Cheap calipers can be adjusted to be more accurate. A few years back, HF was selling their 6" digital calipers for $10 and I bought two pair. They were atrocious. A large part of their inaccurate reads stemmed from the fact that the gib on the scale was loose.
This was due to the poor grind on the beam. The beam was not uniform in width and the gib was adjusted to allow the movable jaw/head to pass over the high spot. The gib was also adjusted loosely to account for additional friction due to any rough spots.
Calipers are essentially a second class lever. The object being measured becomes the fulcrum and force is applied by virtue of the thumb wheel. This action causes the head to pivot around the fulcrum to the extent that the free play in the gib allows. This causes a displacement of the head on the scale proportional to the applied force and dependent on the amount of free play in the gib. The scale resides in the center if the beam so any tilting of the head on the beam results in a change in reading.
the first thing I did was to adjust the gib via the two screws in the top of the head two reduce free play. then I measure the beam thickness at various points along the beam, paying particular attention to tight spots and marking them with a Sharpie. I used a fine stone to smooth the ways , finishing with a final polish with .5 micron diamond paste. Then I readjusted the gib and began work on the upper surface of the beam with the fine stone to reduce the tight spots. I monitored progress with my micrometer and by occasional testing play by sliding the head over the tight areas, As I got closer, I re tightened the gib on the looser areas and proceeded with the stoning and polishing for a uniform drag along the entire beam. When satisfied with the uniformity, I again polished both upper and lower edges with the .5 micron diamond paste and cleaned the calipers thoroughly. A light wipe with light machine oil completed the process. The result is a caliper that operates smoothly and agrees to within .0005 with every comparison with my micrometers or gage pins.
It goes without saying that the jaws on the caliper should be parallel. With the jaws closed, holding them up to diffuse light, no gap should be seen. It is possible to detect a gap as small as .0005" which is the resolution of the caliper. If the jaws are not parallel, a small gap will be observed at some point on the jaw. It is possible to correct this condition but the work is more painstaking to avoid adding any curvature to the jaw.
Even with a tight gib, there will be some displacement of the head with force applied with the thumb wheel. A certain amount of finesse is required to obtain accurate and consistent readings. I try to place the object being measured as close to the beam as possible to reduce the amount of deflection. Another trick I use for small objects is to apple the closing force to the jaws by pinching the jaws directly opposite the object which virtually eliminates any displacement that the thumb wheel causes.
