You asked about holes too small for those telescoping gauges Russell. Yeah gauge pins is one method, they won't give you accuracy's to low 10ths though. Maybe to a 1/2 thou if the hole has a really good surface finish and is round and straight. We all know a 1.00000" pin isn't going to fit in even a 1.0001 hole. To the nearest thou? Then there's Wolfgang's small hole ball gauges that I'd very much agree require practice on already known holes sizes. These are fairly accurate when your getting close and want a fast check when boring,
https://www.starrett.com/category/11140 ... tBy=wp/asc Overly expensive for what they are, but there would be some pretty tight +- limits on the taper angle to get that .001" accuracy. Depending on what you have such as a mill with a good dro, you could use it and something like a 10ths or half 10ths dti to measure holes and part features to at least a few 10ths sort of like a rudimentary CMM. It's a slow and sort of cumbersome method to compare an easily measured out side feature or coordinate to an inside hole or slot along with basic subtraction and those coordinate readings to then come up with a number you can use. One shop made tool I think is very well thought out and pretty clever,
https://www.homemodelenginemachinist.co ... post-48470 scroll down to gbritnell's post that shows the pictures of the tool he came up with. Given both ball tip diameters, there is a limit on just how small you can go. But when making one, the dti itself doesn't even have to be an expensive one, it should have I think 10ths resolution and at least good enough that can be accurately zeroed inside the bore and then removed to be measured with an O.D. micrometer until the dti needle is again at zero and that's your size, or at least to within a 10th or two. It's probably about the cheapest method I know of to get that level of accuracy if your mikes are good. But it obviously can't easily measure out of round or bore taper on deeper holes like proper bore gauges can.
There's telescoping gauges that work just like they should right out of the box and some that can't. They look simple and are, but if the manufacturer cut any corners, accurate hole gauging with those is pretty much impossible. First the telescoping parts need to be very straight so the parts can move with minimal friction each and every time. Then each gauge needs to be disassembled, de-greased and even more important, de-burred with a small stone. Most telescoping gauge designs I know of use some type of machined slot a pin rides in to prevent the gauge from coming apart unless the tightening handle is unscrewed enough. It's generally those slots that need de-burring. That takes quite a bit of time to do properly and why it's almost always ignored on the cheaper off shore sets. So if your using something like that and can't get good accuracy, it might not be your technique, it just might be the tool itself needs a bit of TLC. And these steps aren't optional if you can hear or feel the slightest amount of variable friction or grit as the gauge is collapsed with your fingers. There's no such thing as over oiling machine tools, but there sure is with metrology equipment.I've tried very light coats of dry lube, but still think powder graphite might work as well or better on tools like this. Each tip on each gauge also needs to be inspected for the slightest signs of dings, burrs and even worse any flat areas where the radius isn't constant out to the very center of each gauge tip. The exact radius isn't that important, only that there is one and that it's less than the smallest hole radius that gauge can be used on. I've checked a few off shore sets at tool dealers around here and some showed definite flat spots on most of the tips because the radius wasn't fully formed. Those are useless and impossible to get an accurate measurement without further work to do what the manufacturer should have.
A real machinist probably wouldn't need it, and I've mentioned this before. But I found juggling a mike in one hand, slowly rotating it's thimble with a finger while sweeping the telescoping gauge tip past the mike anvil until I get a size reading to be a bit inconsistent for getting decent and repeatable accuracy. Instead I now use what's known as a micrometer stand to hold the micrometer. With one it's much easier to put one leg of the telescoping gauge against the mike's fixed anvil and gently sweep the other tip back and forth horizontally past the other advancing anvil until I barely get a light drag as the gauge tip sweeps past that mike anvil. Then I also rock that gauge tip back and forth vertically and the light drag feel should be the same in both directions. My repeatability and confidence in the measurements I was getting went up a lot as soon as I started doing it that way. Looking like a real machinist is a lot less important than getting known accurate measurements you can depend on imo.
Mitutoyo, Starrett and a few others make those mike stands, the Mit I think is better designed and should be more stable with larger micrometers, but both are over priced for what they are. Making something to do the same wouldn't be hard. I have seen an almost exact off shore copy of the Mit that was reasonable. But something like a PanaVise 301 with it's plastic jaw liners would do everything my Mit can, and a lot more that it can't.
