Worn Go/No-Go Plug Gauges, a 40 H7/g6 Fit and a Supplier Dispute
A workshop assembles 200 bronze bushes onto steel shafts. The drawing specifies bush bore 40 H7 (+0.025/0.000) and shaft 40 g6 (−0.009/−0.025). Inspection uses Go/No-Go plug gauges last calibrated 14 months ago. A check this week shows the hole Go plug measures 39.996 mm and the hole No-Go plug measures 40.021 mm. During the run, 38 bushes were rejected because the No-Go plug entered them. The bush supplier disputes the rejections and asks for joint re-inspection. The inspector argues that because every accepted bush passed the Go plug, no undersized bore can have escaped into assembly. The production engineer is not convinced and wants a written disposition covering both what may have escaped and what may have been wrongly rejected.
1. Calculate the hole and shaft limits and the maximum and minimum clearance, and name the class of fit.
Read the worked answer
Hole 40 H7: lower limit = 40.000 mm, upper limit = 40.000 + 0.025 = 40.025 mm. Shaft 40 g6: upper limit = 40.000 − 0.009 = 39.991 mm, lower limit = 40.000 − 0.025 = 39.975 mm.
- Maximum clearance = 40.025 − 39.975 = 0.050 mm
- Minimum clearance = 40.000 − 39.991 = 0.009 mm
Since even the tightest pairing gives a positive gap, the fit is a clearance fit on a hole-basis system - a sliding fit. That gives the acceptance rules: a bore below 40.000 mm is undersize and must be rejected, and a bore above 40.025 mm is oversize and must be rejected. Any gauge that misapplies these two limits produces escapes or false rejections, and the 0.050 mm total clearance band is wide, so a 0.004 mm gauge error consumes about 8% of the entire band - enough to matter.
2. Using the measured gauge sizes, explain exactly what each worn gauge does to the accept/reject decision.
Read the worked answer
A Go plug must pass through a bore of 40.000 mm or more. A plug worn to 39.996 mm will pass through bores as small as 39.996 mm, which are 0.004 mm below the lower limit. So the 39.996 mm Go plug accepts undersize bores - the defect escapes into assembly, which is precisely what the inspector's argument overlooks. Passing the Go plug only proves the bore is larger than the gauge, not larger than the drawing limit.
A No-Go plug must not enter a bore of 40.025 mm or less; it should enter only oversize bores. A No-Go worn down to 40.021 mm will enter every bore between 40.021 and 40.025 mm - bores that are perfectly good. So the 40.021 mm No-Go plug rejects good bushes, which explains the 38 rejections and the supplier's complaint.
Note the asymmetry: wear on the Go plug causes escape of bad parts, wear on the No-Go plug causes rejection of good parts. Both stem from the same cause - no calibration for 14 months.
3. Write the disposition: what should be re-inspected, how, and what should be done about the gauges and the drawing limits?
Read the worked answer
The 38 rejects: re-measure with a calibrated plug or a bore comparator/inside micrometer at two depths and two diameters. Given the No-Go was 0.004 mm small, most will probably measure between 40.021 and 40.025 mm and are conforming. Release them on re-inspection; do not scrap on the old gauge's verdict.
The accepted bushes: passing the worn Go plug proves nothing about the lower limit, so run a 100% check of the accepted lot for bores below 40.000 mm, using a comparator zeroed on a setting ring rather than a plug. The number is likely small, but the lot cannot be certified without it.
Assembly risk: worst case, a 39.996 mm bore with a maximum shaft of 39.991 mm still gives 0.005 mm clearance, so parts will assemble but sit below the specified 0.009 mm minimum - out of drawing, not necessarily a jam. Any deviation must be signed off by design, not by the inspector.
System fix: withdraw and recalibrate both gauges, apply a gauge-tolerance and wear allowance so Go plugs are made on the safe side, and add a shift-wise gauge wear check with a setting ring. Replace gauges at the wear limit rather than at failure.