Before anyone measures a first article, each gage on a key characteristic has to prove it can tell good parts from bad. Here the hook fixture fails its first Gage R&R at 33.0 percent of the tolerance because the part rocks in the nest. A locating pin brings it to 9.0 percent. The calipers pass at 6.0 percent, and the calibration log shows both gages in date.
Week 7. Every number from here on comes from a gage. Before anyone measures a first article, each gage on a key characteristic has to prove it can tell good parts from bad ones.
Two gages matter here. Snap hook height, the special characteristic, is measured on a fixture with a dial indicator that reads to 0.001 mm. Width across the lugs, 42.00 plus or minus 0.30, is measured with digital calipers. Each study uses 10 parts, 3 appraisers and 3 trials.
The hook fixture fails first
The hook height tolerance is 0.200 mm wide. The first study gave repeatability 0.0066 mm and reproducibility 0.0088 mm, both as standard deviations. Combined, that is the square root of 0.0066 squared plus 0.0088 squared, or 0.0110 mm. Six times that is 0.066 mm, which is 33.0 percent of the tolerance.
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. Ask your customer what they require. At 33.0 percent this gage fails. Measuring first articles on it would leave a third of the tolerance as measurement noise.
The cause was physical: the part rocked in the nest, and each appraiser seated it a little differently. The team added a locating pin and ran the study again. Repeatability 0.0018, reproducibility 0.0024, combined 0.0030 mm. Six times that is 0.018 mm, or 9.0 percent of the tolerance. The fixture passes.
The calipers and the calibration log
The width tolerance is 0.600 mm wide. The calipers gave repeatability 0.0036 and reproducibility 0.0048, combined 0.0060 mm. Six times that is 0.036 mm, or 6.0 percent of the tolerance.
The calibration log shows both gages in date: the calipers were calibrated three months ago and are due in nine, and the indicator two months ago and due in four.
Read more: Gage R&R explained.
The mistake to avoid
Studying the easy gage and skipping the hard one. The calipers pass without effort. The fixture on the special characteristic is the one that needed proving, and it failed on the first try.
- List the gages on your special and key characteristics.
- Find the Gage R&R for each one. Missing or old means schedule it.
- Check the calibration log for any of them past due.
Open the printable Monday Board
Related on ShopFloorOps
The part, the tool, the program and every number in this book (latch cover LC-5927) 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.