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When I started working on cars, having a stainless steel ruler with sixteenths on it was my idea of tight tolerance. When I needed real precision, I would pay extra attention to which side of the tick mark I was leaning towards. Once I got into college, I started working in a machine shop and suddenly fractions were like math cursive. My compulsion to nail smaller increments intensified when I started doing research for NAS—yes, that NASA—and my life existed way to the right of the decimal point.
When you're building a chassis brace or mounting bracket for a non-critical part, getting your measurements accurate in the hundredths is probably fine, so (0.0X). If you're measuring a bearing surface, you want a micrometer that's accurate into the ten-thousandths (0.000X). There are many jobs in between and something like dial or digital calipers are a good first step into precision measuring. Obviously, operating in the thousandths of an inch requires specialized tools, and a tool's precision is often directly proportional to its price. But deals still exists in this rarified space.
Digital Calipers For The Beginner
I know, Harbor Freight is not the first name you think of when it comes to precision tools. And yet: I have used multiple digital and dial calipers across its product range, and they're as accurate as anything that isn't three or four times the price. These calipers may not be as rugged as some others, and they do eat batteries like candy, but they will last you through the first few years of working with them. I also found that even when I had access to much more expensive calipers, I would grab these if I was leaving the machine shop area or if there was any chance that they could be dropped, smashed, borrowed, misused, etc. Just keep extra batteries on hand, maybe even taped to the case.
All The Digital Caliper You Will Need
The Mitutoyo calipers you see here are found in professional machine shops all over the world. If you need something more precise, you might want to look at something more specialized than a caliper. At the shops I've worked in, they were normally hiding in cabinets and were only to be used by approved hands. It's more of a thing you earned rather than because they're fragile or hard to use. The Mitutoyo's AOS tech uses inductive sensors instead of a gear-drive to measure movement. The sealed system is far more resistant to particle interference malfunctions—they won't get gunked up. It also means you don't have to zero them out every time you turn them on. The jaws are hardened stainless steel, but carbide is available if needed. This model doesn't offer connectivity for downloading measurements to your computer, but if you aren't working in a professional shop, it won't be necessary. Battery life on these is usually around three years, even with daily use.
A Micrometer For Splitting Hairs
You might be wondering why you would need a micrometer if you already have a decent set of digital calipers: resolution. These will normally get you an extra decimal point of precision for starters, but they also are capable of measuring in a more precise location. Calipers have large jaws on them and so can bridge gaps, yes it depends on orientation, but quite often you don't have the luxury of choosing exactly where the probing surface lands on the part. Some mics have flat faced probes, others have hardened points that can get down into grooves to get a true minimum measurement. The downside of micrometers is that to get the extra precision, you give up range. While calipers can go anywhere from a couple of inches to over a foot long, Micrometers are often found in 1-inch (25mm) increments. So, if you are buying these for brake rotors, be sure and check the specs on your car first as front rotors hovering around 1-inch thick are becoming common on vehicles.
Digital vs Dial
I learned how to use measuring tools with analog equipment—the mill and lathe I started on were also fully manual. We didn't even have DROs, and we were happy. Get off my lawn! So you are wondering why I'm not demanding people start the way I did. One, there isn't much of a price advantage to going with dials. More importantly, digital tools can go back and forth from SAE to metric with a single button push.