And the JAWS and the PART or STANDARD must be dead clean, even for the fourth decimal digit (tenths). Even a the oil from a fingerprint will add a tenth.
AND when you use calipers, a lot of difference can be seen when the exact method of closing them is changed. The movable jaw on a caliper rides on a rectangular slide (the beam of the caliper). There WILL be some slop in that fit and the movable jaw can rock by some small angle on the beam. This can and WILL change the reading. Cheap calipers can easily show one or two thousandths of differences while the more expensive ones can be a lot tighter but they will still have this source of error. Most calipers have two adjustment screws that are used to minimize this error and they should be properly adjusted. Most calipers also have a spring between those two screws and the beam: this is to hold the movable jaw against the other side of the beam for maximum perpendicularity.
Many people use the thumb of their right hand, which is holding the beam of the caliper, to tighten the jaws. This applies force at a distance from the object being measured (which is further out in the jaws) and COMPLETELY INSURES that the movable jaw WILL be at an angle. This angle can easily produce the 0.001" error that you are seeing. I like to close the caliper jaws with my left hand by applying force to the two jaws, in line with the object being measured. This allows the spring in the movable jaw to hold it square to the beam and I get more consistent readings this way. This technique works better when the object has two parallel faces, but it can also help with other shapes, like a ball. A micrometer is not subject to this kind of error because the object being measured is usually on the axis of the screw.
On top of that, calipers have both inside and outside jaws. They are ground in two operations and should match at the zero point. BUT, again there will be errors. I have seen as much as a 0.002" difference on name brand, GERMAN calipers. If you need better than that on an internal measurement, then you need to find a calibration factor for your calipers or abandon the calipers and use another technique.
Oh, one more consideration with the internal jaws on a caliper. While the external jaws are aligned with the direction of the caliper's slide, most internal jaws are NOT. Since they are designed to travel to the zero position they must be offset so that they can slide past each other. So, when doing an internal measurement and assuming that you properly wiggle the caliper for the maximum/minimum reading, you are actually doing so at an angle. And that angle will change depending on the size of that measurement: small distances will produce a larger angle and more error while larger distances will decrease the angle and minimize that error. Some calipers have internal jaws that are not designed to pass each other but they will "zero" at a larger reading, like 0.4" and they may have two scales, one for internal and the other for external readings. These are technically superior and the accuracy of the grinding can probably be better maintained in production. But you do not have the ability to measure internal distances under that hard lower limit (the 0.4 inches).
The point is, when talking about hundred thousandths, ten thousandths, and even thousandths of an inch, you can not just grab a caliper or micrometer and slap it on something; oil, grease, dirt and all. You have to THINK about what is taking place and use them wisely. I suspect that the above is much of the reason why calipers, which should be able to measure to thousandths time after time, are often not trusted to better than 0.003" or worse. TECHNIQUE is everything when precision is desired.
I suspect that the adjustment screws on your calipers are a bit loose, allowing a bit of rocking of the movable jaw. I also suspect that you apply pressure with your thumb, ensuring that it is at an angle. I suspect that if you properly adjust those screws and apply the closing pressure properly, that 0.001" error will vanish.
No apologies for the length of this post.
First, I don't see any vernier calipers in your pics. Second, if you are trying to compare a micrometer reading to a caliper then I don't know what to tell you. Third, your mic isn't useful anyways past the 4th digit unless you are inside a temperature controlled room and your hands don't contact the part or the body of the mic.
