Great read on calipers:
http://www.longislandindicator.com/p11.html
Caliper accuracy is only .001" at best, if the gage is not worn or bent. Measuring a 6" part with a 6" caliper starts to get a bit awkward.
I work mostly in the 1-8" range when making parts, so my 6" dial and 8" digital covers it all. Etalon makes the best dial caliper, IMHO, which are 6"
and have smaller tick marks for .005" range measurements. Very accurate. Buying used can be a gamble, especially if parts are no longer available for your choice.
From LIIS: "The
maximum error on these B&S dial calipers is less than or equal to .001" for the first four inches. Above four inches, the maximum error is about .0012" If you need better accuracy, you should be using a micrometer instead of a dial caliper".
All calipers have a max. error. Calipers just aren't made for super accuracy.
Also from LIIS:
Dial Caliper Calibration
These instructions apply to mechanical as well as digital calipers.
Both the inside jaws and the outside jaws need to be calibrated, as well as the depth rod and the step measurement, if these are used. Calipers should be frequently checked for accuracy. They are more susceptible to damage than other tools.
To check for wear in the jaws, do this: clean them and close them. Then hold them up to the light and if they're worn you'll see light shining through the gaps. You can continue to use the calipers if you measure with the unworn surfaces. For total reliability, however, you'll have to send the calipers for servicing. The surfaces can be ground flat again.
For the outside jaws it's a simple matter of inserting a series of gage blocks between them and recording the caliper readings. They must not deviate by more than one graduation (.001") over the first 4" of range. From 4" to 8" the error may be .0015" (one and one-half graduation). From 8" to 12" the error can be .002". Accuracy may vary among different models and the manufacturer's specs should be consulted for this information. Take readings at 1-inch intervals. Three gage blocks (see below) of 1", 2" and 3" sizes will be all you need.
To calibrate the inside jaws you may use a set of ring gages. Do not rely on very small ring gages because the inside jaws can not accurately measure small inside diameters. Suitable ring gages are shown on
page 163. If you invest in just one ring, make it the 2" size. You may also set a calibrated .0001" micrometer to 1" (and higher, if possible) and then use the inside jaws to measure this distance. Since the micrometer has a discrimination ten times that of the calipers, you'll get an accurate reading.
Repeatability means the dial hand returns to the same position on different attempts to measure the same gage block. If this fails, then you'll have to have the calipers serviced.