A lot of things have plus tolerances, holes for bearings and couplings for example
Yes, that's what I meant by QA/metrology vs. actual machining/boring.
If I was a QA inspector, I'd absolutely want expensive, tight tolerance, regularly lab-calibrated plus and minus pins in a temperature controlled lab. For that application, even using two pins to measure the diameter of a hole would be disallowed because of tolerance stacking.
For all of my (and I strongly suspect,
your) hobby machining (vs QA) needs, though, a set of minus pins suffices.
I suggested that if the OP wanted to simply bore holes to accurate dimensions, then a set of minus pins to from 0.061 to 0.0500" would serve him well. I stand by that recommendation, but I'm just some rando on the internet. Use your best judgement.
You guys go ahead and buy the most expensive, highest quality plus and minus pins you can find, measure your holes with three pins and CAD or back-of-the-envelope math, and aim for H7 fits and micron accuracy if you can. Knock yourself out. It's a hobby and sounds like fun. Personally, I have more fun aiming for (and achieving within a few tenths) the closest 0.001". I'm demonstrably able to make parts that work well with good precise feel, which is good enough for me.
Respectfully though, accurately
measuring a bore to an absolute tenths values requires better equipment and environment than I possess, and really doesn't seem necessary in a hobby shop. I wouldn't find that process enjoyable even if I were capable. I am able to measure the diameter of a
shaft to tenths with a good micrometer, but even that is hard to repeat (without temperature control and really clean parts -- an oil film has a dimension, for example). Measuring a bore is significantly more difficult, even with expensive bore gauges and the like.
I'm absolutely capable of machining two
particular mating parts such that they fit
each other with less than 0.0005" radial clearance, but it's a touch/feel thing. My point is that when actually
making parts, you gradually increase the diameter, it never decreases. Making parts that fit is very different than accurately measuring/verifying specific numeric dimensions (the actual numeric dimension is less important). In fact, the actual real-world dimension of a given pin doesn't even matter to me as long as it mics the same (with
the same mic) as the shaft that is intended to go into the hole.
Three pin tricks and the like are fun to think about to get arbitrary tenth accuracy, but tolerance stacking makes me suspicious that the results are truly as accurate as the math might lead one to believe. I've found that even when I do try to manually machine to tenths accuracy, I'm usually fooling myself (and trying to get repeatable results is invariably an exercise in frustration).
The main time I'm particularly persnicketty with bore diameters is when the bore is intended to hold the external race of a bearing, and even then the danger is absolutely overshooting not undershooting. I get about 0.0002" runout from the bearings in the headstock of my lathe, yet I'm still somehow able to make do with my modest minus pins. If I was making a bore to hold a 10mm H7 bearing (up to +18um tolerance) for example, I'd bore until a 0.394" minus pin
just fit into the hole and consider it close enough.
