FWIW,
I've used and own everything from dirt cheap 4" and 6" digital calipers that were given to me as a "gift" for renewing a couple of subscriptions, to Mitutoyo dial and solar powered digital calipers. Due to the built in slop, grinding powder and whatever else is binding up those free calipers, I wouldn't even trust them for woodworking levels of accuracy. All calipers and for sure digital's certainly aren't equal in my experience. No doubt there are other digital calipers just as good as Mitutoyo's or possibly even a bit better that are or would be more common in other parts of the world. But on forums where most of the members are American or at least North American and professional machinists like Practical Machinist, Mitutoyo digital calipers are generally the caliper of choice, and are judged about as good as you can expect. But you also need very consistent technique to get decent accuracy from any set of calipers no matter who makes them. There are some lesser known European brands that are supposed to be very good, but I have zero hands on experience with them.
That thumbwheel on most calipers isn't what you use while closing them on a part, it's designed to be used for larger movements, your supposed to either close the jaws between your fingers or both hands with larger measurements. And you use that consistent feel or method every single time. While most good calipers are only rated at generally + - a couple of thou. High quality calipers should at least make repeatable measurements to quite low tollerance levels. The actual measurement reading may be off that +- .002, but those readings should at least be repeatable to well under .0005 if the user does his part. I can say that while most well respected manufacturer's state there calipers are only accurate to that + - .002 level, mine are well under that with any gauge block I've tested them with so far. That +- .002 number is a maximum allowable tollerance and certainly not the average. Mine so far will do under .001. But all of this is only true for almost brand new equipment. And I still really only trust calipers to that + - .002 level.
But if you start to look around, you'll also see that many calipers and especially the digital ones today don't have those inside jaws for measuring bores. That's because the design of those calipers that have those inside jaws just isn't very accurate, and the smaller the bore the less accurate they are. There's usually a narrow flat ground on each jaw. So your not using point contact to measure bores. And on a lot of calipers, the distance between those inside jaws faces doesn't agree with the distance between the outside jaw faces, so even that measurement isn't all that accurate. But those inside jaws can still be used for something like measuring slots, you do need to cheat and just set the calipers to the slot size, lock that setting and then measure over the jaws with a good micrometer. But your then using the calipers much like a gauge and using other methods to obtain an accurate measurement.
Obviously all this equipment wasn't even available at one time, yet fairly high precision parts were still done using probably not much more than yardsticks or shop built gauges, and the old school inside and outside slip joint calipers that can't give you an actual measurement of anything. Or they used feel and comparing one part against another one. For example the piston fit to the bore. It was probably and mostly due to developing interchangeable threads and parts fit that drove the requirements for accurate measuring equipment. And while I wouldn't want to, I'd bet most engines shown on this forum could be built using nothing more than those slip joint inside and outside calipers and a good 6" or 12" scale along with a very experienced touch. The part sizes wouldn't be exact to micrometer levels, but the engine should still operate just as well.
Your thoughts may differ, but in my shop and for how I work, calipers are by a large margin the most used tool anytime I'm in my shop. For that reason I bought a cheaper but still fairly accurate set of off shore $40.00 calipers for the rough work, and a set of those 6" Mitutoyo solar powered digital's. I can say those Mit. calipers are the very best I've ever used so far. I also own a 6"and 12" set of Mitutoyo dial calipers, but I still very much prefer the digital's for there very fine feel while measuring. But they cost enough that I make a real effort to keep them away from any dirt or swarf, and I make sure they don't get placed anywhere they could be damaged or dropped. But that's just good shop practice, and any good equipment should be treated the same. But even with parts that require measurement's to micrometer accuracy, I'm still using calipers to get me close before I do start using the mikes. So if the tool is going to be used that much and depended on to give accurate readings? Then I think you have no choice but to buy a good set.
Due to that interchangeable parts requirement in industry, they are required in some company's to test and recalibrate there metrology equipment to very very tight levels, and sometimes to do so multiple times per year. At the hobby level and for our own parts were obviously under no need to do the same. But just to reduce the mistakes and the need to remember any fudge factors, it's nice if all our equipment at least agrees with each other for measurement. The actual measurements certainly don't need to meet NIST levels. But I think having them adjusted to read the same, or at least knowing for sure there are inaccuracy's is very important. So buying a set of cheaper gauge blocks as someone already mentioned is a very good idea. And the more you use gauge blocks, the more uses you find for them. I certainly wouldn't want to be without mine. You just use them as your reference standard, and everything gets calibrated to those standards. YMMV, but this sure seems to work very well for me.
Most builds here probably don't need even caliper accuracy levels for a lot of the parts. Yet there are some items that do need everything you can afford for tooling. Checking straightness, lobe lift and position on something like an I.C. cam does need a bit more than the very cheapest tool you can find. Buying measuring equipment at the cheapest possible price can end up being very costly at times. I've got a 1924 edition of a book about building proper master gauges used in industry. Those master gauges were to be used to test the everyday gauges. They talk about setting those master gauges on a lathes faceplate for drilling boring and precision grinding to 5 decimal places back in 1924 using a shop built Dial Test Indicator and shop built hardened and ground machinist buttons. And while I've got the equipment to measure just how far I'd be out from replicating those 1924 efforts, I really doubt my machine tools or my skills could do as well as they managed almost 90 years ago.
My apology's, it seems I went a bit OT to your questions about calipers. :hDe:
Pete
