adjust a good machine to match a wrong number. Before anyone touches the lathe, the gage has to prove it can tell good from bad.
Three checks, in this order
- Is it in calibration? The log shows the digital micrometer was calibrated six weeks ago and is not due for four and a half months.
- Does it read right on a known size? A 24.000 gage block reads 24.001. The 12:00 check piece, measured again on the lab bench gage, reads 24.011 against 24.010 on the floor.
- Can it tell parts apart? A Gage R&R study run last quarter used 10 shafts, 3 operators and 3 trials each.
The study results: repeatability 0.00040 mm, reproducibility 0.00030 mm, both as standard deviations. Combined, the gage variation is the square root of 0.00040 squared plus 0.00030 squared, which is 0.00050 mm.
Six times that is 0.0030 mm. The tolerance is 0.040 mm wide, so the gage uses 7.5 percent of it. A common rule of thumb says under 10 percent is good, 10 to 30 percent is a judgment call, and over 30 percent needs work. Check what your customer requires.
The gage is fit for this tolerance. The drift is real.
Now sort
With a trusted gage, the 114 pieces were measured one by one. The largest reads 24.017 and the smallest 23.998, so all 114 conform. 31 of them read 24.010 or higher, and 7 read 24.015 or higher. They are good parts that were close to the line. The tags came off after the log was updated and the sort record was attached.
Read more: Gage R&R explained. For any term you do not know, see the quality glossary.
The mistake to avoid
Trusting a gage because it has a calibration sticker. Calibration says the gage matches a standard. It does not say your operators get the same answer from it on your parts.
- Find the last Gage R&R for your most important gage.
- If the gage, the part or the operators have changed since, schedule a new one.
- Check the calibration log for any gage used this week that is past due.
Open the printable Monday Board
Related on ShopFloorOps
The part, the machine, the shift and every number in this book (shaft SH-3306 on Lathe 3) are a made-up example. The tool sidebars show ShopFloorOps tools, which I make and sell. Every chapter teaches the step itself, so you can use your own forms. New to a term? See the quality glossary.