Wedg1e
They call me Mr. Bodge-angles
- Messages
- 7,739
- Location
- Teesside, England
Right...! Using calipers to measure bores, hmmm. The so-called 'knife edges' on the internal jaws are not actually knife edges, nor do they taper to a point - if they did they wouldn't last seconds. They have a very narrow flat on them. This means that when you open out to contact the bore of the part, you're not actually using the 'widest' part of the jaws to take the measurement. Consequently everything you measure is undersize... granted, by some trifling amount, but it's there nonetheless.
As part of our metrology accreditation under UKAS I have to show that I've taken the width of those flat faces into account, and relate that to the actual bore, because of course as the bore gets smaller the effect gets worse (below about 20mm I'd be using gauge pins). We're talking microns here, but when you throw in a few more microns here and there such as the basic accuracy of the instrument, temperature coefficient of the part AND the caliper, operator technique, repeatability and so on you end up with an 'uncertainty budget' which you have to take into account. Take the reading three times, calculate the mean, add or subtract the 'expanded uncertainty' and there you go... makes counting paperclips sound exciting, doesn't it
As part of our metrology accreditation under UKAS I have to show that I've taken the width of those flat faces into account, and relate that to the actual bore, because of course as the bore gets smaller the effect gets worse (below about 20mm I'd be using gauge pins). We're talking microns here, but when you throw in a few more microns here and there such as the basic accuracy of the instrument, temperature coefficient of the part AND the caliper, operator technique, repeatability and so on you end up with an 'uncertainty budget' which you have to take into account. Take the reading three times, calculate the mean, add or subtract the 'expanded uncertainty' and there you go... makes counting paperclips sound exciting, doesn't it