Why does my 'clearance' fit end up tight every time?

My money's on the bore being the part you're not really measuring. A shaft is easy, clamp a mic on it anywhere and you know the truth. A hole is not. If you're going off caliper jaws or a mic across the mouth of the bore, you're only sampling two points, and a lathe-bored hole in mild steel is rarely round. It can be lobed or slightly oval, so it reads the same at the mouth every time but pinches down somewhere in the middle. That matches your symptom exactly, slides fine for half an inch then grabs where the bore gets tight. Try a telescoping gauge snugged in the hole and measured with the mic after, or better yet turn a short test plug a couple thou under your target and see if it passes through. If the plug stops partway, there's your answer. Two other things worth checking while you're at it. First, tool pressure: when you're boring, the bar pushes away from the cut, and the deepest part of the hole comes out smallest because the tool relaxes as it exits. So the mouth can be oversize and the back end undersize, which again gives you the slide-then-lock behavior. Lighter cuts at the end, or bore the last pass all the way through in one go without moving anything, helps a lot. Second, and this one bites people constantly: test each bushing by itself over the full length of the axle. If a single bushing slides the whole way no problem, your fit is fine and the real issue is the two bushings not being lined up with each other. Any misalignment between the mounting holes and the plates they sit in will lock up a close sliding fit partway on, no matter how good your diameters are.

And don't run it dry. Mild steel on mild steel with zero clearance to spare will gall and seize almost instantly. A smear of oil on a slow-spinning light load fit is not cheating, it's how those fits are meant to run.

Been down this road more times than I care to count. One trick the old hands taught me before dial bores were common: smear a little machinist's blue on the shaft, slide it in as far as it goes, pull it out and see where the blue rubbed off. Shows you exactly where it's pinching, no guessing. Another thing, if that finish pass was taken with a tired tool, you can leave a glazed, slightly work-hardened skin that grabs even when the size reads right. Take a spring pass with a sharp tool and see if it changes. I've also rescued fits by wrapping fine emery around a dowel and kissing the bore by hand, just a touch, checking between passes.

I guess I never thought about how the tool itself might be doing something odd. When you run the tool across the metal, does the shape of the cutting part change based on how fast I go? Like, if I feed it too fast, does it make the inside of the hole rougher or does it just bite harder? The idea of the bore being slightly round but still tight is confusing because it sounds like such a small thing. Do the type of bits I use matter a lot more than the measurement accuracy? I just want this thing to spin without snagging.

Reading this is making me nervous about a project I've got coming up, making new bushings for a wheelbarrow wheel. I've only ever drilled holes, never bored one, so the idea that a hole can read right at the mouth and still be tight further in is news to me. Question for the more experienced folks here: would running a ream through after the final boring pass sort out the shape problem, or is that overkill for somethig that just spins slowly? I don't own a reamer and I'd hate to buy one for a one-off, but if it saves me a week of head scratching maybe it's cheap.

I stopped fighting this years ago on stuff like cart axles. Instead of turning the shaft to a number, I bore the bushings first, however they come out, then measure the actual hole with a small ball gauge and mic and turn the shaft to match that. Doesn't matter if the bore is a couple thou off nominal, the pair still fits. Did that for my trailer jack bushings and everything slid togather first try. One thing I'd add for the cart: assemble it with the axle in and spin the wheels before you weld or bolt anything final, becasue a rack that's even slightly twisted will pinch a close fit worse than any bore ever will.