If the gauges are rusty they are already out of tolerance, if they are just stained and you clean them with a scotch pad I doubt if you could measure the difference with standard workshop mics. A temperature rise can put sensitive measuring gear out of tolerance as can the temperature of the workpiece. I did not say that plug gauges are turned and any one who has worked in a toolroom would know that although initially they would be turned , hardened then ground. Mostly used on production machines where a large number of the same item are made.
- Thread starter Welderpaul
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Thanks for the imput everyone, as always. Measuring gear is now in his scrap bin!
Wedg1e
They call me Mr. Bodge-angles
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In ultrasonic NDT work, the techies start by calibrating their flaw detector/ thickness gauge against a stepwedge, basically a chunk of the material to be inspected, milled/ground with steps of known thickness (say 1-10mm). The stepwedges are made to a BS/EN/ISO Standard which states that each step thickness should be -0/+0.1mm.
So when we sell the stepwedges they go with a certificate listing the actual measurements I take, in lab conditions, using a micrometer calibrated against traceable slip gauges, externally verified, the whole rigmarole audited by UKAS.
A year later, the customers send the blocks back in and ask for a re-cert, because their QA procedures demand it.
With the stainless steel blocks, not a problem, however the mild steel ones rust, which means I have to clean them. Fine grade Scotchbrite is the usual remedy; it gets them clean enough without removing enough material to worry about - because, of course, my micrometers can only read down to 3 decimal places. Certainly there WILL be loss of thickness, however I can't measure it. But even if I could, going back to the Standard, it will say that blocks should be produced with a surface ground finish of <=2.5microns. As soon as that block rusts, the surface finish is worse than required by the Standard so whether I clean it or not, technically it no longer conforms to the Standard. Yet the micrometer will show that the thickness of the block is still within spec. and the ultrasonic kit is incapable of resolving any finer than the mic, so from a 'fitness for purpose' standpoint, the stepwedges are acceptable. Under the letter of the Standard, they should all go in the bin. If I told the customers that, they'd send them somewhere that wouldn't argue...
So when we sell the stepwedges they go with a certificate listing the actual measurements I take, in lab conditions, using a micrometer calibrated against traceable slip gauges, externally verified, the whole rigmarole audited by UKAS.
A year later, the customers send the blocks back in and ask for a re-cert, because their QA procedures demand it.
With the stainless steel blocks, not a problem, however the mild steel ones rust, which means I have to clean them. Fine grade Scotchbrite is the usual remedy; it gets them clean enough without removing enough material to worry about - because, of course, my micrometers can only read down to 3 decimal places. Certainly there WILL be loss of thickness, however I can't measure it. But even if I could, going back to the Standard, it will say that blocks should be produced with a surface ground finish of <=2.5microns. As soon as that block rusts, the surface finish is worse than required by the Standard so whether I clean it or not, technically it no longer conforms to the Standard. Yet the micrometer will show that the thickness of the block is still within spec. and the ultrasonic kit is incapable of resolving any finer than the mic, so from a 'fitness for purpose' standpoint, the stepwedges are acceptable. Under the letter of the Standard, they should all go in the bin. If I told the customers that, they'd send them somewhere that wouldn't argue...
Wedg1e
They call me Mr. Bodge-angles
- Messages
- 7,895
- Location
- Teesside, England
